Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 042352 AAA AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 652 PM EST Sun Dec 4 2016 .SYNOPSIS... High pressure crests over our region this evening before gradually moving off to the east tonight. A weak low pressure system will bring some light snow to the region on Monday. High pressure will settle in for Tuesday. Low pressure may affect the region with more light rain or snow Thursday with colder air following for Friday and over the weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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650 PM Update... Minor changes to the forecast mainly to sky cover. Stratus is diminishing across the Connecticut River Valley and the Canadian border with dew points in the teens and 20s. While some clouds are already arriving from the SW ahead of the next short wave, feel the timing of the snow showers are fine right now with RHs still in the 50s over southern NH. When tonight`s sounding arrives will not be surprised to still see quite a bit of dry air which will have to be overcome before we can see measurable snow. We should have plenty of time for temperatures to radiate towards the low dew points mentioned above before significant cloud cover/ moisture arrives. Previous discussion... High pressure will crest over the region tonight before moving off to the east late. This will allow for clear skies for at least the first half of the night along with rapidly diminishing winds this evening. Strato-cu in the mountains will also dissipate this evening. This will set much of the area up for good radiational cooling conditions through at least half the night. Many folks will drop into the teens to lower 20s. A quick-moving short wave trough will approach from the west later tonight, and aid in increasing clouds from west to east starting after midnight. Light snow should begin to develop across southwestern NH prior to or around dawn as moisture and forcing for ascent increases ahead of the short wave trough.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... As a short wave trough approaches from the west early Monday morning, light snow will overspread the forecast area from west to east. The snow should begin in much of NH during the morning commute. While the snow will not be particularly heavy, cold overnight temperatures will allow what falls to stick to the roadways. This will likely make the morning commute a slippery one mainly across southern and central NH. The snow then spreads east and northeastward during the balance of the morning, reaching the mid coast of Maine and Augusta area by midday. There is multi-model consensus that the short wave trough is strong enough and just far enough south to induce stronger pressure falls in the Gulf of Maine Monday afternoon. This will aid in slightly enhanced inflow into a weak frontogenetic zone extending northwestward to the Maine coastal plain. These features should enhance the snowfall a little bit more than what we were previously thinking, allowing for a 1-3 inch snowfall even on the Maine coastal plain. So while much of Maine may escape a slippery morning commute, a slippery evening commute is likely in the cards. The snow will come to an end from southwest to northeast around midday across southwestern NH, finally shutting off during the early evening hours on the mid coast of Maine. At this time, it looks likely that the snow will have ended for the evening commute in much of New Hampshire. Gradual clearing is expected monday night. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Upper air pattern remains fairly progressive across North America as larger scale waves move relatively easily from west to east across the continent through the week. Several weak waves will move toward New England with the biggest one arriving on Thursday. Colder air pours out of Canada behind it through the end of the week. The details: High pressure crosses the area on Tuesday with seasonably cold temperatures expected, generally in the upper 30s to low 40s. High pressure pushes east Tuesday night, with the best radiational cooling across the eastern part of the forecast area. Clouds will begin moving in from west to east during the evening in advance of the next minor shortwave trough. The increase in cloud cover will limit nighttime cooling over most of New Hampshire and later into western Maine as well. The wave that moves into the area Tuesday night is a relatively broad but weak feature which connects a northern and southern stream wave. The southern stream wave moves east into the Atlantic south of New England while the northern one moves towards James Bay. Increasingly as this feature moves east, the connection between the two will lessen, decreasing the overall precipitation potential as it moves east into our area. Southwest New Hampshire will have the best chance of seeing precipitation from this, with chances decreasing toward the east. The temperature aloft looks cold enough for this to fall as snow, with near-surface temperatures also complying to a large degree. Amounts are likely to remain in the Trace to 2 inch range, arriving just in time to slicken up the morning commute. The next wave is expected to arrive in our area on Thursday, though models are still not agreeing on the degree to which the southern and northern stream waves will merge. The GFS keeps the southern stream wave separate until it moves into Nova Scotia Friday night and is picked up by the broader northern stream trough. The ECMWF continues to forecast a merging to take place over eastern New England, which would develop a more robust surface low tracking near the Maine coastline Thursday night. The GFS solution would favor colder temperatures (at least aloft, downslope warming might actually mean warmer surface temperatures on the coastal plain) and less precipitation, while the ECMWF produces an advisory level snowfall with the track close enough to the coast to be concerned about a rain/snow line. A compromise approach would favor a surface low track offshore of Cape Cod with light snowfall for southern and coastal areas. This compromise approach was used when preparing the forecast, with both rain and snow mentioned all the way to the coastline and all snow inland. As this wave merges with the broader northern stream trough, it will pull it toward the east and into our area allowing cold air from Canada to pour in behind it. Expect progressively colder temperatures Friday and Saturday with a northwest flow over the region allowing for a chance of snow showers in the mountains. Saturday night could be the coldest night of the season so far as both major models agree that surface high pressure will be moving across the forecast area during the overnight hours, allowing good radiational cooling conditions within a couple weeks of the longest night of the year. In fact, raw ECMWF suggests subzero temperatures in the far northeast of the forecast area. Current forecast is for widespread lows in the teens with single digits in the typical cold spots. While this would be the coldest of the season so far, the cold temperatures available here are not that impressive by typical winter standards. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... Short Term...VFR conditions are expected to be the rule overnight tonight. Snow will advance northeastward across the region Monday morning...likely reaching MHT/CON around 12z...and PWM by 14-15z. MVFR to IFR conditions are then expected for several hours as light snow falls. VFR conditions are likely to redevelop across the region starting in SW NH later Monday afternoon, then progressing northeastward from there during the early evening hours. Long Term...After a VFR day on Tuesday, expect another round of light snow moving in from the southwest Wednesday morning with MVFR to IFR conditions expected mainly in the southwest. Precipitation will dissipate as it moves east, so places like LEB, CON, and MHT are the most likely to be impacted while AUG and RKD may not see any snow. Low pressure moving through the Gulf of Maine Thursday will lead to another chance of accumulating snow especially toward the coast. && .MARINE... Short Term...Conditions are expected to remain below small craft thresholds tonight through Monday. However, visibilities are expected to be lowered in rain and snow on Monday. Long Term...Several weak high and low pressure systems cross the waters through midweek, with winds generally variable at less than 20KT. Low pressure moving near the Gulf of Maine on Thursday will be the strongest of these systems, and will pull in colder air behind it through the end of the week. This will keep a brisk west to northwest flow over the Gulf of Maine into the weekend, likely near advisory level though possibly briefly reaching gale force. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Hanes SHORT TERM...Ekster LONG TERM...Kimble

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