The May 25 hail storm in eastern Connecticut was an odd cluster of storms. The only severe reports in southern New England were of large hail with no reports of damaging wind.
Synoptically, forcing was marginal at best. The main forcing for convection appeared to be differential heating boundaries (storms fired first in Berkshire foothills and Worcester Hills) and some enhancement through sea breeze fronts and outflow boundaries. While there was modest instability present (MLCAPE around 500 j/kg) vertical shear between 30 and 40 knots was able to produce transient supercell structures. Very low wet bulb zero heights – around 7,500 ft AGL, also increased the hail threat.
One such transient supercell produced large hail near the Mass Pike southwest of Worcester and in Thompson and Woodstock, CT. On radar, 2 large three body scatter spikes (TBSS) were present downradial of 2 hail cores along with a side lobe (an artifact of the WSR-88d occasionally seen).
As the storm moved south dozens of dime to quarter size hail reports were received across Windham and New London Counties. The mesocyclone, while never particularly strong, extended in the vertical through the hail growth zone (-10ºC to -20ºC). In addition, the height of the 60dbz level occasionally reached -20ºC when the storm was at its strongest.
Although the was one report of golf ball size hail (that report may be on the high side) most of the hail reports remained around 1″ in diameter. There is little evidence of giant wet hail growth (CC <0.9 in the hail growth zone) and in addition the typical proxys for updraft strength of radar (mesocyclone intensity, storm top divergence) weren’t exactly off the charts.
That said, the storm did produce a classic large hail (>1″ diameter) signal for most of its lifetime in eastern Connecticut.
I’ve outlined the area here where Z>50 dbz and you can see a large chunk of that where the reflectivity is quite high (i.e. >60 dbz in white) is coincident with low ZDR (near or below zero) in addition to relatively low CC (<0.95). This, combined with the very large TBSS, is a good indication for severe hail falling. Many reports of quarter-size or even a bit larger came in under this echo from Plainfield and Griswold including this great video from my buddy Craig.