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08/03/08 FCST: MN/IA/WI

Chase Target for Sunday, August 3.

Chase target:
Winthrop, MN.

Timing and storm mode:
Storms should fire at 6 PM CDT. Supercells with severe wind will be the primary severe threat. There is a significant chance of a bust for diurnal convection.

ULVL ridging dominated the CNTRL CONUS, with a potent H5 S/WV noted over the NWRN CONUS and a 12-hr height fall bulls-eye of 60 m centered over CA-MB. An impressive axis of 16-20C H85 moisture extended along a LLJ from KS/MO NWRD into SD. An H85 moisture bulls-eye of 22C Td was noted over SERN SD. An axis of ST extended from WRN IA into ERN SD and provided a visible indication of deep moisture in place. Models appear to be overly ambitious with LLVL moisture, with the WRF indicating SFC Td’s of nearly 80F over SERN SD, while verification is more then 5F less then that. Very warm mid-level temperatures were noted upstream, with 20C H7 temperatures over CO in association with the EML. The OAX sounding was impressive, while indicating a MLCAPE of nearly 7000J/kg and enormous hodograph curvatures along with a TC in excess of 100F courtesy of the EML. On Sunday, these steep mid-level lapse rates will shift E and contribute to both strong instability and capping.

Very complicated scenario indeed… A nocturnal MCS, sustained by a 50kt LLJ and forcing from a lead H5 wave, will be in progress over MN early in the period. This elevated convection should initiate at the nose of the LLJ over SWRN MN at 05Z, and move or expand SEWRD into ERN IA in a zone of WAA and isentropic assent. By 18Z elevated convection will be ongoing in locations N of US-12 in CNRL MN, and an OFB should exist along a Granite Falls to Mankato line.

A significant concern is capping strength, with 16-18C H7 temperatures nosing into SWRN MN. Forcing associated with a secondary wave with strong UVM, and attendant mid-level cooling late in the period, should be sufficient to increase convergence along the aforementioned OFB. SFC-based convective initiation is dependent on LLVL moisture return, and a bust is a definite possibility. Verification of 70-72F SFC dewpoints seems reasonable. High moisture throughout a 200mb deep layer beneath steep mid-level lapse rates should yield MLCAPE’s in excess of 4000J/kg by late afternoon. Large SFC-4km hodographs will result from veering H7 flow AOA 40 kts over backing SFC winds. Overnight, a veering 40kt LLJ will aid in the maintenance of a large MCS that will track SE into NERN IA and WRN WI by 06Z.

- bill
9:30 PM CDT, 08/02/08
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Target: East of the I-29 corridor. (Elkton, SD to Marshall, MN up to Bird Island, MN) Will be basing out of Brookings.

Thoughts: The CAPE is there, but so is a strong cap. I agree with most of Bill's thoughts as a bust is certainly possible. A chance for tornadoes exists in the area, and if going by my DreamCast somewhere will get a decent tornado chance this afternoon. Clouds in the area could definately inhibit any instability.

Good Luck to those out there.
Just got off work and I must say this is a very unseasonably strong forcing environment. However, the midsummer heat is definitely keeping the CAP in place even with the mid level forcing. I would be playing the warm front currently across SC MN like the two previous posters have said. SPC Mesoanalysis currently showing 3 km EHI > 11 and Supercell Comp > 25 and Sig Tor ~ 4. Looks like there are Cu trying to punch the cap over the rather widespread stratus. If nothing goes in the next hour I'll be worried but otherwise... could be a surprising early August show. Storm mode doesn't seem to be a question as all the elevated stuff that was going on just to the north was supercellular.

BTW: 90/80 in N.C. IA. :) Yay farm-effect.