2/21/07 FCST: TX

[Bill Tabor]

It may be premature, but GFS is indicating possible early season severe / tornadic event in Tx next Wed 21st. Hard to pinpoint area now, but somewhere near central / W TX IMO at the moment.

Discussion: Southwest flow will return at midlevels and bring a closed low / neutral tilt low and substantial PVA ingressing into Tx early in the day and affecting central Tx by 0Z. At the surface a mild / moderately strong low pressure area situated in sw Tx / Mexico will back light surface winds and provide gulf moisture with dewpoints to the lower 60's and temps between 60 to 65 in places by 0z. A warm frontal boundary should extend E/W just south of DFW extending west toward ABI and intersecting a weak trough oriented from ABI to DRT and should be the focus for severe weather. Just above the surface 850 mb winds are forecast from S / SSE at 20 to 40 knots. Above that level directional and speed shear are favorable for supercells. It appears convection will begin from the west influenced by lift from approaching mid level system. Mid level 40 to 80 know jet left exit region and upper level 100 to 125 jet streak divergent areas overlaying surface features should serve to intensify and focus storm intensity.

It's still early, but something to watch as this system develops (or falls apart).

 

[nickgrillo]

Kind of far out, no?

I've been glancing at the mid/long-range GFS for the past few days, and each run has had a tendency to bring a potential setup to the southern plains between the 21st-22nd. It forecasts low-level parcel trajectories to become more favorable around 48hrs prior to the arrival of the main shortwave (advecting enough BL moisture to go along with all of that turning with height -- i.e. southerly/southeasterly flow at the sfc and strong, southwest flow aloft at 300mb).

I think Monday was the first time I've really looked at any forecast models since late last summer, IIRC...

 

[Jeff Miller]

Well

7 Days is a long time indeed to become specific about a severe setup, but I am being sold on the idea of a pattern change for sure. Even though the setup looks decent ATTM, I don't want to get overly excited about anything at this point. It is premature, but one thing this does indicate is the intense longing for ANY sort of a well-favored pattern to develop! We're starving for severe, folks!

I'm not as sold on this scenario as I am of a change in the overall pattern. If anything, this will give us something to discuss and ponder over the next week or so. Let's hope the situation improves instead of dissipates!

 

[Bill Tabor]

Yes, 7 days is a long time, but I used a disclaimer - possibly premature, still early, may fall apart.

I did forget to mention any weaknesses, and I intended to. Here are some problems with the day as I see them right now, so far out.

The biggest weakness is of course the GFS which is notoriously unreliable this far out as far as accuracy is concerned. This thing may look completely different by next Wed, and no longer in sync. The GFS has burned me many times far out. That's why I almost don't rely on the models much until 24 to 48 hours out. But it is picking up something, and there is a chance albeit a small one at this point, that something like this could materialize.

Next, I'd say another problem is climatology. We usually don't get a good system like this in this area in February. Based on this the odds of the scenario panning out are low. Then again, no month is immune to severe weather, and I've seen some good severe in Feb in the past.

Also, short days of daylight will provide less opportunity for diabatic heating. This also provides less time for travel while chasing. I can remember a lot of chases in Feb / Mar where nothing happened until almost dark, and then it was just a drive home in the dark.

Next timing may be an issue as it appears to me that the area of mid level lift will not likely affect the better temps and dewpoints a bit further east until almost 0z and that is little time before dark for chasing (as mentioned above).

This is also only 1 model perception and may be flawed. Weaknesses I see in the scenario for not producing tornadoes would be the weak surface winds currently being indicated, and the fact that the sfc low will not be that potent and deep.

In summary, the scenario I painted previously was what I gleaned from the model interpretation, but all these above are reasons it may not pan out, especially this far out. However the major elements may be correct. A sfc low may form as well as the mid level wave. Timing may be off though. I've seen the GFS change the whole scenario by 1 day by speeding up or slowing down, even though the scenario may come to pass it could happen at midnight - for instance.

As I said before though, it could be interesting. It may be fun to watch. It also gives us (or at least myself) some motivation for getting my equipment checked, maintained, and repaired, and all set for the coming season. It could just be magic.

 

[Mike Hollingshead]

Not that it is worth much, but I swear I mentioned this very system, for this very day, to another chaser about 5 or 6 days ago as a joke, while looking out at the 352hr progs or so. The difference between that run and the one this morning weren't that different on positioning. Now though, tonight's ejects it much quicker.

