2/24/11 FCST: OK, TX, LA, AR, MO

[Donovan Gruner]

Looks like models are beginning to agree more and more on the details of Thursday's possible event. Looks like the center of the low will head over N TX into Central OK by early afternoon. Dewpoints well into the 60's should have the moisture more than covered. Strong jet max over N TX and Central OK and possibly 50+ kt shear, along with some dryline involvement could make this prove to be quite the season opener...

Any other thoughts? I know we're still far out, but it looks pretty damn good as of now.

 

[Kelly Sugden]

Definitely something to watch. Fairly significant shot at severe in a long time. SPC was thinking more of a supercell storm mode rather than linear. I took at look at some forecast soundings per the 20.00Z ECMWF. Looks more linear and like a big squall line but I wouldn't rule out a few sups along or ahead of the front. Anyway, way too early for mesoscale details. Crappy chase country.

 

[Trey Thee]

Definitely something to watch. Fairly significant shot at severe in a long time. SPC was thinking more of a supercell storm mode rather than linear. I took at look at some forecast soundings per the 20.00Z ECMWF. Looks more linear and like a big squall line but I wouldn't rule out a few sups along or ahead of the front. Anyway, way too early for mesoscale details. Crappy chase country.

I agree its definitely too early for details...it's also too early to pin down the exact location. So perhaps we'll see some minor track changes in the coming days. But I think N TX and C OK are pretty good to chase in.

Donovan, I agree things are looking pretty impressive at this point. However, given what I've seen in the past with these early systems, I'm not pinning any hopes up just yet. If we wake up Tuesday morning and still have good instability predicted and along with moisture return and strong jet streak, I will start to believe...until then I'll just monitor the model changes and of course stormtrack

 

[Skip Talbot]

Wow, a very potent looking system that could be one of our first big severe weather events of the plains this season. Looking off the past couple GFS runs the setup is no slam dunk though and has quite the mix of pros and cons:

Pros:

• Strong deep layer speed shear of 50-70 knots
• Ample moisture with dewpoints into the 60's across much of the warm sector

Cons:

• Poor instability
• Poor directional shear

From a chasing stanpoint I would add the additional cons:

• Excessive storm speeds
• Poor terrain
• Poor visibility due to low cloud bases and widespread convection

The GFS looks like its finally starting to settle on a solution for this system after bouncing around quite a bit the last few days. NAM and GFS are in fairly good agreement out to 84 (the day before Thursday) with location and strength of the trough, but the GFS looks like its a little more bullish on the moisture return. We'll still probably have more than adequate moisture return into e TX, LA, and AR, however. My biggest concern is probably the storm mode, along with the available instability. Winds are pretty badly veered from the surface up. It says linear mode to me. The excessive speed shear is probably more than enough to spin up supercells in there, but I imagine that a lot of junkvection is also going to be forced up and many of these sups will be embedded. The models are also forecasting lingering precip and cloud cover to hamper instability. Instability of 750 J/kg is enough for supercells and tornadoes when coupled with these shear profiles, but its still pretty marginal. We may very well see these values drop off on tomorrow morning's NAM run, just as we saw with today's (2/20) setup when the GFS showed some instability when the event was further out. Just given the moisture return and the dyanmics of the upper level system, we may see some severe wind and a few tornadoes in northeast TX into LA/AR, but enough crapvection in between to make them needles in a haystack.

From a chasing standpoint, I'm not really considering this setup. Its setting up over terrible terrain, and the lack of directional shear and excessive speed shear is going to make for screaming storm speeds. The low LCL's due to the cool surface temps (which is also what's limiting the instability) will make for low, grungy bases and a low cloud cover so visibility between the trees and hills will be further hampered. If we could just get this system to slow down a little bit I think we'd be much better off, with the dryline further west over better terrain, and the wind profile not so veered and unidirectional.

 

[Tim Marquis]

There has been a significant slow down in the GFS 06z and 12z runs. Lets hope that this trend continues as the terrain situation drastically improves with the slower system. I still am hesitant with how strong of an event this will be especially since its 5 days out and its the end of February. How fast the gulf warms up and moistens will play a huge role over the next coming days to how much moisture will be there on Thursday. Very strong surface moisture gradient and deep layer shear and moderate instability up to 1000J/kg of cape in some areas. Overall for a February set up not bad at all.

