Midwest Edition-Sunday August 2nd, 2009 severe wx outlook

Plains:
A shortwave rotating around the upper low over Canada will move into the northern Planes tomorrow. The S/W will have a surface reflection and will be dragging a dry-line through the central Dakotas and western Nebraska tomorrow morning, and it will move east tomorrow, reaching NE Nebraska and central MN by tomorrow evening as the surface low moves east-southeast.

The models develop a decent LLJ ahead of the front, so even though a front recently cleared out the region moisture return should be decent, especially up near the low. It should be fairly sunny tomorrow ahead of the system so there will be intense surface heating, with temps getting into the upper 80s ahead of the boundary, and into the 90s behind it. The combo of temps in the upper 80s, and dew points in the upper 60s should provide for good destabilization in the lower levels of the atmosphere. Lapse rates will be fairly steep as well, so MLCAPEs of 1000-1500 j/kg will be realized. With dry mid levels the MLCAPEs are rather weak, while SBCAPEs are in the 2000+ range over some areas near the low with LIs of -4 to -6. This means the most unstable parcels will be surface based, which is a plus for severe convection. 

There will be a half decent 30-40 knot low and mid level jet Sunday near the low pressure, with a 40-50 knot 500MB jet moving over the warm sector. There will be around 90 degrees of turning between the surface and 500MBs Sunday due to backed winds ahead of the low, so shear will be decent. There will be some capping tomorrow, but by tomorrow evening as the dew points become higher and an upper level jet streak begins to move over the warm sector there should be at least scattered convection along the dry line. With dry air aloft, decent shear, and moderate surface based instability there could be a number of severe storms with damaging winds and large hail tomorrow. LCLs will generally be pretty high with a large dew point and temperature spread, but tomorrow evening as temps come down with the loss of diurnal heating LCLs could get below 1000 meters AGL in parts of the warm sector tomorrow. With decent shear an isolated tornado is possible, but it does appear storms will be high based so it will take a rather strong updraft so will only add 2% tornado probs.

With a decent LLJ a MCS could form tomorrow evening over parts of MN, northern IA and possibly extending back over northern SD/southern ND, and maintain itself overnight as the system moves east so will extend at least modest severe probs eastward into parts of WI and eastern IA for Sunday night. With dry air aloft, good shear, and good surface based instability will add a small area of 30% wind/hail probs within a larger 15% area.