11/29/10 FCST: TX,LA,AR,MS

[Zack Cooper]

Looks like a possible severe weather outbreak may take place tomorrow over portions of E TX, North LA, and Southern AR.

looks like around 12Z Monday, a low pressure system will be located near the KS, NE border, with a strong pre frontal trough moving into a rather warm, moist airmass.

As of 18Z Monday, GFS forecasts dews to be in the lower to middle 60's over much of E TX and NW LA... looks like a dryline will be moving East into this area at this time, and while lapse rates look modest at best, CAPE values at or above 500 J/KG and limited CIN, I think thunderstorm development, including a few supercells, appears likely over extreme E tx Monday Afternoon. 40-60 KT effective bulk shear is fcst, so a few strong tornadoes definitely cannot be ruled out over N CNTRAL LA AND S CNTRAL AR.

 

[J Brooks]

Yeah I am in agreement here. LA is looking like the place to be tomorrow during daylight hours, then over into MS tomorrow evening and over night.

Both the NAM and GFS 21z runs seem to be in pretty good agreement on timing and placement of this system. The cold front should be near the TX/LA border by around noon with the surface low sitting over the central plains. An associated warm front expanding northward through east TX, LA, and into AR ahead of the front. That looks to provide enough warm air to allow 500 - 1000 j/kg CAPE to build into the area. Moisture return will be more than sufficient with a dryline sitting right on the TX/LA border. Impressive wind shear over the area, good moisture return, and sufficient CAPE should give everything needed for a classic late November severe weather outbreak.

I won't be able to take the whole day off tomorrow, but I will be headed for northern LA as far west of Monroe as I can get early tomorrow afternoon, and will follow the system back into MS tomorrow evening.

 

[Zack Cooper]

Based off the 00Z NAM, I believe a significant severe weather outbreak may take place over portions of Louisiana and Southern AR.

as of 18Z Monday, NAM forecasts Helicity values of 350-450 over much of Northern LA and South Central AR... Effective bulk shear AOA 55 KTS pretty much area wide, and based off forecast dew's, a pretty significant dryline will be surging into this airmass supportive of tornadoes, so a few supercells could definitely spawn strong tornadoes over NCENTRAL LA tomorrow afternoon. The only issue I see is low instability, with CAPE values only 500-1000 J/KG over most of ARKLATEX region and into the Mississippi valley, only when you get further South in Louisiana does it appear cape values will be above 1000 J/KG.

 

[Jesse Risley]

I do notice that the 00z suite of numerical models really wants to advect the higher surface dews (AOA 60 degrees) to points along and north of US 20 by 00z TUE, though I'm inclined to remain a bit skeptical on this, since that would be a recovery of 25+ degrees in less than 24 hours so far north, taking into consideration current (05:20z) surface observations across that region.

The slight discrepancy between the juxtaposition of the highest CAPE and SRH values is also evident on the 00z models, though there is sufficient overlap of ample instability and favorable shear profiles between US 20 and the Hwy 84 corridor in the vicinity of Alexandria, LA (Sfc - 3 km lapse rates will probably not get much better than 6 C), where the best TOR thread will likely reside (forecast soundings aren't exactly unimpressive).

The evolution of the most favorable CAPE values will ultimately depend on the behavior of those 2m dewpoints, along with whatever pockets of clearing might occur ahead of the main front.

 

[Zack Cooper]

dewpoints this morning in the lower 50's here at SHV. Models want to bring dews up torwards 65 torwards this afternoon but I'm a bit skeptical... so will wait and see..

Dryline still sitting back there West of Dallas... so I think SPC'S outlook may need to be shifted slightly westward... depending on the speed of the dryline.. then again, it is only 4 AM central!

Mid level lapse rates are currently better than anticipated... with a few areas of 7.5.

What really concerns me though for today is that effective bulk shear is forecast to be 50-70 KT... and helicity values may reach 450-500 m2/s2 in some places!

I still think... given that we get at least some moisture return and instability... an outbreak of severe weather... including a few strong tornadoes.. is possible today

 

[Jeff House]

This is not looking like an outbreak to me, but could unfortunately could still include some dangerous nocturnal weather for LA/MS. I’d place the 10% TOR south and west a bit, with greater thermodynamics and low-level turning, which displaces it from the best upper support. Therefore, I’d lose the hatched area. Since I have zero interest in chasing, I’m happy to forecast no hatched for local residents at night.

The trough is positively tilted, which might mean less junk convection. However with the just in time moisture return, a neg tilt would be required for an outbreak. This is the South where a neg tilt could bring bad things embedded in the junk. Fortunately, that’s not the case tonight. I’m a seller of strong tornadoes.

I look for isolated short-lived tornadoes along the warm front lifting from the Gulf and in the warm sector ahead of the DL/CF. This should mainly affect central LA to southern MS late this afternoon into tonight. Otherwise look for a line to organize and promote some straight winds. Good luck and be safe to those chasing.