Tuesday, August 23, 2011

Severe Weather Nowcast - Too Little Too Late?

Today is a perfect example of how morning garbage storms can really mess up an otherwise great setup. Sure, the shelf cloud was pretty but you can thank that decaying crap complex for ruining what would have been something far better. Perhaps I would be less bitter if the complex had held together when it reached Chicago, but instead it fell apart completely and even the shelf cloud was gone, leaving the area in nothing but thundery showers for a whole 4 hours. Finally, a loud pulse storm rolled through around 4pm, and the sun quickly has come out over all of IL as shown by the visible sat:

We are finally destabilizing as shown by the 3hrs CAPE change:

But it might be too little too late. Areas to the west though have had a chance to sit in the sun and cook longer, but of course out there the cap [a warm layer of air aloft that thwarts storm development] is more of an issue. Still, convection allowing models have shown a new band of storms developing shortly around sunset. Most notebly for Chicago, the 12z 4km wrf, which has actually handled todays storm situation quite well so far. So we will see if this trend continues:
Another popular convection allowing model, the HRRR also buys into a similar situation though it keeps the development further west and never really brings it into the Chicago area, likely due to the lack of instability that has yet to materialize over this area. However, this model is run each hour, and the solutions can vary greatly from run to run, below are the 2 most recent runs for the same time [10pm this evening] and you can see a huge difference.

Naturally being a storm lover I want the bottom solution to verify, but I am leaning towards the top one as of right now.

A big negative factor for severe weather is dismal low level lapse rates. This is lower than pathetic and would need to change quickly.
Still though, shear over the region remains impressive, with bulk shear in the 40-50kt range. Low level flow is backing, and there would be both ample speed and directional shear to support severe storms including a brief window for supercells with very large hail and an isolated tornado threat given the high instability out west, After which a more typical transition to a linear mode takes over with attendant wind/hail threat.
Such is always the case when watching an event unfold, there are factors supporting severe weather, and other factors scream bust. I think the forecast I posted last night is on track fairly well though and would expect the area that would see the best chance of severe weather to be from I-39 west towards the Missisippi river. As far as Chicagoland goes it is a tough call, but those in the area still need to be on alert for potential severe weather.

For the latest you can always stay tuned to my facebook page, or follow Convective Addiction here: http://www.facebook.com/aerostorms/posts/10150271094208807#!/convectiveaddiction

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