Saturday, October 31, 2009

May 8, 2009 review: devastating derecho from S Plains to OH/TN valleys

A few weeks ago I put together a simplistic meteorological summary page on the 05/08/09 derecho event, with quite a bit of observational data. This page can be viewed HERE.

Here's two teaser images from the page: a 12Z subjective surface analysis, and the 00Z-12Z RAOB evolution at Springfield, MO.

Wednesday, May 20, 2009

May 13 review: Killer tornadoes strike northern MO

A powerful supercell produced a few damaging tornadoes across portions of far northcentral/northeastern MO on May 13. The first of these, rated EF-1, killed a woman in a mobile home near the town of Milan MO. The second, also rated EF-1, passed near the town of Novinger MO but caused no injuries/fatalities. The third tornado produced EF-1 damage in northern sections of Kirksville MO, also killing two more people to the northwest of town. This tornado strengthened to an EF-2 as it moved toward the town of Clay MO. The storm went on to produce two more weak tornadoes north of Edina MO.

A pair of elevated TS clusters were ongoing over portions of the Midwest on Wednesday morning, to the north of the surface warm front. The lead cluster over much of IL was driven by sharp moisture advection/convergence in the exit region of a 60-70 kt westsouthwesterly LLJ, while the trailing, more intense cluster was crossing the northern third of MO in advance of a potent low-amplitude 7H shortwave trough. An MCV, remnants of severe convective storms across the southern High Plains the prior evening, was also crossing the Ozark mountains at this time… and began to ignite surface-based storms after 15Z across far southern MO.

In the wake of these convective systems, a pronounced EML was overspreading the central Plains amidst weak shortwave ridging…well downstream and equatorward of the primary, strong shortwave rotating into the northern Plains. In fact, the central Plains capping thermal ridge underwent amplification during the late morning and early afternoon, as evidenced by 7H temps increasing from 11°C to 13°C at TOP between 12Z and 18Z (see RAOB overlay below, with extreme mid-level lapse rates by 18Z). Also note, despite the presence of the stout cap aloft, diurnal mixing still took a pronounced toll on prefrontal boundary layer moisture...with the richest moist layer becoming decidedly shallow by 18Z. Observed 100 mb ML dewpoints on the 18Z TOP and OUN RAOBs only ranged from 60-64°F… not a particularly positive sign. Still, it appears that persistent south-southwesterly low-level flow ahead of the cold front, coupled with evapotraspirative effects, helped boundary layer moisture quality to recover again through the afternoon (ML dewpoints increasing back to the 64-68°F range) per 00Z RAOB data.

Short-term guidance and conceptual models suggested SRH would be marginal for a significant tornado threat over the majority of the area east of the cold front, given 1) veering of near-surface winds to SSW amidst deep vertical mixing, and 2) the primary LLJ axis shifting away from the area. The obvious exception to this would be in the raincooled, gradually recovering BL airmass across northcentral and northeastern MO… where guidance suggested 0-1 km SRH AOA 250 m2/s2 would be possible given not only weaker BL mixing (more backed near-surface component) but also the maintenance of a somewhat stronger LLJ (~40 kt) nearer the primary low-level cyclone riding the U.S.-Canada border. By mid-afternoon (see crude 21Z sfc analysis below), surface observations showed the warm front rapidly redeveloping northward toward the I-80 corridor in IA/northern IL; while to its south, modifying outflow air was evident via a dense VFR stratocu deck and south-southeast sfc winds amidst low 70s°F over upper 60s°F. Given sfc winds just to the north (e.g. far southeastern IA) were slighly more veered, it is possible that an outflow boundary remained anchored along and just south of the Kirksville-Quincy-Winchester corridor. Regardless, observational trends clearly supported better-than-progged SRH/LCLs for a sig tor threat over this modifying airmass. The primary question was the strength of the low-level destabilization, given that BL heating would remain modest and a formidable 7H thermal ridge was still cresting not far west of the area via the 18Z TOP RAOB.

Shortly after 21Z, a combination of frontal lift/convergence, weak height falls aloft, and rapid cooling of the capping layer from the west all resulted in explosive convective initiation along the cold front from the KS Flint Hills up toward Des Moines IA (see visible satellite pic below). More importantly, a prefrontal confluence band—coincident with a narrow low-level thermal ridge with sfc temperatures warming into the low 80s°F—initiated a broken line of storms just east of the I-35 corridor, from Kansas City to Princeton MO. The fact that this latter band of storms was able to initiate ahead of the primary surface boundary was probable confirmation of what afternoon RUC model soundings had suggested: that the very warm capping thermal ridge had held primarily west and south of the MO river; while a very deep/moist boundary layer, with essentially no capping inversion aloft, had evolved amidst the recovering airmass over far northcentral and northeastern MO.

Given westnorthwest-oriented deep layer shear vectors atop the cold front and prefrontal trough, discrete cellular mode actually dominated for the first few hours after initiation. Additionally, stronger flow fields aided the northern storms in moving eastward off the boundaries, while markedly rightward-deviant motions amidst very strong CAPE helped southern storms move off the cold front across portions of KS and eventually northern OK. With a consolidating/strengthening supercell emerging across far northcentral MO and crossing the effective outflow boundary near Milan MO… the stage was set for a cyclic tornadic supercell.

