Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 130117 AFDGYX Area Forecast Discussion National Weather Service Gray ME 817 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 snow showers. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Update... This latest update is to adjust grids per current conditions and also allow the winter weather advisories to expire along the coast. Latest mesoscale models suggest, that although there may be a couple pockets of freezing rain, the transition to just rain or snow will continue to occur over the next hour. Prev Disc... Have extended the advisories along the coastline as mixed precipitation continues away from the immediate shoreline communities, but still over the interior portions of the coastal counties. The mixed precipitation remains in the form of freezing rain, however sleet becomes more predominant with the stronger echoes. WSR-88D CC values show the advance of the mixed precipitation into central and northern areas is slowing, and not advancing quite as quickly with time as compared to early this evening. Have extended the coastal advisories until 8 pm. Northeasterly winds continue to bring cold air into the region of a strong coastal front. The surface low (992 mb) will continue to move northeast this evening. At 23Z this system was located near the New Hampshire Seacoast. As the low goes by and winds switch to the northwest, temperatures may actually go up over southern and coastal areas with the additional mixing. Made adjustments to winds, surface temperatures, dew points and temperatures aloft. Otherwise, no other significant changes. Prev Disc... High Impact Weather Potential: Heavy snow accumulations in the foothills and mountains with snow changing to mixed precipitation to the south and east. Pattern: Early afternoon water vapor imagery shows impressive trough digging south and east across the Great Lakes region...taking on a negative tilt. Primary low pressure decaying over northern New York with 3 hour pressure falls favoring a new low pressure over southeastern New England. This low will strengthen rapidly given the impressive dynamic forcing arriving from the west...with precipitation likely to increase in intensity for a time before rapidly ending south of the mountains tonight as this low moves north and east of the region. Through Evening: KGYX 88D correlation coefficient showing warm air aloft advancing north and west with the coastal front lagging behind. Complex scenario through this evening with a period of mix likely from EEN/CON/IZG/AUG south and east as temperatures aloft continue to warm east of advancing low at H8. Expanded advisories a bit further up the coast with FZRA at IWI and Phippsburg in the last hour...but will hold off further east as RKD has gone to rain. Rates will increases as convective elements noted over southern New England rotate north. However...radar presentation suggests periodic...rather then steady heavy precipitation rates. Tonight: Low pressure moves to vicinity RKD by midnight and then into far northern Maine by daybreak. This will bring the heavy precipitation rates to an end overnight with activity increasingly becoming confined to the mountains. High temperatures will likely be reached around midnight before falling through daybreak. Headlines: Outside of the advisory expansion mentioned above...will make no other changes to ongoing warnings/ advisories. Northward moving mix line may jeopardize warnings for Merrimack/Belknap...but will let them ride given ongoing moderate/heavy snow over the northern portions of these counties.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... High Impact Weather Potential: Westerly wind approach advisory criteria on Wednesday with additional snow accumulation in the mountains. Wednesday: Low pressure near KCAR at daybreak will deepen substantially as it moves north and east during the day with robust cold advection taking T8s from the minus single digits in the AM to the minus teens towards evening. Biggest story will be the winds with robust cold advection likely taking full advantage of 5kft winds around 40kts. Isallobaric component is not very impressive...so while I do expect some gusts to 40 mph...expect that we`ll likely fall a bit short of widespread advisory criteria. Will mention the winds in the afternoon Hazardous Weather Outlook. As for precipitation chances...nice setup for bountiful mountain snow showers with ample moisture in the 0-10kft band which intersects well with the dendritic growth zone. Thus...will continue inherited likely/categorical pops in the terrain with a few inches of accumulation possible. A few snow showers possible south and east of this...but do not expect any accumulations in these areas. The robust cold advection mentioned above will keep temperatures generally steady through the day in the teens/20s...falling back into the single digits/teens through the evening. Wednesday Night: Cold advection comes to an end with gradient gradually weakening behind departing low. With low level moisture continuing to decrease...expect mountain snow showers to wane in coverage and come to and end. Guidance is in agreement with a fast moving clipper-system moving south of New England by daybreak Thursday....with some clouds and possibly a flurry over southern areas. T8s will be at their coldest values thus far this season...but only represent a -1 to -2 sigma departure from climatology. Expect lows to fall into the single digits in the mountains...to the teens along the coast. Wind chills in the 0 to -10F range everywhere. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Cold west northwest flow continues on Thursday, but not as strong as Wednesday. Highs will only make it into the teens north and 20s south. High pressure building in Thursday night will allow for winds to die off and let radiational cooling take over. Not an ideal set up, but good enough for lows falling to around their dewpoints which puts things in the single digits above and below zero. Continued cold in the teens and 20s on Friday. Another trough swings by Friday night into Saturday. Not much moisture with this (aside from lake moisture), so not expecting much snow shower activity, but it is possible. Models indicating some hints of an inverted surface trough reaching back through the Gulf of Maine which if it materialized could bring some light snow to the coast, but this is low confidence. High pressure builds in again for Saturday night and Sunday with temperatures gradually moderating a bit, but still 5 or more degrees below normal. Models not in good agreement on the evolution of an ejecting cut off low over northern Mexico and its interaction with a northern stream kicker trough diving in from the northern Rockies. This impacts the forecast for New England early next week. This ejecting wave will make its way toward New England Sunday night into Monday, during which time we will see a chance of snow. Still a fair amount of uncertainty with this considering the lack of model consistency and the history of poor handling of cut off lows. At this time, the precip potential from this is rather light, with only a few inches likely if it does move through. && .AVIATION /01Z WEDNESDAY 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. Long Term...Could hang on to MVFR ceilings and occasional light snow north of the mountains through Thursday, but should even begin to see improvement to VFR even there by Thursday night and Friday. May see mountain snows again Saturday. && .MARINE... Short Term...Small craft conditions will continue through tonight with winds increasing on Wednesday as low pressure departs to the north and east with gale force gusts likely all areas on Wednesday and Wednesday night. Long Term...Strong west northwest flow continues through Thursday night with advisory level wind speeds likely. This finally slackens off Thursday night into Friday as high pressure moves by. Wave heights will be highest further from shore on the offshore fetch. Could see northwest winds increase again on Saturday and possibly reach advisory levels again for a day or two. && .GYX WATCHES/WARNINGS/ADVISORIES...
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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 9 AM Wednesday to 11 AM EST Thursday for ANZ150>154.
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&& $$ JC

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