5/6/08 FCST: SD/NE/IA/KS/OK/TX/NM

[Amos Magliocco]

12z GFS and 0z ECMWF are in fair agreement regarding the position of the upper low over south central Arizona on Tuesday afternoon and attendant sfc features on the high plains as stronger upper flow overspreads the region. Two areas of interest for potential SVR: ahead of the nose of the upper jet in the eastern TX PH / NW Texas, and east-northeast of the surface low in NW Kansas or even SW Nebraska. The latter area suffers weakness aloft (WRF's 0z Wed 500mb winds in NW KS are ~20kts) and the potential for anemic SR anvil winds, and the former shows surface flow more parallel to the DL, though capping doesn't look like an issue. For what it's worth, WRF agrees with bringing lower 60F Tds up to the DL in both targets and depicts morning precip to the east, raising the premature hope for boundary interactions. For now I'll go with the flow and plan for the southern target until there's more support aloft up north.

 

[Darrin Rasberry]

Absolutely zero upper-level support for Iowa, from what I see on the GFS. Nonetheless the SPC has seen it fit to place a SLGT over the area, and KDMX has been predicting thunderstorms since we chased last Friday. I am going to watch this a bit more closely tomorrow evening, hoping for lined-up hailers in my area and expecting nothing more than a few towers toppling over in otherwise clear blue skies. I will, more than likely, base out of home once again for this SLGT event. I've been disgusted with the central Iowa showings the last few days, so I've only given the models a cursory glance - any other input on the Iowa situation would be appreciated, but a few clicks on DuPage don't really have me all that interested.

 

[Rich Thompson]

One big concern for this next system (Tue 5/6 - Wed 5/7) will be the structure of the wave and the quality of the low-level moisture. Soundings and surface obs look great in S TX right now, but I fear that will change as subsidence over the Gulf results in a shallow marine boundary layer, and there's no surface heat flux to drive deepening of the marine layer (i.e., the air mass is fully modified). This dry air above the surface will move nwd/nwwd into TX the next couple of days, supporting the dreaded "moisture holes" Tue and Wed with daytime heating/mixing. That could hurt an otherwise promising scenario across W TX on Tuesday.

The system itself looks rather unusual with the initial lee cyclone hanging back under (or almost behind) the mid level trough axis by Wed, which is a signature of a weakening baroclinic wave. The stronger low-level flow and shear will be east of the trough axis, though most of models depict the stronger instability along the mid level wave axis where lapse rates are steepest, well west of the more favorable wind profiles. Perhaps the NE TX into AR area will see some tornado threat Wed afternoon, but this will be strongly dependent on the existence of a secondary corridor of richer moisture return from LA into AR. It won't happen if the NAM/GFS are correct with only a narrow corridor of 64-68 F dewpoints across E TX during Wed afternoon.

Rich T.

 

[Mike Hollingshead]

I *think* I'm going with a bit of an off the wall target tomorrow. I'm seriously leaning towards chasing somewhere near Chamberlain SD. I like the colder mid-levels there with that wave and wnw 40 knot mid-levels. What has intrigued me for awhile is when you get decent backing from 500mb on up. Get the anvil going well left of the motion. Shows 50 knots or so of 300mb winds from the sw up there. Low level winds won't be nuts, but I bet there are some flying eagles up there tomorrow. I hope *some* moisture can work up there by tomorrow afternoon.

Chamberlain SD at 21z

Huron SD at 0z

 

[Bill Schintler]

Chase Target for Tuesday, May 6

Chase target:
7 miles south of Perryton, TX (in the northeastern corner of the TX panhandle).

Timing and storm mode:
Supercell storms capable of all modes of severe weather, including a few tornadoes, will be likely. Convection will be ongoing in the morning hours, which should exit the area to the east into OK by 11 AM. Renewed convection should fire near the intersection of a dryline and numerous boundaries left over from nocturnal convection by 4 PM CDT. Overnight, another MCS should develop as the shortwave trough approaches, and chasers may consider staying overnight along I-40 in the eastern TX panhandle for a good nocturnal lightning show.

Synopsis:
UA pattern features ridging between mid/upper level low near the NRN Baja, and the ERN CONUS trough. A 70kt H5 jet max is FCST to eject from the base of the Baja low and overspread the TX panhandle by 00Z 05/07/08. Several MCSs are ongoing in SRN KS, the panhandles, and in NWRN OK.