A representative thermodynamic environment for this tornadic supercell was attained by modifying the 00Z DVN RAOB alongside the 00Z TOP RAOB (Kirksville sits about halfway between each). The surface warm front lifted through DVN (~120 miles NE of Kirksville) just prior to the 00Z RAOB launch, which means its sounding--characterized by 2100 J/kg MLCAPE and no MLCINH, despite a saturated boundary layer--was strongly representative of the modifying outflow pool. Meanwhile, just behind the low-level cold front, not only was TOP’s capping inversion aloft (having cooled very markedly the past few hours) similar in magnitude to DVN’s… but the entire thermal profile above the frontal inversion was nearly identical to DVN’s as well. It’s possible that mid-level thermal ridging near Kirksville wasn’t quite as suppressed as at TOP (and yet hadn’t advanced as far NE as DVN)… but this is assumed to be negligible. Below is the modified DVN sounding to represent the Kirksville event. Note moderate-bordering-on-strong instability (MLCAPE = 2455 J/kg); a very deep/moist boundary layer, with remnant EML above 650mb; and strong boundary layer RH (MLLCL < 800 m). Although MLCINH is weak to nil... note that low-level instability is not particularly strong (0-3 km MLCAPE = 41 J/kg), given the nearly saturated, weak-lapse-rate environment below 3 km.

Creating a single, strongly representative sr-hodograph for this event is more difficult given the gradual veering of low-level winds observed in the modified outflow air between 21-00Z. Regardless, the DVN RAOB and Winchester IL profiler were ideally situated to sample the vertical shear within this airmass, and both indicated a very strongly sheared low-level environment for the duration of this event--with very large 0-1 km SRH. Hodographs #1 and #2 below are those observed on the Winchester profiler (N of the effective outflow boundary and ~100 miles ESE of Kirksville) at 21Z and 00Z. Note that veering 0-1 km flow by 00Z is offset by acceleration of the synoptic LLJ to near 50 kt... actually resulting in an increase in SRH, given low-level winds were still strongly backed in a storm-relative sense. The third hodograph, from the raw 00Z DVN RAOB (120 miles NE of Kirksville)--likely launched around 23Z--is a good "medium" between the two... and also produced the largest low-level shear of the three, with 0-1 km SRH at a whopping 460 m2/s2. [Short-term model guidance vastly underestimated the size and pronounced "sickle shape" of these hodographs within the modifying outflow air... with NAM and NAM-KF models forecasting more veered near-sfc winds and a weaker LLJ.] Deep layer shear also appeared favorably strong via these hodographs, with around 50 kt of 0-6 km bulk shear. One final thing to note is that mid- and upper-level storm-relative winds on the three hodographs are pretty modest (20-35 kts)... and when coupled with anomalously rich PWATs (> 1.5"), this probably contributed to HP supercell processes that acted to temper the strength of these tornadoes. [Given that a very rare combination of ~2500 MLCAPE and ~400 0-1 km SRH existed amidst ideal low-level thermodynamics (albeit modest 0-3 km CAPE)... it would seem that the tornadoes from this storm could have been far more violent than they were.]

Finally, below are representative profiler hodographs for the tornadic supercells affecting the Burlington KS and Lamont OK areas, respectively. The latter tornado was a surprisingly long-lived and nearly stationary cone. The thermodynamic environment in these areas was characterized by higher LCL/LFC, but very strong instability (MLCAPE 3500-4000 J/kg) and steep low-level lapse rates/no CINH. This may have facilitated strong near-ground stretching to help in the production of tornadoes. Also, low-level hodographs remained a bit more kinked than forecast... and given the strongly deviant storm motions (almost too strongly deviant in the Lamont case, given the increasingly parallel nature of the storm motion with the near-sfc shear vector)... 0-1 km SRH was actually pretty favorable, in the 140-170 m2/s2 range. Note that deep layer shear is solidly on the low end for supercell storms though (~30 kts of 0-6 km bulk shear)... with the large instability likely playing a role in allowing for sustained supercells.

Thursday, April 30, 2009

April 29 review: mesoscale magic in the TX South Plains

A potent supercell tracked between the Plainview and Matador TX areas yesterday, producing a couple substantial/photogenic tornadoes northeast of the town of Floydada. Surprisingly, not terribly many local chasers were on it... perhaps due to its relatively short tornadic period before going OFD... or due to other sucker storms existing farther E. SPC handled the forecast well IMO. Model signals were pretty subtle for a significant event... though knowing that OFBs might come into play later, coupled with the amount of BL moisture lurking to the southeast, was a sign that some mesoscale magic could definitely occur.

During the morning hours, an elongated shortwave trough tracked eastward thru KS-OK, supporting scattered thunderstorm activity. The airmass feeding into these storms was characterized by anomalously strong precipitable water values and effective parcels already rooted near the surface. A strong MCS was thus gradually born over the Red River valley, on the southern flank of the convective activity. The far western segment of the MCS outflow appeared to settle southwestward, and by early afternoon curved E-NE-N along a Crowell-Turkey-Wayside-Amarillo TX line (see 1945Z visible satellite image below). A second surface boundary was also evident on satellite, extending E-W from Crowell-Matador-Plainview and intersecting the dryline near Springlake. The latter E-W boundary was not apparent via the TX mesonet, but gradually appeared as a fineline of reflectivity on Lubbock's radar... its signature probably becoming blended with an increasingly well-defined RFD gustfront off the supercell-to-be. Also of note, a convergence line/surface trough was present in the moist sector a few counties east of the dryline, generally straddling the I-27 corridor.

A trailing shortwave trough, low-amplitude but well-defined in nature, approached west TX by mid-afternoon. Persistent isallobaric forcing, likely a result of both an upswing in QG ascent and increased heating of the outflow air, produced a well-defined low-level mass response along and east of the dryline between 21Z and 23Z. (
See sfc obs below... no frontal analysis done as my usual analysis software doesn't have the TX mesonet data.) Enhanced downward mixing of westerly momentum forced the dryline eastward, where it merged with the convergence line near I-27; meanwhile, surface winds in the moist sector increased and backed substantially to the ESE. The supercell in question developed near the intersection of the E-W boundary and the effective dryline by 21Z, and tracked E to ESE along or just north of that boundary.

