Area Forecast Discussion
Issued by NWS Portland, ME

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232 FXUS61 KGYX 091402 AFDGYX Area Forecast Discussion National Weather Service Gray ME 902 AM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure developing south of Nantucket today will move northeastward into the Gulf of Maine and eventually into the Maritimes tonight. This low will produce a plowable snowfall for a good portion of the forecast area. Dry weather is expected Sunday through Monday in the wake of the low. The next low pressure system will affect Maine and New Hampshire Tuesday with another round of wintry precipitation. Cold and windy conditions are then expected midweek. Another low pressure system could affect the region at the end of the week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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900 AM...Morning KGYX RAOB shows dry air in place below 700mb. The forecast question today will be how efficiently we overcome the dry layer and precipitation begins reaching the ground. I slowed the timing of precipitation onset slightly for this ESTF update and ingested the current mesonet into near term grids. Prev disc... 645 AM Update...Just a few minor changes to the timing of the front edge of the precipitation shield...just a tad slower. Otherwise, the rest of the forecast and associated advisories remains unchanged. Previously... No real changes in the morphology of low pressure that is expected to develop south of Nantucket today and move through the Gulf of Maine tonight. The 00z deterministic model suite is in pretty good agreement - with the 00z GFS being an eastern outlier. Have used a blend of available guidance for PoPs, temperatures, QPF, and snowfall today and tonight with the most weight given to the ECMWF and 12z ECMWF ensemble. This has resulted in the conversion of winter storm watches to winter weather advisories for a general 3-6" snowfall. A few places may limp over 6" in the end. However, the southwest flow regime aloft with little in the way of mid level low development off the coast looks to limit inflow and aids in less than spectacular dendritic snow growth. This should keep most locations below the winter storm warning threshold for snow (6"+). However, if trends dictate otherwise later this morning, the next shift has the option to upgrade to warnings if necessary. Snow will begin midday in the southernmost tier of zones, gradually advancing northward throughout the afternoon hours. The advisories have been timed accordingly. On the coast, relatively warm temperatures may allow for some rain to mix in for a time, especially on the midcoast. Eventually, cold air is expected to win out, even on the Islands. However, will start the advisory there (Midcoast) later on in the afternoon. Roads should become slippery in many locations by mid to late afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Snow continues this evening, possibly moderate at times with snow continuing to accumulate on all surfaces. The low pressure system will be a fast mover, so the snow will likely taper off and end from southwest to northeast around midnight, finally ending just prior to sunrise on the Midcoast. Chilly gusty winds will kick in Sunday with variably cloudy conditions. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... High impact weather: Accumulating snowfall expected Tuesday Overview: A trough remains over the eastern portion of North America through next week. Short waves will move through this major trough every few days with the resulting surface lows tracking through New England brining several rounds of snow. Monday: Monday will be a quiet day between the storms. Cyclonic flow will continue with a few scattered snow showers persisting in the mountains through the morning. Tuesday- Wednesday: Confidence is now moderate to high that accumulating snow will impact Maine and New Hampshire from Tuesday into Wednesday. Both deterministic and ensemble tracks are converging. A short wave is expected to dive south out of the Canadian rockies triggering surface low development over the Great Lakes. This low will subsequently move northeastwards through New England bringing snow to the region. While overall spread amongst low tracks has decreased since yesterday there is still a bit of a duel ongoing between the main low headed up the St. Lawrence river valley and a secondary surface low tracking just barely off shore through the Gulf of Maine. In particular the coastal low is what gives this forecast some wiggle room, wanted or not. An intensification of the coastal side could pull the entire system towards the coast, or alternatively an westward move would help to pull more warm air in aloft putting southern New Hampshire and extreme coastal Maine into a potential changeover to rain or sleet. Either scenario leads to widespread precipitation through the day on Tuesday and thus have kept PoP at likely. This system is now also favorable for a strong coastal front to setup and even the global scale guidance hints at that putting as much as a 20F spread in temperatures along the coast. With a prolonged period of colder temperatures preceding the arrival of the low expect the cold air to be fairly deep and entrenched which will keep the warm air further offshore. Would expect a coastal front to set up off shore of Maine except for possibly Matinicus and extend southwestwards into central Massachusetts. With this in mind have kept temperatures colder over land through the day on Tuesday resulting in precipitation type as almost entirely snow. The area most likely to see a changeover to sleet or rain would be coastal Knox county Tuesday afternoon as the low moves overheard allowing the warmer air to move inland. A strong coastal front could also help to focus snowfall just inland. Have held off from incorporating this sort of mesoscale effect at this point in the forecast because there is still a lot of potential for the larger scale dynamics of the system to change. Thursday - Friday: Thursday we return to the general cyclonic flow with scattered snow showers. A break between storms won`t last long as yes another system tried to intensify on Friday and move up the coast. It`s at about this point in the forecast that confidence decreases. Models will try to progress the long wave trough which had persisted all week. Progress is often ahead of its time and thus have low confidence in any sort specifics of a storm for end of the week although the snowy pattern is likely to continue. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Short Term /through Sunday/...VFR to start the day, but conditions eventually drop to MVFR this afternoon, then to IFR this evening in snow. Improving conditions are expected after midnight tonight as the snowstorm moves away. Mainly VFR Sunday except in the mountains. Long Term...Monday will see cyclonic flow with just a chance for MVFR in snow showers in the mountains. Low pressure will move through brining snow to most of the region on Tuesday. Expect conditions to become IFR, especially Tuesday afternoon and evening. By Wednesday all but the mountains will return to VFR. && .MARINE... Short Term /through Sunday/...Low pressure developing off the coast tonight will bring SCA conditions tonight through Sunday night as gusty NW winds develop. A very brief period of gales will be possible later Sunday but confidence too low to post warnings at this time. Long Term... Winds will come around to southeasterly ahead of the low passing through the St. Lawrence valley on Tuesday. Expect conditions to increase to small craft advisory with gales possible. Northwesterly cyclonic flow sets back up for the end of the week. && .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...Winter Weather Advisory from 1 PM this afternoon to 5 AM EST Sunday for MEZ018-019-023-024. Winter Weather Advisory from 2 PM this afternoon to 7 AM EST Sunday for MEZ007>009-012>014-020>022. Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Sunday for MEZ025>028. NH...Winter Weather Advisory from 1 PM this afternoon to 5 AM EST Sunday for NHZ005>010. Winter Weather Advisory from 2 PM this afternoon to 5 AM EST Sunday for NHZ002>004. Winter Weather Advisory until 3 AM EST Sunday for NHZ011>015. MARINE...Small Craft Advisory from 4 AM Sunday to 1 AM EST Monday for ANZ153. Small Craft Advisory from 6 PM this evening to 1 AM EST Monday for ANZ150>152-154.
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&& $$ ES

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