Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 250030 AFDGYX Area Forecast Discussion National Weather Service Gray ME 830 PM EDT Mon Apr 24 2017 .SYNOPSIS... Today was the last warm, sunny day for a while. An easterly onshore flow begins tonight and will transport cool, moist air in off the Gulf of Maine. By Tuesday drizzle and rain will begin to overspread the region with rain becoming heavier Tuesday night into Wednesday as low pressure moves north along the East Coast. The cool, moist maritime air may linger into Thursday east of the mountains, but warm air will begin to move into western New England and eventually even make it to the Maine coast this weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 800 PM Update... Quick update to adjust temp/td/sky grids. High clouds will be on the increase from the south and west through the remainder of the evening and overnight as a storm system to the south slowly lifts north. Area of stratus sitting offshore is also slowly moving northward and will impact mid coast areas with low clouds and possibly some fog by midnight or shortly after. May see a few stray showers by morning in southern zones but the main event will come tomorrow afternoon and overnight Tuesday. No other changes planned attm. Previous discussion... High pressure lifts into Nova Scotia tonight while low pressure drifts north along the Outer Banks of North Carolina. In between the two, an easterly onshore flow will develop over New England. This will begin transporting cool, moist air into the area overnight tonight. Clouds will be on the increase and by morning there will be a chance of rain and drizzle in southern areas. Out in the Atlantic right now there are a few patches of low clouds lurking well offshore, and these may get pulled westward into New England by morning. Temperatures tonight will fall only into the 40s across the southern half of the area where the easterly flow will have the best moisture associated with it. Further to the north across interior and northern Maine, temperatures will fall below freezing as dry air remains in place there and the sky will stay clear for a longer portion of the night. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Increasingly moist onshore easterly flow will promote thickening clouds and a threat of drizzle or light rain from south to north on Tuesday. Daytime temperatures will remain stuck in the 40s, but on the northwest side of the mountains a downslope component to the wind will allow things to be a bit warmer, in the mid 50s. Slow moving cut off low pressure system begins arriving in New England Tuesday night, bringing better synoptic scale support for rainfall Tuesday night through the day Wednesday. With a good moisture feed off the Atlantic, expect a more significant rainfall to begin especially on the coastal plain east of the mountains. Total rainfall amounts could exceed 2 inches along the coast where some enhancement along a coastal front could occur. The temperature will gradually warm out of the 40s and into the 50s on Wednesday as moist advection forces the temperature upward. Rain may come to an end during the day Wednesday to the northwest of the mountains where temperatures may pop up into the 60s. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... High Impact Weather Potential: Residual minor flooding /both river and coastal/ to start the period. There is some threat in the Friday-Saturday period for thunderstorms...although severe weather appears unlikely at this time. Pattern: A look at the pattern across the Northern Hemisphere early this afternoon reveals a potent central/eastern Pacific jet largely forced by an impressive cutoff low over the Aleutian islands /+EPO/. Weak troughing exists along the west coast of North America...which would imply downstream ridging to the east...but this is only part of the story as a cutoff low meanders south of this over the southeastern Untied States. The primary feature controlling our sensible weather will initially be this cutoff...but as this feature is eventually pushed east...we/ll see more impacts from developing southwesterly flow aloft ahead of western trough with embedded impulses providing sensible weather impacts. Thus...after the initial impacts of the cutoff through Thursday...the Friday-Monday period should be dominated by southwesterly flow aloft /suggesting temps at or above seasonal norms...teleconnecting well with the +EPO/ although the position of the trough near the Rockies this weekend may allow some continental polar air to sneak in from the north between shortwaves. Model trends/confidence: Rather good agreement in the pattern exists in the deterministic/ensemble guidance for Wednesday night through the start of the weekend...with the largest differences showing up to end the forecast period Sunday-Monday particularly in the details of amplifying trough over central North America...with downstream impacts on our region. Recent run to run changes have also been rather small through the end of this week...although longer