Another MidWest Edition-August 3rd, 2009 severe wx:

August 3rd, 2009 severe wx:

Great Lakes, Upper Mississippi Valley:

A low pressure will continue to move east through the northern Lakes into western Ontario tomorrow, continuing to drag a cold front through the area. By Monday morning the cold front should be draped from central WI, possibly far SE MN, northern IA, through central Nebraska. South of this boundary the airmass will be moist with dew points in the upper 60s into the low to even mid 70s over Missouri. It will not take much to get decent instability in this airmass, but will debris from the overnight convection ruin this?

Right now based on where the clouds are moving with the current storms…to the ESE it appears that most of the debris will be from central IA and northern IL points northward. South of there it will be mostly sunny until cu start going up, so we should get temps into 90s over MO and possibly southern IA ahead of the front tomorrow. 700MB temps will be 10-13 across IA and Missouri tomorrow, and 13-15 over Nebraska. So, there will be some capping over the region tomorrow. The cap over Nebraska will be pretty strong and it will basically take a sledge hammer to break it. If that area sees almost full sun all day which it very well could, temps will get into the upper 90s to locally above 100. This could be enough to break the cap, so there could be some storms develop over parts of NB/KS tomorrow evening. The cap over Missouri will be a little weaker, but there is always the risk of a cap bust, but again as long as debris stays north of that area and there is decent sun most of the day the low level lapse rates should steepin up enough to weaken the cap. No guarantees the cap will break with 700MB temps as high as there are, but the temps up there definitely aren’t in solid cap territory that will always kill storms, and the surface temps are progged to get very warm so there is a decent chance the cap will break. 

This cap is definitely a “cap and shoot” type situation where the cap will limit convection most of the day and allow intense destabilization of the boundary layer which would result in explosive convection once the cap weakens. So, it may take until late afternoon or early evening but once things get going conditions will be very conducive for severe wx. SBCAPEs will be 3500+ in a large area over southern IA, northern MO, possibly into eastern Nebraska. Even over portions of IL, where there aren’t significant debris clouds SBCAPEs will easily reach 2000+ j/kg. As I said earlier most debris clouds will be over northern IL, as a general rule I-80 northward. South of there is will destabilize. North of there instability may be a question. The NAM wants to bring moderate CAPEs as far north as southern WI. Not sure about this but it will need to be watched. Models show around 90 degrees of turning between the surface and 500MBs over a large portion of the warm sector Monday late afternoon and evening with a 30-40 knot 500MB flow over northern portions of the warm sector. This shear is enough for super cells. 

So, in summary expect scattered convection to break out along and ahead of the cold front from eastern Nebraska, southern IA/northern MO/central and possibly northern IL by late afternoon. Shear and instability are sufficient for super cells. The low level flow isn’t the strongest and LCLs are up at around 1000 meters AGL, so the tornado efficiency of super cells Monday definitely won’t be the best, but with the very intense instability and decently curved hodos there will be super cells tomorrow and some will produce tornadoes. Right now am thinking about where to cap probs for tornadoes. Think tomorrow evening as the LCLs come down some and the LLJ begins to pick up some there could be a brief period that is rather favorable for tornadoes before the temps go down too far and the cap takes back over. Because of this will add a small 10% tornado area over southern IA and northern MO. Will go with 30% wind/hail along with hatching for potential for significant wind/hail in this same general area is environment is favorable for super cells, it will be very unstable, and to boot there will be dry air aloft so there will be a number of wind and hail reports across the area tomorrow. Almost capped probs at 15% in case of a cap bust but if the cap weakens enough which again is very possible the enhanced probs will pay off. Am thinking will extend slight risk as far north and east as the IL/WI border…instability is a question over northern IL and especially southern WI depending on debris from overnight…again if it destabilizes there could be severe up there which is why I’m reluctant to not put in some sort of severe probs for them. Subsidence to the east will probably limit the severe threat east of IL so will not bring a slight risk into IN or MI for tomorrow, but if activity develops over IL and moves east overnight there will be a modest severe threat so will add 5% wind/hail probs for IN and parts of MI for Monday night. 

Northern Plains:

Another S/W will move through and could tap the moderate instability and cause some storms to fire along the front over the Dakotas, Montana, and western NB so will also add probs up here for severe. Instability will be weak to moderate, but with 40 knots or so of bulk shear and half decent lapse rates there will be a modest severe threat up here so will go with 5/5 wind/hail probs.