Severe Weather 2/28/12

An active pattern is back, and we have severe rounds of severe wx to watch out for in the coming days. In this post, I will focus on the round today, which has been pegged by most in the weather circles as a forgettable precursor to a bigger event on Friday. Now I’m going to go out on a limb and say this has potential to be bad on its own. The usual disclaimers apply – i.e. my reasoning can be off sometimes, I’m not an expert, listen to official sources, etc. But I did want to share my two cents, even knowing I could be completely off.

Let’s say we had nothing but observational data and H5 and surface model progs available to us. The first thing I would notice is that we have an incredibly dynamic system on our hands: at H5 0Z tmrw we have SW winds of 90-100 kt, temps of <-15C north of I-40; at San Diego 0Z today we had hail. The moisture is there: 60F isodrosotherm to I-40, 65F isodrosotherm to I-20. Without any other data, such as CAPE/SRH/EHI/STP/even H7/H85/H2 data… this looks like a MDT/HIGH risk day.

But nobody is considering it one. Why?

What’s wrong is that the thermodynamics are not there supposedly. The moisture is there, the cold upper air temps are there, but the surface temperatures are cool. H5-sfc lapse rates SUCK. CAPE SUCKSx2. But wait. Let’s look at it more closely:

^A forecast sounding valid for 0Z tomorrow, for Hutchinson, KS.
Lapse rates are HELLA steep below H7 – nearly dry-adiabatic. Sure, we could use some better directional shear – and for that reason I don’t expect tornadic supercells to explode and multiply in srn KS – but the thermodynamic profile isn’t all that bad. The warm layer above H7 is the limiting factor. Erase that, CAPE values are okay, and regardless, 0-3 km CAPE values look pretty good from an initial look. We all know how bad models can be in regions of strong temperature advection, so it’s hard to promise that the warm layer will be modeled correctly at game time. I wouldn’t be surprised to see some tornadoes from low-topped supercells between extreme nrn OK and srn NE, but only initially in their storm cycles (the paltry directional shear will hurt storm modes over time) and close to the dryline where there will be solar insolation. Note that many models do attempt to break out precipitation in this area.

Such a threat would initially remain near and west of I-35. Because of the veered nature of the flow regime and the drought in the srn Plains, I think the dryline will mix a bit further east than the NAM indicates. Further east, stratus will rule the day, and models keep some type of warm front over central AR due to ongoing convection or some cold pool. Basically, north of the warm front is stable land. Furthermore, models keep a strong cap in the area through 0Z.

But after 0Z things change bigtime. Between 0Z and 6Z temperatures actually go up as the LLJ veers into AR and transports warm air into the area. Either (a) storms move in from the west, or (b) storms blow up along the LLJ axis as height falls overspread the region, but if these can become discrete in Scenario (a) or root to the surface in Scenario (b), I wouldn’t be surprised to see some sig tornadoes in wrn AR or ern OK tomorrow. Note that because of the capping and weaker forcing to the south the threat will remain north of the Red River.

(Compare 0Z and 6Z surface temps over AR below.)

This is the scenario I’m envisioning. Some low-topped supercells or a marginally severe QLCS develops along the dryline between 21Z-0Z, from extreme nrn OK to NE. These may produce a few isolated tornadoes and a few marginal severe reports. (5% tor, 15% hail, 5% wind) After around 0Z, the LLJ axis veers and intensifies, transporting higher theta-E air to the Ozarks and Ouachitas of ern OK and wrn AR. In this regime the enhanced lift from the LLJ will initiate storms starting around 02Z. Of course the storms will be elevated in the beginning, but as CINH is eroded the strongest cells in the new batch plus possible convection left over from the west could become tornadic around 04-05Z. (10% hatched tor, 15% hail, 30% wind) These storms will move east and slowly morph into a QLCS as surface flow veers. The tornado threat will decrease accordingly around 08-09Z.

Again not a HUGE threat day, but I think we might get some notable tors by the end of the day if the latter scenario verifies. And that may end up surprising some people who will overlook the setup tomorrow.

…Added post-script….
As a final note, and to tie this thing up, because this trough is so beautiful and dynamic, and because we have the low-level moisture that is so often missing in these setups, it might not hurt to be slightly more bullish than usual in this situation. There are instances that my intuitive feel from the most basic maps end up being more correct. And in this case, the intuitive look of this trough is MDT/HIGH risk.


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