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Severe Thunderstorms Likely in New England on Sunday

On Sunday, a stalled frontal boundary over southern New England will lift northward as a warm front. This will happen as a surface low tracks to the northwest of the region. This will put most of New England into the system's warm sector with heat and humidity surging once again. This setup will be favorable for severe thunderstorm development across the region.

Rounds of showers and storms will continue to move through New England tonight into Sunday morning. Any storm that develops in the morning will struggle to become severe. A majority of this system's widespread rainfall (which will be north of the warm front) will remain to the north of New England and into northern Maine.

HRRR showing potential weather Sunday morning:

After the morning round, there will likely be a period of much more isolated activity before the main activity begins to erupt in the afternoon and evening. The main time frame for severe storms will likely be from noon to 5pm for western areas and 3pm to 8pm for areas farther east. With that said, this timing is still subject to change, so be sure to keep an eye on trends tomorrow morning.

HRRR showing potential weather Sunday afternoon. Remember this just one model's depiction, the amount and placement of storms may vary:

The greatest chance for severe weather will be southern Vermont, New Hampshire and western Massachusetts. With that said, all of New England outside of eastern Maine will have a chance to see a stronger storm. The areas mentioned before will likely see the most favorable timing of storms combined with the strongest severe weather ingredients.

The greatest threat from severe storms on Sunday will be potentially damaging wind gusts. This will come as an unusually strong (by summer standards) low level jet crosses the region. This jet will aid in pushing the warm front to the north. The jet will also create gusty southwest winds outside of storms. Strong to severe storms will only enhance these already gusty winds. 

The tornado threat is elevated by New England standards. The Storm Prediction Center has put southern Vermont and much of New Hampshire in the 10% zone for tornadoes. This is only the third 10% tornado risk issued for this area since 2010. Shear profiles over this area are supportive of discrete rotating supercells. With that said, it is far more likely that if any tornadoes develop, they would most likely be of the more typical New England variety, meaning non-significant or weaker tornadoes. 

Storm Prediction Center tornado outlook for Sunday:

Large hail will likely be the most limited threat of the three main severe threats (wind and tornadoes being the other two). Lapse rates (the rate of temperature decline with altitude) will be mediocre for hail development and the freezing level will be around 14,000 to 15,000 feet, which will make it difficult for large hail to reach the ground. With that said, strong updrafts would allow for larger hail, but this threat should be pretty isolated. 

As far as the four main severe ingredients go, deep layer shear is expected to be around 40-50 knots, which is sufficient for supercell development. Lift will be provided by the system’s fronts. Moisture will be ample with high humidity levels in the warm sector.

The big question, as always for New England, will be the amount of instability. The heat and humidity of the warm sector will provide instability, but if it remains cloudy for the entire day, CAPE values will not be able to climb. The southwest flow should help clear clouds out, but clouds will try to be stubborn and stick around. That will be the battle when it comes to severe weather. 

Euro expected CAPE values around mid-afternoon Sunday:

With all of that said, this is New England, and there will be factors working against a severe weather outbreak. The following is why we didn’t use the term “severe weather outbreak” in the title of this article. The northward extent of the warm front will be critical. Areas north of the front will not see much severe weather. Models can often move warm fronts through northern New England too quickly. If the front moves slower than expected or does reach as far north as expected, the severe threat will be reduced.

Also, as stated before, if clouds linger for much of the day, instability will not build as high as expected and therefore reduce the threat. The morning activity may also rob the atmosphere of energy for the afternoon storms. If the atmosphere can’t rebuild this energy (mainly due to a cloudy sky), afternoon activity will not be as widespread. If the morning activity lingers later into the morning or early afternoon, this would also impede activity later in the day. Widespread severe weather is not a slam dunk forecast tomorrow.



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