Midwest Edition-August 6, 2009 severe wx outlook:

August 6, 2009 severe wx outlook:

Northern Plains:

Thursday will be the first of three days that a ring of fire setup will bring thunderstorm complexs to somewhere in the Midwest. With confluence behind a trough over the northeast and Great Lakes there should be a high over the upper Mississippi valley Thursday…with the heat ridge centered over Texas this should cause the ring to setup from Montanna through the Dakotas and south through Nebraska, Kansas, and possibly Oklahoma. The ring should shift east slowly during the 12z-12z forecast period as the ridge begins to pull out and the warm front is pushed north and east. This evening the ring of fire storms seem to be between the 584dm 500MB height line and the 592dm height line. Following this general rule the ridge will expand far enough east by tomorrow night to possibly get some action as far east as MN, IA, MO, AR, and possibly western WI/IL although I am somewhat skeptical that action can make it that far into WI and IL by Friday 12z…

There are two big forecasting issues with these setups…the models suck at forecasting how quickly heights will rise and how far northeast the warmfront and ring will get…and timing the S/Ws. 24+ hours out from the end of the forecast period you kind of have to give a general guess at how far east the severe threat will get by Friday morning and I just did that…however we are definitely close enough to time the pieces of energy well and take a good guess at where the best severe threat will be and where MCSs may eventually develop…

Will go from the south and then north…first wave is a with a little piece of upper level energy currently located over western CO/eastern Utah. On the nose of this small “jet streak” if you can even call it that there is currently a complex of thunderstorms…there is an area of moderate to strong instability ahead of it along with 40-50 knots of shear, so this complex should hold together some overnight. It will likely be somewhere over Oklahoma or southern Kansas at the start of the period. It will likely weaken as Thursday wears on as higher mid level temps advect in, but will paint in small hail/wind probs over KS/OK for a small severe threat still lingering with this feature in the morning.

The next area farther north will likely be more significant…and will be associated with a jet streak rounding the trough on the west coast and moving over the ridge…there is already some convection developing over South Dakota on the nose of this jet streak. There is a surface reflection with this currently over Washington state that will be over SW Montana/NW Wyoming by morning. This surface reflection will generate a decent LLJ with 30-40 knots at 850MB likely by Thursday afternoon. There will be a 40+ knot 500MB jet on top of this with good large scale ascent with the right entrance portion of a departing jet streak. The surface reflection will also back surface winds to the south-southeast under a westerly flow at 500MB. 

So shear and dynamics will be good across the northern Planes with this wave Thursday afternoon and evening. With dew points into the low 70s it won’t take much to generate a stipe of 1000+ j/kg of CAPE over the region. This should lead to numerous storms breaking out over eastern Montana, the Dakotas, Nebrask Kansas and possibly western MN Thursday afternoon and evening. With decent shear and some instability there will without a doubt be a severe threat with this Thursday afternoon onwards. Instability will be somewhat of a question. Tonights complex over the Dakotas will probably stay rather weak with the blowoff shearing off to the east with a strong WNW flow aloft, but the strong LLJ will setup decent overrunning to the northeast of the warm front tomorrow. So, expect widespread storms to break out. PWATs will be 1.5-2” across the area tomorrow so all these storms could weaken the overall instability and hurt the overall severe threat. Will without a doubt need at least 15/15/2 probs but am not sure if enhanced probs will be needed. Then the LLJ kicks in…

Best instability will develop over far western SD/NB/KS where capping will limit overrunning…however that area will be capped…so, with capping/instability issues may not see widespread severe threat until the strong nocturnal LLJ kicks in. By this point the ring will be more over MN/IA, so this may be the best severe threat. Will paint in 30% wind/hail probs here as the strong LLJ with weak to moderate instability should allow these storms to grow upscale into an MCS with a more widespread severe threat, from the edge of the higher mid level temps tomorrow evening eastward some over SD/MN/NB…will add 5% tornado probs with some instability and a strong LLJ, could be a small window for some tornadoes in the evening. Will likely see this activity develop into some sort of a complex/MCS tomorrow night and trek ESE towards parts of MN/IA/western IL/WI overnight with some of a remaining severe threat due to the strong LLJ.