3/17/08 FCST: TX/OK/KS/MO/AR

[Chad Cowan]

GFS-based Ensemble and ECMWF have been consistent in showing an amplified trough digging into the Southwest over the weekend indicating the possibility of severe weather on St. Patrick's Day into Tuesday. There are timing differences between these two models as to when the trough will eject, but there will be significant surface cyclogenesis over the central plains when it does. The last few operational GFS runs have put the sfc low in W KS at 18z Monday and most of these progged lows have been <995 mb.

Deep layer and low-lever shear will be quite sufficient for tornadoes with the 100+kt 500mb jet streak atop a 60+kt 850mb LLJ. Moisture may be hard to come by with only a short period of return flow in the wake of the initial shortwave, but it appears that the 60 degree dp's could make it up into KS and MO.

It's too far out to nail down any specifics, but it looks like spring is here and La Nina is going to provide us with some major buckles in the jet stream for this upcoming season.

 

[Derek Weston]

GFS continues to show a relatively strong low ejecting NE over Kansas. Lately the GFS has also been rather optimistic about the dewpoints as well -- very small dewpoint depressions over E. Texas and Central Oklahoma. 250 & 500 speeds are good, but unidirectional. Best 850 winds a bit off to the east -- still worried about overall unidirectional sheer. Tough call.

As of latest run, I'd say NE Tx and south central OK.

 

[Tony Laubach]

I'm wondering if we shouldn't add Central America to the forecast list...

Latest GFS has the 500mb low into Mexico... this will save the front range from big snows, but kill any reasonable severe chances... models do show good moisture return as far north as south-central Kansas with CAPE values increasing with each run. However, the bulk of the goodies appear to be too far south to do much with the enviornment.

We'll see if this holds true, but what was once promising now has some "ehs" to it.

 

[Dan Dawson]

The GFS and NAM are still in disagreement on this morning's runs in the speed of the trough and the details of how it ejects from the mountains. Both models have a closed low at 500 mb early on in the day on Monday, but then split it into two main shortwaves. In the NAM, however, the northern wave ejects out rather quickly, veering the low-level flow over most of OK and north TX and taking the surface low quickly off to the northeast into northeast KS by 0Z. In the GFS, the northern shortwave is weaker and much slower, leaving the surface low in NW OK by 0Z. This is quite a difference, to say the least. Both models veer the 850 mb flow from S to SSW during the day, but the GFS is less aggressive than the NAM due to the aforementioned slower and weaker northern shortwave. On days where this has happened (and when I had the presence of mind to note it), I have had little success chasing, even though other parameters look good. I attribute this to dry air being advected from the W over the warm sector and diluting updrafts/mixing dry air down into the moist boundary layer, not to mention making the dryline more progressive. However, both models have a second low associated with the southern part of the wave in extreme SW TX/NE Mexico, and this may be a second chase target for those that live down that way or are willing to head down there (not I). I'm putting my hopes on something like the GFS solution to a slower northern shortwave for a chase target closer to Norman.

 

[Jonathan Whitehead]

Wow surprised this thread is as short as it is with tomorrows chances of severe storms.
Looking at the 12Z models, appears SPC has a good handle on tomorrows severe weather setup. Models keep the better moisture (>60) confined to south of Red River until about 18Zish when a nose of 60 dews advects into south-central OK translating NEWRd into eastern OK. Unfortunately Im not seeing good supercell chances in OK as convection from tonights activity in the panhandles spreading east throughout the day combined with significant low level stratus will greatly limit instability.

As far south of the Red River....
Im really liking the chances of surfaced based rotating storms along the dryline in west central TX. Moisture will not be a problem at all but there are some things im concered about with regards to this region. First, Im concerned about the possibility of the dryline mixing east too early firing off convection (ala central OK last April, who could forget that day). I say this b/c some of the models wanting to mix the dryline east by 18Z. Secondly is the cap. Im little concerned about the strength of the cap. Some of the forecast soundings Ive looked at, esp N of I-20, show a weak cap at 18Z or no cap at all (see first concern).

As far as a chase target, still too early to nail down an exact location but Im eyeing a region btwn Abilene and San Angelo.
Will look at 18Z runs this evening.

