Stormtalk

Monday, 8 March: 10:oo a.m.

Central Texas looking a little more favorable but not great, cold core potential continues in TX/OK panhandles…

This setup, while still not impressive, is a bit more interesting this morning.  We are still dealing with a close upper low system that would normally be most favorable for cold core supercells over the Oklahoma and Texas panhandles.  That setup still looks alive and probably holds the best opportunity to see a decent, chasesable storm today.

The other area is over central Texas in the warm sector ahead of the dryline.  Despite the shortwave being a close low, deep layer shear is more than supportive for supercells since the 500mb winds are so veered (straight out of the southwest) and quite strong (60-65 knots).  This is on top of 850mb flow out of the south-southwest around 30 knots. But the main problem – which has been stated here for about a week – is low level cloud cover. The good news is the cirrus deck is already moving out of central Texas, so all we need is daytime heating to overcome the low and mid level warm air advection.  And maybe enough mixing will occur east of the dryline to allow for some sunshine to break through the clouds west and southwest of Dallas to build enough CAPE to support severe storms.

The evolution of the ‘clearing’ this afternoon across western to central Texas should be interesting and will determine whether or not supercells can develop.  If they do develop low level shear is supportive of an isolated tornado or two with dewpoints in the upper 50s to near 60 and surface temperatures in the upper 60s to 70s.  Otherwise the cloud cover may simply remain largely in place, keeping CAPE values quite low.  This would result in mostly elevated convection and basically a big mess.

The SPC only has a slight risk for the cold core setup in the TX/OK panhandles, with just a See Text region over Texas.  Like I said before this setup isn’t great, in fact it’s pretty marginal.  But if conditions come together just right things could get interesting later this afternoon and early into the evening.

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Friday, 5 March:  10:00 a.m.

Mostly a messy severe potential for Monday across Texas and Oklahoma continues….
If anyone is planning on chasing feel free to leave a comment.  I’d love to hear your thoughts/plans.

I’m happy to see the GFS/European models have trended in the way I expected yesterday.  The GFS is lower to eject the shortwave into the southern Plains on Monday.  Plus the European sped up the system a bit and now both models bring the upper low to the Texas panhandle by 00z Tue 9 March (Monday evening).  Details with the wave are still subject to change since, again, we are dealing with a closed upper low over the Pacific.  But at least now we can have confidence in this wave impacting the southern Plains on Monday.

With a surface low expected to deepen during the day over the Texas panhandle expect a dryline (albeit difuse) to mix eastward throughout the day from west Texas into central and western north Texas by the evening.  Moisture is going to be an issue, with the GFS bringing lower to possibly mid 50s dewpoints to the I-20 corridor and lower 50s to the Red River and southern Oklahoma.  But despite that lapse rates will be quite steep, especially later in the afternoon/evening as the upper low moves in and 500mb temperatures drop to -18/-20C.   If the low level cloud deck can be mixed out by sunshine 500-1000 j/kg of CAPE is possible across central to western north Texas.

One  good thing I can say is the GFS does show a clear slot in the upper levels clearing our the high level clouds by midday across western and central Texas.  But the model doesn’t actually show the low level cloud deck mixing out.  But this will be a key player in the forecast across the warm sector.  Should that cloud deck mix out, the 500-1000 CAPE could be realized and support the instability necessary for severe thunderstorms, some possibly supercells. But it’s going to be an uphill battle with intense warm advection over central Texas.  Although maybe enough dry air will mix in just east of the dryline and bring a small clear slot….we’ll see.

The 850 to 500mb directional shear looks quite poor.  And there will be a lot of forcing by the afternoon and evening from intense vorticity advection.  So I expect mostly linear storms/bow echoes across the warm sector in Texas initially.  As storms move rapidly north-northeast or northeast a squall line should develop.  It really just looks like a messy setup still across Texas unless some things change.  Across central Oklahoma there should be a lot of rain with all the warm advection.

The cold core setup looks further west over the Texas or Oklahoma panhandles, if such a setup exists.  I’m personally not impressed with it right now, but things could certainly change.

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Thursday, 4 March:  2:00 p.m.

Well the focus for the first potentially decent chase setup has shifted from Sunday to Monday across the southern Plains.  Computer models insist that a shortwave that is currently over the eastern Pacific will be slower than previously depicted to eject into the desert southwest and then southern Plains Sunday into Monday.

Below is the latest (12z March 4) GFS run, showing the 500mb shortwave over Texas and Oklahoma midday Monday.  This scenario would present a warm sector severe thunderstorm threat across much of central and eastern Texas.  A strong 850mb jet out of the south underneath a very strong southwesterly 500mb jet will provide great shear for supercells.  But the problem – like Sunday – will be extensive low level cloud cover.  If a clear slot can develop in the afternoon then things could get interesting in Texas, but right now that setup looks messy.  Don’t worry – these details will probably change, so stay tuned.

Another possibility is for a cold core setup of low topped supercells and possibly even tornadoes across western Oklahoma. This type of setup is much more difficult to pin down this far in advance, but the potential is there.

I do want to point out that the slower European solution has been leading the way with this shortwave.  The latest 12z March 4 European remains slower than the GFS, bringing the closed 500mb low to the northwestern Texas panhandle by 00z Tuesday, March 9 (Monday evening). So how slowly this shortwave ejects eastward remains an issue in the models.  This usually happens when a closed upper low is involved coming out of the eastern Pacific.   I think models will continue to have problems with the timing until the system is actually measured with upper air data in California (Sunday morning).  Until then expect continued model disagreement.

So I would expect the GFS to continue trending slower with the eastward ejection of this shortwave, but how much more is the question.  At some point the European will have to speed it up a bit as well.

Stay tuned……

- Jim