The Jayton profiler, located about 50 miles SE of where the storm became tornadic near Floydada, captured the mass response quite dramatically. Note in the 22Z and 00Z Jayton profiler hodographs below, the 500-1000 m flow backing and increasing to near 30 kt. Accordingly, given such a strong sr-inflow component and a perfectly kinked and curving hodo below 1 km... strong to very strong 0-1 km SRH resulted. Using the Plainview storm motion, 0-1 km SRH on the 00Z Jayton profiler was > 330 m2/s2 (with continued strong curvature through 3 km contributing to > 550 m2/s2 of 0-3 km SRH).
Also note, 0-6 km bulk shear was moderate to strong, in excess of 50 kt. In comparison, the 00Z Amarillo RAOB hodograph (third hodograph below) showed 0-1 km SRH of ~200 m2/s2... though it's possible the strongest low-level mass response hadn't yet occurred by the time of balloon launch (~23Z) and/or occurred farther south only. With neither OFB evident in surface data and likely becoming increasingly diffuse given increased mixing/heating and mass response... it is tough to know whether or not the boundary enhanced horizontal vorticity available to the storm. Regardless, I would think the shear sampled by the 00Z Jayton profiler is a reasonable estimate for the Floydada area tornadoes, with any changes in the shear due to increased elevation on the caprock escarpment probably marginal.

The thermodynamic environment available to this storm was likewise very impressive, due to the influx of upper 60s surface dewpoints up onto the caprock combined with fairly modest capping aloft. In order to achieve the correct elevation/pressure surface for Floydada (which is a good 500 feet lower than AMA), I used a base NAM-WRF model sounding with a 900mb pressure surface (ideal given relative pressure surfaces in the region that evening) and applied the AMA RAOB data to it, blending the BL using TX mesonet data. The resulting dewpoint lapse rate is dry adiabatic (similar to that observed at AMA)... but this is known to happen when anomalously rich near-surface moisture exists. Bottom line, note the strong instability (MLCAPE > 3100 J/kg), very strong 0-3 km MLCAPE (~150 J/kg), and sufficiently low MLLCLs (1200 m). The second sounding below is the raw 00Z AMA RAOB, which likely got "munched" by anvil-level convection just above 200 mb.

In reviewing this data, it is probably a good thing for TX residents that the storm quickly became outflow dominant (with the gust front apparently undercutting the updraft and encouraging evolution to a small bow)... because the mesoscale environment was truly primed for strong to violent tornadoes. If I had been free to chase on this day, I hope that I woulda taken the chance on it.

Sunday, April 26, 2009

April 25: underwhelming OK storms, and tornado near KC metro

Observed a couple near- and after-dark supercells yesterday south of Clinton, OK between 830 and 930 PM. The structure of the lead supercell (first pic below) was pretty trashy... high-based and dominated by HP processes. The trailing supercell initially had a stacked plates look to it... and briefly developed an increasingly low, compact, and rounded base beneath its vigorous/hard updraft (second pic below)... before raising its base again and looking pretty underwhelming after dark. (In hindsight, I probably should have exited my vehicle to ascertain whether outflow air from the lead supercell had contaminated the environment of the trailing supercell-- which actually tracked farther NW). Heard some radio reports of wall clouds with moderate rotation from these supercells, and was surprised... I didn't see anything of that sort... in general they were high-based and non-impressive.

Similar to what happened the day prior (April 24), afternoon boundary layer moisture mixed out a good bit yesterday across far southwestern OK (see 23Z surface map below) in vicinity of the storms... with surface dewpoints falling to the 58-61F range. The 12Z NAM suggested this might be a problem... and BL trajectories emanating from the strongly/deeply mixed moist sector airmass around Childress/Wichita Falls didn't help any. While it appeared that more substantial moisture survived mixing and was lurking immediately to the east, that was not really the case... with the richest moisture being rather shallow on the 00Z OKC sounding (surface dews 64F but 100mb ML dews 60F)... not much improvement from the afternoon before. This was a far cry from what was forecast in previous model runs, and the "real-world" result was not surprising: high-based supercells amidst modest SRH before dark... transitioning to more low-based but increasingly elevated trash with diurnal cooling. Bottom line, by the time diurnal cooling/decoupling dramatically improved LCLs and SRH... the BL was probably too stable.

Interestingly, it appears that richer BL moisture pooled ahead of the quasistationary front from northcentral OK to northeastern KS, possibly due to less vigorous vertical mixing. This, combined with cooler thermal profiles through the low- and mid-troposphere, resulted in strongly surface-based environments but with far lower daytime LCLs than in SW OK. This may have been a factor in allowing a supercell to initiate on the front near Lawrence KS, move east a bit, and produce a beautiful 15-minute duration tornado in Leavenworth county around 630 PM. Another tornadic supercell moving off the front affected the Enid area of northcentral OK shortly after dark as well.

The 00Z Topeka RAOB (sounding #1 below) was launched just to the cool side of the quasistationary front and ~40 miles west of the Leavenworth co. storm. Given the shallow slope of the frontal system (frontal inversion only ~350 meters deep) topped by a moist layer/cap aloft almost identical to that observed in the warm sector well SSE on the SGF RAOB... it looked perfectly feasible to modify the Topeka RAOB to represent the Leavenworth co. tornado (sounding #2 below). Given relatively rich moisture just above the inversion, again the assumption is that somewhat richer moisture (ML dewpoints around 62F) survived mixing ahead of the front in comparison to in southwestern/southcentral OK. The modified Topeka sounding cranks out moderate instability with MLCAPE of 1689 J/kg; a strongly surface-based environment given MLCIN of 23 J/kg and 0-3 km MLCAPE of 59 J/kg; and favorably low MLLCL at 973 meters.

