Heat and Thunderstorm Potential

A couple days before the unofficial start of summer it looks like some hot and summery weather is moving in. This warmth will be impressive and possibly close to record breaking on Wednesday but it’s certainly not unheard of. The all time record in May at Windsor Locks is 99º set back in 1996. We will not break that record.

As I mentioned in my previous entry meteorologists like to use the 850 mb temperature as a starting point for making a temperature forecast. Generally 850 mb temperatures above 20ºC support temperatures well in the 90s. Here’s what our computer models are forecasting Wednesday afternoon.

– 18z NAM +19ºC
– 18z GFS +18ºC
– 12z Euro +20ºC

850 mb temperatures around +18ºC correlate to temperatures around 90º especially given full sun, deep mixing, and downsloping, the latter of which won’t be present Wednesday. With that in mind I expect temperatures away from the water to approach 90º and possibly as high as 93º or 94º at Bradley Airport if the Euro temperature forecasts are realized.

A cold front advancing from east to west or what we call a backdoor cold front may produce enough lift to trigger some storms. Here’s a forecast sounding from the 18z NAM on the left. Our models are indicating a VERY unstable atmosphere with CAPE values approaching 3000 j/kg. This seems a bit overdone likely because dew points will be a bit lower than this particular model is forecasting (lower dew points mean less instability).

The area between the red line and yellow line on the left is a graphical representation of how much CAPE there is. The more area means more CAPE. Less area means less CAPE. For calculus buffs out there, like myself, CAPE is computed by taking and integral between the level of free convection and the equilibrium level. In this case it will be from 850mb and 175mb. CAPE is important because it represents the amount of energy available to a thunderstorm owing to the atmosphere’s instability (air parcels are warmer and more buoyant than their environment that means they rise and can produce convection or storms).

Shear is important for storm organization and that appears to be lacking Wednesday. In addition it’s unclear whether the backdoor cold front will provide enough “lift” to spur organized thunderstorm development. Given the amount of instability, however, we will have to watch for severe thunderstorms Wednesday. At this point I’d say the threat is “low” for widespread severe weather but that may increase as we draw closer. What will be unusual in this setup is that the storms will likely be moving northeast to southwest.



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