Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 200351 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1151 PM EDT Fri May 19 2017 .SYNOPSIS... Much cooler air will continue to move into the area tonight behind a cold front. Cool high pressure builds across New England on Saturday. Some areas of frost are possible at night this weekend. High pressure shifts east on Sunday. Low pressure will move in from the west on Monday bringing widespread rain showers. Unsettled weather will hang around through the rest of the work week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 10pm... winds are still gusting along the coast but are beginning to subside in the interior. Temperatures will continue to fall overnight under cold advection. Expanded early morning frost slightly as winds calm in the mountain valleys just before sunrise. 6pm update... Clearing skies through the north country now behind the front. Expect to see winds decrease in the next few hours as the sunsets and the front moves through. No major changes to the forecast at this time. prev.. Cold advection continues behind the front this evening. It is quite remarkable to see temperatures fall during the middle of the day at this time of year, but it has been occurring not just in the mountains but even in southern areas. Clouds have accompanied the front and will remain lodged against the mountains for the next few hours before dissipating in the dry northerly flow. Pressure gradient behind the front will prevent winds from going completely calm tonight, so it will not be ideal radiational cooling conditions. Radiational cooling will contribute to the cooling tonight, but the low levels will remain mixed and prevent even cooler temperatures from developing. However, cold advection alone appears to be enough to bring northern areas down to around the freezing point. The NWS defined growing season does not start until May 21 up there, so we will not be issuing frost or freeze headlines tonight. Tomorrow will be another story, though! && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... The peak of the cold air aloft will be overhead on Saturday. 850MB temperatures of about 0C translates to temperatures warming into the upper 50s to low 60s. However, given the strong sun this time of year (we are only a month from the solstice), we should be able to produce enough heating to mix even higher than this, modifying the air mass to yield highs in the mid to upper 60s in southern areas. Deep mixing should allow very dry air to mix down, with dewpoints falling into the 20s and low 30s. Surface high pressure settles into the area Saturday night, becoming centered over eastern New Hampshire by morning. This will set the stage for ideal radiational cooling conditions, about as ideal as it gets this time of year. Dry air in place in the low levels should prevent fog formation and promote frost instead. As a result, we will likely need frost and/or freeze headlines Saturday night for all of the north and a good portion of the interior. Even interior York and Cumberland Counties could get a frost as well as most of interior New Hampshire. Will hold off on issuing this product until after we see how cold it gets tonight and get a better feel for what this air mass is capable of. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Ridge axis will be overhead to start the day Sun...but changes are on the way for the rest of the extended. Ridging slides Ewd...and will be replaced by a general tendency for Ern CONUS trofing. This will mean temps fairly close to normal...and unsettled wx. With lower heights aloft any sunshine will tend to produce cloud cover and maybe afternoon pop up showers...even on the so called dry days. There are two windows for higher chances at rainfall thru the end of next week. One will be late Sun into Mon...where a cold front will approach from the W. The s/wv trof responsible for the bulk of the lift is forecast to shear out into Canada...allowing the front to tend to wash out against the departing ridge. I expected some showers...but not a wash out. The next window is a little more broad...from late Wed to Fri. The 19.12z GFS and ECMWF differ on the timing of trailing s/wv trofs and resulting precip. CMC-global solution leans more towards the GFS...delaying until Thu-Fri with the stronger wave and heavier rainfall. That will be important for any onshore or NE flow however...as a 12 foot tide is coming up for late Fri. && .AVIATION /04Z SATURDAY THROUGH WEDNESDAY/... Short Term...MVFR ceilings lingering north of the mountains will dissipate this evening leading to VFR conditions area wide. Winds diminish a bit tonight but do not go completely calm. Expect a lighter northerly flow on Saturday, but still perhaps gusting to 15 KT. Long Term...Generally VFR conditions expected to prevail thru the period. With high pressure overhead early Sun...VFR will certainly prevail thru much of the day. Late in the day and overnight a cold front approaches from the W. The best forcing for ascent will be N...and so scattered to 50/50 SHRA are expected. The heavier SHRA may bring MVFR conditions. Front will be off shore by Tue...though SHRA may pop up during the day behind the front. && .MARINE...
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Short Term...Have dropped the SCA as winds have diminished. High pressure builds in behind the front through the weekend and leads to lighter winds. Long Term...Initially high pressure in place Sun. Thru the day flow turns onshore as a cold front slowly approaches from the W. By Tue waves may build to near 5 ft on the outer waters due to persistent flow direction. Another...stronger system moves thru Thu into Fri and may bring another period of SCA conditions to the waters.
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&& .FIRE WEATHER... Canadian air mass brings in very dry air for Saturday. As the May sun does its work on the dry air mass, relative humidity values will fall to below 30 percent. However, wind speeds are expected to remain fairly light and should prevent dangerous fire growth conditions. && .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...None. NH...None. MARINE...None.
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&& $$ NEAR TERM...Marine SHORT TERM...Marine LONG TERM...Ekster

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