Area Forecast Discussion
Issued by NWS Portland, ME

Home |  Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
678 FXUS61 KGYX 131818 AFDGYX Area Forecast Discussion National Weather Service Gray ME 218 PM EDT Sun Jul 13 2025 .SYNOPSIS...
-- Changed Discussion --
There is a chance of a few showers or a thunderstorm during peak heating this afternoon and early evening, mainly in the mountains. A cold front will near the region on Monday with widespread showers and thunderstorms, some of which could produce locally heavy rainfall. Drier weather returns Tuesday, with heat and humidity building in through late week. Chances for afternoon showers and thunderstorms return late week.
-- End Changed Discussion --
&& .NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Skies have thinned through the morning with a mostly sunny day away from the immediate coast and portions of the White Mountains. Low stratus and fog is clinging to the land/water interface, indicating the likely resurgence of low clouds as the afternoon wanes. Expect one more night of foggy and low cloud conditions tonight before cold front brings drier air behind it Monday. Some instability has built over western areas today, but kept shower chances on the lower end with the outside chance of a rumble of thunder overnight.
-- End Changed Discussion --
&& .SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...
-- Changed Discussion --
Monday, the main concern will be heavy showers and thunderstorms, again bringing enhanced rainfall rates and the threat of localized flash flooding. Moist airmass has been no stranger of late with a string of 00z soundings at or above the 75th PWAT percentile (around 1.36"), and a few cases above the 90th (around 1.6"). A break is around the corner, but lies on the other side of a slow moving cold front approaching Monday. Northern New England will be positioned along the trailing end of the cold front, below the apex pushing through central Quebec. This tail segment has been consistently modeled slow, and its velocity will allow a final pool of PWAT (upwards of 2") to build ahead of the primary forcing. Combined with warm cloud layers of 12-13 kft, and high freezing levels, precip production/efficiency will be well represented. Concern for flash flooding will be where these moisture dynamics overlap supportive instability. Believe WPC`s Slight and Marginal ERO outlines this corridor well. This is draped across the western half of the CWA, including much of NH and southwestern ME. With another night of stratus and fog tonight, the periphery from the foothills west should see a more rapid increase in daytime instability w/ less low cloud in the AM. Another factor will be ample MUCAPE expanding eastward as the day goes on. This should provide fuel for storms ahead of later forcing to carry into otherwise stable surface layers through late morning and the afternoon. Resultant rainfall rates in stronger cells will likely fall between 1-2" per hour. Should areas of training or backbuilding take place in the absence of mismatched cloud layer vs. LLJ velocities, would expect the outlined precip efficiency to quickly manifest localized flash flooding. While there is plenty going for this setup, there remain a few barriers that limit coverage and greater confidence. First, is the SBCAPE footprint and how quickly morning clouds thin. HREF ceiling probabilities keep the Midcoast and Kennebec Valley overcast through mid afternoon, and thinning will just be occuring around noontime for the southern half of NH through the ME foothills. There will be heavy reliance on lift from the incoming front to initiate showers/storms NW of the mountains where instability builds first. Second, will be the progressive lift provided by the front. While slow, this should be enough to keep deeper showers/storms moving vs. anchoring to terrain like we saw the past few days. Current coverage is not enough to warrant a Flood Watch for Flash Flooding at this time, but scattered cases of flash flooding will be possible. Shower and thunderstorm coverage wanes in the evening as daytime heating is lost. This will keep Monday night mostly dry with thinning skies west to east.
