8/6/07 FCST: SD/MN/IA/NE

[Michael O'Keeffe]

Surprised there isn't much talk about tomorrow's severe weather event! As a shortwave should enter parts of the N Plains it will be associated with a low and a wf. Tds will be very high once again causing CAPE values souring close to 5000j/kg. Favorable deep layer shear will help contribute to a few supercells with tornadoes. I'm unsure of a target at this point, but a general area would include SE SD, SW MN, & NW IA possibly streching into far NE Nebraska. I most likely will chase this setup as it looks fairly good for tornadoes if you believe the WRF.

 

[Bill Schintler]

Chase target for Monday, August 6

Chase target – northern target:
20 miles W of Ellendale, ND (35 miles NW of Aberdeen, SD):

Timing and storm mode:
Ongoing elevated convection will move in from the west and become surface-based by 3 PM CDT. Storm mode will be primarily multicell, however embedded supercells are possible along the southern edge of the convective complex.

Chase target – southern target:
Benedict, NE (25 miles E of Grand Island)

Timing and storm mode:
Storm initiation at 4 PM CDT. A few supercells will be possible early in storm evolution, with a rapid transition to a multicell complex.

Synopsis:
This evening, the ULVL flow was becoming increasingly SWRLY over the NRN tier of states in the WRN CONUS, while a number of S/WVs were embedded within the jet stream and large-scale assent was spreading through MT as noted on WV imagery. Strong storms over CNTRL SD appear to be associated with a couple of mid-level impulses, and these should rapidly diminish in intensity with the loss of daytime heating. Upstream soundings (RIW, BIS, RAP, and GGW) indicated steep lapse rates below 600 mb. The GFS initialized 10 degrees too low with regard to SFC dew points in SRN ND/NRN SD while the WRF seems to have a better handle on LLVL moisture.

Discussion – northern target:
Somewhat of a complex FCST, as the exact timing and location of S/WV features and attendant SFC reflections are not obvious. Also of concern is the degree of overcast early in the period. A SFC trough will approach from the W, and it appears as though low pressure along the SD/ND border early in the day will redevelop over SWRN KS later in the day due to strong heating. The timing of this ULVL wave will likely be several hours too early for the realization of the strongest instability. A WF along the SD/ND border should provide the focus for SFC-based convection ahead of ongoing elevated storms as the second of two S/WV’s approaches and mid-level temperatures cool several degrees. Extensive cloudiness over ND during the period will sharpen the temperature gradient along the front as well as slowly push it to the S. Marginal mid-level lapse rates of 6.5-7 C/km will couple with dewpoints near 70F with a moist layer 50mb deep to produce MLCAPEs AOB 1500J/kG. 50 kts of bulk shear along with SFC-3km helicities to 200m2/s2 should partially offset the modest instability.

Discussion – southern target:
A compact mid-level wave, well to the S of the stronger westerlies, will move E through NEB during the afternoon hours. Lift in association with this feature along with SFC temperatures in the low-90’s F should be sufficient to overcome warm mid-level temperatures. A meso-low should strengthen E of the main SFC trough, and this will interact with a northward-moving OFB from an advertised MCS that will develop in NERN KS Sunday evening. Strong SFC-based instability will couple with weak shear to produce mainly multicell storms.

- bill
[FONT=&quot]9:45 PM CDT, 08/05/07[/FONT]

 

[Darren Addy]

I'm a relative newbie at this stuff, but I have never seen these kinds of numbers:

Same time frame/model seems to have the cap breaking around Lexington, NE and the lift mechanism (trof) looks to be arriving between 8 PM - 9 PM local time.