04/25/05 FCST: TX/OK

I'll be up front here and say that I'm clutching at straws.....'cause I am. It's really the only way to keep from going into hibernation at this time of year, when you step out the door to temps in the 50s, dewpoints in the 30s and a Canadian airmass spilling down through the Plains.

Would anybody like to speculate about the severe potential for next Monday April 25th? The flow will be screwy up until then, but the ETA (today's 12Z isn't out yet) forecasts a trend to southwesterlies at 500mb come 0Z Tuesday 26th - centered around Wichita Falls. It forecasts a 999mb surface low out by CDS, too, by 12Z Monday.

The Gulf fetch is not-too-good-but-could-be-worse, and the models are consistent with breaking out precip across the southern Plains at this time.

Still very sketchy - but thought I'd open up this thread for anyone who wants to throw some ideas out there.

KR
 
The 12z eta/nam is quite aggressive with moisture return sunday and monday. I'm not sure if I buy it, but its 84-hour forecast for monday night appears to be quite an event in central and northern Texas, possibly starting along the I-35 corridor. Current eta forecast of 2500 cape + 300 helicity + 50 kt midlevel jet + good upper jet placement + deepening sfc low + advancing dryline = :shock:

The GFS though has been weaker with the central US longwave and thus not as aggressive with lowlevel moisture and other features. 12z run also appears to be holding it back a bit.

Still, this will be fun for us Texas residents to watch since we've been pretty much skipped over by this year's severe weather season. So it's hard for me to not get excited by the eta despite its 84-hour opportunity to back out and trend more toward the gfs, especially with that moisture return.
 
Looks like Texas and extreme southern Oklahoma will get something on Monday and into Tuesday morning. Upper level Low Pressure moving in from the west, with the Low progged at 995mb could make due for some interesting severe weather in the Southern Plains states.

Only major problem I have with the 12Z NAM is what is the problem with the dewpoints! Looks like it will be a struggle to fuel anything to develop, dewpoints will stay pretty much low until the afternoon on Monday where they could reach up into the 60's. Dewpoints are going to remain in the low 40's during the day Sunday, so unless something major changes, good luck. All of the other severe parameters are there for something major to happen, but the dewpoints and moisture could be the killer.

Personally, I do not see a good chance of severe weather coming into the Central/Southern Plains until late next week (Friday/Saturday)
 
The new 0Z NAM shows the Upper Level Low Pressure (995mb) at 0Z Tuesday centered near Dallas, TX. Dewpoints near the Dallas/Fort Worth area at 0Z Tuesday will be between 60-65 degrees. 12 hours earlier at 12Z Monday show dewpoints only in the 40 degree area. Previous to that, moisture will have a hard time returning to anywhere in the Plains states (We are going to have a probable frost or freeze here in the Kansas City area this weekend.)

To me, this looks like a minimal severe weather producer for any area in Texas from Dallas/Fort Worth, southeastward. The lack of complete moisture return from the Gulf of Mexico will prevent the development of anything major in the SE Texas area. I think the best chance for severe thunderstorm development will be along and east of the dryline which should be between Interstate 35/Interstate 45 at 0Z Tuesday. The closer to the Gulf, the better the chances. Moisture return is the problem, here.

I still believe the better chances for severe weather anywhere in the Plains states will hold off until a period between the 28th and the 30th of the month.
 
After this big mid-continent scouring job over the weekend, looks to me like Monday is kind of a quasi-March setup. As for the precip forecast over much of the central plains for Sunday night and Monday - especially if you look at some of the forecast accumulation totals over a fairly wide area - looks like more of a general overcast rain event rather than precip emanating out of any sharp convergence.

The exception, of course, looks to be E TX as Gulf air simply won't have enough time to advect much further north than the Red River before the surface low slides underneath OK. I do agree Monday's event will be confined to E TX and perhaps even in to LA. However, the event could be significant severe. ETA model shows pretty definite area of 300-500 m2s-2 helicity. As is often the case with supercell development in the deep south in early spring patterns - into which it looks like we have temporarily retrograded - this kind of helicity is sufficient to initiate very strong updrafts given just moderate instability. We also see pretty decent crossover of 850mb and 500mb winds, along with 50 to 60kt deep layer shear. So, probably not a classic chase day on the plains, but E TX / W LA could be in for some rough stuff.
 
