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Breaking Down Sunday's Thunderstorm Threat for New England

A warm front associated with an area of low pressure tracking to New England's north has begun to slice through New England this morning. Once this warm front passes, New England will be firmly in the system's warm sector with heat and humidity spiking. This heat and humidity will provide the foundation for a line of thunderstorms this afternoon.



Storms will begin to fire up early this afternoon (or even late this morning) across western New England, particularly Vermont, as the cold front begins to cross the region. As storms begin to erupt, they will likely be more spread out and individual (or discrete) rather than in a more organized line.


HRRR showing potential weather early this afternoon:



As the storms advance eastward, the individual storms will likely form into a more traditional line of storms ahead of the cold front thanks to the linear forcing along the front. This line should be rather progressive and push eastward at a decent pace, pushing offshore by around sunset.


HRRR showing potential weather around mid to late afternoon Sunday:


The ingredients for thunderstorms to turn severe will be in place. At this point, shear looks to be the strongest of these ingredients. Shear will likely be on the order of 40-50+ knots. This is more than enough to allow for storms to not only strengthen, but hold their strength as they cross the region. The cold front will provide lift and looks to cross much of the region at a favorable time to take advantage of daytime heating. There will also be ample moisture as heat and humidity build heading into the day. 



The big question is instability. It does look like CAPE values will build toward 2,500 j/kg, with the most unstable CAPE across interior southern New England. This is more than enough when combined with the other ingredients to produce scattered strong to severe thunderstorms. The question on instability will come down to cloud cover. If it remains mainly cloudy, instability will not be able to build as much and therefore lower the threat for severe weather. With that said, breaks of sun have begun to form this morning in places.


HRRR expected CAPE values this afternoon:


The one big obstacle for severe storms will be unimpressive lapse rates (the rate of temperature decrease with altitude; when this temperature falls quickly, it is known as a steep lapse rate and is an indicator of instability). With the other ingredients in place, scattered severe storms should be able to form, but the conditions are not absolutely perfect for development.



Given the setup, the biggest threat from thunderstorms will be strong straight-line winds. This is reinforced by strong shear and a linear storm motion. Large hail is a secondary threat and will be more likely earlier in the day when more discrete supercells are possible. Once the storms form a more organized line, this threat will drop, but not totally go away.


Quick spin-up tornadoes are also possible in the supercells, but this threat is low, far lower than last Sunday when rotating storms were the paramount concern and a handful of tornado warnings were issued.



While the entire region could see a stronger storm today, Vermont and northern New Hampshire have a lower risk as the front crosses there earlier in the day. This will bring the highest chance for storms before peak daytime heating and instability can build.


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