This weekend, a system more typical of the winter months will be moving through New England. A primary low pressure system will track well to the north of New England with a secondary low forming and passing through the region (while this setup is more common in the winter, the track this system is taking would produce a winter rainstorm, not a snowstorm, but in late June, that's beside the point). A warm front will be pulled through New England tonight with a cold front being dragged across the region later Sunday.
Showers across northern New England this morning will help to moisten the atmosphere for a period of more steady rain this afternoon and headed into the evening. Showers during the daylight hours Saturday will be most likely across Vermont and northern New Hampshire. Southern New England will remain largely dry today, outside of some isolated showers that drop south.
HRRR showing expected weather around mid-afternoon today:
Heading into the evening, the showers will drop southward into southern New England, bringing periods of rain from around 6pm onward. At this time, showers should begin to wane across northern New England. While most of this activity should come in the form of showers, a few thunderstorms will be possible, including overnight storms. Thunderstorms that develop are unlikely to turn severe. With that said, gusty winds and small hail will be possible across Vermont.
The overall threat for flash flooding has decreased in the last 24 hours as total rainfall has trended downward. The atmosphere will be primed for locally heavy downpours to develop within the showers amid an increasingly moisture-rich environment. The greatest chance for isolated issues to pop up will be across Vermont. The highest threat for this is looking to stay in upstate New York.
HRRR showing potential weather just after midnight tonight:
The system will also be able to produce gusty winds, mainly over Vermont. This will come as a strong (for summer) low-level jet crosses the region. This jet continues to look like it will be strongest over the northern tier of New England. Winds should remain below damaging levels, but it will be a gusty storm at times, especially for Vermont, with 40+mph gusts possible. Any thunderstorm activity will help bring these winds down to the surface.
850mb (about 5,000 feet above sea level) winds this evening. The red indicates winds of 55-65mph. While winds this strong won't make it to the surface, it will be enough to produce some gusts of 35-45mph, with potentially stronger gusts in any thunderstorm:
This strong southwesterly low-level jet is helping to transport the very moisture-rich atmosphere into New England as the warm front crosses the region tonight. This will help maintain the threat for thunderstorms to develop through the overnight hours. It will also usher in very humid air for Sunday.
On Sunday, New England will be within the warm sector of the storm as the warm front finishes pushing through the region. This will set the stage for a very warm and very humid day, similar to last Sunday. A cold front will cross the region later in the day, providing forcing for thunderstorm development. This would come after scattered morning showers become more and more isolated as the morning goes on.
The ingredients for severe thunderstorms will be in place. At this point, shear looks to be the strongest of the ingredients. Shear will likely be on the order of 40-50+ knots. This is more than enough to sustain thunderstorms and allow for strengthening. 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 the most unstable CAPE values will build toward 2,000 j/kg. 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.
Strong winds will be the greatest threat with storms. With a moisture-rich environment, some isolated flooding issues could arise in places as well. Large hail is not looking to be much of a threat, but it’s certainly possible in places, it just shouldn’t be widespread among storms.
A brief, spin-up tornado can’t be ruled out as the environment does favor possible rotation, but this is a very low risk, much lower than last Sunday. Vermont and northern New Hampshire will see the lowest chance for severe weather as the front crosses these areas a bit earlier in the day, limiting the amount of daytime instability.
Next week will likely see a general warming trend as we head to the fourth of July with Monday a bit below average in the post-frontal air mass. Cold air aloft will support cloud development in the afternoon and maybe some showers and a thunderstorm or two as well. Shower chances look to increase again heading toward the end of the week.