Overview High pressure offshore will maintain warm and dry conditions today, with southwest winds drawing in increasingly warmer air. Heat and humidity will peak on Friday ahead of a cold front, with heat indices surpassing 90 degrees south of the mountains. Scattered strong to severe storms are likely on Friday afternoon as the front moves through the state. Cooler, drier air is expected to arrive on Saturday as high pressure returns. Another system approaches on Sunday, bringing the next chance for showers. Heat and and storms for Friday The week ends hot and humid for areas south and east of the mountains, with an approaching cold front from Canada expected to cross the region Friday evening. Heat indices will reach significantly higher levels in the south compared to the north, where values may exceed the mid-90s. However, northern areas will remain warm, with heat indices ranging from the mid-70s to the low 80s. Dew points pushing into the 60s will contribute to muggy and humid conditions, especially in southwest Maine. Portions of southern New Hampshire and southwest Maine are currently under a heat advisory, where prolonged exposure to high temperatures could lead to heat-related illnesses.  Latest model guidance continues to show uncertainty regarding storm coverage and the potential for severe weather on Friday. A key factor remains how far south the cold front can progress into New England before boundary layer destabilization occurs. That said, the front over New England and a pre-frontal trough extending across the Mid-Atlantic are likely to serve as a focus for at least scattered thunderstorm development. The SPC has placed the southern portion of Maine under a Marginal Risk for severe storms. Furthermore, the WPC has issued a Marginal Risk for excessive rainfall throughout much of the state, excluding the southwesternmost part of the region. Localized flash flooding is possible, especially in areas that see repeated rounds of heavier rainfall.  Friday 6 AM to 9 PM - The returning heat and humidity will provide sufficient fuel for scattered showers and thunderstorms along the southward-moving front. Model guidance suggests a faster frontal passage, which may reduce the overall severe weather threat, particularly for higher elevations. There remains a chance for isolated severe storms south of the mountains. A narrow corridor along the coast and adjacent interior may see CAPE (Convective Available Potential Energy) values reach or exceed 1000J/kg. Convection will likely organize into short lines or clusters where brief but intense storms may develop. Given sufficient instability, some storms could produce large hail and gusty winds. While the front’s forward speed should limit the flash flood risk, PWAT (precipitable water) values will be elevated, and any storm could deliver locally heavy rainfall. Dry air rushes back in Friday night into Saturday Friday 8 PM to Saturday Noon - Starting Friday night, dew point temperatures (above) will begin to decrease as drier air moves in behind the advancing cold front and high pressure crests over the region. This will lead to improving conditions overnight, with lowering humidity and a more comfortable air mass settling in across the region. Dew points will begin dropping in the north and move south throughout the day on Saturday, creating a much more pleasant feel compared to previous days. Shortwave passes to the southwest on Sunday High pressure shifts eastward in response to an approaching shortwave aloft. Model guidance varies on the exact placement and strength of this feature, resulting in low confidence in pinpointing which areas will experience the greatest coverage or intensity. That said, ensemble trends currently favor southern and central New Hampshire, as well as southern Maine, as the areas with the higher potential for activity at this stage. Outlook through midweek Sunday 11 PM (03z Monday) to Tuesday 11 PM (03z Wednesday) - High pressure will build in and quickly shift to the south and east of the region by early next week. This pattern will allow heat and humidity to return, with heat index values potentially reaching the lower 90s in some areas ahead of an approaching frontal boundary expected to arrive by midweek. While Monday may feature isolated showers or storms, chances for more widespread precipitation increase on Tuesday and possibly into Wednesday as the front draws closer. A downward trend in heat and humidity may begin on Wednesday, but that will depend on whether the front has fully cleared the area by then. Temperature outlook through Wednesday Today's update prepared by Penn State graduate |
Mike Haggett
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