Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 181630 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1130 AM EST Sun Feb 18 2018 .SYNOPSIS...
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High pressure will build across the region through tonight before moving east of northern New England Monday. A warm front will move north of the region Tuesday and usher in very warm air for the time of year. A cold front will move through Thursday ushering in some cooler air for the end of the week.
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1130 AM...Minor ESTF update to reflect the current mesonet and satellite trends in near term grids. Prev Disc... 905 AM...the back edge of the snow shield is exiting the remainder of the coast and the winter weather advisory has been cancelled there. For this ESTF update...I adjusted near term grids to reflect current radar and satellite trends as well as the latest mesonet. Added pops and light accumulations for light upslope snow evident on KGYX Radar. Prev disc... 645 AM...Snow is rapidly ending across the area from west to east as the ocean low and associated shortwave impulse exit to our east. Winter headlines have been dropped for all but mid coast Maine zones with this update. Prev Disc... A broad area of diffluence aloft exists in the left exit region of an upper level jet streak nosing thru the Northeast. A fast moving...but amplifying...S/WV trof is also driving low pressure development in the Gulf of ME. The broad lift over the area is producing widespread light snow. This is combining with a more narrowly focused band of occasionally heavy snow due to frontogenetical forcing related to cyclogensis off the coast. That stronger forcing for ascent is mainly located in the snow growth zone...leading to efficient production of dendrites...and relatively warm near surface layer leading to clumps of aggregates. This is likely what is leading to the relatively higher visibility in heavy snow as well. With little wind this is accumulating nicely across Srn NH and SWrn ME. I expect fairly widespread 4 to 7 inch amounts in that zone at daybreak...which will compact and begin melting as temps warm. Expanded the winter wx advisories NWwd...and contemplated expanding warnings...but I expect impacts have already occurred and only an hour or two of appreciable snow accumulation left. Readings are expected to warm nicely in the wake of low pressure today. Mix down tools bring readings well into the near 40 near the coast. I did nudge the forecast down from those values just a little bit for melting effects today however.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... High pressure builds in to our SW tonight...with surface ridge extending Nwd thru the forecast area. It will cool quickly especially in any areas that keep a good deal of fresh snow. I did not get too aggressive on radiational cooling though...because return flow begins overnight and some higher clouds may encroach on the forecast area. But with readings below freezing we will have to watch out for refreezing of melted snow and icy spots on area roads. By Mon mixing to H9 will lead to widespread 40s across the forecast area. This will begin a period of very mild wx across the region. In that return flow...increasing moisture will also lead to increasing clouds and perhaps some precip moving into Wrn zones late in the day Mon. Some snow is possible at elevation...but with mild temps most precip...if any...should fall as rain. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Model guidance continues to show ridging aloft developing off the southeast coast of the United States. This will likely remain in place through the week. This will allow some very warm air to surge north as the storm track will remain to the west of new England. A warm front will move north Monday night which will bring in some light precipitation. Precipitation will likely begin as a mix in the mountains but change to rain Tuesday. Tuesday and Wednesday will be very warm with temperatures on Wednesday approaching 70 degrees in southern New Hampshire. ***Records for PWM & CON*** TUE WED PWM 61/1994 59/1953 CON 66/1930 64/1872 Also expect at least patchy fog Monday night into through Wednesday. A cold front will slip through the forecast area Thursday with cooler weather but still above normal temperatures for Thursday and Friday. Low pressure again moves into the region Friday with rain showers or snow showers over the weekend. && .AVIATION /16Z SUNDAY THROUGH THURSDAY/... Short Term...Widespread IFR and LIFR in SN this morning will quickly end from W to E as low pressure develops in the Gulf of ME. NW flow developing will bring VFR quickly back to areas S of the mtns...while some lingering upslope moisture will likely keep a MVFR CIG around HIE. A few surface wind gusts to around 20 kts are possible this afternoon in WNWly flow at all terminals. VFR conditions expected tonight and for most of Mon. Late in the period some MVFR conditions and possibly -SHRA may move into Wrn zones around HIE and LEB. Long Term...Expect IFR Monday night in fog and rain and Tuesday night in fog. MVFR ceilings possible Tuesday improving to VFR Thursday. && .MARINE... Short Term...SCA remains in effect for all waters except Casco Bay for this afternoon in the wake of departing low pressure. Winds and seas gradually diminish late today and tonight as high pressure builds in from the SW. Return flow over the waters is forecast to increase quickly Mon...but model guidance tends to over forecast Sly flow over the cold water. I preferred to lower wind speeds and also took the lower WNA wave guidance. Still some marginal SCA conditions are possible outside the bays late Mon. Long Term...Expect SCA will be needed Tuesday and Wednesday because of an increasing southwest wind and increasing seas. && .HYDROLOGY... We will have to monitor the rivers closely next week as the warm weather and some rain may lead to enough runoff to cause ice jam flooding. There are several locations with ice jams in Maine and New Hampshire. Looks like a 72 hour period or longer with temperatures remaining above freezing. It is possible that the warmer weather will weaken the ice so that any rise in river levels will just move the ice harmlessly downriver. QPF should be fairly light with around one quarter to one third of an inch expected Monday night into Tuesday. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ150>152-154. && $$ ES is the U.S. government's official web portal to all federal, state and local government web resources and services.