Discussion:
Once again a challenging FCST due to several large areas of ongoing convection and attendant OFBs. Looking first at the large picture, low pressure will move E through the OK panhandle on Tuesday while a DL mixed E into the TX panhandle. The WRN edge of a band of ST will push E to the TX/OK border by 17Z, and this will allow for strong SFC heating and evapotranspiration owing to recent widespread heavy rainfalls. This should allow SFC dew points to locally reach the mid-60s along boundaries where strong SFC convergence takes place. MLCAPEs should reach 2500J/kG beneath 8C/km mid-level lapse rates.

Now for mesoscale details associated with the ongoing convection. The current reasoning is that outflow from the line of convection in the OK panhandle at 4Z will push south to about Canadian by 18Z. The large line of storms extending from near Clarendon SWRD to Whiteface will produce a NWRD moving OFB that will reach I-40 by early afternoon. Meanwhile, storms N of Amarillo will produce one or more boundaries. When things are all said and done, a likely area for initial convection will be near Canadian along a boundary parallel to US-83, to the NE of which SFC flow will be sharply backed beneath a SRLY 30kt LLJ. Deep-layer shear should increase to 50 kts as a lead H5 streak overspreads the region.

- bill
11:30 PM CDT, 05/05/08

 

[Shane Adams]

I've been waffling all evening between the two target areas of SW Kansas and the TX pan. They both seem reasonably decent, considering it's May 5 but still really March. I'm leaning towards TX because I like the better h7 winds down there (more southwesterly than SW Kansas which are mostly due west). Initially I was worried about dewpoint depressions in the southern target, but if the advertised initiation condition of mid 70s/60-62 can be realized, I think the anvil shadows/cooled temps will create more favorable DDs for tornadic chances.

Should be interesting at the surface tomorrow with all the ongoing stuff. Definitely could be a needle-in-a-haystack boundary type day. For now I'm content to hug the area east of that surface low in the w TX pan.

 

[Sam Barricklow]

Target Area - Haskell, Floydada or Pampa?

Just did a quick review of the UCAR RUC and noted the dryline bulge between Lubbock and Midland this afternoon, along with a forecast CAPE maximum and strengthening winds aloft.

Target area #1 for 21 Z this afternoon would be bounded by Abilene, Snyder, Plainview, Childress, to Abilene. Exact position to be determined by the location of the dryline. With the unknown quality of the post MCS airmass this afternoon, I'd rather be on the higher elevations of the Caprock. As far as starting points, Haskel or Rule might be a good place to monitor developments during the afternoon, or in the parking lot of that isolated, large Catholic Church that's somewhere in nearby (anyone recall the name of the church or a nearby town)? If the dryline is far enough west, then the Dickens, Floydada to Plainview area might be a good starting point.

Target area #2 is the Pampa to Canadian area of the eastern Panhandle. However, storms will be moving away from the better winds aloft in this area.

As others have mentioned, the southern target is forecast to have stronger winds at 700 mb, which is a key ingredient.

I will probably not chase today, but will watch from work as time permits. Good luck to all who chase, and stay out of the big hail!

 

[Darrin Rasberry]

As far as starting points, Haskel or Rule might be a good place to monitor developments during the afternoon, or in the parking lot of that isolated, large Catholic Church that's somewhere in nearby (anyone recall the name of the church or a nearby town)?

That's St. Mary's in Windthorst. I think Throckmorton might be a bit closer to where the models are pointing their fingers, but then again, I wonder how much the ongoing crapvection is factoring into that.

As far as Iowa is concerned, the RUC's projected morning convective march isn't going to materialize. Temp/dews are approx. 55/50 in southern Iowa with a slightly higher spread here near Ames, so the moisture might get here, although the RUC is not so confident. It's also putting one heck of a warm pool over us, with temps expected in the eighties.

 

[Paul Knightley]

A couple of regions look interesting to me - one is INVOF the Caprock, west or SW of Childress - this morning's outflow boundary should lift back north through today, as surface heating commences - increasing lift should weaken the cap this afternoon, as should surface convergence. Storms should then move eastwards this evening along the Red River valley, and with a strengthening LLJ, tornadoes could continue after dark, with relatively small CIN.
Another area of interest is western Kansas - several models depict a surface low developing later this afternoon/evening in response to increasing upper support. Scattered supercells could develop here later in the day, perhaps tornadic, and then tend to increase in coverage overnight as the LLJ increases, although these should eventually congeal into one or more MCSs - wind/hail will then be the primary threat above a cooling boundary layer.