 

[Greg Blumberg]

Pretty impressive setup being put together by the 12 UTC GFS. I haven't been paying much attention but now with the SPC outlook and the recent trends I've been seeing here in Norman (and synoptically) over the past few days, I have good feelings about Thursday. My short term forecast is speaking wonders for later on this week. I can't see the current western OK/KS (maybe further into Colorado) making it far into the Gulf to wipe out the good moisture. It really only should just push the moisture back, stall and wait for the southerly winds to advect it. Also, considering what I've been seeing so far with the Gulf's buoys, I really don't find moisture that much of a concern.

I will add that I was unimpressed with the 12 UTC NAM's performance handling the current surface low pressure system in extreme western Iowa. The model had the low significantly further south and not as deep. The 12 UTC GFS did have the correct track, but also failed in seeing the deepening. These two things make me seriously question the validity of both the NAM (if we're using the NAM to look for short term model solution convergence) and GFS Thursday solutions. Additionally, I don't see the GFS handling the building of the downstream Mississippi River ridge today well. I believe it's going to build northward much more than advertised by the models. Having that could push the target area westward and lead to more "slowing down" as Tim mentioned, which is a plus for those who don't want to chase in the jungle.

 

[Jesse Risley]

I will add that I was unimpressed with the 12 UTC NAM's performance handling the current surface low pressure system in extreme western Iowa. The model had the low significantly further south and not as deep. The 12 UTC GFS did have the correct track, but also failed in seeing the deepening. These two things make me seriously question the validity of both the NAM (if we're using the NAM to look for short term model solution convergence) and GFS Thursday solutions. Additionally, I don't see the GFS handling the building of the downstream Mississippi River ridge today well. I believe it's going to build northward much more than advertised by the models. Having that could push the target area westward and lead to more "slowing down" as Tim mentioned, which is a plus for those who don't want to chase in the jungle.

This would stand to reason given where the best zone of baroclinicity will lie thanks in part to the temperature patterns. The 12z ECMWF has hopped on board with this reasoning, slowing the system down a bit and deepening the low pressure system as compared to last night's 00z run, with it now showing the low deepening (<1000 mb) and trekking roughly along the I-44/I-70 corridor from NE OK - STL - C IN by 6z FRI as opposed to the earlier advertised Ozark/Ohio Valley track.

As was previously mentioned, while it's too early to be talking about specific parameters, especially mesoscale details, the GFS is far more magnanimous in both areal extent and degree of CAPE, with a broad area of 700 J/KG values advertised across parts of TX/OK/AR, whereas the ECMWF advertised a far less impressive area of lower CAPE values AOB 500 J/KG mainly confined to the Middle and Lower Mississippi Valley where the best shear will co-exist with whatever instability eventually materializes across the region where PVA is found downwind of the H5 vort max, featuring optimal upper level divergence.

It will be interesting to see how the shortwave itself evolves, as this will impact the evolution of the SVR threat on Wed evening and into Thursday. Future model runs should better refine the pattern, but the GFS, with its slightly further south and slower surface feature progression, naturally has the shortwave with associated region of higher vorticity further south as well, entering C OK by 00z FRI; notwithstanding, the ECMWF is progging the mid-level impulse further north across central KS/OK (EC High Plains/Plains Border physiographic regions) by 18z THUR. Finally, I decide to take a passing glance and noticed some significant differences on the H7 synoptic scale VVL chart. The GFS has some very significant values (> 10 mb/s) across parts of the Ozarks THUR evening, whereas the ECMWF is more modest with values (app. 2-6 mb/s) across much of the same region.

I suspect further timing and placement of mid and upper level features will be resolved in the next 48 hours, which will give a better clue as to how, when and where the best SVR parameters will evolve on THUR assuming everything comes together as progged.

 

[Andrew Ryan]

This is a situation that definitely needs to be monitored. A lot of people are dismissing the potential because the GFS has all the potential in eastern Oklahoma and Arkansas but, as Mike said earlier, the GFS has a known SE bias and this case is no different.

The ingredients (moisture, baroclinic instability, longwave trough) are there, we just need something to get it going. Someone is going to see severe weather, it's just a matter of when. I made a blog post about the upcoming potential and went into a little bit more detail about the ingredients http://www.tornadopix.com/627/severe-weather-possible-in-oklahoma-texas-and-arkansas-on-thursday/

 

[Connor McCrorey]

The 12Z NAM is showing lapse rates that are nearly dry adiabatic all the way from the surface up to almost 700 mb in parts of NE OK. With temperatures in the upper 60s along with dewpoints in the upper 50s, this is leading to some awesome 0-3 km CAPE, even with relatively disappointing instability as a whole thanks to warmer temperatures aloft. I may end up chasing Thursday, and I'm thinking NE OK might be the place to do it. The better speed shear is out in Arkansas, but it's certainly not bad in OK, with 850 mb winds at over 40 kts in NE OK. Directional shear is terrible everywhere, so unless this slows and we see some more backed low levels winds, I'm not too excited about this chase... But with the vort max cut off and not shown coming onshore for another 48 hours, I have a feeling the models aren't handling this one well and we may see something very different when the day actually comes.

