3/18/08 FCST: TX/AR/LA

[Bill Schintler]

Chase target:
Marshall, TX (25 miles west of Shreveport)

Timing and storm mode:
Storms will be ongoing and a large convective complex will move into the target area between 1 and 2 PM CDT. A full spectrum of severe weather is forecast including embedded supercells and tornadoes. Chase conditions will be poor, however, due to rapid storm motion to the northeast at 50 mph, a poor road network, and limited visibility because of low cloud bases and an intervening stratus deck.

Synopsis:
The H5 low will sink SE before turning towards the NE as an 85kt streak ejects from the trough base. A large MCS will be ongoing early in the period owing to a SRLY 60kt LLJ. Ahead of this convection, SFC-based instability will increase during the day despite widespread overcast over ERN TX.

Discussion:
Modest instability will be juxtaposed with very impressive shear parameters and low cloud base levels which will create an environment favorable for embedded bow echoes and supercells within the leading edge of a large convective system. Additionally, discrete storms are probable ahead of the MCS during the afternoon hours. Deep layer shear will increase to 80 kts as the aforementioned H5 flow spreads E over the region. Very large hodograph curvatures with SRHs AOA 500m2/s2 will be present as the LLJ increases to 70kts and overspreads backing SFC wind fields. Individual cells will race rapidly to the NE at up to 60mph due to strong flow in the H9-H4 layer.

- bill
8:45 PM CDT, 03/17/08

 

[Jim Bishop]

GFS hasn't come out yet. But from the last couple runs and the latest WRF this looks like a classic Southeast and East Texas tornado outbreak. In particular it looks like the Houston area will be under the gun in the late morning hours.

Unfortunately storms will be moving 50+ mph to the north to north-northeast. So I'll setup on a state highway and let the storms come to me.

With the backed surface flow under a 60knot southerly 850mb jet hodographs look amazing for tornadoes on any storm that can move northeast. Any discrete supercells with have high tornado potential. Also a squall line will eventually form and cell within the line with have good tornado potential.

This will be tough and messy but I'll give it a shot since it's so close to home.

 

[Bill Tabor]

Today is interesting with some fairly strong parameters per mesoanalysis showing in the area near and east of AUS (both to north and south) toward LA and coastal region HOU. The sfc low is in place as I recall just sw of AUS and sfc winds are backed a bit with all overhead jets in place. However so far seems we are primarily getting heavy flooding rains through training line segments. Anyone have any thoughts on this congealing and becoming something more respectful given SPC's PDS? If so, what mechanism do you see focusing this? Right now, to me it appears it will continue to do this. Scratching my head trying to figure out how to make supercells of all this. Thoughts?

 

[Bill Tabor]

David Douglas reminds me there isn't much Helicity near AUS. Checking mesoanalysis to some degree that is true. Most of it is still east of here which may explain why current new line passing through Williamson and Travis aren't doing much.

I was checking 850mb to 500mb crossover winds and was surprised to find they are really a bit backward near AUS. You almost have to go to the LA border to get good crossover. SO....I suppose that implies that as the day and night wears on these storms should get into a more organized environment for creating supercells. We'll see how it goes. Unfortunately I am stuck here in AUS for another hour at least. Will be tough to catch up east if anything decent develops. Probably it will nearly be in the tall pine trees anyway before anything decent develops. Perhaps you folks down near HOU metro may have a look at something near home or perhaps just to your north as the day wears on.

 

[David Douglas]

Agreed. Right now I'd be interested in the area just west of Houston. The 1 Km SRH values are in the 700-800 range, ML CAPEs in the 1500 range, dewpoints around 70, and the mid-level jet is punching into that region.

 

[Bill Tabor]

I meant to add much earlier today that another reason for the ongoing difficulty with severe (tornadic), etc is the line of storms earlier today (perhaps still now) were actually behind (west and northwest of) the trough and sfc boundary. This was mentioned by SPC in an MSD and they also mentioned they were elevated. SPC also mentioned there was some vigorous storms ongoing ahead of the boundary (east of it) but when I checked an hour later I didn't see any.

However it now seems to me that perhaps the primary reason for lack of severe is too much forcing and upper dynamics releasing the atmospheric energy quickly without being focused on a particular boundary. This is causing numerous widespread storms which is releasing the instability and stabilizing the atmosphere over time.

So far it appears that storms in LA (south of the trough / boundary) are primarily remaining non-severe. Some of the the storms (eastern half) are even in reasonable 850/500 crossover winds. This southern area may continue this way tonight until a factor changes...