Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 230009 AFDGYX Area Forecast Discussion National Weather Service Gray ME 709 PM EST Mon Jan 22 2018 .SYNOPSIS... A frontal boundary will lift back north tonight and spread some light snow across the area. This will gradually transition to some drizzle overnight...which could get icy where temperatures remain below freezing. By Tuesday morning steady precipitation will be moving back in from the west. A period of sleet or freezing rain will gradually transition to rain as the cold front approaches the area. Once the front comes through we will turn colder...but precipitation will come to an end. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 7 pm Update: Input latest mesonet data and ingested into ESTF. Light WAA overrunning precipitation now developing and overspreading the area. The center of the cold high pressure system is centered over downeast Maine allowing a deep layer of cold air damming and drainage winds to continue from the north and northeast at low levels, so light snow or mixed snow and sleet expected this evening over central and northern areas of ME/NH. Over extreme southern and southeast areas of NH ptype will be primarily freezing rain as temps above the boundary layer were warmer although shallow cold air remains at the surface. Only minor adjustments were needed to forecast. Previous Discussion: Mid level warm front is moving steadily towards the forecast area at this hour. Precipitation shield is fairly ragged currently...but is expected to fill in as frontogenesis occurs between the leading edge of WAA and cold dome in place. At the moment however...precipitation has mostly come to an end across the area. There may be some patchy drizzle or very light snow...but I feel the most significant wx will move in around the 22 to 23z window. Most temps have fallen below freezing across the forecast area...with parts of Srn NH and the CT River Valley hovering around freezing. I expect this will be about the extent of the coldest air. But that does leave room for accumulating snowfall with this mid level warm front. Generally speaking 1 to 4 inches across the area...less in the S and more in the Wrn ME mtns. Behind the mid level front we lose saturation and precip may change to drizzle or freezing drizzle depending on surface temps. The higher terrain may keep enough saturation to continue light snow. Any ice amounts will be light from the overnight. The main concern this evening will be watching to see if mid level temps are warm enough to start as a mix right away across Srn NH. In addition...the moist and mild air moving into the colder air mass may lead to some areas of dense fog. No advisory has been issued...but Srn NH into coastal Wrn ME would be the most likely areas to see dense fog. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... The cold front and another area of steady precipitation will be approaching from the W Tue morning. I expect significant parts of the forecast area to remain below freezing at this time. This round of precipitation will likely be freezing rain and sleet for much of the area...before gradually transitioning to rain. This will occur most rapidly along the coast and across Srn NH...where strong Sly winds aloft help shove the surface front Nwd. The interior will likely hang on to frozen or freezing precip until around 18z...when ageostrophic flow becomes more SEly. Given the precip rates and more marginal temps...ice accumulation will mainly stay around a tenth to two tenths of an inch. Enough to make some roads icy...but unlikely to cause significant disruption to the power grid. The rainfall amounts could be enough to contribute to some rises on area rivers. This could be enough to cause ice movement and additional ice jam flooding. See the flood watch for more information. As warm front moves into coastal area...especially Midcoast ME...some of the stronger winds aloft may mix down. A small window of near 50 mph wind gusts are possible if the warm front can make it appreciably onshore around RKD. A wind advisory has been issued. Cold front sweeps thru the area late Tue...mixing out the dug in low level cold. Precip will quickly end from W to E...with upslope flow contributing to snow showers in the mtns. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... The deterministic model solutions are in decent agreement on the long wave pattern through day 10. The pattern remains progressive and begins to amplify with time. A steady progression of troughs and ridges across the forecast area will result in several sharp swings in temperature between well below and well above normal readings. In the dailies...low pressure and associated shortwave impulse exits into the maritimes on Wednesday with lingering upslope clouds and snow showers for the higher terrain. A second shortwave impulse and reinforcing shot of cold air arrives for Wednesday Night and Thursday. The cold begins to ease on Friday as ridging and rising heights arrive. A warm front lifts north across the area Friday night with a southerly flow and well above normal temperatures for the weekend. The more progressive GFS was discarded as an outlier with the approaching frontal system in favor of the ECMWF/CMC-Global solutions which brings the front and steadier band of precipitation across the forecast area late Saturday Night and Sunday. Colder air filters into the area by Monday behind this system along with a return to below normal temperatures to start the new work week. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... Short Term...Widespread MVFR and areas of IFR CIGs remain across the forecast area in light NE to onshore flow. We are seeing a break in precip at this time which will gradually fill in from the W starting around 22-23z. CIGs will continue to lower as warmer air aloft moves in. I expect widespread IFR or lower conditions...with maybe HIE remaining MVFR to VFR in downsloping this evening. Precip this evening is expected to be mainly SN...transitioning to DZ or FZDZ behind the mid level warm front between 03 and 06z depending on surface temp. Between 12 and 15z Tue another area of precip will move in from the W. I expect this will begin as a brief period of FZRA or PL before changing to RA at all terminals...with CON and AUG most at risk of hanging on to colder surface temps the longest. This is also about the time strong winds aloft move overhead and lead to LLWS conditions. Winds will be strong close to the surface so shear will begin in the 1000 to 1500 ft range. Cold front sweeps thru the region late Tue...and winds become Wly and conditions improve to MVFR and VFR. Long Term... Wed...Sct MVFR in mtn -shsn. NW sfc wind gusting to 25 kt. && .MARINE... Short Term...NE flow over the waters will gradually become Sly overnight as a warm front lift Nwd. Some patchy dense fog is possible as that warmer air moves Nwd. Winds increase Tue...and gales are likely over the outer waters and Penobscot Bay. Gale warnings remain in effect. There is a chance gale force gusts may occur over Casco Bay...but confidence was not high enough to include in the gale warning. Winds become Wly behind the cold front later Tue...and SCA conditions will continue overnight. Long Term... Wed - Sat...Small Craft conditions are likely...with gusts approaching gale outside the bays. && .HYDROLOGY... A Flood Watch is in effect for the southern half of New Hampshire as well as western Maine south of the mountains. These areas are expected to get enough rainfall during the day on Tuesday to cause ice movement and possible ice jams on area rivers. This could lead to minor flooding especially near areas already affected by ice jams...including the Ashuelot...Piscataquog...Saco...and Kennebec. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Flood Watch from Tuesday morning through late Tuesday night for MEZ012-018>028. Winter Weather Advisory until 6 PM EST Tuesday for MEZ007>009- 012>014-018>022. Winter Weather Advisory until noon EST Tuesday for MEZ023>028. Wind Advisory from 11 AM to 9 PM EST Tuesday for MEZ027-028. NH...Flood Watch from Tuesday morning through late Tuesday night for NHZ003>015. Winter Weather Advisory until 6 PM EST Tuesday for NHZ001-002- 004-006. Winter Weather Advisory until noon EST Tuesday for NHZ003-005- 007>015. MARINE...Small Craft Advisory from noon to 11 PM EST Tuesday for ANZ153. Gale Warning from noon to 11 PM EST Tuesday for ANZ150>152-154. && $$

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