Hodograph #1 below is that observed on the 00Z Topeka RAOB; while hodograph #2 below shows the 22Z Lathrop MO wind profiler (in the warm sector ~60 miles ENE of the tornado). While near-surface winds backed slightly 22-00Z just ahead of the front, moreso than at Lathrop... the Lathrop profiler also showed more curvature through 1 km (~1 km flow more veered) than the warm sector RAOBs at SGF and OKC, which looked suspect. So it may be a wash... would estimate 0-1 km SRH of around 140 m2/s2 with 0-6 km bulk shear of 35 kt for the Leavenworth co. tornado. (Mid- and high-level sr flow was obviously pretty poor as well, though that didn't seem a major factor).

This was probably the 7th or 8th time I've been chasing well away from KC, only to have a semi-unexpected substantial tornado event occur close to home. Interestingly, it seems that many of these tricky far northeast KS-northwest MO "missed events" occur amidst shear environments that are not particularly impressive, with tornadoes probably occurring equally if not moreso due to processes associated with strong near-ground vertical stretching rather than low-level shear; the May 29 2004 Camden Point MO F3, June 4 2005 Hiawatha KS F2, and June 8 2005 Oskaloosa KS F0 come to mind, all of these being gorgeous (and in the former two cases, very strong) tornadoes. Mother nature likes to keep it interesting I guess, and I don't see that ever changing. Looking back at the model data from yesterday morning... I probably should have given the frontal boundary in that area a "closer look" with regard to chasing... esp since I was due back at work today.

Congrats to Dick and Darin for catching a gorgeous stovepipe tornado today not far from yesterday's action area... Roger Mills county OK. Sfc dews had no problem holding in the mid 60s today in the warm sector.

Friday, April 24, 2009

April 25 forecast

Gonna be as brief as possible here... in between getting off of work at 11 PM and charging my camera batteries while seeing if I can rid myself of a migraine-like headache.

Tomorrow's severe weather forecast is a bit complex, but at least should be centered over a fairly localized area over the southern High Plains. 12Z RAOBs this morning at BRO/CRP/DFW showed that the deeper BL over the Gulf of Mexico had finally retreated inland, with impressive moisture quality (100 mb ML dewpoints ranging from 61F near the Red River to 68F along the gulf coast). This moisture mixed out a bit this afternoon while spreading northward over north TX and OK... but remained at an impressive depth of 150-175mb on the 00Z RAOBs.

Model concensus is for the cold front to stall across the northern TX panhandle, far northwestern OK and southeastern KS by early afternoon tomorrow... with the effective dryline becoming pronounced near the TX panhandle-western OK border via diurnal mixing to its west and rich moisture flooding northwestward to its east. Previous model forecasts of ML dewpoints in the 63-65F range in the target area now look fairly realistic... far more impressive moisture than we've seen lately... have to go way back to the Feb 10 event along the Red River to find moisture like that.

Timing and location of initiation is uncertain. Upper air analysis and NAM/RUC initalizations suggest a possible low-amplitude impulse over the CO river valley this evening that could potentially overspread the southern High Plains tomorrow aftn/eve... but think it will be of little consequence, and that the vertical motion field on the NAM may be more a function of the convection explosion forecast to begin around 00Z. FWIW, the flow at jet level is forecast to become increasingly difluent and divergent through the afternoon and evening, which may offer some support for upward vertical motion. On that note, I expect a fair amount of cirrus overspreading the area tomorrow... anyone's guess what impact it will have on mixing/temps/initiation.

The triple point eastnortheastward along the cold front is the most obvious place to expect initiation by around peak heating, where convergence will be strong. Despite the front likely lifting back north after dark due to considerable pressure falls overspreading the cool sector... still not sure I'm crazy about the triple point target... will have to evaluate that more later.

The dryline itself is forecast to be weakly convergent by mid- to late-afternoon. Interesting to note the NAM is forecasting a very deeply mixed moist sector tomorrow afternoon (sfc temps 85-90F or more) as far north as far southwestern OK and northwestern TX. It's possible this could enhance low-level baroclinicity/cyclogenesis and more vigorously mix a dryline bulge into southwestern OK during the afternoon. Equally as possible if not moreso... diurnal backing of the low-level flow and revving of the LLJ could result in a more strongly convergent
retreating dryline after 00Z... model signals suggest it's quite possible this could be the mechanism for initiation. This evolution might actually be "better" given SRH will be weak to nil prior to 00Z.

It's tough to trust anything in the NAM in the 00Z-06Z time frame given the convective explosion that is forecast. However, given the anticipated quality/depth of BL moisture, capping aloft that shouldn't be out of control (~7H temps not exceeding 8C), the added benefit of elevated terrain in the High Plains, and LLJ expected to accelerate to 40-50 kt after 00Z... it's realistic to expect a window of strong surface-based instability (MLCAPE 2000-3000 J/kg with very weak CINH) for a few hours after dark, coupled with 0-1 km SRH easily exceeding 300 m2/s2. Thus, if storms fire, potential exists for tornadic supercells and a strong tornado or two prior to 03-04Z. Mid- and high-level SR-flow is quite modest, but I can't help but be reminded of the loose analog case of 5-12-04... which featured modest flow aloft as well but had no trouble producing a dry-side-of-classic tornadic supercell.

Preliminary target = Allison, TX in far eastcentral portions of the panhandle. Haven't chased TX in three years!

Saturday, April 18, 2009

cold core tornado event occurring over SW KS!

The usual cold core goldmine of SW KS is producing again... with 6+ tor reports in the DDC area including reports of two on the ground at once. I finished a string of five midnight shifts at 7 A.M. and wish I had looked closer at this potential target and considered trying to chase after work, rather than driving home and crashing for some much needed sleep... but that's fatigue for ya.

