Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 121819 AAD AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 119 PM EST Tue Dec 12 2017 .SYNOPSIS... Low pressure over New England will strengthen and lift north and east of the region tonight and on Wednesday...with heavy snow and rain through this evening giving way to mountain snow showers and strong westerly winds on Wednesday. The coldest air of the season will move into the region behind this system for the remainder of the week...with a few chances for additional show showers. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 115pm Update: Added coastal Rockingham/York/Cumberland to the advisory as correlation coefficient off of GYX 88D matches with observations showing warm air aloft moving ahead of coastal front at the surface...resulting in a period of sleet and freezing rain. No other significant changes at this time. 945am Update: No changes to the headlines with this morning update. Initial around of snow has produced 1-3 inches across the region. Biggest changes are to match most recent local WRF and GFS LAMP data which indicates a much slower transition to above freezing temperatures...with nearly all inland likely remaining below freezing through the event. Primary low continues to lift through New York with surface pressure falls not really yet pin pointing the location of secondary development. Coastal front is well evident on GYX 88D as a fine line that is now beginning to lift north and west. Already seeing precipitation filling in to the west in response to strengthening mid and upper level forcing. Thus...snow will redevelop with coastal changeover to rain during the mid afternoon...with some mix likely north of this as warm air aloft briefly lifts north before precipitation ends. Have to watch interior York/Cumberland for warning level amounts...but amounts thus far have been less than previous while rates will shortly increase...inherited snow totals still looking good. 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. && .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 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 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 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 /18Z TUESDAY THROUGH SUNDAY/... Short Term... LIFR/IFR in snow changing to rain MHT/PWM/RKD with a period of freezing rain/sleet at these locations during the transition. Mix also expected CON/AUG...with all snow HIE/LEB. Precipitation ends this evening with slow improvement in conditions overnight. VFR outside of the mountains on Wednesday with westerly winds 20-30kts. Continuing SHSN at HIE on Wednesday with IFR/MVFR restrictions. Winds gradually diminish Wednesday night with VFR conditions expected. 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. Winter Weather Advisory until 6 PM EST this evening for MEZ023- 024. NH...Winter Storm Warning until 1 AM EST Wednesday for NHZ001>009. Winter Weather Advisory until 1 AM EST Wednesday for NHZ010>013-015. Winter Weather Advisory until 6 PM EST this evening for NHZ014. MARINE...Gale Warning until 1 AM EST Wednesday for ANZ150>154. && $$ SYNOPSIS... NEAR TERM...Arnott SHORT TERM... LONG TERM... AVIATION...Arnott MARINE... is the U.S. government's official web portal to all federal, state and local government web resources and services.