Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 220453 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1253 AM EDT Thu Mar 22 2018 .SYNOPSIS... A coastal storm developing south of southern New England tonight will spread snow over parts of southern New Hampshire and coastal western Maine. Snow will stay mainly south of the mountains, with light to moderate accumulations near the coast. Amounts of 2 to 4 inches will taper quickly to around an inch in the foothills. Snow will come to an end Thursday, tapering off to showers. A trough of low pressure will linger over the area into Saturday with scattered snow showers in the mountains. High pressure will build in from the north Sunday and Monday and will hold over the area on Tuesday. && .NEAR TERM /THROUGH TODAY/... Update... Have removed any mention of moderate snow from the most recent gridded package and replaced with light. Have backed off on pops for the next few hours as dry air continues to win out. Precipitation on latest Radar imagery continues to sublimate before reaching the ground. Will likely need to back off on pops and snowfall amounts for the morning hours as well. Prev Disc... As of 9 PM... Current forecast generally on track. Preponderance of the high res numerical guidance suggests leading edge of the snow moving into far southern NH and southwest coast of ME by around 04-05z. Have only made some minor tweaks to the QPF/snow forecast through 06z to better reflect current conditions. Winds have been a little slow to pick up but have been slowly increasing over the last couple of hours along the coast. Remainder of the forecast looks fine for now. Previous discussion... S/WV trof exiting the Southeast coast will cyclogenesis off the Mid Atlantic coast this evening. Low pressure will move NEwd towards the Canadian Maritimes by Thu. Confluent flow aloft is helping to keep low level flow Nly...advecting dry air down thru much of the forecast area. This has been eroding the leading edge of snowfall for most of the day. It is just now that stronger advection from the Southeast S/WV is forcing the main precip shield Nwd. That slow and steady advance will continue this evening...reaching Srn NH by about 8 PM. Snow may hold off until closer to midnight in the White Mtns and parts of Wrn ME. Despite a fairly unconventional and complicated about 2 AM tonight deep saturation and lift thru the snow growth zone should allow for a band of moderate snow roughly parallel to the coast of Wrn ME. Snowfall rates from midnight to 4 AM may approach a half inch an hour at times. Hi-res model guidance continues to be quite aggressive with snowfall amounts...pushing warning criteria for the Seacoast and some of the islands and necks in Wrn ME. On the other end of the spectrum the 21.12z ECWMF and EPS remain steadfast on meager QPF. Probabilities for 3 or more inches of snow are about 10 percent for the extreme SErn parts of the forecast area. Given the ECMWF held serve...I have not changed the snowfall amounts from the midday update. Winter wx advisories remain in effect...though the Nrn parts of those zones will be on the marginal side. The intensity of the snowfall is forecast to rapidly weaken by around 8 AM. && .SHORT TERM /TONIGHT THROUGH 6 PM FRIDAY/... A weakening band of frontogenesis and deformation NW of departing low pressure will lingering precip into Thu. It will gradually become more showery into the afternoon...with rain possibly mixing in as temps warm. Some forecast guidance does indicate a saturated snow growth zones lingering over the forecast area well into the afternoon. So PoP may need to be increased for showers in later updates. Only light additional accumulations are expected. Nly winds will remain gusty in the wake of low pressure...with some gusts up to 25 kts possible. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Upper level low will remain in place over the region through much of the weekend. Friday will be mostly cloudy with light northwesterly flow as the upper level trough remains to the west. High temperatures will be near average near 40 in the south and mid 30s in the north. Saturday the low shifts overhead bringing an increased of snow showers through the mountains. By Sunday a stronger upper level vort max will move in allowing the trough to have a positive tilt as it moves out. The cold air will push in possibly accompanied by a few snow squalls. High temperatures will barely reach freezing. High pressure moves in for the start of next week with clear skies and near average temperatures. Another oceanic low will pass out to sea in the middle of next week. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... Short Term...Expecting VFR conditions for another few hours while dry air erodes the leading edge of the -SN. After 04z Srn NH will likely quickly go to IFR conditions in SN...closer to 06z across coastal Wrn ME. In that 06-12z window...SN may become moderate at times with LIFR conditions possible. SN tapers off mid morning Thu...though SHSN may linger into the afternoon...especially across Nrn NH and Wrn ME. NNE winds will also remain gusty...with surface gusts approaching 20 to 25 kts at all terminals. Long Term... VFR conditions will prevail through the weekend under mostly cloudy skies. Upslope snow showers may briefly reduce ceilings to MVFR in the mountains. && .MARINE... Short Term...NE winds will continue to steadily increase thru tonight ahead of approaching coastal storm. Expecting gales outside the bays...with marginal gales in Casco and Penobscot Bay. Seas are expected to increase to 15 to 20 ft tonight. Winds and seas will slowly diminish Thu. Long Term... Seas and winds will subside as the midweek storm moves out to sea. Winds will again increase to near Gales by Sunday morning as another front moves through the waters. An oceanic storm may generate large swell in the middle of next week. && .TIDES/COASTAL FLOODING... A tidal anomaly at close to a half foot and an additional storm surge up to two feet are expected tonight near the time of high tide. This will combine with building nearshore waves of 10 to 15 feet to produce generally minor beach erosion and splash- over flooding. Pockets of moderate impacts are possible, due in part to the damage of sand dunes and infrastructure from the storms earlier this month. Astronomical tides peak as well tonight. There will also be coastal flooding with up to a foot of water on the side streets near Portland Harbor and near Marginal Way. Flooding can also be expected along Granite Point Road in Biddeford and the Backbay area of Hampton. Some streets in Portsmouth may have minor flooding. Areas that have splash-over will have also some flooding due to the pooling of water landward of the dunes. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Winter Weather Advisory until 11 AM EDT this morning for MEZ023>028. Coastal Flood Advisory until 5 AM EDT early this morning for MEZ023-024. NH...Winter Weather Advisory until 9 AM EDT this morning for NHZ012>015. Coastal Flood Advisory until 5 AM EDT early this morning for NHZ014. MARINE...Gale Warning until 5 PM EDT this afternoon for ANZ151-153. Gale Warning until 5 PM EDT this afternoon for ANZ150-152-154. && $$ JC is the U.S. government's official web portal to all federal, state and local government web resources and services.