 

[Shane Adams]

It's starting to become a familiar picture IMO. Morning convection continues to track southeastward across W/NW Texas. With airmass recovery times needing longer the further north you go, it seems obvious to me now that today will be yet another I-20 shuffle in and around the Abilene area. RUC breaks out new convective precip in this spot by 21Z, after hours of no precip (during which time, hopefully in theory, the airmass will destabilize once again). Today is starting to look like another April 9/23 IMO.

Will be leaning towards this target 95% while keeping one eye on the area futher northeast of there as well, southwest of Childress.

 

[Tim Marshall]

TARGET: TURKEY, TX TIME OF DEPARTURE: 10 AM. Don't let a little morning convection get you down. I believe it was a good thing -in part. I anticipate low level moisture will recover in West Texas to give us round #2 tonight. PROS- Clearing skies (thank you overnight convection) will provide plenty of surface heating. Upslope flow of the modified air will become fuel for afternoon storms. Great directional shear profiles with east-southeast winds at the surface (thanks again overnight convection) with southwesterly winds aloft (increasing by tonight). Large upper cold pool associated with closed low will be approaching the area late today -the air mass can only become more unstable. CONS - OK, so overnight convection did mess up the quality of the low-level moisture. But, I believe that is temporary. I am concerned that the dryline will not move eastward very much today due to lower pressures with the approaching system to the west. This means, no westerly winds or push behind the dryline or convergence - as the system is 12 hours out of phase. I am also concerned about clouds (low and high) returning over the target area today. Finally, the logistics aren't great in intercepting storms as the road network off that cap is poor. TM

 

[Amos Magliocco]

Tough choice currently about which direction to take west, I-20, 380, or 287. I'm nervous the boundary won't lift north as quickly as I'd like, and it seems easier to follow it from the south. 12z RUC adds to concerns about a sluggish DL, holding it at the border probably because of pressure falls on the western side. Agree completely with Shane that this feels like a repeat of two weeks ago: let the boundary do the work. I'll split the difference and go west on 380, initial target of Jayton to Crosbyton. Not where I'd hoped we'd be chasing...again.

 

[Angie Norris]

If I could head out today, target would be Matador, TX. Shear is quite sufficient with SSE winds at the surface later this afternoon and W winds aloft by 0Z. CAPE looks good at about 3500 J/kg by 0Z. It does worry me about the dryline holding way back in NM, but I think the upper support may be enough to get things to pop.
Y'all have fun, try to stay out of the hail, and keep safe today.

 

[Chris Lott]

I won't be heading out today either, but if I was I would be leaning toward SWW(Sweetwater). Already some clearing going on over that area, with shear increasing over the area ahead of the approaching system. Forecasted CAPE in the area to be near 3000 j/kg with LI's approaching -8. My only concern is the possibility of more cloud cover approaching the area before the best parameters come together, therefore limiting some of the instability. However, it does appear to get better as the day goes on, just hopefully not another night time event for many chasers. Good luck to all who head out today.

 

[Rocky Rascovich]

Pouring over the data, it's looking like the area from CDS to LBB for later today around 23Z for CI as the airmass recovers from the subsidence left in the wake of the convective blob from earlier today. Upper level dynamics from encroaching shortwave intermingling with 3500 j/kg cape and 250 m/s helicities, I'm fairly certain they'll be some good storms, but I suspect they'll turn HP in a hurry with more bows and line segments and eventual MCS for N.TX. and S.Okla.

HOWEVER, take a look at the latest NAM and 15Z RUC. Surface pressures really deepen in SW.KS. The NAM has it dropping by some 7 or 8mb. in just 6 hours. Can this be overdone a tad? Helicity and cape values don't seem near as good that way up toward Gage to Perryton and on north into SW KS. I would think with such deepening, a surprise should be in order for those area's. Currently that area is in the clear so the atmosphere should be recovering and percolate the instability as the day progresses. I'll be heading out with Michael Ratliff and likely go west on I-40. We'll assess the options from there.

Rocky&family