Tornado Central

Well, this one is sort of creeping up on me. The 12z NAM is forecasting a relatively decent chance of chaseable storms across parts of the southern Plains Thursday afternoon. As a shortwave trough swings into the area overnight Wednesday into Thursday, both the GFS and NAM forecast the trough to cut-off through at least the mid-levels (e.g. 500 mb). The orientation of the upper-level jet looks nice, with northeastern OK and adjacent portions of KS, AR, and MO located in the left-exit region, which will result in a strengthing (per NAM) or quasi-steady-state (in a system-relative standpoint per GFS) low pressure system that moves eastward from the panhandles to northeastern OK by evening Thursday. Associated with the trough and mid-level low is very cold temperatures; 500 mb temps of -18 to -22 C are forecast to move atop the warm sector by afternoon.

Closer to the sfc, the NAM is a bit more aggressive in returning more substantial moisture to the area NE through SE of the sfc low, indicating sfc Tds >60 F possible east of the dryline by afternoon; the GFS primarily is forecasting mid-upper 50s Tds N of I40, with >60F Tds S of I40. The warm front associated with the sfc low looks like it'll be located (and moving slowly Nward with time) along the KS/OK border into southern MO. The GFS is forecasting more in the way of QPF during the day compared to the NAM, which, I assume, is why the GFS 2 m Ts are lower compared to the NAM (thereby impacting CAPE forecasts). Even so, both models are indicating >1500 j/kg E of the dryline Thursday afternoon.

This upcoming system is presenting a couple of target area opportunities. First, there's the usual warm-sector play E of the dryline. If we can get upper-50 Tds up here, the cold mid-level temps should result in workable instability. Another target would be ENE of the sfc low along the warm front, where one would have the benefit of backed sfc winds and a good baroclinic boundary. Surface moisture may not be quite as good (given advection is occuring from the south), but mid-level temps are likely to be cooler. The NAM is currently forecasting the best deep-layer shear along the dryline S of I40, as weaker mid-level flow resides in NE Oklahoma.

Of course, I think the biggest source of caution right now, for me at least, is the realized degree of moisture return. Current analyses indicate northerly winds of 30-40 mph across the western Gulf of Mexico behind the cold front that is current located from near a Tampa Bay, FL, to Yucatan peninsula line, and it will continue it's march southeastward into, at least, the northwestern Caribbean. In other words, the GoM is going to be completely scoured (heck, even part of the Caribbean will!).However, as Rich T points out this time of year, the temperature of the air behind the cold front is very important in determining the amount of airmass modification that occurs. In this case, the air behind this CF is actually rather cold With such cool air moving over the GoM, there should be more in the way of vertical moisture flux off the GoM owing to more a substantial difference between the sea surface the overlaying air. We are going to need everything we can get, I think, since current Tds along the upper Texas coast are in the 20s. Yes, the 20s, with temperatures in the upper 50s! *edit: Actually, Tds in deep south TX are even lower -- e.g. CRP currently has a Td of 19F. I remain quite skeptical of >58F Tds returning by Thursday, but there is a chance that we will see more rapid airmass modification than we saw ahead of previous systems this year for the reasons mentioned above.

Right now, I'm hoping this slows down a bit, since I'd rather be be targetting far eastern OK. Given the bias in the NAM/GFS to be a bit fast (after least they seem to be too fast more often than being too slow), I'm hoping we can back this up to the I35 corridor. The area along the OK/KS border immediately E/NE of the sfc low looks tasty to me.