Tuesday, May 1, 2012

SPC Widens Threat Across Great Lakes for Tomorrow, May 2nd

The Storm Prediction Center has widened the area expected to receive severe thunderstorms from the Plains to the Mid Atlantic. This move essentially covers the entire Great Lakes region, with the exception of New York.

The area being watched for this situation includes the areas of northeast Nebraska, northwest Iowa, extreme southeast South Dakota, as well as southern Minnesota. Those are the areas most at risk, as of now. The latest 12z SPC NAM Parallel model shows an abundance of showers and thunderstorms breaking out across the areas mentioned above. These storms look to be in a cluster, so a tornado threat does not appear incredibly high for now. The proximity to the warm front does have me concerned, however.
Considering that the low pressure area will be in western Nebraska, I am watching for the wind field in that area. It does appear that a counterclockwise wind field is setting up, primarily in Iowa and southern Minnesota, as well as eastern Nebraska. This increased speed and different direction may provide a leg up for tornado threat in comparison to other areas, where the lower level wind field looks to be mainly south to north in accordance with the warm air being pulled towards the areas to the east of the highest risk. Looking over bulk shearing values is indicating to me that there will be a narrow corridor of fairly high shearing rates in the late evening into the overnight hours. I am warning this- small, mesoscale areas are not to be trusted. Yes, they are to be watched, but models are not exactly 'spot-on' when you are looking for a small area's tornado risk.
The main areas I would project the most intense convection to be would definitely include Wisconsin, extreme north Illinois for the overnight hours. I don't need to mention the higher risk areas, as they are already higher risk. Anyhow, judging by 500mb relative humidity values, it is clear that the areas mentioned above will have ample supplies of moist air that may be able to be significantly lifted to the point of severe convection. Should this happen, residents in the mentioned areas should prepare for an intense evening, primarily in Wisconsin.

I'm sorry that there is a lot of writing here, but I'm trying to get as much information out without clogging it up with multiple images.


Thursday Severe Threat Similar to Wednesday

Probability of Severe Weather
The Storm Prediction Center has outlined an area of severe weather from the central Plains to Michigan for Thursday in the midst of a frontal boundary closing in on a moist and warm area of high temperatures brought north by the warm front associated with this event. Instability values on the order of up to 3000 j/kg will be present in the Great Lakes for this event, but shearing will be a problem. There is expected to be a weak swath of winds in the lower levels that may help to energize the storms, but with little help from the Lower Level or main jet streams, there are limits to this event.

I'm sorry for the shortness and vagueness of this briefing, but I am rushed on time this morning. I will make it up this afternoon with both Wednesday and Thursday discussions. Both risks are over my area, so I will be sure to get pictures if I can.