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Some Storms This Afternoon for the North; Unsettled for Everyone After

A warm front has begun to lift through New England this morning as an area of low pressure slowly moves to the north of New England. This has sparked showers across the north that will continue moving from west to east as the morning goes on. Later this afternoon, the ingredients will begin to come together for the development of thunderstorms across portions of northern New England. Southern New England will see far less activity this afternoon.



Scattered thunderstorms will likely begin to fire this afternoon across the northern tier of New England before dropping southward toward the evening. Being in the warm sector of the system, there is plenty of moisture and lift for thunderstorms. The other two main ingredients for storms are more questionable. There won't be much by way of strong shear and any lingering cloud cover from this morning's showers may limit instability.


GFS showing expected weather around mid-afternoon:




With a lack of shear and questionable instability, thunderstorms are not expected to become severe, however, any storms that are able to develop could produce brief torrential downpours, gusty winds and small hail. This comes as CAPE values (a measurement of instability) are expected to climb over 500 j/kg this afternoon and into the evening across Vermont and northern New Hampshire. You typically need CAPE values to climb to at least 1,000 to have enough instability for severe thunderstorms.


CAPE values are expected to stay at 200 or less across central and southern New England, which would limit activity south of Lake Winnipesaukee, but an isolated shower or brief storm can't be completely ruled out heading toward the evening. This comes as more sunshine in the afternoon will provide plenty of daytime heating.


GFS showing expected CAPE values (j/kg) this afternoon:


At this point, it doesn't appear that enough storms will develop to create too much storm training (when multiple storms move over the same area), but there is a chance areas in northern New England see more than one thunderstorm. Highs today will be in the 70s to low 80s for most, with 60s across the south coast (due to a southerly flow creating a sea breeze) and eastern Maine (due to the warm front stalling out).


In typical spring fashion, the system will stall across New England for Wednesday. At the same time, another area of low pressure will emerge off the Mid-Atlantic coast. We noted yesterday that models have recently been trending systems to our south as the event draws closer, leading to a drying trend in the forecast. This has occurred with the Mid-Atlantic system. It has trended farther south and therefore southern New England has trended drier for Wednesday.




This setup will keep the most numerous showers across areas that are closer to the stalled frontal boundary in northern New England. This front will be weakening as it remains stuck over the area Wednesday, so it is unlikely to be a washout across the north. The system to the south will be too far removed to produce much by way of showers activity for southern New England on Wednesday. Still, enough moisture will be present for brief, spot showers.


Euro showing expected weather for the early afternoon Wednesday, showing showers more concentrated near the cold front, with most activity from the southern system staying south and west:


Thursday will see a similar setup to Wednesday, only the stalled cold front will begin to drop southward as the Mid-Atlantic system rises northward. This will increase shower chances for southern New England. It's still unlikely to be an all day rain (we mentioned in yesterday's "weather this week" article that washout potential would be low despite unsettled weather). The system will bring back an onshore flow, leading to a cooling trend through the weekend.



The system will likely cut off and linger around due to the aforementioned block. Cut off lows are difficult to predict, but generally they keep the clouds around with isolated to scattered shower chances. It does look like Friday will be a dry day. Things become more questionable heading into the weekend as the next frontal system arrives from the west while the cut off low lingers offshore.


Keeping with the trend this week, washout chances this weekend are low, but unsettled weather will persist. The extent of showers on Saturday will come down to the level of interaction between the cut off low and approaching frontal system. Total rainfall through this weekend will remain under an inch for most, with potentially over an inch in areas that will see thunderstorms today.



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