All the ingredients seemed to be lining up for a significant severe weather event, particularly in western Connecticut and adjacent parts of New York. What did we see? Sporadic damage reports and a dying squall line that moved out of the Poconos.
There was certainly a lot of potential on Thursday. Models had been showing moderate amounts of instability and very strong shear. Forecast and observed hodographs indicated the potential for tornadoes in any discrete cells that developed or along squall lines.
While deep layer (and low level) shear was sufficient for supercells a strong cap near 850mb prevented convection from developing ahead of the main line. Here’s a 23z RAP sounding (initialization) for HVN and you can see right off the bat the problem.
Yikes!!! That’s a lot of CIN. The LFC is nearly 775mb with a large negative area on that sounding. While the squall line looked exceptionally impressive over Pennsylvania it began to fizzle as it worked east. The large amount of convective inhibition became problematic as the squall line’s cold pool was unable to force surface based parcels to the LFC.
Our models had shown that the CIN present during the day would weaken but apparently that was not the case. While low level moistening helped eliminate some of the negative area it was not enough. The cap was also quite low. Typically we see caps around 700 mb… not 850mb! The exceptional warm 850mb temperatures (over +22c) effectively served to cap convection.
With a convective temperature of 95F we just weren’t going to get rid of that CIN. The warm layer at 850mb (NAM did do well with this) was just too much. With stronger surface heating and more low level moistening things may have been different.
The storms were most certainly a bust. The Storm Prediction Center issued the first ever (that I can remember) day 2 moderate risk for Connecticut. The probabilities for severe wind and significant severe were some of the highest you’ll ever see in Connecticut. In fact while the event was ongoing the SPC sent out a discussion saying they contemplated an upgrade to “high risk” which would have been the first time since May 31, 1998 parts of the state have been in a high risk! As the squall line was leaving the Poconos the SPC expected the storm to strengthen and called it a derecho.
In the end a lack of synoptic forecasting (500mb heights were neutral from 18z-00z) and strong CIN thanks to a stout low level cap did our severe chances in. Given the potential it was important to mention the possibilities. Severe weather is a challenge to forecast – certainly much more challenging than a winter snowstorm!