 

[Bill Tabor]

It seems to me one of the biggest issues facing chaseable tornadic weather tomorrow during the day is the fact that the NAM shows the 500mb wave and associated vorticity to be far west of the area of concern in Tx. The whole vortmax heads south and as I recall doesn't actually make it through Tx until 0z the next day (Tuesday). This should put much of Tx in subsidence during the day. Toward 0z the low strengthens in sw Tx and backs the sfc winds. Perhaps there will be enough wave action overnight to stir the pot and justify SPC's 30% sig severe. For some reason right now (before tonights 0z run) I have my eye on SJT near 18z perhaps near the trough/cold front intersection. Will I chase? I'd say that depends on how things continue to develop and what I see overnight and in the morning concerning convection.

 

[Bill Schintler]

Chase Target for Monday, March 17

Chase target:
Vernon, TX (35 miles west of Wichita Falls).

Timing and storm mode:
Widespread convection will be ongoing in areas generally east of I-35 during the early afternoon hours. New convection will develop further west along an advancing dry line between 3 and 4 PM CDT. A few supercell storms will likely, along with a chance for a tornado or two.

Synopsis:
The H5 chart showed 12 hr 30m height falls concentrated over SRN NM, with a 100kt lead streak rounding the trough base. SFC analysis indicated that the richest moisture remained S of I-20 in TX where SFC dewpoints of 60F were confined. This moisture will continue to increase and push NW overnight with SERLY SFC flow along with a 60kt SRLY LLJ. Soundings on NM indicated steep lapse rates in the SFC-500mb layer, while the 00Z TUS sounding was apparently missing.

Discussion:
Elevated convection will be ongoing during the early afternoon hours, with the WRN edge of the precipitation pushing E of a Wichita Falls to Abilene line by 16Z as a push of dry air in the H7-H4 layer surges in from the SW. Partial clearing will take place, with an axis of broken CU indicative of LLVL moisture E of an advancing DL. By mid-afternoon, inhibition should erode as SFC temperatures climb into the upper 70s F along and W of the DL and cooler mid-levels advance ahead of the trough. A narrow axis of MLCAPEs AOA 1500J/kG will push NWRD ahead of the DL, which should begin to bulge or advance first in the Red River area. The details of the ULVL forcing are not obvious. A number of waves in the H6-H5 layer will eject from the trough base and ripple through the TX panhandle in the wake of the aforementioned lead wave, and forcing from these features should remain W of the target area though 00Z.

Shear in the SFC-6km layer will be sufficient for storm organization AOA 40 kts although the degree of LLVL hodograph curvature is in question as the strongest LLJ flow may be E of the area along with weak SFC flow. MDLs suggest a compact, secondary LLJ developing in the TX panhandle by late afternoon. In the event that this feature makes headway E of the DL into the instability axis, SFC-3km SRHs AOA 300m2/s2 are possible. This, along with LCLs in the 800-1000m AGL range, would support a localized diurnal tornado threat. More likely, however, conditions for tornadogenesis will improve after dark as the BL decouples and LLVL directional shear increases while LCLs decrease. Overnight, a 40kt LLJ will aid in the maintenance of a large convective complex that will develop and move S and E through TX.

- bill
9:53 PM CDT, 03/16/08

 

[Shane Young]

Haven't been following this event at all, but the discussion here reminds me of a couple events...
1. The moderate risk bust in Central Texas (and then GA into the Carolina the next day) on 2/16 where the stronger upper level winds lagged back to the west and really eliminated any threat of deep rotation/tornadoes.
2. One of quite a few days, actually... one of which is October 9, 2001. Not a very scientific study, but I've had great luck with heavy precip tapering off not long after dawn (typically from the LLJ) giving way to great events. Even one of the best setups of recent here in FL (12 days ago) had heavy rain timing.


Now, looking at the actual specifics of THIS event.

I am quite confident that, although the pattern shape is quite similar to 2/16... it won't play out the same.
Considering
2/16 250 mb at 12Z
vs
12Z GFS for today
Yes... the pattern is quite similar...
But the trough isn't quite as elongated this time... and is better embedded in the flow (compare winds up in the Dakotas)... plus the speeds are consistently at least 20-40 knots stronger for this event.

It does look similar on first glance... but I think this is the more potent setup. There's also a quite well defined shortwave rounding the southern edge of the vort max (remember that a month ago the primary SWs were weaker and were actually overshadowed by an earlier SW in the STJ). I still don't think the vort max will move very quickly east... but it should be enough with the well defined SW and better speed shear.


And the surface looks supportive enough.
Dryline bulge suggested likely by the 700 and 850 flow in both the GFS and NAM (the position of the weakening low at these levels -> westerly flow).
Moisture axis right now is from Junction coming on up into near Lubbock.
With the 300 and 500 jets suggested to be overhead as the day goes on, I think the setup looks very good.


