A large, sprawling cut-off low that will originate over Lake Superior and western Ontario is going to lead to a few days of weather trouble here in New England. The great synopticians at SUNY Albany and the NWS in Albany with their CSTAR program have some great info on warm season cut-off lows here. It appears that we will see a several day period of several hazards including flash flooding and severe weather.
Already a flash flood watch is in effect for southern New England. This time of year when an anomalous cut-off low digs south the presence of strong ascent, unusually high levels of moisture (tropical connection), and convection can lead to problems.
Initially, Thursday’s weather will be dictated by a strong upper level disturbance swinging around the closed low. The shortwave here at 00z Friday is hard to miss – swinging through central Pennsylvania well east of the cut-off located over the western Great Lakes. The good news is that the system remains far enough “west” that with any luck we should manage to avoid a significant severe weather episode. The best forcing (and shear) will arrive after sunset when instability is waning. That said, by dark Thursday, wind fields increase dramatically and any convection will have to be watched for rotation.
The bigger threat late Thursday and especially Thursday night may be flash flooding. The ground is very wet and rivers are very high. With strong moisture transport and a quasi-stationary surface boundary over New England we may be able to wring out quite a bit of rainfall.
On Friday the flooding threat, and potentially a severe weather threat, will continue.
The setup here is a classic signal for heavy rain in New England. Per CSTAR research by Scalora (see here) there is the potential for excessive rainfall.
The progged low level jet on the GFS by late Friday exceeds 50 knots in southeastern New England. The exact location of this jet and how transient it is will determine the severe weather threat and extent of flooding. Any surface-based instability that develops could also lead to tornadoes. A setup similar to this in July 2008 lead to a very destructive and long track F-2 tornado in New Hampshire.
The evolution of all of this is going to be tricky. These aren’t easy to forecast a few days ahead of time. At this point the best bet is that we will see multiple rounds of thunderstorms, some severe, that could lead to flooding. The signals are there. The potential for significant flooding and tornadoes is there as well so this bears watching through at least Monday as this cut-off low shows no signs of wanting to move along or dissipate any time soon.