The set-up fits the classic Jon Davies pattern pretty well, though the low-level pressure/height field (and thus low-level shear) is very weak. Cells fired first near the intersection of the slow-moving occluded front and a weakly defined west-to-east boundary... then built southsoutheastward along the occluded front. Only the cells near the subtle boundary intersection and/or just to the cool side of the west-to-east boundary have been tornadic. While ambient horizontal and vertical vorticity associated with the boundary intersection and mid-level low are probably playing a role, low-level stretching--associated with steep low- thru mid-level lapse rates and very large low-level instability--are probably dominating w.r.t. tornadogenesis processes. The 12Z DDC sounding this morning sampled 700mb and 500mb temps of -5°C and -25°C respectively... more typical of a late winter cold core set-up (though low-level theta-e is modest, an offsetting factor). Had I seen that 12Z sounding before going to sleep, I may have reconsidered whether to chase.

Anyway, big kudos for John Hart & SPC for going with a tor box on this one in spite of very marginal low-level flow/shear!

Saturday, April 11, 2009

April 9 eastern KS-OK border chase

Dick, Darin, and I agreed on a target in the Coffeyville KS-Bartlesville OK area on Thursday. Given relatively underwhelming moisture return across the board and comparatively poor roads/terrain in the obvious warm sector target, it was definitely a day to give close consideration to the southcentral KS cold core potential ahead of the mid-level low center... but we shared too many concerns about that target being productive.

We moved as far west as Sedan KS 3-4 PM, letting a few marginally impressive severe storms trail by well to our west. We gradually moved east, staying near Highway 166 to keep our options open. With nothing really becoming dominant and with storms farther south largely taking a linear configuration, things were looking a bit grim as we reached Edna KS around 545 PM. Suddenly, a semi-discrete and intensifying storm along the OK turnpike caught our eye. Moments later it received a tor-warning, and that sealed it--we headed farther east on 166 for an intercept. Given the torrential rain and getting caught behind a semi... it took us longer to get there than it should have.

Eerily, we intercepted the storm in the town of Picher OK (struck by a killer EF-4 tornado in May 2008), with tornado sirens blazing as cityfolk looked warily off to the SW to try to catch a glimpse of the storm.
The supercell was in the process of rapidly contracting, and completely dissolved not 15 minutes later. We got a good look at the compact rainfree base (and a couple nub funnels... see pix below, view looking south at 6:37 PM) before it wrapped in rain. Lots of dime hail, with maybe a few quarters, in the core... impressive considering the storm was going downhill so quickly. Interesting that an EM reported a tornado near Miami only a couple minutes prior to our close intercept of the base, but concensus from numerous chaser/public videos is that the funnel cloud in question did not touch down.

Significant tornadoes managed to occur farther south in the Mena AR and Shreveport LA areas. It's possible that the environment was more strongly surface-based in a wider corridor in those areas due to stronger moisture return. Clearly, had our dewpoints been 5°F higher across the board, tornado potential would have increased considerably. There also may have been a few problems with the local shear profile in northeast OK/far southeast KS (e.g. initially modest 0-1 km SRH, or a veering-backing pattern in the vertical), but wind profiler data wasn't much help in conclusively gauging that one way or another.

As for the cold core target along the trailing surface trough/psuedo-occluded front, surface dewpoints appeared to verify about 2°F lower than forecast from the 12Z NAM and as much as 4°F lower than from the 12Z RUC. Given the related LCL heights and 0-3 km CAPE had been forecast to be only modestly favorable to the cool side of the front, I can only assume that the parameters took a further hit given the poorer-than-forecast moisture. Pressure rises in the wake of the primary surface low, with the wind shift pushing southward as a cold front, may have also acted to minimize the width and/or duration of the unstable boundary layer along and N of the front... if not undercutting updrafts altogether. The 00Z/12Z NAM and 09Z/12Z RUC did a good job with forecasting the evolution of this surface boundary, despite my having worried that the southward push from the NAM was a result of modeled cold pooling associated with very vigorous convective initiation forecast along the boundary. The progressive easterly motion of the mid-level low not far behind the surface low, also noted by Dick, probably had a role in the isallobaric forcing and frontal evolution. In the loose analog case of 10-26-06, the cyclone did not appear to be as vertically stacked, probably favoring the more stationary nature of the occluded boundary.

Wednesday, April 8, 2009

April 9 forecast

The 00Z models show a slight southward shift from previous runs, which is a real downer given the poor chase territory over much of northeast OK. I think the potential for a pseudo cold core play looks better in some regards on the 00Z NAM, as previous runs had 35-55 kt 500mb flow (southeast of the mid-level low) overspreading the surface low and adjacent warm front. The 25-35 kt 500mb flow forecast off the 00Z run looks better for keeping storm motions slower (lessening the risk of them either weakening or quickly becoming elevated), and is in the range consistent with a lot of other cold core tornado events (particularly those tied directly to the cool side of a baroclinic boundary). I'm a little leary of sticking close to the surface low tomorrow though, given the progressive nature of the low and strong low-level cold advection forecast in its wake. I'm also not sure how "tornado friendly" the cool side of the effective warm front will be, given the rather late arrival of the substantial low-level moisture (if it in fact arrives as forecast) and 700-500mb temperatures that are not especially impressive for an early April cold core event. My initial hunch is to play the northern end of the warm sector, east of the low a bit (say, the far eastern KS-OK border), and hope something goes up on the northern sections of the dryline and gets its act together in far northeast OK or far southeast KS before outrunning the narrow instability axis. It is certainly possible that a better cold core play might evolve farther west, tied to the surface low/warm front intersection, and surface obs and visible satellite later on tomorrow will help with figuring that out. If we can't get upper 50s dews up toward the KS-OK border before dark, the sfc low target (w/ the coldest temps aloft and strong PVA) may be the only viable option.

The just-in 00Z NAM-WRF-NMM 4km WRF from Matt Pyle has no trouble initiating an arc of storms across northeast OK between 5 and 7 P.M. local time, which is interesting.

This would be a much tougher forecast (from a chase strategy standpiont) if it was occurring smack dab in the middle of KS. As it is... I refuse to mess with "Ozark" country (especially around, if not after, sunset), where parameters are more typically "stacked up" ... i.e., stronger deep layer shear, stronger moisture/CAPE, and thus bigger EHI/STP values than points farther northwest. Thus, the cold core or pseudo cold core (i.e. open warm sector ESE of the sfc low) looks like the most practical target at this time.

