Monday, August 22, 2011

Potential for Severe Weather in Chicago Tuesday and Wednesday.

Bored and checking over the 0z NAM as it continues to trickle in. A NW flow type event could bring multiple rounds of severe weather to the area. These are quirky setups that occur in this area from time to time. Basically if you take a traditional setup and rotate the shear profile clockwise 90 degrees you get a NW flow event. They have dealt some surprise tornado events in the past, and are worth watching.

Tomorrows setup looks to be one of those cases as decent directional shear will be in place. Surface to 500mb crossovers show a good 90 degrees of turning, though speed shear is a bit lacking near the surface which leads me to believes storms will puke on their own outflow before they get a chance to utilize their full potential. Instability will really need to be in place to compensate for this, and it looks like the best instability will be to the west, with a modest 1500 CAPE in the N IL area.

The other big question is what will happen with a morning round of storms progged to form in E IA and track over N IL. Latest HRRR runs as well as the 0z NAM support this idea, though the NAM is much later with initiation. There are a couple scenarios that will play out.

1- Storms form in E IA earlier than later, move through Chicago by early morning, and N IL has sufficient time to recover. Outflow from this complex could spark a new round of storms, potentially supercells early on given ample directional shear before eventually congealing into a large bowing MCS. This would be the most ideal scenario chase wise.

2- Storms form later than sooner, and move through N IL and the Chicago area in late morning or early afternoon. If early afternoon they could potentially be severe if enough instability can develop ahead of them...or we get a decaying MCS with moderate rain and a CG or two. Despite whichever scenario plays out, a later passing would likely ruin the threat for the remainder of the day. I would then place the favored area of initiation further north and west into Wisconsin, this new round would potentially reach Chicago after dark.

The other trend I noticed is the fropa seems to have slowed down a bit. This intrigues me as well since severe storms will likely redevelop ahead of the front Wednesday. Slower frontal passage gives the Chicago area a greater chance of getting in on this action as well as opposed to what normally happens where the front passes through here at noon and storms form near I-65 and flip me the bird as they slide away. So I am all for a slower frontal passage.

Basically, I don't know exactly what will happen with the threat these next two days, but it will probably be one of the scenarios I mentioned above, so I am going to be nowcasting the event, and head out if I deem it worthwhile.

Right now my target triangle for potential supercells would be Rochelle, Madison, Dubuque.

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