Area Forecast Discussion
Issued by NWS Caribou, ME

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000 FXUS61 KCAR 210312 AFDCAR Area Forecast Discussion National Weather Service Caribou ME 1012 PM EST Tue Feb 20 2018 .SYNOPSIS...
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A strong ridge of high pressure will remain anchored off the Southeast coast through Friday. A cold front will drop south into northern Maine early tonight then lift back north as a warm front late tonight. A cold front will cross the state Wednesday afternoon. .NEAR TERM /THROUGH WEDNESDAY/... 930 PM Update: The 00z upper air sounding indicated a sandwich of sub-freezing air between a narrow column of sfc-1000ft and 4000 to 7000ft abv fzg layers. Since then, sfc-bl temps have cooled to at or just below fzg spcly ovr far Nrn and far Nern Aroostook county with adjacent areas immediately to the south and west close to fzg. The Region has been in a break of precip, but latest hrly meso model sim radar indicates another batch of lgt precip movg into Nrn ptns of the Rgn within the next couple of hrs...lasting to the pre-dawn hrs...associated with the main surge of warm air alf with the building massive mid and upper ridge ovr the Ern U.S. Conditions for ice accretion are marginal in low trrn vly areas...but better ovr high trrn which is more in the elevated sub-fzg layer alf. Eventually, strong warm advcn both at mid and llvls heralded by a mdtly strong llvl jet will deplete this elevated sub-fzg layer by daybreak Wed. Otherwise, we made upwards adjustments to hi trrn wind going into Wed when the llvl SSW jet makes its presence known. Lastly, we made adjustments to fcst hrly temps, dwpts, and ovrngt fcst lows based on trends seen from erly to mid eve sfc obs. Orgnl Disc: The freezing rain advisory has been lifted since temperatures are remaining above freezing. However, a few far northern spots in the St. John valley may get some patchy freezing rain later this evening. The advisory will be reissued if the colder air presses in later tonight. A strong, warm ridge of high pressure will remain to our south tonight as high pressure over eastern Canada pushes some cooler air into the far north. A frontal boundary will be across the area between the two air masses. Moisture running along this front will bring occasional light rain to the region tonight. Some freezing rain is likely over the far north as subfreezing air from Canada seeps into the low levels. The frontal boundary will lift north as a warm front late tonight. Warm advection over the cold ground surface will likely produce fog across much of the region late tonight and some locally dense fog is possible. A winter weather advisory remains up across northern Aroostook County where a bit of freezing rain is likely. Wednesday will be very warm as the strong ridge to our south combined with low pressure tracking north of the area pulls some very warm air up across the area. Highs will approach 60 Downeast and 50 over the north. Northern areas will likely remain mostly cloudy while Downeast areas have partial sunshine.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... On Wednesday night, cold air will filter into the area with a fairly rapid freeze in northern zones that could produce some slick roadways. Further south, temperatures will remain above freezing until later in the night. The relatively short duration of the record warmth and lack of significant rainfall mitigates hydro concerns at this time and no Flood Watch will be issued now, but rivers will continue to rise into Friday. The biggest concern will be ice break ups and subsequent jams, but projected river rises are less worrisome than yesterday. On Thursday, high pressure builds with temperatures still above normal, but substantially cooler than Wednesday. Most guidance is trending colder and have lowered Thursday`s highs. The big H500 ridge will remain off the Carolina coast and a weak disturbance will shoot northeastward on Thursday north of this high...and south of the big surface high building over the state. This means a good deal of high clouds in the southern half of the forecast area on Thursday and maybe a brief sprinkle for the outer islands of Hancock County. On Thursday night, the 1040mb high pressure crests over the area. Lows were adjusted downward due to good potential for strong radiational cooling. On Friday, another weak disturbance in the southern stream will rapidly approach from the southwest. Clouds increase later in the day with a weak overrunning event that will produce mostly snow Friday night. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... For Friday night, sub-advisory snowfall of no more than 2-3 inches is expected with precipitation ending as rain in southern zones. Temperatures will be warmer behind this system on Saturday with highs in the upper 30s to lower 40s. Somewhat cooler air arrives Saturday afternoon behind a cold front. High pressure building Saturday night presents another good opportunity for radiational cooling...especially in northern zones. Clouds will increase quickly on Sunday ahead of yet another shortwave moving in the southern stream out of the lower Mississippi Valley. This system will have better upper level support and is expected to phase with a northern stream shortwave. A secondary low will develop along the coast and cut off warm air advection. As a result, this system should be all snow except along the coast. Most areas should receive around 4 to 6 inches as it stands now. The exception would be the coastal zones where P-type issues will cause a significant decrease in accumulations. High pressure gradually builds Monday into Wednesday with seasonal temperatures. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/... NEAR TERM: IFR conditions in low clouds are expected tonight and some freezing rain is likely in some far northern spots. Conditions should improve to VFR Downeast around mid morning and improve to MVFR across the north around midday Wednesday. Some wind shear is likely Wednesday, especially over the north, for SW winds at 2K feet around 35kt. SHORT TERM: Expect VFR conditions Wednesday night until later Friday afternoon. IFR conditions due to snow and cigs will occur Friday night into early Saturday morning. Cigs will lift to VFR by Saturday afternoon and all sites will remain VFR until later Sunday afternoon when IFR vis in snow returns. && .MARINE... NEAR TERM: A SCA will be up for Wednesday for southwest winds gusting up to 25 kt. Warmer air moving over the colder waters will likely produce some areas of dense fog over the waters tonight through Wednesday. SHORT TERM: An SCA will be needed Wednesday night into early Thursday morning for the outer waters. The outer waters will likely need another SCA Friday night. A brief gale is anticipated later Sunday. && .HYDROLOGY... Will be watching the rivers closely on Wednesday night into Thursday for some possible ice movement. Decided not do issue any flood watch headlines, due to a few reasons. First, QPF still looks light for this event with generally .50" or less expected. Second, is the longevity of the thaw expected. Even though temps are above freezing for parts of the Penobscot river watershed today, they won`t be that warm with most areas staying in the mid 30s. The real warmth will come on Wednesday and we are expecting rapid snowmelt with dewpoints surging to 50 degrees and temps will also reach into the upper 50s for the lower Penobscot watershed and low 50s for the upper part of the watershed. Fortunately, the cold front will quickly push through Wednesday evening and night dropping temps quickly back below freezing. The longevity of the warm spell will be too quick for areas that still hold a significant snowpack. We do expect river rises due to run-off, but current forecasts are only showing a few feet of a rise, not enough to really break up the ice. There will be weakening of the ice pack though, especially on the edges, so some ice will be moving around, but no significant ice jams are expected. Overall threat is there, but current thinking is the threat is low. Users with interests on area rivers across the lower Penobscot watershed should keep a river watch Wednesday through Wednesday night. && .CAR WATCHES/WARNINGS/ADVISORIES...
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ME...Winter Weather Advisory until 5 AM EST Wednesday for MEZ001- 002. Dense Fog Advisory until 5 AM EST Wednesday for MEZ010-011- 015>017-029>032. MARINE...Small Craft Advisory from 7 AM Wednesday to 7 AM EST Thursday for ANZ050>052.
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