SREF values don't look nearly as good, with low CAPE values and diminishing 0-1 km SRH as the day goes on over Texas.
But it looks like it's not resolving the moist axis very well. I think you'll see Td near 60 all the way up to almost AMA.
The only question is where can we get a little clearing? That elevated moist layer could be the decider today.


I think you'll find some gaps in the upper clouds and temperatures will get up regardless... I'd really want to focus between the southern side and center of the dryline bulge... though I think the western Oklahoma border area has some potential as well.
If I were in the Plains and had open day, I'd head out. If I could choose a spot, I'd probably be staying the night in Childress or Abilene.
Then again, the night is almost over!

Anyways, that's my 2 cents. As typical, be prepared for epic failure.
Good luck to all to day!

SY

 

[Jonathan Whitehead]

Well looking at the this mornings model runs, NAM & RUC both show a dryline bulge south of Abilene southward to east of San Angelo. Appears SPC has picked up on this in the 8am SWODY1 update. If this does happen and with the modest CAPE/shear in the area, looks as if the Hill Country up into SW portions of the FWD CWA could be under the gun. As for a chase target, still up in the air. Originally I had planned on Abilene but I want to warn/spot more for the DFW metroplex so Im considering a more southern target ATTM.

 

[Darrin Rasberry]

Craig and I are currently sitting at my parents house (enjoyed a great breakfast, had a severe thunderstorm warning issued earlier with heavy rain and frequent lightning plus got plenty of rest) The sun is currently out and the storms have moved out of the area. Dryline is steadily moving toward Abilene and Vernon with a 28F dewpoint in Big Springs while just to the east in Sweet Water the DP is 61F! Will wait to until at least noon to take off and will probably head just west and south of Wichita and sit near the dryline. Golfball to baseball sized hail and storm structure are our main goals today but a tornado would always be nice! Slight risk is going to be upgraded now to a moderate just south of where we are in the next outlook

 

[Jeff Snyder]

Not an easy forecast, but it looks good enough to make the trip south... From SPS northward, I'm worried about the marching cold front overtaking any discrete attempts in the form of a squall line. From SPS southward, this should not be a significant issue, at least before sunset. The NAM and RUC, however, show abysmal 850mb flow west of I35 at 00z, with relatively weak sfc winds. If this verifies, weak low-level shear may limit the strength of any low-level mesocyclones. It's certainly nice to see some 62-63F tds in far southwestern OK and western N TX, and vis sat imagery indicates potential for strong insolation this afternoon assuming cirrus doesn't thicken too much.

Not much time to make a more in-depth written forecast.... The area S of SPS towards Graham looks fine, with things looking better farther south. Since I don't feel like heading S of I20 today, my prelim plan is to make way for the area between SPS and I20. 850mb flow is much better even a little farther east, so I may try to hedge that-a-way if convection can develop there (i.e. closer to DFW/I35).

 

[Tim Marshall]

TARGET: BROWNWOOD-STEPHENVILLE, TX TIME OF DEPARTURE: 11:30 AM DISCUSSION: I like the fact that the low and upper clouds are breaking over central Texas now and that the squall line over northwest Texas has lifted north into Oklahoma. Ample surface moisture is already in place. Expect supercells to initiate along the dryline later today from Abilene to Junction and race north-northeastward to I-20. I'll chase back to metroplex tonight. TIM MARSHALL

 

[Mikey Gribble]

I don't have much time now so I just posted a target and a brief forecast here http://loadedgunchasing.com/blog.html
I wish I was chasing today. I always get my pants pulled down on these first few setups.
I like the area between Graham and Granbury Texas in the 00Z time frame. Any discrete storms coming through this corridor will have a good tornado potential this evening IMO. If I was chasing I would target directly between these two towns. Actually I'd go closer to Granbury, but whatever. Good luck to anybody going out today.

 

[Jason Montano]

Currently debating whether I am going out or not. I had a hunting trip planned for today but may have to cancel it. I am slightly concerned about the ETA forecasting precip to hold off until after 0z south of 1-20. However, both RUC and GFS have precip breaking out in that area before 0z. I am going to put off the hunting trip until after the 1630z swoody1. If I do head out it will be west of where I am in Killeen.

 

[Paul Redmond]

I am favorable for Hamilton to Stephensville. It looks like a 281 chase for me today up from san antonio. I may have to chance it leaving late today as I have work commitments. Glad I brought the kit today.