Of all the tornado warnings that are issued in southern New England – it’s hard to think of one more perfectly executed than the one issued Monday morning on August 22nd for Middlesex County, Massachusetts by Hayden Frank at the National Weather Service in Taunton.
This was a classic low probability/high impact severe weather event that we deal with with some regularity over the summer. What the models did show was some minor instability and a moderately sheared environment in the lowest levels of the atmosphere. With very low cloud bases (low LCLs) and a moist flow off the ocean (where sea surface temperatures are at their climatological maximum) these are always worth watching but frequently don’t produce much of anything.
The meteorologists at Taunton mentioned this possibility in their 4 p.m. Sunday forecast discussion.
there is a very low probability of a brief tornado or waterspout developing. However, this is a very low probability.
Hayden mentioned it again on NWS Chat to media and Emergency Managers later in the evening.
The 12z models on Sunday weren’t overly impressive – but they were enough to keep things interesting. Here’s the 12z 4km NAM which shows an impressively veered wind profile and a bit of surface-based instability (about 500 j/kg of CAPE).
The 00z 4km NAM had a similar story to tell with some surface-based CAPE and a similarly impressive veered wind profile in the lowest part of the atmosphere. What is most impressive here is that there’s over 200 m2/s2 of SRH in the 0-1km layer!
On radar you can see a hint of an MCV that forms just north of Worcester and tracks northeast toward the New Hampshire Seacoast. It’s certainly conceivable that low level shear was enhanced even further near this system. The storm began to look interesting around Marlborough.
A line of higher reflectivity echoes is moving northeast and there appears to be several embedded mesocyclones. None are particularly tight but the one on the Marlborough/Southborough line is concerning with ~59 knot of Low Level Delta V (LLDV) and nROT > 1.00. These values are higher than the median for recent northeast tornadoes – though the LLDV is not gate-to-gate and is relatively broad.
At 701 UTC some of these embedded mesocyclones persisted and the first Tornado Warning of the morning was issued. nROT continued around 1.00 as the line moved northeast with several mesocyclones appearing and disappearing until one really went to town over Concord. nROT peaked at 1.40 and LLDV peaked at 73 knots.
For a night where there was no tornado outlook from SPC (they did mention the threat for an isolated rotating storm, however) and an uncertain evolution this was a very impressive warning performance by the NWS. The warning was issued at 3:01 a.m. and the tornado hit Concord, MA at 3:20 a.m. That’s an impressive 19 minutes of lead time!
Kudos to the NWS to recognizing the threat and watching the radar so closely during the overnight shift. A job well done and a good reminder to watching things very, very closely this time of year when you have strong 0-1km shear, low LCLs, and even a small amount of instability.