08/01/06 FCST: IA

Chase target:

Ida Grove, IA (between Denison and Storm Lake).

Timing:

Renewed convection will develop in the target location at 6 PM CDT.

Comments:

Ongoing convection in western IA will weaken and move out of the area by early afternoon. Supercell storms will be likely late in the afternoon with large hail to golf-ball size and a tornado or two. Storms will evolve into an MCS by mid-evening, which will track ENE through NRN IA. By that time, severe weather potential will transition to that of wind and heavy rainfall.

Discussion:

A SE/NW-oriented outflow boundary, currently located along an YKN/OAX line in NE, will lift NEWRD today and provide the focus for renewed convection as the RRQ of a 45kt H7 speed max lifts through the area. Late afternoon appears to be the best time for surface-based convection as localized forcing becomes sufficient to overcome convective temperatures in the low-100’s F. A window of time will exist when storms should lift over to the NE side of the OFB where a more backed SFC flow and cooler temperatures will be in place from rain-cooled air along with a partial cloud cover. Very high instability, with SBCAPE’s to 4000J/kg, will be juxtaposed with modest but adequate shear parameters. SRH (SFC-1km and SFC-3km) should locally exceed 200m2/s2 while deep-layer shear (SFC-6km) should reach 40 kts. A 45kt LLJ will maintain storms into the overnight hours, when very heavy rainfall totals will be likely where storms train in an area with precipitable waters in excess of 2.5â€￾ along with NWRD advection of a 16C H85 airmass into NCNTRL IA.

- bill
 
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