04/25/05 FCST: TX/OK

[Jeff Snyder]

Hmm... the 18z NAM is pretty similar to the 12z... Still showing >55 Tds to the Red River by 18z, and >60 tds to near MLC and southward by 0z. I'm not so sure about how the NAM moves the 60 Td isodrosotherm northward like 160 miles between 18z and 0z. I'm sure there is some general evapotranspiration and/or regular evaporation that's creating a general 3-5 degree Td rise, which, in addition to some advection, is leading to the surface in the 60 degree Td line... At any rate, the very strong low-level shear and moderate instability are creating STP (Significant Tornado Parameter) values >3 from Durant southward, and >7 from I20 southward, which is the best we've seen so far this season for warm-sector activity.

IF we can get the NAM-forecast Tds to verify...
IF we can get some insolation and resultant destabilization across the area tomorrow...
I certainly would expect a MDT risk, 15% hatched type of event from ADM-Durant southward into TX. We shall see, however...

 

[Jim Bishop]

Current bouy sfc dewpoint obs over the wester Gulf (east of the U.S./Mexican border) are in the mid to upper 50's, as they were around 00Z. The NAM's initalization shows a tounge of 60's stretching quite a bit further north than that. So finally I can say that the NAM is probably overdoing the moisture return for tomorrow. But probably by no more than 5 degrees.

Either way, 1500-2000 j/kg of CAPE should be present in the late afternoon with upper 50's to low 60's surface dewpoints stretching to DFW, and upper 50's making it to the Red River.

Forecast hodographs by NAM show great low level curvature for areas east of I-35 and south of DFW. Hodographs near the Red River look good too, but they look better for tornadoes further south. What's really grabbing my attention are the 350-400m^2/s^2 0-3km SRH values, and of course the 200-250m^2/s^2 0-1km SRH values are entising as well!

I am a little concerned about the CAP strength south of Waco, TX. However, the best low level shear for tornadoes will exist between a north south line from DFW south to the Austin line and extending east to past I-45. But, tornadic supercells will be possible from the Red River valley south all the way down to near Austin, and advancing east from there.

If I could chase I'd be sitting in Waco, Texas tomorrow waiting for crap to hit the fan. Of course I'm still concerned about the midday precip NAM continues to predict.

Give the degree of shear that will be present, and the possiblity for 2500 j/kg of CAPE if sufficient evapotrasporation occurs, if the crap convection clears early enough and that amount of CAPE is present ahead of the dryline, we've got a significant tornado outbreak on our hands! I'd expect the SPC to issue a high risk at the 3:30pm CDT update if that were the case!!

 

[Jeff Snyder]

I'm turning quite pessimistic again, as I have a very hard time believing that >60 Tds will make it any north of DFW. Winds currently in central TX are rather disorganized, and many locales are still in the 30s. The >50 tds are into southern TX, with the 42020 buoy (50 miles east of CRP) near 61-62F for Td. The low-level shear looks awesome, but, as has been the case numerous times this spring so far, the previous front has completely destroyed the Gulf in terms of high-octane air... I just can't believe the NAM Td forecast tomorrow afternoon north of DFW, and thus I can't believe the resultant CAPE forecasts. I certainly hope I'm wrong, but I think this is an event that I'll be able to wonder about later this summer when I think of what could have been... This is like 3-21 in that we have fantastic low-level shear in the warm-sector, a relatively rare event in itself, but we just can't muster the juice to really get the event off the ground.

Now, I've dedicated my forecast to north of DFW, since that's the farthest I'm able to go tomorrow. It looks better south, obviously, in the better ll moisture, but I haven't examined it much to the south...

 

[Rich Thompson]

With a net northward motion of ~20 kts for the next 21 hours, surface Tds may approach 58-60 F as far N as Dallas. Moisture transport will be greater just off the ground through about mid morning Monday, and then you could see a northward jump in the surface moisture with vertical mixing.

A big concern will be the impact of any precipitation. The low levels are quite dry across TX, and any elevated convection could lead to a rain-cooled boundary layer, with lower surface temperatures than shown in the NAM forecasts. Regardless, we'll likely be dealing with a narrow corridor of instability, and a limited window of opportunity.