 

[Skip Talbot]

Good points about the low level instability, and I agree with your target Connor.

I think the timing on this system is pretty bad from what the 12z NAM is showing. Its more progressive than I thought it would be, and maybe there is still time to slow it down some, but that trough ejects out across the warm sector by late morning. The forcing from this is going to kick up a ton of cloud cover and precip in the warm sector, which is going to severely limit surface heating and instability. Right now its looking like northeast TX into AR will feature a strongly forced, low topped linear line. You could still get some strong winds and embedded supercells out of that, but I wouldn't consider this at all a favorable chase setup.

Further north across eastern Oklahoma. It looks like a dry slot is showing up at 750 mb. This could punch in from the west, clear out those clouds, and provide some last minute surface heating for the steep lapse rates Connor mentioned. I'm worried about storm mode here too though. Low topped storms may be stuck in a linear mode due to badly veered surface winds and undercutting from the fast moving cold front/dryline. Still, I think this area shows the best potential for low topped, discrete supercells provided that we get airmass recovery behind the morning crapvection. Perhaps a tornado or too if we can pool some of that low level instability along the warm front in northeast OK/southern MO, where low level winds back strongly.

 

[ChristianTerry]

Gah!!! Couldn't this come on a weekend when I could actually chase!!! Anyways, moisture is looking very rich coming up from the Gulf. NE Texas and SE Oklahoma as well as Arkansas is in very good deep moisture. Instability levels are a little low with 500-750KG showing up on the models. Mother Nature however doesn't always need tons of instability. Shear looks okay for the time being. Arkansas should be in the better for the chances.

 

[Mikey Gribble]

I'm not very impressed with the tornado potential for Thursday. I didn't spend much time forecasting yet and I only looked at the eastern Oklahoma into western Arkansas target, so take this for what it's worth.
Some noteable differences between the NAM and GFS with instability and convection. The GFS is more aggressive with instability and fires convection along the surface low, dryline and frontal boundary early in the afternoon. The NAM on the other hand has weaker instability and although it shows crapvection north of the front, there isn't any real precip near the surface low during daylight hours. I think it is important we get good surface heating and instability building early in the day if there is going to be any real chance for tornadic storms. 850mb winds are veering at 18Z, but they start veering even worse as the afternoon progresses. Once they start veering badly I think you can forget about tornadoes. The only window I can see for a tornado threat is if the GFS is right and we have adequate instability (and storms fire by early afternoon), then there may be enough turning in the lowest 1km to get it done during a short window before 850mb winds veer hard. I'm not a fan of setups like this where you have somewhat backed surface winds and then unidirectional shear above that. It rarely bodes well for tornado potential.
Really the only good things I see with this setup is the potential for seasonably good moisture and decent deep layer shear (which is to be expected this time of year). Other than that I don't see anything to get excited about, but maybe I'm missing something.
With fast storm motions, questionable chasing terrain and mediocre at best tornado potential I will likely be sitting this one out. I won't make that call until I have a chance to see a few more model runs though.

 

[Connor McCrorey]

Based off of the NAM alone, I'm seeing quite a few similarities to March 8, 2010. Big severe squall line moving through early, then by mid afternoon a dry slot helps clouds clear out, allowing for rapid heating and destabilization of a previously worked-over atmosphere. This heating under very cold air in the low levels allowing for nearly dry adiabatic lapse rates up to 700 mb (as mentioned in my last post) should yield some insane 0-3 km CAPE values. 700 mb temperatures should actually be almost identical to those near Hammon on 3/8, at about -3C (hard to believe that in a few more months we'll be looking at cap busts with 13C or higher temps up there!). One difference is that surface temperatures and dewpoints should actually be higher than 3/8 this time around, with even smaller T/Td spreads.

Differences? The Hammon tornado and the two after it were located significantly SE of the sfc low, like a more classic warm sector setup (no, the Hammon event was not cold core). This Thursday, the area I'm referring to looks to be located farther northwest relative to the low, where low level winds will likely be more veered. One thing to remember, though, is that while the low level winds on 3/8 were only slightly veered, with surface winds actually out of the south (<--I think that's correct), mid and upper level winds were much more meridional, with 500 mb winds out of the S/SSW over W OK. Flow at that same level this Thursday should be pretty much due southwesterly.

