3/28/2008 FCST: S Central TX

[Bill Tabor]

Looking at the digital atmosphere today I see the cold front is currently E/W just north of AUS. Checking SPC Mesoanalysis products shows a fairly stout cap in place; however checking UCAR Sfc Tornado Alley Overlay shows some larger areas already mostly uncapped. Does anyone have any idea why the discrepancy?

http://www.spc.noaa.gov/exper/mesoanalysis/frames.php?sector=2
http://www.rap.ucar.edu/weather/surface/overlay.php?region=ABI&metar_over=Go

I was checking out this forecast yesterday and appears some chance toward DRT late afternoon / early evening as of last 0z NAM. I may have a shot here in AUS of seeing a nearby severe hail storm later today which could be worthwhile.

 

[Bill Tabor]

Ok, perhaps already answered my own question. Appears that the Ucar overlay shows RUC Analysis data for cape/cin whereas the SPC product apparently is plugged in a bit differently with perhaps other info. However it almost appears to me the Tornado Alley Overlay Cape/Cin image looks a bit different than their RUC Cape/Cin if you go to the model, but perhaps that is only because it is quite a bit bigger and easier to see.

 

[Bill Tabor]

Still trying to get this forecast out, but I keep getting distracted with other issues / tasks. After reviewing this morning's NAM I like the target Cotulla/La Salle, TX by or near 0z. The forecast sounding was reasonable even for a possible torn showing turning in lower levels, decent helicity, cape, and only 25 cinh. I would expect the mid level wave to help kick this area off late this afternoon / early evening. Earlier I kind of had my eye on the Kerrville, San Antonio, Uvalde triangle, but NAM forecast soundings seemed to show a lot of capping later, and with a kink in hodographs on lower level winds.

The problem I see with this scenario now, as in the process of reviewing the RUC it seems to indicate decent 700mb winds are not to be had around COT. To get those you have to go further north. Likely further north and west.

I'm still looking at it. The RUC seems to be pushing me back toward the triangle mentioned above, or perhaps a hair further north or west. I like what I see in this quadrant of TX though for tornadoes as far as sfc, 925mb, and 850mb winds are concerned. It shows good backing around the low in Mexico, although I'd like to see a little more strength speedwise.

After I finish more RUC and mesoanalysis I will likely update these thoughts.

 

[Bill Tabor]

Hmm...tough call. I was trying to make something work (chase wise) in S TX, but doesn't look so good. The RUC has very little 700mb flow forecast in south TX. For torn potential per RUC that would kill it IMO. Alternatively further north in better 700mb winds it is likely behind the progressing front although it may be slowing down a bit or going stationary. I still see a very heavy capping inversion and cloud cover over south TX with fairly robust 700mb temps also for this time of year. Currently I see some increase in Cape across the border from COT in Mexico building with possible CINH eroding. Due to upslope storms will likely start over the Mexican mountains. They may be able to break east later today with support of the expect mid level wave. Mesoanalysis 700-400mb Diff Vorticity Advection seems to indicate a N/S area from near Rosenfield, Tx which may later today help touch off convection assuming CINH is not too great. I wouldn't expect much until near 0z.

I expect hail / wind storms, but can't pull together a storm really capable of producing a tornado. So far sfc to 850mb frontogenesis is fairly weak, but seems to be strengthening. Perhaps the boundary can aid and act to focus helicity... and help 700mb storm relative winds. Seems it is the only hope generally for a torn. I'll have to examine the boundary and see what's possible.

Anyway, still seems to me best chance for anything would be near the sfc boundary, closer to the Mexico side, somewhat near Cotulla. At the moment I don't see a reason to head out the door to reach that area.

 

[Bill Tabor]

Update: Still desk chasing . There is now a storm over the mountains in Mexico with very high capes uncapped in that region. Capping is still pretty strong in TX so far though. 850-250mb Diff Divergence shows a little action near COT though. http://www.spc.noaa.gov/exper/mesoanalysis/s1/ddiv.gif Not sure it's enough to do the trick. 21z RUC update shows some convection breaking out near COT at 0z and increasing along the frontal boundary to the ne by 3z.