I pretty much agree with what Ben said. Everything is there except the moisture is the big question mark. If the 00Z NAM is telling the truth, I think the southern end of the dryline could be in business. I am optimistic strictly because it is almost May.
 
I must be missing something or it wouldn't be this quiet on here. From what I have looked at I think Monday has potential. The dryline should run right down the middle of Texas. The NAM has dewpoints 65 extending all the way up past Austin 70 nosing on shore around Houston. The GFS is a little more conservative with 65 slightly farther South. SPC mentions...
SEA
SFC TEMPS ARE WARM ENOUGH TO MODIFY AIR MASS THETAE CONSIDERABLY BY
LATTER PART OF PERIOD.
They do mention they are refraining from higher probability because of questions concerning the extent of the moisture return. It is almost May now and I think this is the time of year when the trend changes from things not coming together to actually coming together.
Nam has CAPE at 2500-3000 in SE Texas. This area coincides with the stronger helicity. 500mb winds should be at 60+kts. SE Texas will also have a 100-120kts jet streak at 250mb. Shear is not going to be a problem. Surface winds are out of the SSE. One thing I didn't quite get was pecipitation showing up so early on the models. The models don't show the low level jet kicking in until Sunday night so moisture is still a question mark, but I think it could happen.
 
Michael, it looks like what is going on with the "early" precip has to do with the southerly flow (at surface and 850 mb) begins to return late Sunday over what will be seasonably cool surface temps. With the cold air covering most of central CONUS surface tonight under clear skies, we will head into Sunday with still cool surface temps. By overnight Sunday, moist air will be advecting over western plains. For example, western KS will have both temps and dewpoints almost equal in the mid-40's overnight Sunday. So, air will become saturated having as much to do with the low temps as the rising dewpoints. In the meantime, looks like negatively tilted area of low pressure at the surface is digging southeastward, providing just enough lift with the saturated air to result in widespread precip (beginning as light rain) over, for example, much of KS and northern OK by Monday morning. Not a warm front, per say, but kind of the same effect.

As for what this means for position of severe weather later Monday, my concern is with the resultant widespread cloud cover forecast over much of central plains Monday. However, full clear skies are evident in W TX, with only partial cloud cover over central TX into S central OK. At any rate, I believe northern extent of severe activity will be defined by dryline/overtaking trough, which will in turn be defined by how far S the center of surface low pressure stands mid-day Monday: right now, it looks like north central TX. As I stated earlier, I believe the rather emphatic forecast of high helicity will be sufficient to trigger severe, with the moisture and instability that is available. Most likely, the Dallas area will be northernmost limit, but with a little wobbling of that low perhaps we could see brief window of opportunity in S Central OK.

BTW, congratulations on your chase Thursday - very good photos, especially considering you were chasing from the west.
 
According to the 0Z NAM, most of north and east Texas will have at most 12 hours of moisture to work with. At 12Z Monday, dewpoints will be at 45 degrees or less north of Texas Highway 7. I feel that any area north of Texas Highway 7 will have very little moisture to work with for the development of severe thunderstorms. I do see the nice moisture surge northward from the Gulf of Mexico during the day, but the rate of moisture return from the Gulf of Mexico is not sufficient enough to juice up the atmosphere north of Texas Highway 7. However, south of Texas Highway 7 is more apt to see severe thunderstorm development along and ahead of the dryline. The area should have 45-55 degree dewpoints coming in as early as late Sunday, and the question is if that will be enough moisture to help sustain severe thunderstorm development. I still believe that the best chances for severe thunderstorms are nearer to the Gulf of Mexico, the farther south, the better the chances. A good target from what I have been seeing so far is Schulenburg, Texas.
 
Maybe I'm just optimistic, but Monday looks like it could be a pretty significant event for central into east Texas. I am not concerned about the moisture return, as low 60's surface dewpoints seems reasonable by the late afternoon, even though the NAM is being more aggressive than that.

I am concerned about morning convection associated with an initial impulse moving through the area. Assuming that mess moves out of the way and destabalization of the airmass can occur, then things are looking good.

