Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 121156 AFDGYX Area Forecast Discussion National Weather Service Gray ME 656 AM EST Tue Dec 12 2017 .SYNOPSIS... Snow will overspread the region this morning as low pressure wraps up over western Pennsylvania. This low will quickly strengthen today and move towards Downeast Maine through the day. Some warmer air will try and push inland from the coast and change the snow to rain for a time. Precipitation will quickly come to an end from southwest to northeast this evening. Snow showers will continue in the mountains through Wednesday...with snow showers moving south of the mountains ahead of an Arctic cold front around midday. Bitter cold and gusty winds will move in behind the front. Expect lows to fall into the teens and single digits Wednesday night...with wind chills approaching zero to ten below. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7am update... Snow has started across most of southern Maine and New hampshire. Visibilities are decreasing as snow increases to the southwest. The Cold air damming has continued to strengthen with even all buoys continuing to report NW flow, this may keep the precipitation as snow even longer along the coast. Prev Disc... ***Widespread snowfall expected **** Low pressure will move through the region today bringing heavy snow, mixing with and changing to rain along the coast. Currently: At 3am a low pressure center was located over Georgian bay, with a secondary low forming over Pittsburg PA. Pressure falls have been increasing to the southeast over the past few hours with the PA low expected to take over as the main center by daybreak. Ahead of this low a broad region of light precipitation extended into New Hampshire. Across Northern New England a strong region of cold air damming is in place, with a 1020mb high extending from New Brunswick south across coastal Maine to the New Hampshire/Massachusetts line. Within this region temperatures have continued to cool overnight despite advancing cloud cover from the approaching storm. Morning: Snow will continue to move into the region this morning, overspreading the entire CWA by mid-morning. The low levels remain quite dry with dewpoints in the single digits north and near 20 south. Thus the initial precipitation may take a while to reach the surface. This has been reflected in upstream observations over New York state. Snowfall will increase this morning as the low intensifies and the low levels saturate. With the cold air damming expect ptype to remain as snow through the morning for all but extreme southern New Hampshire where a bit of mixing is possible just before noon. Expect a quick 2-3" of snow as the initial round of precipitation moves in. Afternoon: By afternoon we see two things occur. First the heavier qpf makes its way into maine and encounters the strong coastal front. Expect this to result in a maxima in precipitation rates just inland of the front. Snowfall rates of 1"/hr are possible in this area, extending across the Foothills. Meanwhile the low center begins to track across southern New Hampshire and up Penobscot bay. As it does it will erode some of the same surface cold air that`s helping to enhance snow totals. This will result in a very tight gradient in snowfall between the haves and the have nots. The area with the most uncertainty remains Kennebec and Waldo counties. Snow: 6-10 inches of snow across a broad brush of the mountains and northern portion of the area. The sticking points remain the coastal zones where rain can mix in. With the warm air coming in all at once as the low passes overhead, expect a minimal amount of sleet or freezing rain to occur during the transition. Headlines: A few changes to the existing headlines this morning. The Winter Weather Advisories have been expanded to Eastern Hillsborough and Interior Rockingham counties. This reflects the current colder temperatures there which should allow them to pick up near advisory level snowfall. There may also be a brief burst of sleet or freezing rain as the precipitation type changes midday which will result in slick roads and warrants an advisory expansion. Winter Weather Advisories have been converted to Winter Storm warnings for Sullivan, Merrimack, and Belknap counties in NH. This reflects slightly higher snow totals overall for these areas, especially in the higher terrain.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... As the low moves off into Eastern Maine this evening cold air will come rushing back in behind it as the precipitation cuts off. For the northern Mountains, this will mean another few inches of snow on the backside as the precipitation wraps around behind the cold front. At the coast precipitation will come to an end with a chance for some light drizzle or freezing drizzle to take over at the end. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Upper low pressure is expected to cross the forecast area Wed. This will bring several sensible wx impacts to the region. The two higher confidence impacts will be much colder air and gusty winds. As the cold front crosses the area behind the trof axis...temps will likely hold steady or fall during the day Wed. In addition...with the strong CAA winds will become gusty...and could approach 30 to 35 kts into Wed evening. Gradient flow is expected to continue overnight...so I used raw model 2 m temps for lows in the well mixed environment. As readings fall into the teens and single digits...wind chill values will also drop to near zero...and as cold as ten below in the Nrn zones. The more uncertain part of the Wed forecast will be coverage and intensity of snow showers. The upper low itself should provide some support for snow showers as it swings overhead...so I have at least slight chance to chance PoP for part of the day Wed all the way to the coast. Behind the trof axis plenty of low level moisture will remain...but flow will become Wly and downsloping commences off the higher terrain. This may dry out some of the areas downstream of the mtns. Upwind of the mtns however...very cold temps will have the prime snow growth zone in the same region of the atmosphere where orographic lift will be maximized. That will support widespread snow showers thru the day...possibly heavy at times...and efficient accumulations due to high ratio snow. It is possible some of the higher terrain will pick up an additional 3 to 5 inches or more beyond what they receive today. A trailing S/WV trof looks to pass S of the forecast area and out to sea Thu. But the pattern remains active...and fast moving waves thru the Nrn stream will have the potential to phase with the Srn stream and bring precip to the area. There are two windows worth keeping an eye...one being Sat and another early next week. At the moment the S/WV model guidance is keying on for Sat is over the Pacific Ocean and some 24 to 36 hours from moving over land observations. For now I see no reason to deviate from a chance of snow near the coast. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Short Term... MVFR ceilings and snow showers are now moving into southern New Hampshire and will continue to extend into southern Maine. Expect conditions to deteriorate to IFR in snow by mid morning with periods of LIFR in heavy snow possible across the foothills. The storm will move out with conditions returning to MVFR late tonight. Long Term...Upper low pressure will cross the forecast area Wed with areas of MVFR CIGs. Widespread SHSN expected...which will bring local IFR conditions...most likely at LEB and HIE. In addition Wly wind gusts near 30 kts possible Wed and Wed evening. SHSN linger near HIE into Thu...with VFR conditions expected elsewhere. Ocean low pressure will develop Sat...and at this time looks like it will remain out to sea. There is a chance this spreads some -SN back towards coastal terminals...but confidence is low in that scenario. && .MARINE... Short Term... Low pressure will cross the Gulf of Maine with winds briefing increasing to Gale force this afternoon as the low moves through. Long Term...Strong CAA Wed will allow a Wly gale to develop. The bays should be more of a marginal gale...but outside the bays may gust to near 45 kts Wed evening. Winds and seas will gradually diminish into Thu...but SCA conditions will likely linger into Thu night. Low pressure looks to develop in the Wrn Atlantic to start the weekend...and may prolong SCA conditions...especially seas. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Winter Storm Warning until 1 AM EST Wednesday for MEZ007>009- 012>014. Winter Weather Advisory until 1 AM EST Wednesday for MEZ018>022. NH...Winter Storm Warning until 1 AM EST Wednesday for NHZ001>009. Winter Weather Advisory until 1 AM EST Wednesday for NHZ010>013-015. MARINE...Gale Warning from 10 AM this morning to 1 AM EST Wednesday for ANZ150>154. && $$ SYNOPSIS... NEAR TERM...Curtis SHORT TERM...Curtis LONG TERM...Legro AVIATION...Curtis/Legro MARINE...Curtis/Legro

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