Hail in Connecticut, 10 inches of snow on Mount Washington, and at least three tornadoes in southeastern Massachusetts and Rhode Island. Tornadoes weren't in the forecast but the atmosphere was primed for tornadoes. Plentiful low level instability and wind shear produced a swarm of very shallow supercells that starting spinning almost immediately.
Based on radar, storm surveys, and eyewitness video I'm able to confirm tornado touchdowns in North Providence/Lincoln, RI, Norton, Mass. and Sandwich, Mass. There may have been more.
To measure instability we use something called CAPE which is an acronym for Convective Available Potential Energy. Values about 2,000 or 3,000 get our attention but today they were almost 10% of that! What was different, however, was that all the instability was in the lowest 10,000 feet of the atmosphere. We can look at just the CAPE in the 0-3km layer and that was between 100 and 200 units which is very impressive. When coupled with strong low level shear we were set for supercells and tornadoes.
Two of these tornadoes we were able to confirm in real time as lofted tornado debris was detected by Doppler Radar. A Tornado Debris Signature is a confirmation that a tornado occurred as lofted tornado debris has a distinct signature that's different from rain, snow or hail. We noted two large, but brief, tornado debris signatures in Lincoln, RI, and Norton, Mass.
Today's tornado event was extremely unusual. Tornadoes are rare in Rhode Island and even more rare on Cape Cod. Today's tornado in Sandwich was the first tornado in Barnstable County since 1977! A "perfect" combination of low level shear and low level instability wound up producing a widespread severe weather event.
This event was not forecast at all. It was a big miss. Looking back I see a few signals that showed up last night in some computer guidance but at the time they didn't raise any big red flags to me. By early afternoon, however, it was clear that we had trouble on our hands. A truly unique and unusual day of weather in New England.