Area Forecast Discussion
Issued by NWS Portland, ME

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461 FXUS61 KGYX 111459 AAB AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 959 AM EST Mon Dec 11 2017 .SYNOPSIS... A cold front will cross the region today...continuing snow showers in the higher terrain. The cold will settle across the area by tonight...and set the stage for the a wintry storm Tuesday. Precipitation will overspread the area from west to east early Tuesday...mostly starting as snow. Slowly warmer air will work inland from the coast and change precipitation from snow to rain. A narrow band of mixed precipitation is possible between the rain and snow. Across the foothills and mountains precipitation is likely to remain snow for the majority of the event...and up to 6 inches or more is possible. Precipitation will quickly come to an end Tuesday evening from west to east. Behind that much colder air will move into the region and linger through the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 AM Update... Minor changes to up PoPs and QPF for current upslope showers ongoing in the mountains and foothills. No other major changes at this time. Will be interrogating new model data as it comes in for tomorrow`s event. 7am update... minor adjustment for current obs. A cold front will move through today, with surface high pressure will building into the region behind it while the core of the upper level trough brings cold air in overhead. Temperatures will remain fairly cool with highs just above freezing along the coast an in the mid 20s north. The cold air mass an initially clear skies will allow temperatures to drop quickly after sunset, however increasing cloud cover through the overnight in advance of the approaching storm will keep lows from dropping below zero in the north.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... *** Widespread snowfall expected Tuesday *** Low pressure currently diving south through North Dakota will intensify as it moves into the Great Lakes and cross Northern New England on Tuesday bringing snowfall to the region. Confidence is high in the timing of precipitation but only moderate for snowfall amounts due to the potential for a changeover to rain/sleet/freezing rain along the coast and thus the winter storm watch will remain in place. Precipitation will initially begin around daybreak on Tuesday and spread from west to east across the area during the morning. During this time the low will remain to our west over Lake Ontario. This position will allow the cold air which settled into the region overnight Monday to remain in place. A coastal front will form with the cold air dammed against the mountains and the above freezing temperatures remaining off shore for all but the outer islands of the Maine coast. With this expect the entire region to see snow as the warm advection ahead of the low center brings moisture into the region. Greatest snowfall in this time will be through interior southern New Hampshire where the greatest moisture exists, along with good frontogenesis aloft. As we move into the afternoon the setup becomes a bit more tricky. The low center begins to reform and is expected to be right along the Maine coast by evening. This sets up two key features of the forecast. For the early to mid afternoon we`ll see the area of greatest forcing and qpf move east into Maine. In combination with the existing coastal front, this has the potential to create a region of high snow totals just inland of the front, with snowfall rates above 1"/hr possible. The issue of course is exactly where that coastal front is going to be. At the same time as the forcing for higher snowfall totals is moving east the surface low is also reforming and trying to bring the warm air further inland, likely changing a portion of the midcoast over to rain with a mix of sleet and freezing rain along the boundary. Meanwhile almost as soon as the low can form and bring the warm air, the dry slot will also arrive bringing an end to the precipitation in the evening. The net result of all this is a region of higher snow totals, around 6-10" extending from Sullivan county in New Hampshire up through the Maine foothills towards Bangor. To the north, along the Canadian border we may see lower amounts just due to the main forcing being more towards the coast. From Manchester, NH and up along I-95 in Maine through to Augusta is really the linchpin in terms of the warm air. This area could see high snow totals if they remain on the cold side or low amounts if the warm air is able to penetrate. Finally along the coast, mixing with rain and sleet is expected, and here snow totals will likely result in just an advisory level. While there is a portion of the area where confidence is high enough for a warning, with so much still in question have opted to just keep the entire region in the watch. The low center will move off to the northeast overnight Tuesday with just upslope snow remaining along the Canadian border by Wednesday morning. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... In the wake of surface low pressure...the main upper low will barrel across the region Wed. In addition to bringing the real colder air...it will also provide the instability for widespread snow showers. While the focus will be in the higher terrain where orographic lift will enhance vertical motion...the Arctic front dropping H8 temps from near 0C to -20C over the course of the day Wed will also allow for snow showers to organize along the boundary. Given the high low level RH...sharp frontal boundary...and low static stability...some of this activity along the front may be in the form of snow squalls. In addition to the cold and snow...winds will be quite gusty Wed. Deep mixing in CAA will support frequent gusts around 30 kts...and possibly higher as some forecast soundings try mixing to a 40 kt Wly LLJ around H9. Forecast soundings also indicate the boundary layer will remain mixed into Wed night. I favored raw 2 m temps for overnight readings...along with winds/gusts higher than the multi-model consensus blend. Temps falling into the teens and single digits coupled with the wind will produce wind chill values below zero...to as cold as the teens below zero in the Nrn zones. The deep...cyclonic flow lingers Thu but will begin to relax into Fri as the next S/WV trof approaches. This wave will race thru the Great Lakes and lead to development of an ocean low pressure Fri night into Sat. With the main baroclinic zone well off shore this low shows no threat to the forecast area on the 11.00z model suite. That being said...the S/WV in question will move into the Gulf of AK this afternoon. A sample of this feature with 12.00z raobs will likely impact model solutions in the coming 24 hours. Additionally...ensemble sensitivity really begin to grow in the 48 to 60 hour window...suggesting that they may not stabilize anytime soon. I have hedged towards a chance of snow for the SErn third of the forecast area...higher than the multi-model consensus blend for that reason. The active Nrn stream continues into the weekend...with another potential wave arriving sometime Sun or Sun night. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Short Term... A weak front will move through the northern portion of the area today with MVFR in occasional SHSN expected, elsewhere VFR. Low pressure will move through the region on Tuesday. Expect conditions to rapidly deteriorate to IFR on Tuesday morning with periods of LIFR in heavy snow expected for Southern New Hampshire between 12Z and 18Z and into southern Maine from 18Z- 00Z. Long Term...Deep cyclonic flow will persist over the forecast area into Thu. Moist...Wly flow will keep MVFR CIGs mainly upwind of the mtns...along with upslope SHSN. HIE will see periods of MVFR and possibly local IFR in SHSN. The upper low crosses the area Wed afternoon...and that may allow for SHSN to maintain outside of the high terrain. Local IFR conditions will be likely in any SHSN that move downstream of the mtns. Otherwise VFR conditions expected thru Thu. An ocean low pressure will develop Fri and move NEwd paralleling the coast. That may spread -SN to areas near the coast...though this is low confidence at this time. && .MARINE... Short Term...Have extended the small craft advisory as winds continue to gust to 25-30kts. Expect a brief break in conditions late Monday as high pressure moves in. By Tuesday the intensifying low will bring Gale conditions to the waters and a Gale watch has also been issued. Long Term...As low pressure deepens over Quebec...a strong pressure gradient will set up over the coastal waters. A strong Wly gale will continue into Wed night. Winds and seas gradually diminish Thu afternoon. Another low pressure developing near the Gulf of ME may extend SCA conditions into Fri. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Winter Storm Watch from late tonight through late Tuesday night for MEZ007>009-012>014-018>022. NH...Winter Storm Watch from late tonight through late Tuesday night for NHZ001>013-015. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ150>154. Gale Watch from Tuesday morning through late Tuesday night for ANZ150>152-154. && $$ NEAR TERM...Hanes SHORT TERM...Curtis LONG TERM...Legro

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