Monday, March 23, 2009

March 23 aftn update

I woke up before noon to consider whether to prepare for a chase. Not unlike many set-ups while I'm on nights... I'm still sitting at home, considering going back to bed to get some more sleep!

Color me surprised, but convective initiation has already occurred along the dryline, in an arc from Brady NE east-southeast to Kearney NE and then south-southeast to Osborne/Lincoln, KS. In hindsight, this is likely a result of deep mixing in the moist sector with sfc temps rising well through the 70s (5-10F warmer than forecast). Deep convection is also trying to form farther south, from Medicine Lodge KS down toward Clinton OK... though its been unsuccessful thus far... probably will have a better shot as mid-level cooling/lift continues to overspread the dryline.

For the northern stuff, surface obs show T/Td spreads of 22-25F... so for now the tornado threat would seem to be low.

Sfc winds are sustained at 39 kt at Hutchinson KS! Local WFOs have issued high wind warnings for a good chunk of central KS.

This is just as tough a forecast as expected, so my confidence is pretty low on the details. If the mid to upper 50s dewpoints can back northwestward up to the dryline and discrete convection develops and persists, still could certainly be some tors southcentral KS/northcentral OK, mainly near/after 00Z as the boundary layer decouples.

March 23 final forecast

I don't have time to get too in-depth with this... actually haven't even looked at as much data as I'd have liked to. I may try to chase after a 5-hour sleep following work this morning, maybe targeting the dryline in southcentral KS. Here are some random thoughts for tomorrow.

-Even if the pseudo-cold core play (occluded/cold front in NE) was within reach, I'm still not sure I see it as being better than the dryline play. I don't see a baroclinic boundary, and while surface winds may be isallobarically backed up there, low-level shear doesn't look any better than points south given a more backed storm motion and the likelihood of a weaker LLJ. Still, wouldn't be surprised to see a few tornadoes up there (especially if they heat up well into the 60s°F)... though my hunch is that convection will assume the form of line segments. One thing that is better about the N target is a very high chance of daytime initiation, though.

-The first thing that concerns me about the dryline target is the extremely strong surface pressure gradient across southcentral and eastern KS... surface winds are no doubt being underforecast by the models, and that could easily be somewhat of an SRH-killer tomorrow afternoon. As I alluded to in previous posts, 30 kt sustained winds are generally "not your friend" on a chase day. The NAM-WRF indicates the pressure gradient and surface winds should begin to relax by 00Z and more certainly by 03Z, which should naturally allow for an increase in low-level shear. My hope is that sr-hodographs as forecast (250-350 m2/s2 of 0-1 km SRH, with rightward deviance not having a whole lot of net effect on the SRH magnitude in this event) are still large enough such that stronger-than-forecast surface winds still allow for fairly large SRH (> 200 m2/s2).

-Another thing that concerns me about the dryline play, particularly given that I work tonight, is the timing of convective initiation. Interestingly, the 00Z NAM-WRF weakens the forward-side-of-trough 5H wind max between 18Z and 00Z as it rotates atop the KS dryline... fragmenting the vorticity into a piece north (closer to the mid-level low center) and a trailing piece back in the dry air. The attendant vertical motion field is similarly fragmented, not organizing over and then intercepting the dryline until after 00Z... and I have to wonder if it is related to the weakening of the mid-level jet. (I've had a good number of chases in the past, some involving a cold core and some not, where I've been left high and dry under the mid-level jet max... while initiation and tornadoes occur farther N, just east and southeast of the primary vort center). High-res simulated radar reflectivity from both the 00Z NMM and ARW don't initiate dryline convection until between 00Z and 03Z, either, and the 00Z and 06Z operational NAM-WRF and 03Z NAM-KF all indicate initiation starting right around 00Z (7 P.M. ... not really worth trying to chase given my working tonight). It's anyone's guess whether this guidance will come to fruition as described, but regardless, it is supported by the background mass fields and overall it lessens my confidence in getting storms to pop earlier rather than later. One would think that with a strongly convergent dryline and lifting/cooling of the residual cap from the west... initiation would be possible by 4-6 P.M. We shall see. We certainly can't have our moisture mixing out if we want to achieve dryline initiation. Which leads me to...

-My last concern, obviously, is the magnitude of moisture. The 00Z NAM-WRF forecast is a bit better than prior runs, indicating surface dewpoints solidly in the upper 50s°F over southcentral KS by peak heating. 100mb ML dewpoints on the 00Z RAOBs were already around 50°F, so we don't have far to go... with the 55°F surface isodrosotherm about near the Red River as I type. I worry about the effects of the strong vertical mixing, though, and whether we will end up with poorer boundary layer moisture than is forecast. It certainly isn't a good sign that boundary layer moisture didn't improve any over deep south TX during
the last 24 hours, per the 00Z Brownsville and Corpus Christi RAOBs.

One final thing which will be interesting is the evolution of convection as the plunging cold front overtakes the dryline, which wasn't addressed much in the otherwise excellent 06Z SPC Day 1 Outlook. The large scale pattern bares some resemblances to May 1, 2008... with an extremely intense band of high-level difluence/divergence forecast to overtake the sharpening cold front between 03Z and 12Z. Could this lead to another very damaging QLCS with embedded circulations/F3 tornadoes? Hopefully not, but an organized squall line with some damaging gusts seems like a good bet.

It's also good to see that an extreme snow event is looking less likely for central ND, as the upper low is now forecast to be weaker (with filling low-level cyclones) as it lifts northeast and gradually opens into a shear axis. My family back home is dealing with flooding due to tons of melting snow already, and they really don't need another 18" on top of it. Hopefully they skate by with 6-8" in Bismarck.