I think the front and high crashing into the gulf this weekend will kill any hope the system has. Just looking at the 60-84hr progs on the nam it doesn't look good for the 21st. My hope is for the system after the one on the 21st, so long as it(first one here on the 21st) doesn't ride the coast and send another front hard to the south(which it very well may).

Edit: Looking at the gfs, this weekends gulf trashing doesn't look as bad as the nam has it.

 

[Bill Tabor]

Did I mean to say Tuesday, or has the GFS moved an entire day? If I was looking at the model run and it said 0 UTC Wed 21 Feb, I think I may have for some reason looked at my calendar to see what day the 21st was - obviously Wed. But if this is the case I meant Tuesday 20th. Did this run change this much from yesterday already?

 

[Mike Hollingshead]

Did I mean to say Tuesday, or has the GFS moved an entire day? If I was looking at the model run and it said 0 UTC Wed 21 Feb, I think I may have for some reason looked at my calendar to see what day the 21st was - obviously Wed. But if this is the case I meant Tuesday 20th. Did this run change this much from yesterday already?

It was showing it on the 21st all along but has been speeding it up the last day. Going to need a boat to chase it on the 21st now.

 

[Bill Tabor]

Ok, so I wasn't crazy. The GFS sped up by a whole day and is passing the low through Tx 24 hours earlier. It looks all screwed up to me now. I doubt that will give it enough time for recovery of moist gulf air or increased temperatures. The features don't look as well lined up now either.

Guess this is a good example of the accuracy of the GFS that far out. Guess I'll wait until a couple of days before to start trying to figure what exactly is coming. Chance are it will change 1 or 2 times before then so hard to say what day or what we'll end up with. Maybe I should be looking at a longer term Ensemble instead. Still probably wish casting 7 days out though.

 

[Jim Bishop]

The 12z Feb 16 GFS is looking much more interesting for East Texas/Louisiana for Tue, Feb 20. It has a stronger shortwave along with a stronger and more southerly (rathern than southwesterly) 850mb jet. Also it now indicates that a Pacific front could be the initiator of sever storms, rather than general warm air advection. So now we're talking about much better shear for supercells, and a boundary to initiate them. Looks interesting...maybe supercells in a broken squall line. Could be bad for Western Louisiana.

 

[Bill Tabor]

The 12z Feb 16 GFS is looking much more interesting for East Texas/Louisiana for Tue, Feb 20. It has a stronger shortwave along with a stronger and more southerly (rathern than southwesterly) 850mb jet. Also it now indicates that a Pacific front could be the initiator of sever storms, rather than general warm air advection. So now we're talking about much better shear for supercells, and a boundary to initiate them. Looks interesting...maybe supercells in a broken squall line. Could be bad for Western Louisiana.

I agree it looks a little better again. IMO the area now would be se TX (Gulf area) and southern LA. Srfc winds particularly at 0Z suck - almost nil though for tornadoes, and 850's are mostly veered except in southern LA. Does look like vorticity and upper jets will affect this area though.

Guess we'll have to see what fiction it comes up with on the next run.

EDIT: Also, I agree...this could be bad for Louisianna.

 

[Bill Tabor]

Tuesday is now on the NAM radar screen as well as the GFS. GFS is now showing it similar to yesterdays run near the Tx gulf coast into southern LA, but doesn't actually show precip here (as I recall) during the daylight. Climatologically that area is somewhat correct.

The NAM on the other hand has a bit more positive view (for chasing) as it is a bit further west during the same time frame and shows 60 degree dewpoints by 0Z up into central Tx along with mid level wave impacting this area around the same time. Srfc winds look marginally ok to me direction wise, but are still weak. 850mb winds are mostly less veered along the Tx coast, but still a bit too sw oriented IMO. A stronger srfc low could back those winds, but I suppose that would only happen if the system slowed down and the mid level low influenced the area more and earlier.

Still remain pessimistic about the quality of this day with regards to chasing. My intuition tells me that something might happen down toward the HOU area late and on into southern LA. That is what usually happens there this time of year. It will probably be another one of those murky bad visibility days. Still it might be an opportunity if someone was so inclined assuming the NAM maintains or improves.

 

[Bill Tabor]

Amazing how this day has really changed since it was originally 7 days out on the GFS. Now just one day before it looks nothing like it did a week ago. Now the low which was to be in sw TX and Mexico and back winds will be in OK headed for AR, and solid, discrete mid level low is now headed quite a bit further north too and is sheered out positively from sw to ne.

This is a completely different day now, and looks like mostly a non-event for Tx, which is what I was interested in. Oh well. Guess I'll take a look at later in the week that a few people have mentioned, and see what's up.