Tuesday severe weather potential – Still uncertain

Good afternoon and Happy Father’s Day to all the dads out there!

The next big weather story for the Northeast is the disturbance in the High Plains which the Storm Prediction Center has already tagged as an enhanced risk of severe weather for both today and tomorrow (with further upgrades likely tomorrow). There is some potential for a severe weather event in the Northeast on Tuesday associated with a unseasonably strong low pressure system over northern Ontario and Quebec, with some models even showing a high-end event possible. However, there are many factors that may mitigate the threat, possibly greatly.


CAPE forecast for Tuesday afternoon (GFS run at 12Z June 21, 2015). Greatest instability from western Massachusetts to central Pennsylvania.

First of all, time of day will be critical. A faster system (as supported by the 12Z NAM) would send the main frontal boundary offshore before peak daytime heating, limiting the severe threat to the immediate coastal areas (and perhaps northern New England – the 12Z NAM clearly shows a strong tornado threat in NH/ME and especially central Quebec). In addition, the strength of the shear is crucial, yet the models show (thanks to the strong low to the north) shear that is at least conducive for tornadic activity across much of New England (location depends on the timing factor), and very powerful shear closer to the low near and north of the Canadian border which, if combined with strong instability, is capable of more widespread or significant tornadoes. South of approximately 40-41°N latitude, shear becomes much more marginal for tornadoes, but damaging wind and large hail are certainly possible as well across the mid-Atlantic.


Low-level helicity forecast for Tuesday afternoon (12Z NAM run on June 21, 2015). Above 200 m^2 s^-2 typically supports more than isolated tornadic activity – notice the extreme values near and north of the Canadian border.

Those are just two of the model runs and there are multiple factors involved. The SPC has only posted a slight risk for Tuesday, and understandably given the uncertainty. It takes numerous factors to bring together a severe weather outbreak, and some may come together, but in less populated areas. In late June, shear tends to often be a limiting factor as strong low pressure systems are less likely to form – this one is unseasonably strong. Some comparisons may also be made to the May 30-31, 1998 system, although that didn’t affect most of New England at peak heating (so time of day may be different). What happens tomorrow in the Great Lakes (there is a significant severe weather threat there tomorrow, including strong/long-track tornadoes possible) could play an important role.

Forecaster Craig Ceecee

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