Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 222303 AFDGYX Area Forecast Discussion National Weather Service Gray ME 703 PM EDT Sun Oct 22 2017 .SYNOPSIS... High pressure will hold over the maritimes tonight through Monday. A slow moving cold front will approach from the west Monday night and Tuesday and will slowly cross the region Wednesday through Thursday. A shallow ridge of high pressure will build over the region on Friday and will shift offshore on Saturday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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7 pm Update: Weak sfc meso-low sitting just off the coast allowing a light moist northeast flow producing an area of low stratus along and off the coast. This area of low clouds should linger the rest of the night. Otherwise updated mesonet data and only minor near term tweaks needed for the rest of the evening. Previous Discussion: High pressure sliding into the Maritimes has set up a bit of a backdoor cold front across the area...with PWM at 57 this hour...and EEN 82. Surface ridging still remains in control of the region...so after dark we should see big drops in temps. Dew points again are higher than previous days...so only an isolated reading of upper 30s is expected in the coldest of valleys. Otherwise mainly 40s tonight. The increased moisture should allow for a little more ground fog...though dense fog is a little more uncertain. Model guidance is bullish on widespread marine fog tonight. The tendency is for models to rush this in on return flow...which is just beginning to develop. However there is a band of MVFR clouds just off the Seacoast and SWrn ME coast. The warm temps inland is helping to erode the leading edge of this...so I have at least some concern that at nightfall this may surge inland. I have trended this way in the sky forecast...but I have not gone heavy on the fog given the moisture is fairly shallow in nature. That will be the forecast challenge for tonight. Otherwise valley fog is expected farther inland from coastal influences.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Another very warm late Oct day on tap for Mon. With mixing to H9...we should see widespread upper 60s to lower 70s. With deeper mixing in Sly flow this should bring some uniformity to high temps. Mon night deeper moisture arrives. With it I expect marine fog and stratus to develop and move inland. With WAA ongoing above the marine layer...some drizzle or an isolated shower is possible. Temps should also remain more or less steady after sunset. Overall I think Mon night the forecast area will remain in the WAA in the warm sector...but steady precip will hold off until later in the day. For that reason I backed off the likely PoP that model guidance was spitting out. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Slow moving front will continue to inch toward the region on Tuesday. Strong onshore flow will mainly produce areas of drizzle and scattered showers during the day as main focus of heavier rain remains across eastern New York and western New England. High temps will generally range through the 60s. Heavy rain will gradually shift into western New Hampshire Tuesday night. Strong low low level jet will combine with tropical moisture to produce 1 to 2 inches of rain across the western half of the state by Wednesday morning with lighter amounts in eastern New Hampshire and western Maine. Lows overnight will range from the mid 50s to lower 60s. Heavy rain will gradually shift east on Wednesday affecting most of the forecast area through the day. Looking for additional rainfall amounts of 1 to 2 inches in eastern New Hampshire and western Maine by the end of the day. With area rivers at record low levels for this time of year not anticipating any flooding problems but poorly drained areas and urban street flooding could become an issue by late in the day. Rain will continue Wednesday night as upper trough to the west of the forecast area goes neg tilt slowing surface trough over central Maine and wrapping considerable moisture back to the west. This will result in another 1 to 2 inches across much of the forecast area overnight and may result in problems on small rivers and streams through out the forecast area. Lows overnight will range from the mid 40s to lower 50s. Focus of heavy rain will gradually shift into eastern Maine on Thursday. Expect occasional showers through the day with some minor additional accumulations possible...mainly in eastern zones. Highs on Thursday will range from the mid 50s to lower 60s. Upper trough will swing northeast through the region keeping showers in the forecast through the 1st half of Thursday night. Downsloping winds behind departing low and a shallow ridge of high pressure building in from the west will bring clearing to the region after midnight. Lows overnight will dip into the mid 30s to lower 40s. Quiet weather returns to the forecast area on Friday as high pressure builds south of New England. Highs on Friday will range from the mid 50s north to the lower 60s south. Fair weather will extend into Saturday as another deep trough and slow moving cold front approaches from the west. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... Short Term...Tricky forecast this evening and overnight along the coast. There is a band of MVFR CIGs in marine onshore flow running from near PSM to PWM then off shore. Model forecasts have this becoming widespread LIFR tonight. However model guidance does tend to rush this...and guidance already suggests this should be widespread LIFR CIGs at the moment. Keeping in mind that moisture is still relatively shallow...30s dew points around 1000 feet...and this is only the first night of return flow I have not gone IFR or lower at coastal terminals. I did however include some MVFR CIGs...as some convergence along the coast could allow this band of clouds to persist. CT River Valley should also see fog tonight...with LEB seeing LIFR conditions after 08z most likely. With E flow...HIE may remain mixed enough to avoid fog again. Mon will return to VFR...followed by increasing moisture and clouds Mon night. More widespread IFR marine fog/stratus is likely...along with developing -RA or -DZ towards morning. Long Term... Widespread IFR/LIFR ceilings and MVFR vsby Tuesday through Wednesday. MVFR/IFR ceilings Thursday...improving to VFR Thursday night. VFR Friday. && .MARINE... Short Term...NE flow along the coast will stay below 25 kts this evening before weakening and turning more onshore. Then winds and seas will remain below SCA thresholds thru Mon. Mon night increasing Sly flow will start to build seas towards 5 ft late...leading into stronger winds Tue. Will also have to watch for some marine fog/stratus tonight...but especially Mon night. Shallow moisture should limit coverage and density...but deeper moisture Mon night should allow for more widespread low visibility. Long Term... Gales likely Tuesday through Tuesday night. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Marine

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