Weather for New England and the Northeast

Area Forecast Discussion
National Weather Service Gray ME
4:05 PM EDT Mon May 18 2026

Synopsis
A powerful ridge of high pressure will allow unseasonably hot air to surge into northern New England from the southwest, setting up a brief period of near record warmth. An approaching cold front will trigger scattered strong to severe thunderstorms Tuesday afternoon and evening before finally sweeping through the region on Wednesday. Cooler, drier high pressure will build in behind the front for Thursday and Friday.

What Has Changed

A Heat Advisory has been issued for portions of far southern New Hampshire Tuesday. Temperatures will climb into the lower 90s into the early afternoon, with heat index values up to 95. A Marginal Risk for severe thunderstorms has been expanded to include all of New Hampshire and much of southwestern Maine Tuesday.

Key Messages

Hot, well above normal temperatures move into the region for Tuesday and Wednesday. Widespread highs in the upper 80s to lower 90s are expected Tuesday, with warm overnight lows in the upper 60s to around 70. Another round of hot temperatures is possible Wednesday, mainly focused from the interior to the coast.

An approaching cold front will bring chances for thunderstorms Tuesday and chances for showers and storms Wednesday when the front crosses. Any thunderstorm that develops Tuesday will have the potential to become severe.

High pressure builds in Thursday and Friday for mostly dry and cooler weather. Uncertainty exists going into the weekend as moisture approaches from the southwest while high pressure may keep things dry through Saturday.

Discussion

Hot, Well Above Normal Temperatures (Key Message 1)

The upper air analysis at the 850 millibar level, which is a standard meteorology measurement used to analyze weather conditions about 5,000 feet up in the atmosphere, shows a broad zone of warm air expanding across the eastern United States. The upper level jet stream remains far north of the Great Lakes, allowing low level winds from the southwest to pull warmer air into our area tonight.

A brief period of high pressure moves overhead tonight, bringing mostly clear skies and calm winds. As this warm air flows in just above the ground, it will create a very steep low level inversion, which is a technical term for a layer of the atmosphere where the temperature increases with height rather than cooling down. This inversion will trap cooler air at the surface, especially along the Maine coast where winds blowing off the ocean may push areas of fog inland late tonight.

By Tuesday morning, clear skies will allow full sunshine to quickly heat the ground. This incoming air mass is remarkably hot, sitting in the 99.5 percentile compared to climate history for southern New Hampshire and coastal or interior Maine at the 5,000 foot level. Daytime mixing, which happens when warm air rises from the heated ground and mixes with the air above it, will easily pull this intense heat down to the surface. Expect daytime temperatures to soar into the upper 80s, with lower 90s across the southern half of New Hampshire.

Fortunately, this will not feel as sticky or humid as a typical mid summer heatwave because moisture near the ground is thin. However, because this is the first real heat of the season, a Heat Advisory has been issued for Hillsborough and interior Rockingham counties in New Hampshire, where heat index values, a measure of how hot it actually feels when combining air temperature and humidity, will approach 95 degrees in the afternoon.

A southwest breeze will keep temperatures elevated through Tuesday night. Lows will only drop into the upper 60s to around 70 degrees for much of the interior and the coast, offering little relief overnight. This sets the stage for another warm day on Wednesday, though forecast highs look a bit lower than Tuesday. The hottest temperatures will be focused over coastal and interior Maine as well as southeastern New Hampshire ahead of an approaching cold front.

The mountains and foothills should see more normal seasonal temperatures, though that depends heavily on convective activity, which is the scientific phrase for the lifting of warm air that creates showers and thunderstorms. As the front pushes out to sea Wednesday evening, a much more refreshing night is on the way, with lows dropping into the upper 30s to mid 40s.

Thunderstorm Potential and Severe Weather Risk (Key Message 2)

A low pressure system and its connected warm front will move northeast through Quebec on Tuesday, leaving a trailing cold front stretched out to the southwest across the Great Lakes. This keeps northern New England firmly in the warm zone Tuesday and Tuesday night, with the cold front scheduled to pass through on Wednesday.

With surface temperatures reaching the upper 80s and low 90s and dew points around 60, the atmosphere will generate surface based CAPE, which stands for Convective Available Potential Energy and measures the amount of fuel available for thunderstorms, up to 1,500 Joules per kilogram Tuesday afternoon. Deep layer wind shear, which describes the change in wind speed and direction at different heights in the atmosphere, will be around 25 to 35 knots. This is strong enough to organize thunderstorms into organized lines or clusters.

The biggest limiting factor for widespread severe weather is that the cold front, our main source of atmospheric lift to kick off the storms, will stay far to our west through Tuesday night. High resolution computer models suggest that widely scattered storms are still possible even without that strong frontal lift, and a few could develop rotating updrafts. The best chance for these storms will be during the afternoon across the interior, though a few computer simulations show storms tracking all the way to the coast.

The primary danger will be damaging winds. Computer forecast models display inverted V profiles on their atmospheric soundings, which is a graphic representation of a vertical cross section of the atmosphere. An inverted V shape means the air near the ground is very dry with dew point depressions, the difference between the actual temperature and the dew point temperature, greater than 20 degrees. This dry lower air causes rain to evaporate before hitting the ground, cooling the air rapidly and creating intense, rushing downbursts of wind. While mid level lapse rates, the rate at which temperature decreases with height, are not overly impressive, hail cannot be ruled out in any storm with a strong rotating updraft. Storm activity should decrease Tuesday night, but a few overnight showers or storms remain possible.

The cold front finally arrives in western New England Wednesday morning and exits off the coast by Wednesday afternoon. Computer models show a narrower band of energy ahead of the front, with available storm fuel around 1,000 Joules per kilogram and wind shear around 35 knots. Because the front is moving through early in the day before the sun can fully heat the ground, thunderstorm potential will be limited mostly to areas south of the mountains. Dry air moving in aloft will also make it hard for storms to stay strong, and the Storm Prediction Center has kept the severe weather risk south of our area for Wednesday.

High Pressure and Cooler Weekend Outlook (Key Message 3)

High pressure will build into the region from the west northwest Wednesday night into Thursday, delivering a period of gusty northwest winds. Skies will be mostly sunny on Thursday, with cooler high temperatures ranging from the 50s in the north to the mid 60s along the coast.

This high pressure system will center itself directly over southern New England by Friday, causing winds to calm down across southern areas. Meanwhile, a short wave, which is a compact upper level atmospheric disturbance that can cause shifting winds or cloud cover, will move through eastern Canada and keep conditions breezy across the mountains and central Maine. Friday highs will range from the upper 50s north to the upper 60s south of the mountains under fair skies.

Looking toward the weekend, energy spinning out of a larger weather trough in the central United States will begin pushing moisture toward New England.
Global weather models suggest that the strong high pressure over the Northeast should help block or suppress this moisture through Saturday and into Sunday.
The National Blend of Models generally keeps the precipitation probability, the mathematical chance of measurable rain at any given point, below 50 percent through the weekend, which is a reasonable estimate this far out.