Thursday, March 10, 2016

Exceptionally deep Mexican mid level low!

Been forever since I've had a chance to post... this was extracted directly from my e-mail to colleagues 24 hours ago at 5AM Wednesday.

"What a whopper of a system digging through Mexico overnight tonight...  This area is notoriously datasparse with respect to upper air, however.  As the system rocketed just south of due sewd AOA 40kts from 18Z/07 to beyond 12Z/08 given the formidable high level jet energy on the back side of the large scale trough... a 545 dm 5H height was observed at San Diego 00Z/08 at about the time the shear axis had wholly evolved into a closed circulation which later passed E of the site; while the following morning Tuscon observed a 548 dm 5H height 12Z/08 in the nern semi-circle of the mid level low.  Neither of these obs actually cracked 2.5 standard deviations below normalized climo--while in itself respectable--given their latitude this time of late winter.  The 00Z/09 EC, NAM, RAP, and GFS initialized quite closely in showing the 549-550 dm 5H closed low centered over the srn coast of Sonora... with the former two models a tad deeper and more in line with UA obs (on the edge of the nrn semicircle of the system, of course).  On that note, this is not a model verification review or deterministic model discussion... but trends in the EC/GFS look quite similar during the next 36 hrs though again the former remains stronger, and farther west during the system's digging phase. 

Local Time, between roughly Tuesday afternoon and Thursday afternoon--i.e., 21Z/08 through 21Z/10--the anomalous nature of this mid level low... a whopping 6 to 10 standard deviations below normalized climo... will max out as the motion of the cyclone curves from sewd to nnewd during the final ~18 hours of that period.  First... I attached the 00Z/09 GFS ensemble mean forecast at 18Z/09--whose 00Z initialization FYI matched the operational GFS quite well re: depth/geometry of the 5H low/nearby trough.  The colored part of the image shows the large area of exceptional height anomalies on the 5H low's southern periphery (not an atypical location whatsoever)... this is about as large as the 6+ standard deviations below climo appeared to get, so I grabbed the earliest frame of that period which was 18Z or noon today LT.  I have only seen mid-level systems this anomalously deep twice operationally in the last 8 yrs (when I regularly began including climo in my forecast process).  Secondly... I also included a now-roughly-valid-time SFCOA (SPC mesoanalysis) "diagnostic" plot of the 5H heights/winds--at 09Z/3 AM LT--showing the location/depth of the low at the time of this writing.  Though not directly correlated with the anomalously deep mid-level low of course, significant aviation hazards (primarily in the form of severe turbulence) occurred on both sides of the northward-trailing mid through high level trough axis (not shockingly, primarily on the back side), underlining how assessing anomalies--aside from hastening UA analysis and setting the stage for the forecast funnel process/development of a conceptual model--quickly indicates the unseasonable degree of anything from a high precipitable water plume, to a surging cold air mass, to a powerful ~southerly LLJ, all with objectivity... the anomalies naturally having some correlation with significant weather hazards."

Additional info from tonight's shift: 5H low was still moving SSE between 00Z-06Z/10... but as of 09Z/10 has turned due E and is about to rapidly redevelop northeastward, likely as a separate/new circulation... while the fwd-side-of-trof gradient tightens in response, and the mid level jet now occupying the srn semi-circle of the central Mexico 5H low concurrently redevelops to the east side of the 5H low--in NNE-to-SSW fashion from GGG-BRO-Mexico City Mexico to the cstl wtrs of Guerrero by 18Z/10.  Am astounded at the 00Z NAM's fcst of a 70-80kt 750-1250 meter low latitude LLJ-let forecast E of the surface low to develop and lift northward through the wrn Gulf of Mex 06-12Z... the srn extent of which initially evolved in the tropics near 22.5°N!  Truly unusual for a powerful LLJ, even of somewhat small expanse.  Meanwhile, said surface cyclone has, in advance of the mid-level low's evolution (both longitudinally and temporally), redeveloped rapidly from the waters just northeast of Tampico Mexico to the waters ~100nm SE of BRO in only ~3 hours as of earlier this Thursday morning (around 06Z/10).

Finally, here's a link to a nice surface analysis valid at 00Z/10, including PMSL OA, via Tim Vasquez... showing said sfc low's position just northeast of Tampico Mexico: