Saturday August 1st, 2009 severe wx outlook: Great Lakes, Mississippi Valley, Southern Plains!

Another upper low moving east towards the northern Great Lakes will drag a shortwave/cold front southeast through the area…tomorrow morning the cold front should be draped from central Wisconsin south through eastern Iowa, northwest Missouri, southeast Kansas, and into northern Oklahoma. 
There is still a frontal boundary over the Deep South which will block moisture return to some extent. However, dew points in the low to mid 60s should be realized ahead of the warm front, with higher values over KS/OK. 

There is currently a MCS along the front over northern KS, and this will propagate east-southeast overnight, bringing its clouds and precip with it. This will not only limit surface heating some over parts of MO, KS, and IA and tomorrow but will help further block moisture return north of there. So, we will have moisture issues limiting instability north of the MCS, and a lack of good heating limiting instability where the MCS impacts. Instability across a lot of the warm sector should be limited to under 1000 j/kg of MLCAPE, with higher values possible south of the MCS over OK/AR. 

Despite a lack of strong instability we will have a decent front coming in, steep lapse rates, and cold air aloft which will cause storms to break out along and just ahead of the front tomorrow afternoon. Stronger storms will have an isolated damaging wind/hail threat so will need to add slight risk for that. Am debating whether to bring slight risk north of areas MCS will impact due to moisture issues. Today despite weak instability there was half decent coverage of severe reports up north, likely due to the cold air aloft and decent jet streak. Right now LIs are modeled to generally stay positive or only go weakly negative from central IL north which is a big turn off, so will only add 5% probs for wind and hail from central IL north through the western lakes. South of there do believe there will be enough instability and shear for decent storms. This combined with dry mid levels should provide for an isolated hail/wind threat from central IL SWward so will add 15% wind/hail probs there. With LCLs dipping below 1000 meters in this area tomorrow with half decent shear will add a 2% tornado risk in this area as well. A relatively weak low level flow should preclude widespread or strong tornadoes so will not go higher than 2%. Another MCS may form tomorrow night over MO/AR with a heavy rain threat.