Significant low level CAPE, rapid clearing behind an early squall line, really cold temps just above the surface, and great speed shear but a lack of turning with height are all things that I see in common between the two days. I'm not saying Thursday will turn out like 3/8/10, but hey, it would be fun

EDIT: I should mention that I'm referring specifically to a confined area near the OK/KS border. To see the kind of lapse rates I'm talking about, check this out - http://www.twisterdata.com/index.ph...iew=large&archive=false&sounding=y&sndclick=y

 

[Scott Hammel]

Based off of the NAM alone, I'm seeing quite a few similarities to March 8, 2010. Big severe squall line moving through early, then by mid afternoon a dry slot helps clouds clear out, allowing for rapid heating and destabilization of a previously worked-over atmosphere. This heating under very cold air in the low levels allowing for nearly dry adiabatic lapse rates up to 700 mb (as mentioned in my last post) should yield some insane 0-3 km CAPE values. 700 mb temperatures should actually be almost identical to those near Hammon on 3/8, at about -3C (hard to believe that in a few more months we'll be looking at cap busts with 13C or higher temps up there!). One difference is that surface temperatures and dewpoints should actually be higher than 3/8 this time around, with even smaller T/Td spreads.

Yeah, I was thinking a lot along these lines as well but I'm extremely worried about timing. I agree that there should be some clearing of the worked over air behind the squall but I'm afraid it might happen too late in the day to really get adequate heating and enough CAPE to work with. Granted, we need less instability and temp rises than a typical setup this early in the season but it could be an issue. And I agree that the speed shear is there and I like the same area on the OK/KS border as of now. There's some good lift forecasted in that area too, so who knows? Still 50/50 on this but my eyes will be on the models. BTW, nice seeing you on Saturday!

 

[Bill Tabor]

Appears to me the arrival of the systerm has been delayed a bit by perhap 6 hours or so as compared to yesterday's 12z run. GFS and NAM are mostly in agreement. The position of the sfc low was showing in the western OK panhandle at 12z, but now is showing in nw TX perhaps near ABI (from memory) at this time. The sfc frontal boundary now shows extending E/W across the middle of OK. A dryline bulge is shown transitioning during the day along the Red River. The mid level vort max begins at 12z in southern OK and is near Ardmore by 0z. The sfc frontal boundary, sfc low, and mid level vort are all further south than previous and slowed down a bit as mentioned.

Certainly OK will be under the gun all day. Most likely the area I mentioned previously beginning ne of CDS (over across the OK border to the frontal boundary) at 12z moving east along the sfc boundary during the day could be a decent storm zone. (Note: although this beginning position and time may be too far west and too early). It may begin to look best in OK as it nears I35.

Seems north TX has some potential. The DFW area no doubt will get storms. The dryline bulge will be along the Red so DFW and east toward Paris and Texarkana could be interesting by 18z - 0z. Just across the Red into OK during this timeframe may prove better.

Problems: While I do see some fair LI's later in the day I see very little showing in Cape, and EHI. Lift from the vort max can likely be expected to help - but I'd still rather see Cape, and EHI. Looking at crossover winds (lower level directional wind shear) I am not that impressed in OK, or TX for tornadoes. After 18z when the vort max arrives and instability is higher the lower level winds are becoming more unidirectional. There is a bit of shear sfc/500 850/500 but it is somewhat minimal and quickly moves east toward AR/LA.

How to play, what to play, is it worth it: Well, it will be interesting to see how it changes again by tonight's 0z run and on Wed. My inclination is skepticism as before with these early systems - particularly with regard to tornadoes. My action area during the day would extend from south of DFW (Hillsborough) pre-18z and north of OKC to the frontal boundary and then east to the AR/LA border. That would be my Slight/Moderate area. Ardmore might be one decent place to start around 15z, or perhaps near or just north of DFW near 17z. Best areas may be se OK later in the day, or ne Tx later in the day.

Probability of Severe in OK: 20% Chance of Torns in OK 5% to perhaps 10% near triple point vort intersection - north Tx 5%

The above area is based on chaseable terrain (OK/TX) during the daylight. Most of the storm (and possibly tornado) threat will be further east and at night.

Disclaimer: all the above is subject to the fanciful whims of the models - LOL!