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Wednesday, July 25, 2012

BIG Severe Weather Threat

With sunshine, low humidity and a gentle breeze, you would never guess that a significant severe weather threat is looming for tomorrow.

The Storm Prediction Center (SPC) in Norman Oklahoma has the state in a “moderate” risk of severe weather.  Such a high risk on a “Day 2” forecast is rare for us here in Connecticut.  The greatest threat tomorrow is damaging winds and large hail.  But flash flooding is also possible and there is an elevated risk for tornadoes too.

There will be two rounds of storms tomorrow.  The first round will come from a dying complex of thunderstorms that can currently be found in the Great Lakes.  This cluster of storms will ride along a warm front through New York and arrive in Connecticut tomorrow morning.  Heavy rain, lightning and gusty winds are all a concern.  Even though the morning storms could be strong, the main threat for severe weather will be tomorrow afternoon.

Clouds, and showers will break for some sun midday and the dew point will take off.  That means it will get warm and humid around here with temperatures soaring through the 80s. The more sun we see, the worst the severe weather will be tomorrow afternoon.  Round two will begin anytime after 2 P.M and last until 8-10 P.M. Any storms that pop have the potential to turn into rotating supercells with torrential rain, flash flooding, frequent cloud to ground lightning and isolated tornadoes. 

The threat for severe storms exists for the entire state.  There is a slightly higher risk in western Connecticut, but everyone should be prepared to see strong storms tomorrow.

Why tornadoes?  I know, we don’t often mention the “T” word in our forecasts here in Connecticut.  Not only is the atmosphere extremely unstable and moist, but there is also a lot of wind shear.  Wind shear means the wind direction is changing with height.  Shear is one of the key ingredients to the supercell thunderstorms that spawn tornadoes.

Check out this screen shot of one of our forecasting tools called the BUFKIT. On the left side you can see all the severe weather indicies are the color RED, indicating that they are in the high range.


One index that surprises and alarms me the most is the EHI or Energy Helicity Index: This is an index that combines instability and helicity (how wind speed and direction change with height) into one number for forecasting supercell storms.   Anytime this is greater than one, supercells are possible. The 12Z NAM is forecasting BDL (Windsor Locks) at a 4.37 for tomorrow at 4 PM!

Here is a link to a website showing you a key for some of the other indicies on the BUFKIT screen shot.

2 comments:

Dianne Z from Marlborough said...

Thanks for the informative info. My daughter is part of a high school regional CERT team and she has already been instructed about tomorrow's possible severity and told to keep cell phone charged. So it appears no is taking the forecast lightly!

Dianne Z from Marlborough said...

S/b "no one" is taking the forecast lightly!