Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 210338 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1138 PM EDT Tue Mar 20 2018 .SYNOPSIS... High pressure will gradually move away from the region tonight through Tuesday. A low pressure system will pass to our south late Wednesday and Thursday and will likely bring some accumulating snow to at least southern and coastal areas. An upper level low will cross over New England this weekend. A storm may form well out to sea early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Update... Have updated the forecast based on current conditions. Minor adjustments made to temperatures and dew points before we shift our focus to a developing system to our south. Plenty of moisture will be heading north, while subbingly dry air remains in place over northern New England per 00Z GYX sounding. Prev Disc... As of 930 pm...current temps not falling off like a rock the last few nights. Some thin cirrus from roughly the White Mountains in NH to south central ME keeping temps under control somewhat this evening. Have made some adjustments to temps overnight across the board bringing them up a few degrees. Still a cold night, but not quite like the past few. A couple notes on the 18z model guidance regarding the storm across southern areas- 18z guidance suite has not brought any more fidelity to the snowfall potential across southern Maine and New Hampshire. Several inches still appear possible across the southwest coast of Maine and coastal New Hampshire. Some things to note that the 18Z guidance across the board is very slow to saturate the low levels and not bringing measurable QPF until late in the day tomorrow. Model diagnostics suggest a strong signal, most notably the 18Z NAM, that a mesoscale band will be near/just offshore at the peak of the event. The frontogenetic forcing/stability parameters suggest it will be there, yet the QPF associated with such a feature is just not there. At this point I have no strong reason to make changes to current QPF, but the area will be very close to an axis of much higher QPF. Will hope that the 00Z guidance will help to build a better consensus as to whether the band will remain offshore or will come close enough to affect our coastal areas. Previous Discussion... Fair weather will continue through tonight with high pressure ridging nosing into northern New England from the north. Went lower than guidance for lows tonight since we should be able to radiate at the better radiators. Otherwise, expect an increase in high clouds from south to north.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... The main forecast concern tends to revolve around potential accumulating snowfall Wed night and Thu with low pressure developing well to our south. The 12z model suite didn`t give much clarity to snowfall amounts so therefore confidence remains to low to issue any watches, warnings, or advisories at this time. First things first, on Wednesday, the day will be dry. We`ll see skies become cloudy, but any precipitation will likely remain to the south of the forecast area during the day. The real forecast problem involves how far north strong forcing for ascent progresses in association with developing low pressure to our south. Various 12z models have differing solutions on the strength and positioning of forcing mechanisms...and therefore big differences in precip amounts and timing. Several hi-res models such as the ARW and NMM including to an extent the RGEM are too far south to bring our region much snow. The 12z ECMWF has joined that camp. Otherwise, the NAM and GFS are more robust. When uncertainty like this this continues at near point blank range the best course of action is to stay on the conservative side. We used a blend for QPF and snow amounts, but kept them well below warning thresholds. We thought about issuing a winter storm watch for portions of southeastern NH and York County ME but confidence is not high enough as we feel there`s not yet a 50/50 chance in verifying widespread 6+ inches at this time. Updated snowfall map reflects our thinking and doesn`t really offer too much change from the previous forecast. Temperatures should warm up pretty quickly Thursday so whatever falls Wednesday night and Thursday morning will melt on roadways quite quickly. && .LONG TERM /THURSDAY THROUGH TUESDAY/... The week ends with a fairly high amplitude pattern over the CONUS with a trough over the northeast US and a strong ridge building through 105W over the Rocky mountains. After stalling over the Northeast the upper level trough tries to progress near the end of the weekend. The high amplitude nearly blocked flow means fairly low predictability as upstream blocks inhibit the progression and suspect the upper low will linger into the start of next week. While the upper low will bring an extended period of below average temperatures with some shower activity, overall no major storm systems are in the future through the start of next week. Friday the upper level low will be over head. Without much flow to drive showers only a few very isolated showers are possible in the mountains. Skies will remain mostly cloudy with temperatures topping out near 40 degrees to the south, above 5 degrees below average. Saturday the trough axis will shift east slightly. This will allow the mountains to affect the weather. Upslope showers will increase slightly through northern New Hampshire and along the Canadian border in western Maine, while downsloping will bring decreasing cloud cover to the coastal plain. Sunday features a bit of a backdoor cold front as another upper level vort max pushes through aloft and switches surface winds around to northeast. The result is Sunday is the colder day of the weekend. The same upper level forcing will help to destabilize the atmosphere with widespread snow showers, or even brief snow squalls as the colder air moves in early Sunday morning. The northeasterly flow sticks around into the start of next week as another oceanic coastal low develops well off the coast of North Carolina. Currently the forecast progression of the upper level blocking pattern would allow this storm to pass out to sea however this merits watching as a slowdown in the eastward progression of the pattern, which is fairly typical in blocked scenarios, would result in a threat for rain/snow along the coast by the middle of next week. && .AVIATION /04Z WEDNESDAY THROUGH SUNDAY/... Short Term /through Thursday/...VFR tonight. MVFR conditions develop across southern NH around 00z Thursday, with light snow developing Thu night most areas, especially coast and southern NH, with IFR conditions possible through early Thu morning. Long Term... Upper level low will keep a cloud deck in the region through the weekend, but ceilings should remain largely VFR. Sunday showers will develop likely dropping to MVFR through the mountains. && .MARINE... Short Term /through Thursday/...Gales have been issued for Penobscot Bay and the ocean waters for Wed and Thu. SCA for Casco Bay. Long Term...Winds and seas will subside on Friday in the wake of the low passing out to the Maritimes. On Saturday night an upper level vort max will help to push a backdoor front through the region. Have increased the winds a bit over the waters as the core of the upper level low moves over early Sunday when brief wind gusts to Gale force are possible. && .TIDES/COASTAL FLOODING... The highest tides of the month will combine with an increasing storm surge and building waves Wednesday night to allow for beach erosion, splash-over and possibly minor coastal flooding. Total water level tables have been adjusted for this tide due to the increasing northeasterly winds. Models are suggesting the possibility of a large and intense storm to develop well out to sea next week. The forecast suggests this storm may form a small loop and retrograde Monday night into Tuesday. This may produce large, long period swells next week which may lead to some beach erosion. Note, astronomical tides are relatively low next week. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 7 AM Wednesday to 4 PM EDT Thursday for ANZ153. Gale Warning from 11 AM Wednesday to 4 PM EDT Thursday for ANZ150-152-154. Gale Warning from 4 PM Wednesday to 4 PM EDT Thursday for ANZ151. && $$ JC is the U.S. government's official web portal to all federal, state and local government web resources and services.