-- End Changed Discussion --
&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
Pattern Overview: Tuesday and Wednesday look mostly dry as the 500mb pattern is zonal over the region with surface high pressure in control. Thursday and Friday are more unsettled as a pair of shortwaves look to cross the region. We may return to dry weather for the weekend as global models suggest a flip back to zonal flow aloft with surface high pressure nosing back into the region. Impacts and Key Messages: * Hot and humid temperatures will make a return Tuesday, becoming cooler towards the weekend. Details: Tuesday: High pressure noses into the region Tuesday with subtle 500 mb height rises allowing 850 mb temperatures to climb to around 17C. With mostly sunny skies and good mixing this will amount to surface temperatures in the upper 80s and low 90s south of the mountains, and low to mid 80s to the north. We will be fresh on the heels of a frontal passage so dewpoints won`t be overly oppressive, but even with them in the mid to upper 60s temperatures will feel a couple degrees warmer. Clear skies prevail Tuesday night but the dewpoints will keep low temperatures in the mid to upper 60s south of the mountains and in the low 60s to the north. Wednesday and Thursday: Wednesday looks like the hottest day in regards to actual temperature as shortwave ridging peaks, allowing temperatures to climb a few degrees higher. Much of the area south of the mountains likely ends up in the low 90s, with upper 80s to the north. Dewpoints remain similar to Tuesday, but with the warmer surface temperatures some locations may reach heat index values around the mid 90s. Flow turns southwesterly Wednesday night which will begin to drive up the dewpoints and make for a warmer night, with many locations south of the mountains only bottoming out around 70, and upper 60s to the north. An approaching shortwave trough may bring showers to New Hampshire and the western Maine mountains overnight. This will also increase cloud cover keeping surface temperatures a few degrees cooler Thursday, but the southwesterly flow acts to increase dewpoints further. Much of the area likely sees dewpoints in the low 70s which will make it feel very similar to Wednesday with heat indices approaching the mid 90s south of the mountains. The region will be sandwiched between frontal boundaries with a shortwave trough moving through, so heating of this moist airmass should lead to afternoon showers and thunderstorms. CSU machine learning is already showing low probablilites that some stronger storms are possible, so this will be a good period to keep an eye on. Elevated dewpoints once again keep low temperatures around 70 south of the mountains and in the mid to upper 60s to the north. Friday-Sunday: Global models are in pretty good agreement that Friday features a frontal passage which would bring relief from the humidity and kick start a cooling trend. As usual in these situations the front will take some time to clear resulting in southern zones still holding on to some heat and humidity. A hot and humid air mass plus lifting from a front usually results in more widespread thunderstorm activity, again CSU machine learning suggests some stronger ones, but timing will be key and it`s too far out in time to speculate on coverage. Will stick with the afternoon hours being the best chance for thunderstorms to develop. Behind this front we should see much more comfortable dewpoints and temperatures for the weekend with high pressure taking control and keeping things mostly dry other than chances for pop-up showers and thunderstorms in the afternoons.
-- End Changed Discussion --
&& .AVIATION /17Z SUNDAY THROUGH THURSDAY/...
-- Changed Discussion --
Short Term...Ceilings will again thicken and lower tonight with IFR to LIFR conditions when compounded with rebuilding fog. Monday morning will see clouds thin west to east, but Midcoast terminals like RKD and AUG may well stay IFR/LIFR through Monday morning and early afternoon. SHRA and TS develop in the afternoon for NH and western ME terminals. TS may contain erratic winds and heavy downpours, causing local MVFR/IFR. These dissipate after 00z, with conditions slowly improving towards VFR overnight. Long Term...VFR likely prevails on Tuesday and during the day Wednesday. Increasing moisture Wednesday will likely lead to fog development overnight and again overnight Thursday. Showers and thunderstorms are also possible Thursday afternoon, but outside of these ceilings should remain mostly VFR.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
Short Term...Conditions remain below SCA. Fog and marine stratus will continue to hamper visibility for a good portion of the coastal waters and harbors. A cold front will approach the waters Monday afternoon, passing overnight into Tuesday. Long Term...Favorable boating conditions continue as the waters remain on the periphery of the Bermuda High. Some showers and thunderstorms may move over the waters late week with a frontal passage. Increasing southwesterly flow midweek may also lead to areas of fog.
-- End Changed Discussion --
&& .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR/SHORT TERM...Cornwell LONG TERM...Baron