A surface low should form over western Texas and track east into north central Texas by 00Z Tuesday with an associated dryline extending south into southern Texas. The associated convergence and upper support from the 500mb vort max should be enough to break the cap, given the ~16C 850mb temps.

Minus the early day convection and the uncertainty behind that, I'd say Monday looks like a good tornadic supercell setup. Forecast hodographs for central into east Texas look great, and 0-3km SRH values are forecast to be between 300 and 400. Shoot, 0-1km between 150 and 200. 1500-2000 j/kg of CAPE will be fine. I hope I can find a way to chase on Monday.
 
The 00Z run has made only a few slight changes from previous runs. Moisture is looking about the same with dewpoints 60-65 throughout the target area. Stongest 500mb winds have shifted a bit farther South which doesn't matter a whole lot anyways(60-50kts). Perhaps the most significant change from the 12Z run was the decrease in CAPE. It is now showing 2000J/KG. Shear is excellent. I noticed surface temps had a noticeable rise between 18Z and 00Z which could be a result of clearing and insolation. One other significant change from the 12Z is the better helicity with the 00Z run. I pretty much feel the same way Jim does. If we can get the crap to clear out I think Monday could have good tornado potential. I am anxious to see SPCs thinking on this. I wouldn't be suprised to see a moderate risk tomorrow.
 
The 12z runs go not look so good to me. The GFS has the low tracking through S. OK and NAM through RRV. Both models have a broad area of precip through 18z across the warm sector. The models have been consistant with this feature. I really see this as the killer of severe weather across the N half of the warm sector closer to the sfc low. For the S half, I see possible clearing of the morning/afternoon precip, this will allow further destabilization of that area. My only concern for this area is the fact that being this far south from the sfc low, the sfc winds might be veering (SW to S). This would really limit sup activity. I might be a little pessimistic only bc I can't chase, hope thats true! :wink:
 
Originally posted by Justin Walker
Both models have a broad area of precip through 18z across the warm sector. The models have been consistant with this feature. I really see this as the killer of severe weather across the N half of the warm sector closer to the sfc low.

Generally, we share the same concern. As long as the area of precip (likely associated with an initial upper impulse) can move out of the central/north central Texas region with enough time to allow sufficient destabalization, Monday looks amazing.

Assuming it does, the low level and deep layer shear, along with the shear vector's orientation to the dryline, supports descrete supercells. And given the low to mid 60 surface dewpoints, 2000 j/kg CAPE, and incredible low level SRH values, not to mention the curved forecast hodographs, Monday looks like a tornado outbreak. Red River(ish) south to central Texas, maybe Waco or possible even further south.
 
Originally posted by Justin Walker
For the S half, I see possible clearing of the morning/afternoon precip, this will allow further destabilization of that area. My only concern for this area is the fact that being this far south from the sfc low, the sfc winds might be veering (SW to S). This would really limit sup activity.

That crossed my mind too, though if the NAM is correct in deepening the sfc low by 3 mb between 18z and 00z, we should have nicely backed winds all the way to the gulf as forecast, possibly influenced by that secondary sfc low near Monterrey, MX as well.

My biggest concern is how the best upper jet placement and midlevel vort advection appear to be across eastern TX at 18z. That would only help influence that morning junk while possibly suppressing BL mixing and any further convective development along the dryline through the afternoon. Thankfully on water vapor I don't see a Pacific plume headed for the target area which has ruined many chases in the past with mid/high clouds.

I would agree w/ the SPC's lean toward the southern half of the outlook area due to the possiblity of better BL heating, closer proximity to gulf moisture and greater midlevel shear, putting my very preliminary target somewhere near Round Rock, TX. That is very likely to change depending on where we can get some clearing.
 
A bit further moisture analysis is leaving me fairly skeptical. As of 18z, the highest dewpoints I can find in the gulf are from buoys 42001 and 42002 in the central gulf w/ dewpoints of 55, likewise at Brownsville...all locations forecast by the 12z NAM to have dewpoints of 60 at this time.
 
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...
 
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!!
 
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...
 
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.
 
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...
 
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.
 
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
 
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.
 
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