Tuesday, June 10, 2014

06/05/14 damaging wake low in KC metro

Copied & pasted straight from an e-mail and the images I shared with my colleagues last weekend:

Below are a few images of a 16Z (11AM CDT) sfc analysis I created (in NMAP) of the remarkable surface observations during our damaging wake low event late Thursday AM.  This is the first one I can remember seeing operationally in a long while, and absolutely the first one I've witnessed "in person."  (I was in between mid shifts, and the loudness of the southeasterlies and the odd "occasional waves" that the wind arrived in locally at my house, was super-interesting).  Now, numerous times I have observed and hand analyzed MCS-induced rain cooled mesohighs that actually have pressure falls in their wake--and often even a co localized MSLP minimum in association with the falls--but this being in large part simply due to the bubble high having shifted eastward, IMO.  A true wake low event like on Thursday I have not witnessed anywhere near as often.

I used linear interpolation of the sfc obs to create the isobars in east KS and western MO, given the extreme pressure gradient that existed.  No surprise, in two cases I had to "throw out" the 1004-mb-isobar interpolation that arose from two pair of interpolated obs (1: IDX-MKC and IDX-MCI, and 2: OJC-SGF and IDX-SZL); each pair created an excessively wiggly/pointy portion of the isobar when playing "connect the dots" after the interpolation was complete... which again didn't shock me given the localized dense observations among which I was interpolating--amidst an extreme pressure gradient.  I used the mean of each pair of 2 obs to create a single interpolation point for each pair, in effect smoothing them out... and I'm quite satisfied with the result.  Diagnostically, SFCOA/SPC's Mesoanalysis, as well as MSAS, no surprise could not "catch up" with the evolution of this event, let alone even begin to represent a sub-mesoscale/wake low if it tried (comparatively broad bubble highs, MSAS usually does pretty nicely with; as for the wake low, the mesoanalysis as well as MSAS each only fell to ~1006 mb, basically showing a weak inverted trof stemming NNEward from ncntrl OK).  As for the rest of the region outside of east KS and western MO, I just eyeballed the sfc obs to create the isobars, as with a typical hand analysis.  Finally, FWIW, on a typical hand analysis I probably wouldn't have drawn the "squall line" symbol (red line with occasional red dots; I instead use it more as a means of delineating a "leading edge gust front")--quite as far north as I had, particularly given 1) the increasing distance into the cool sector (where I rarely analyze gust fronts/outflow boundaries--and, in fact, there was no well-defined leading edge gust front whatsoever via sfc obs), and 2) the comma head was becoming largely stratiform in nature.  However, I did so essentially to identify the rough N-S location of the entire MCS structure on the surface analysis, given that there was no overlaid radar data.

Image 1 above (click to enlarge): regional, "zoomed-out" version.  You can see a couple 1005 mb synoptic sfc lows near Clinton OK and Tahoka TX, but the most important features along the KS-MO state line are hard to read or discern--as expected; the point partly was to exhibit the extremeness of the pressure gradient near the interface of the bubble high and meso low.

Image 2 above (click to enlarge): zoomed-in version.  You can far better see the sub-synoptic highs and lows, and the isobars, on this one.  Two 1002 mb meso (wake) lows exist near OJC and STJ respectively; and a pair of meso highs exist near Odessa MO and way down near SGF.  (The former meso high resulted from raincooled air beneath the MCS' comma head; and the latter meso high was associated with and immediately behind the severe bow echo that was raking swrn MO).  Also notice that the distance between the 1001.7 mb Olathe KS wake low and the 1015.3 Odessa MO bubble high is only 35 nm, with a pressure difference of 13.5+ mb between the two features!

Image 3 above (click to enlarge): I removed the surface obs so it wasn't so messy, and having  computed the 2-hour pressure changes I put the ones on the map that met or exceeded +/- 3 mb per 2 hr.  Note that 8-12 mb falls per 2 hours are common in the area in which the wake lows bottomed out.  Synoptically driven mid-latitude systems over the CONUS, in my experience, typically produce 3 hours pressure falls in the 9-11 mb range at the largest (pressure rises meanwhile can be a tad stronger yet, as you'd expect).  But this was a 2-hour fall I'd computed (equivalent to 12-18+ mb 3-hour falls... 3 hours being the time-window with which I'm used to gauging isallobaric adjustments/forcing); granted, again, this was sub-mesoscale/stormscale in nature, and thus kind a different kind of animal--I simply brought that up for comparison purposes.  FYI the reason I used 2 hour falls was simply because, based on the motion of the bubble high and the developing wake low, and the rate and time at which the wake low developed... 2-hour changes best showed the pressure rise/fall couplet, while 3-hour changes washed out its "significance" to some extent.

Finally, kudos to SPC, who put out the watch for KC metro at 8 AM (2 1/2 hours before the wind damage got going via the wake low, and 3 hours before it affected KC metro).  IIRC the discussion included mention of potential wind damage on the leading edge of the increasingly stratiform comma head, though that appeared unlikely even as morning wore onward; regardless, the watch did not extend very far downstream, so it is obvious they weren't excited about that particular threat or at best wanted to take a wait-and-see approach with it.  More importantly, they identified in the associated watch text the potential for wake low development with time--completely prior to it happening.  Based on the previous info and watch box structure... the main thrust w.r.t. feeling a watch was necessary must have been recognizing somewhat early on the potential for a wake low.  Pretty slick IMO.

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