Lots of questions that can only be answered with observations from mid morning to early afternoon tomorrow.

 

[Matthew Havin]

This is for the late night/early morning types....

The 6Z NAM is still trying to come out with the same results as the 0Z run.
997mb low west of DFW at 0Z, with 2000+ CAPE east and south of
I-35. To support the moisture return forecast, Austin has gone from 36Td to 54 in a matter of 9 hours (6P-3A).

Precip-EHI colocation east of I-35 at 0Z reaches 4+ a tick NE of Waco.
This will be an active day if the clearing verifies...

 

[Shane Adams]

Latest RUC is showing a bullseye in southern OK by 21Z. Modest CAPE values (1000-1250j/kg) but sufficient shear and helicity (300-350) to make up for it. Oklahoma Mesonet has been showing a slight rise/steady state Td trend in the upper 40s/near 50 along and just north of the Red River. I'd like to see these climb into the mid-50s at least (if not upper), but with the helicity and shear in place I have the feeling something magical could happen and a few freak tornadoes are possible. True, the moisture we do have to work with will be shallow, but I've watched too many dynamic systems produce tornadoes in less-than-desirable surface conditions to ignore this. Most encouraging factor for me attm is the forecast precip, breaking out in a small isolated blob near Ardmore by 21Z and shifting eastward to Durant-Hugo by 0Z. A glance at the broadscale precip map shows nothing, which further suggests this activity will be isolated. UVVs are spiked and stacked in the classic isolated storm type pattern in these areas as well. To my eye, Ardmore, OK is the place to be around 3pm, with adjustments eastward along the US70 corridor throughout the evening. It's a longshot of sorts, but I feel there's a chance for an isolated tornado or two from any sup that can form in that area at that time. We'll be there to find out either way, barring some major collapse in the forecast data between now and departure.

 

[Bill Schintler]

Chase target for today, April 25

Target:
30mi SE of Wichita Falls, TX

Timing:
Expect renewed convection in the wake of the ongoing elevated convection by 4 PM, with majority of severe WX occurring after 6 PM.

Storm type and intensity:
A small tornado risk, along with hail to 2â€￾. Storms will be relatively high-based.

Discussion:
12Z analysis showed nose of a 100kt streak at 300mb approaching the forecast area. Embedded in this flow were two shortwaves as evident by 700mb, 500mb, and WV analysis. The first of the two, apparently associated with convection along and E of I-35 between OUN and FWD, was lifting through ERN TX. Meanwhile, a second wave was approaching the CDS area. A good shear environment was in place: OUN and FWD hodographs both indicated nice loops below 600mb.

Guidance: 12Z NAM and RUC initialized with regard to surface and UA temperature, dewpoint, and wind fields. Models agree to bring a DL towards the FA through 21Z, with a narrow swath of Td’s approaching 60F just E of this feature. A compact area of surface low pressure will develop/translate into the FDR/SPS areas through 22Z.

Deep-layer shear around 40kts and SRH (0-3km) around 150 m2/s2, along with MLCAPE’s near 1500 J/kg will support storm organization and rotation updrafts, while high LCL and LFC levels AOA 1500m will tend to keep storms high-based and minimize tornado threat.

- bill

 

[Mike Johnston]

Note also the 9-hr. RUC is now showing a bullseye of 400-500 SRH values in extreme N Central TX and S Central OK, although the area is relatively small so I'm not sure if it is just an anomaly or not. NWS short-term cloud cover forecast shows clearing over SW into S Central OK between 11am and 2pm. Deep layer shear in this area, however, may not be quite as strong as areas further to the S. The other item of note: an earlier SPC Outlook mentioned uncertainty of LCL heights as a reservation in their slight risk outlook. Looking at the latest NAM run, LCL's ahead of the surface convergence are quite low at 200-700m with LFC's about the same - although there appears an sharp eastward bulge of much greater heights over E central TX. Not sure if this indirectly indicates a dryline bulge....maybe someone else can chime in on the significance of this.