August 4th, 2009 severe wx outlook:
Lower Great Lakes, Ohio Valley, Missouri Valley:
It will be a complicated set up tomorrow with multiple weak S/Ws riding through the region…each with possible convection ongoing in the morning. The main cold front in the morning will be draped from the northern LP of Michigan through southern WI, central WI into central NB. What complicates thing is there is currently a vort max over NW Iowa. It is stirring up a large area of convection. This convection will likely grow upscale into an MCS over the next few areas as it moves into an area of relatively untapped instability. The convection will be elevated due to a low level inversion (cap) but will likely remain fairly intense overnight. With the northwest flow aloft the vort max and associated MCS will likely travel WSW overnight…through Iowa, northern Missouri, central/southern Illinois by morning, and possibly into Indiana. So, basically expect ongoing elevated convection with a small severe threat Tuesday morning over northern/central Missouri, far southern Iowa, central/southern Illinois, and possibly farther east through IN and even western Ohio along the WAA wing of the MCS.
The issue here is that the vortmax/MCV (small low) and outflow boundaries the convection with it lays down may act as the effective cold front…so where this feature tracks is critical to the forecast because north of it moisture will be choked off, so even along the main cold front to the north of this system it may not get that active. Tuesday morning expect the MCV to be located somewhere over the northern half of IL possibly approaching the IN border. The upper flow over this area will be more westerly as opposed to NWrly due to modest ridging over the eastern US. This will cause the MCV to track pretty much due east tomorrow during the day, taking it across northern IN, and northern /central Ohio. From the track of this points south is where the best severe threat will be.
It will be very moist ahead of this feature with dew points easily into the 70s ahead of it. So, it won’t take much to get pretty unstable across the Ohio valley tomorrow. However, there will be a lot of ongoing convection across the region. Anywhere from northern Missouri back east-northeast into central Ohio there will be areas of elevated convection ongoing. This will limit instability some across the region. Again though, with the high dew points it will probably only take temps the low to mid 80s to get MLCAPEs approaching 2000 j/kg across a lot of the area, so there still should be moderate instability ahead of the convection with this MCS. With 45+ degrees of turn between the surface and 500MB and 30-40 knots of bulk shear and a large area of 2000+ j/kg of MLCAPE ahead of the morning MCV and associated convection will definitely require a slight risk. Instability will not be as extreme as yesterday and shear will be marginal so will for now not add significant hatching on maps. Am debating whether or not to go with 30% for wind or hail tomorrow. The vortmax and the convection it produced today did end up producing fairly widespread reports. I think coverage today didn’t meet 30% criteria…close but not quite. Tomorrow with better instability for this system to work with it’s possible that coverage may reach that 30% criteria. For now will hope marginal shear will prevent storms from being that organized and will go against 30% wind or hail. May add it in Tuesday morning if I feel it is warranted. Will go with 2% tornado probs as one or two tornadoes are possible but with a weak LLJ don’t think there will be to many tornadoes. Shear and instability could support super cellular structures tomorrow, as well as multicellular clusters or even bowing line segments, so again may need to add some areas of enhanced probs but that would be done in the morning.
With the main cold front pushing through Michigan into northern IL/IN/OH by tomorrow evening…MCV to the south will rob moisture and instability from areas farther north. However there will be 40-50+ knots of bulk shear so there will be a threat for a few isolated severe storms. Am not expecting anything too organized so will cap probs at 5% wind/hail north of the MCV.
Plains:
Another S/W moving into Montana tomorrow morning may be the focal point for more storms tomorrow afternoon…with the front draped from northern Nebraska/eastern SD into central MT will likely see some storms develop in an area of 50+ dew points ahead of this front. There will 45+ knots of bulk shear over the region with dry air aloft so there will be a severe threat with these storms. Will go with 15% wind/hail probs. These storms again could form into a nice MCS tomorrow night as they move into Nebraska/Iowa/Missouri where there will be better moisture and instability. Because of this I will just connect the two slight risk areas. Since it is currently 4z the night before am not confident enough about higher risk zones which is why I am just broad brushing the 15% risk areas for now, however one or more enhanced risk area may be issued in the morning.
With both the Midwest and Northeast forecasts there is a lot of unpredictability, mainly due to the vortmax/MCV that I think will end up tracking from northern IL/somewhere in Ohio to western PA by evening. The predictability of these things is low. Because of this I will likely come out with an update tomorrow morning where I may add enhanced probs or switch things around some.
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