The severe threat for Wednesday afternoon was isolated in southern New England as the four main components needed to produce strong to severe thunderstorms were in place. These components are shear, lift, instability and moisture. The main question mark was the amount of instability, but the atmosphere was able to de-stabilize for portions of the region after the morning shield of rain used much energy.
In the afternoon, it was becoming clear that hail would be the primary concern among any severe thunderstorms. This came as temperatures at the 500mb (about 18,000 feet above sea level) dropped to around 0°F. This resulted in a steep lapse rate, which is basically a rapid decrease in temperature with height. Combine this with effective shear to allow thunderstorms to strengthen, and you have a recipe for hail.
Severe thunderstorms began to form in western Massachusetts around mid-afternoon. Storms were quickly able to take advantage of an atmosphere set up for storms. The threat was isolated in New England because only a small portion of southern New England was primed with favorable conditions. This smaller area (western and central Massachusetts, northern Connecticut, Rhode Island and into the South Shore) that was primed did get hit hard.
So, the atmosphere was primed for hail, but in order for large hail to form, you need strong updrafts. Updrafts transport water droplets upward into a thunderstorm, where it is below freezing. These droplets freeze together and form hailstones. Hailstones fall back to the ground when they become too heavy for the updraft to keep them suspended in the storm. So, the stronger the updraft, the bigger the hail can get as it will take longer to fall back to the earth.
This chart shows what updraft speed is needed to create the different sizes of hail. The largest hail seen Wednesday needed updrafts of 60-70mph:
With that said, the largest hail size came from Narragansett, Rhode Island, where a hailstone measured up to two inches in diameter, which is comparable to an egg. There were reports across portions of Massachusetts, Connecticut and Rhode Island of 0.75-1.75 inch stones. Hailstones well over an inch are uncommon anywhere, but in New England, it is very rare to see sizes this big.
Hail reports from Wednesday:
There were no immediate reports of injuries, although one report did mention dented car hoods in Rhode Island. Numerous smaller hailstones fell during the storms, blanketing the ground in places. Torrential rain and frequent cloud to ground lightning was also reported. A lightning strike was reported to have caused a fire in Duxbury, Massachusetts.
Hail from southern New England Wednesday:
Credits: @axlroseamoji; Benny Barber; Ryan Benoit;