Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 182020 AFDGYX Area Forecast Discussion National Weather Service Gray ME 320 PM EST Sat Nov 18 2017 .SYNOPSIS... Low pressure moving through the Saint Lawrence Valley will push a warm front northward through northern New England tonight. Expect mostly rain as this moves through, but northern areas could begin as light snow or freezing rain before the temperature warms overnight. As the low moves east on Sunday it will drag a cold front across the region from west to east, causing temperatures to fall through the day and winds to become gusty. Expect cold conditions continuing on Monday before a warm up on Tuesday. Another cold front moves through on Wednesday with cold air spilling in for the last half of the week. && .NEAR TERM /TONIGHT/... As low pressure moves through the Saint Lawrence Valley tonight a warm front will lift northward through New England. Already seeing precipitation beginning across southern New England, and this will spread northward into New Hampshire and Maine this evening. The leading edges of this could begin as snow, but expect this to be primarily a rain maker. The only exception will be in interior parts of western Maine and northern New Hampshire where low level temperatures may stay below freezing for a while as the rain begins. Low level dry air is still fairly impressive, and it is likely that as precipitation begins it will cause temperatures to fall more through evaporational cooling than radiational cooling. As the night goes on expect temperatures to warm, and by morning it should be above freezing just about everywhere. Have issued a Winter Weather Advisory for areas that are most likely to see some light freezing rain tonight. Do not expect much accumulation, but it will be just enough to make roads slick. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... A cold front will move from west to east across the forecast area during the first part of the day on Sunday. This will shut off the precipitation and shift winds to the west. Pretty good cold advection signal behind this front suggests good low level mixing and falling temperatures during the day. 850/925 MB winds are forecast to be 40 to 50 KT, so expect with good cold advection these will mix down with high efficiency especially to the east of the mountains where downsloping conditions will aid in the process. Have issued a Wind Advisory for this area, though wind gusts will be primarily on the low end of Advisory criteria. This could be just enough to down a few limbs and cause a few isolated power outages. Winds stay up into the evening hours, but will begin to lose their gust potential overnight. With the low levels well mixed expect low temperatures to be driven more by cold advection than radiational cooling, and will generally be in the 20s. && .LONG TERM /MONDAY THROUGH SATURDAY/... High Impact Weather Potential: * Gusty winds on Monday approach wind advisory criteria. Pattern / Implications: The pattern across North America has been rather progressive over the past week with shortwaves impacting our region every two to three days. Current and recent runs of the deterministic and ensemble guidance suite continue this idea through this long term forecast period...although with the NAO beginning to shift negative and the AO following suit...its not surprising to find both the GFS and EC /and their respective ensemble means/ carving out a more long-lasting trough over the eastern United States by the end of this forecast period into next weekend. With the fact-paced flow in place...expect a bit of a roller-coaster ride in terms of temperatures...with a few shortwaves and associated precipitation chances. Model Trends / Confidence: Deterministic and ensemble guidance is in good agreement on the large scale pattern through Tuesday of next week before diverging somewhat for the middle and latter portions of the week. The primary differences concern the handling of digging shortwave energy Tuesday night into Wednesday. Recent GFS/GGEM solutions drop significant energy along the western flank of advancing trough...closing off a mid level low over the southeast on Wednesday while the 18.00Z ECMWF favored more of this energy remaining with the northern feature with a stronger H5 trough reaching the east coast on Wednesday. 18.12Z ECMWF has trended closer to the GFS/GGEM camp...and given much better ensemble support for this solution will lean heavily in that direction with this forecast package. Forecast agreement increases after this period with cool northwesterly flow enveloping the northeastern United States. The Details... Monday: -1 to -2 sigma PWAT anomaly moves overhead under robust northwesterly flow with high pressure arriving from the west towards evening. Residual moisture and cyclonic flow will allow ongoing mountain snow showers to continue through the morning with decreasing clouds expected to rule the day. T9s generally -5 to-10c indicate highs in the mid 20s north to mid 30s south...nearly 10 degrees below normal for the date. Winds: With the core of the H5 trough overhead...model soundings suggest a great llevel mixing setup...with mixing height reaching above 6 kft. This should take full advantage of winds at this height around 40kts. Thus...expect winds 15 to 25 mph sustained...with gusts of 30 to 40 mph likely. Can/t rule out a few gusts around wind advisory levels. Future shifts will certainly need to watch for the potential for additional wind advisory headlines. A combination of coupled llevel wind flow /1000 mb geostrophic winds around 20kts/ and a period of SCT/BKN mid level cloudiness should help lows from getting too out of hand...but a few isolated locations may be able to radiate. However...teens and 20s look most likely for lows...close to model consensus. Tuesday: Return flow with rapid warm advection moving T8s back above freezing. Given the setup...a return to warmer than normal temperatures is expected. It again looks breezy...but now from the southwest as backing flow aloft strengthens to +1-2 sigma at the H8 level. T9s reach back towards 5C which indicate that given good mechanical mixing...southeastern NH will likely reach into the lower 50s...but it won/t feel quite this warm given the breeze. Wednesday-Thursday: As described above...forecast confidence decreases somewhat for the important Thanksgiving travel period...although the 18.12Z guidance suite is trending towards a consensus. This would favor a moisture-starved northern stream shortwave and associated frontal passage on Wednesday followed by high pressure briefly building into the region for Thanksgiving. Temperatures near climatology on Wednesday ahead of the front Wednesday moving back below normal for the Thanksgiving holiday. Friday-Saturday: Have to keep an eye on the cutoff energy over the southeastern states as next northern stream shortwave arrives from the west. There have been varying signals for some semblance of low pressure along the east coast Friday...but with the most likely solution keeping precipitation offshore. Thus...would await the surface low associated with the northern stream shortwave for any sensible weather impacts. No signal for significant departure of temperatures from seasonal norms...with chance PoPs for Saturday. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... Short Term...VFR conditions give way to IFR ceilings and rain this evening and overnight. Winds shift to the west behind a cold front tomorrow morning. By tomorrow afternoon strong cold advection will allow these winds to gust up to 35 or 40 KT before diminishing a bit overnight. Long Term...Improving to VFR Monday with continued strong northwesterly wind gusts to 35kts. Expect VFR conditions to largely dominate the week...with a cold frontal passage Wednesday bringing the potential for some restrictions in shra/sn on Wednesday. && .MARINE... Short Term...Expect gale conditions as a cold front moves through early on Sunday. There is a chance that southerly winds reach gale force ahead of the front, but the big show will be behind the front when winds shift to the west and gust up to 45 KT in a cold west wind. Gusts should diminish below gale force by around midnight Sunday night. Wave heights will be highest further from shore. Long Term...Another period of gale force gusts over the outer waters is likely on Monday with SCAs elsewhere...with winds diminishing...but waves remaining elevated Monday night. Winds shift southwesterly and strengthen on Tuesday with SCAs likely to continue into Wednesday before diminishing winds and waves are expected for the end of the week into the weekend. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Wind Advisory from noon Sunday to midnight EST Sunday night for MEZ012-018>028. Winter Weather Advisory from 8 PM this evening to 6 AM EST Sunday for MEZ007>009-012>014. NH...Wind Advisory from noon Sunday to midnight EST Sunday night for NHZ004-006-008>010-012>015. Winter Weather Advisory from 8 PM this evening to 6 AM EST Sunday for NHZ001-002-004. MARINE...Gale Warning from 6 AM Sunday to midnight EST Sunday night for ANZ150>154. && $$ NEAR TERM...Kimble SHORT TERM...Kimble LONG TERM...Arnott is the U.S. government's official web portal to all federal, state and local government web resources and services.