Saturday, March 21, 2009

March 23 forecast update

Just a dismal forecast update for Monday's severe weather potential. It's really too bad we don't have another day of return flow ahead of this system, or rather, that trajectories don't improve sooner than forecast. It looks like the boundary layer air mass over the southern two-thirds of the Gulf of Mexico has modified further, with moisture deepening considerably. The Brownsville TX 00Z RAOB looked far better than I would have expected, with a 1.5 km deep moist layer and a 100mb ML dewpoint of 63°F! In fact, dense MVFR stratus has already materialized over portions of the Rio Grande valley and up as far north as the TX Hill Country. But none of this seems to matter, as our trajectories just aren't forecast to improve in time to get low-level moisture of anywhere near that quality into the central Plains prior to darkfall Monday. I'll do one last forecast tomorrow night when the new data is in... with the system being onshore the models should have a very strong lock on it by then. The 00Z GFS tonight still seems to be having issues with keeping the warm sector socked in with clouds and cooler temps. Anyway, my hunch is that this could be the type of system where dewpoints on the north end of the warm sector aren't terribly weaker than well to the south... e.g. broad mid 50s°F at the surface. If so, though again the impressively cold 7H-5H air will probably be lagging the warm sector a bit, there could still be some enhancement to the low-level destabilization process up north. Any cold core potential looks like a "warm sector" variety up there, as a synoptic warm front will not be present (rather just an inverted trough extending N of the surface low). That target is out of my reach regardless, but it will still be interesting to watch how that area may evolve... especially w.r.t. low-level destabilization and ultimate convective mode. The NAM is actually showing strong crossover of the 0-6 km bulk shear vectors with the bent back/occluded front up there, though they back with time. The models are really still all over the place on many mesoscale facets of this system though, and hopefully tomorrow night things will look clearer.

March 23 forecast

Using the term "forecast" loosely, as I'm at the end of a mid shift and just coherent enough to throw together a discussion...

The first potential severe weather maker in the Plains in the past several weeks (aside from the bird fart of an event on March 9) is pretty clearly going to arrive on Monday. While many local and even national media outlets have, oddly, been advertising the potential for a considerable tornado outbreak, I remain unimpressed at worst and "mildly interested" at best w.r.t. tornado potential.

A large upper level trough, with 500 mb temps of -36 to -40°C, is currently over portions of the northern Pacific and Gulf of Alaska... with two embedded closed lows apparent off the coast of British Columbia and a third fast-moving perturbation ejecting into the forward side of the large scale trough. Digging mid- and high-level jets on the trough's back side will continue to carve out the trough as it consolidates southeastward into much of CA and the CA coastal waters Sunday night... and then turns east toward the central plains on Monday. The ECMWF and GFS are coming into very close agreement on mass fields with the 00Z run of each model. Expect a mid-level low to close off over eastern CO by 12Z Monday within the neutral tilt rough, and then lift northeast into western NE by 00Z... with an occluding, sub 990 mb surface low preceding it. The synoptic cold front will begin plunging through western NE and into northwestern KS during the day, probably forming a triple point with the occluding dryline in southcentral NE and far northcentral KS by afternoon.

A plethora of questions exist as they relate to the ultimate magnitude and type of severe weather. They include:

- Moisture/instability: although 65-72°F surface dewpoints are still hanging on essentially across the southwestern half of the Gulf of Mexico, the moisture is likely very shallow. Moreover, the sprawling surface ridge over much of the eastern seaboard and deep South will not budge a lot the next 48 hours, which means easterly trajectories persisting across the Gulf of Mexico... and with warm boundary layer air over the same areas, considerable moisture flux from the gulf would seem unlikely. Trajectories won't become more favorable for northwestward advection of higher moisture until sometime Sunday night. The GFS/ECMWF/NAM all agree on 100 mb ML dewpoints of 54-56°F spreading into central KS and central OK in advance of the dryline by Monday afternoon... which considering the above, looks reasonable. This is pretty poor moisture quality for a "warm sector" event in late March. The GFS generates/advects some very cold air atop the BL by 00Z (resulting in wickedly large low-level CAPE), which reeks of convective feedback. Also, the GFS keeps the warm sector socked in with low clouds and cool surface temps. Preferring the ECMWF/NAM thermodynamics, would expect at least filtered March sun ahead of the dryline to allow temperatures to climb to near 70°F. Although the impressively cold upper tropospheric air will lag well to the west, 0-2°C at 700mb overtaking the dryline by mid-afternoon should aid in eliminating the cap, with MLCAPE approaching 1000 J/kg and modest amounts of low-level CAPE.

- Storm mode / tornado threat: A strongly convergent dryline should evolve by mid-afternoon given the strength of the low-level cyclone and intense downward mixing of westerly momentum in the high plains. This coupled with strong DCVA on the inside of the mid-level jet max would seem to point toward fairly vigorous/widespread convective initiation in a narrow band from north to south. 0-6 km bulk shear vectors on the ECMWF have consistently shown a healthy crossover with the dryline (45-60 degrees), while on the 00Z GFS the angle is a bit smaller (though still respectable). Generally expect a mix of line segments and discrete cells given the favorable shear vector orientation coupled with very strong synoptic scale/mesoscale lift and modest instability. Given the ambient surface winds will probably be quite strong (25 kts or more), the 50-60 kt LLJ maintained by the ECMWF/NAM would be preferred in order to maintain strong low-level shear for a substantial tornado threat. The 00Z GFS hodographs looked a bit poorer than in previous runs with the primary LLJ core shifting east away from the dryline, though the 06Z run looks better again. Most of my experiences with very strong ambient surface winds have been bad ones as they are notorious for putting a big kabosh on low-level shear... but given a 50-60 kt LLJ is maintained during the afternoon and storms are successful in becoming rightward-deviant, that won't be an issue on Monday. We'll see.

As usual, mesoscale details should be clearer by the morning of the event. My biggest hope is that KC can get some more rainfall out of this one...

Thursday, February 12, 2009

Feb 10 review: significant tornadoes strike Edmond and Lone Grove, OK

February 10 looked to be the first potential severe weather outbreak of the slow-to-start 2009 severe weather season. I had planned to chase the event from a week out as long-range models developed a strong lock on the general pattern, but was later scheduled to work a day shift the following morning (February 11). Given the tornado threat initially looked somewhat marginal before dark and probably would be solidly on the TX side of the red river (a 7-hour+ drive back home after the chase), I decided I should sit it out. Guidance the morning of suggested severe storm potential might shift a bit more to the NW, but I still wasn't sure how significant the daylight tornado threat would be given initially semi-modest low-level shear just E of the dryline and the rather expansive nature of convection expected to develop by mid-afternoon. My virtual target was Ardmore (just E of Lone Grove), but we all know how targets change and how one can be suckered and swayed through the course of a chase day. I definitely wouldn't have thought to target the I-44 corridor from OKC to Tulsa, that's for sure, where the "daytime storm-of-the-day" ultimately trekked.

Below is the modified 18Z OUN RAOB and 21Z Purcell profiler to represent the environment of this storm as it produced an EF-2 tornado that struck Edmond, OK around 300PM. This storm fired on the northern flank of the returning deep BL moisture (another reason I'd probably have never targeted this region, all things considered), and had no trouble becoming a strong supercell producing baseball hail for a few hours. The rapidity with which the storm became such a nasty little beast is slightly surprising given very strong deep layer shear (the Purcell profiler sampled 6 km flow as high as 88 kts!!), as I usually like to see stronger low-level shear for such set-ups than was in place for this one. However, moderate CAPE was present (approaching 1500 J/kg...certainly stronger than forecast), and low-level CAPE was very strong (150 J/kg below 3 km) given seasonably cold temps above the BL... these factors probably played a role in allowing for the long-lived supercell.

Sadly, after dark, the second and final significant tornado of the day struck portions of Lone Grove, OK and caused 9 fatalities. By around sundown, the synoptic cold front was overtaking the dryline across west OK and northwest TX, encouraging upscale convective growth to a solid squall line. Convective initiation concurrently took place farther east in the warm sector, with a supercell emerging south through east of Wichita Falls, TX. Pretty much everything was in place for this storm to become a violent tornado-producer as it approached the western edge of the nocturnally-accelerating LLJ core. Even if surface temperatures had fallen toward the mid 60s after sundown in advance of this storm, the quality of the boundary layer moisture (surface dewpoints near 64°F) and absence of a significant EML would have allowed for a sufficiently strong surface-based environment to persist. As it was, surface temperatures managed to hold in the 70-72°F range in the storm's inflow environment through at least 8 P.M... resulting in the maintenance of near-zero CINH and very strong low-level instability (150-200 J/kg 0-3 km MLCAPE). In addition to the absolutely ideal low-level thermodynamic environment after dark, MLCAPE was > 2000 J/kg, with quite strong deep layer shear and 0-1 km SRH exceeding 300 m2/s2 (see modified 00Z DFW sounding and its hodograph blended with 01-02Z VWP data from the FWS 88D). The tornado was on the ground for over an hour, and preliminarily has received a rating of high-end EF-4. This case continues the unnerving trend for nighttime killer tornadoes in the Plains the past few years... more often seen in the deep South. Luckily the storm was swallowed shortly thereafter from behind by the squall line... and despite the prefrontal warm sector remaining mild and weakly capped all night, additional convective initiation did not take place there.

Sunday, February 8, 2009

Feb 9 forecast

Just a brief post for tomorrow's system. Looks like it could be an interesting day for the middle/lower MO river valley, at least by almost-still-the-middle-of-winter standards. Strong upper level trough crossing northern Baja this morning will eject negative tilt through the central Plains tomorrow. The associated surface low will seemingly have no trouble undergoing bombogenesis as it lifts NNE overnight tonight through tomorrow, potentially making a run at 982 mb by peak heating tomorrow. This isn't a classic Davies cold core set-up by any means, as the mid-level system (a negative-tilt shear axis) is not closed off... and will be moving at a high rate of speed. Showery precipitation, driven by strong high-level difluence/divergence preceding the upper trough, will spread through the middle and lower MO river valleys tomorrow morning. The potential for mini-supercells will follow in the warm sector of the surface cyclone (potentially initiating in the thermal ridge near a prefrontal confluence band), as the dry slot attempts to wrap northward and northeastward into the base of the negative-tilt trough. The farther north the dry slot can advance, the better, as my sense is that cells just to the cyconlic side of the mid-level jet will struggle to organize amidst 700-500mb flow of 50-80 kts, given SBCAPE will likely be AOB 500 J/kg. Respectable low-level CAPE (700 temps around -5C!) and healthy low-level shear (0-1km SRH potentially as high as 200-300 m2/s2) will aid in organization and the potential for mini-supercells and tornadoes. We've certainly seen past low-CAPE/super high-shear settings produce tornadoes (e.g. 11/12/05), but those cases seem pretty rare. Tomorrow's system has a huge bust potential, particularly if it races through the area more quickly and/or farther north like the GFS has been hinting at.

EDIT @ 930 P.M. : am liking the looks of the latest models even better. They continue the trend of mid-level dryslotting into the mid/lower MO river valley by early afternoon, heating the surface well into the 50s. Also, models actually bring dewpoints in the low 50s into the area--definitely looks at least possible given the deep (2 km), saturated boundary layer observed on the DDC and OUN RAOBs with ML dewpoints of 7-11C. Finally, the wind gradient in the cyclonic side of the mid-level jet is forecast to be far more diffuse, with non-outrageous 700-500mb flow (40-55 kts)... I'm actually seeing some semblences of a "warm sector" cold core pattern emerging.