term trends favor a slower exit to departing cutoff low. Overall confidence is therefore above average for this forecast package. The Details.... Wednesday Night - Thursday Night: Cutoff low actually makes it/s closest approach during this period with weak surface low expected to be just off the southern New England coast 12Z Thursday. The surface feature becomes absorbed in the lowering pressure field /due to approaching shortwave over the Ohio Valley/ through the day Thursday with mid level low slowly pushing east towards southern Nova Scotia Thursday night. Ample moisture will remain and with cyclonic flow gradually falling apart in the low levels...expect numerous showers Wednesday night to gradually lessen in coverage before ending by Thursday evening. Rainfall totals will not be substantial at this range with little support for anything other than broad ascent on the synoptic scale...with the likely presence of a coastal front / cold air damming signature across coastal Maine potentially providing assistance on the mesoscale. With weakening precipitation forcing...expect conditions a good bit warmer with some sunny breaks likely north of the mountains and down the Connecticut valley. Temperatures aloft will be rather warm and so there is certainly bust potential on temperatures depending on where any sun is realized. Expect most locations to reach the 60s...but in the aforementioned sunnier spots...lower and perhaps even middle 70s is possible. With the warm airmass aloft and ample cloudiness... overnight lows both nights look to remain well above /+10F/ seasonal norms. Friday: While our cutoff low will finally move east on Friday...our weather remains potentially unsettled as low pressure moving north from James Bay pushes a cold front into the region. T8s ahead of this front will be +11 - +13C which indicates the potential for some summer-like temperatures. Upstream frontal boundary will be in a weakening phase as it arrives late in the day or on Friday night...with rather lackluster forcing despite a narrow PWAT plume around 1.25 inches /about +2 sigma/. Will carry a chance of showers and possibly thunderstorms...with the greatest threat for rain being over the mountains and points north...closer to the best mid and upper level forcing. Would not be surprised if locations over southern NH make a run at 80 if any clouds/storm activity hold off until late in the day. Wind fields are fairly robust aloft...so a strong storm isn/t out of the question but the lack of forcing precludes much concern about a significant severe threat. Saturday - Sunday: Given the deep layer west southwest flow across the region...the aforementioned cold frontal boundary will struggle to cross the region this weekend...and will eventually pull back to the north by early next week as low pressure takes shape in the Mid Mississippi valley. This leaves the front somewhere across New England with modest agreement on a fast-moving impulse moving through the region Saturday. Given timing uncertainty and the frontal proximity...can/t rule out a shower either day...with thunder possible south. As for temperatures...there/s a better signal for warmth Saturday with the front sinking south of the region on Sunday likely keeping us closer to climatology. Certainly no reason to expect a washout...but feel that at least some precipitation is likely for much of the area during the 2 day period. Monday: Despite it being a week away...there is overall agreement on a large Great Lakes low pressure system to end this forecast period on Monday with a very warm signal over our region as warm front pushes north of the area. At this range...plenty of things can change...but starting this period with above normal temperatures and a chance for showers/thunderstorms with the nearby warm front in order. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... Short Term...VFR conditions this evening will begin to transition to MVFR and then IFR conditions from south to north during the day Tuesday as low level moisture spills in off the Gulf of Maine. There could be an area of lower clouds arriving along the immediate coast as early as tonight which would bring a threat for LIFR conditions. But the broader forecast is for worsening conditions on Tuesday with just about all areas IFR or worse by Tuesday evening in drizzle and light rain which will likely last through Wednesday. To the northwest of the mountains up in Whitefield, conditions may stay VFR to MVFR until the better forcing for rain arrives Tuesday night. Long Term...Lingering MVFR/IFR ceilings likely through Thursday with nearby low pressure system. Improvement to VFR likely for Thursday night into Friday...with occasional restrictions in SHRA and possible TSRA possible Friday afternoon. Frontal boundary remains across the region through the weekend with VFR likely dominating...but scattered MVFR/IFR restrictions in SHRA possible. && .MARINE... Short Term...Expect an increasing easterly flow tonight into Tuesday, with winds as high as 25 KT beginning Tuesday afternoon. Winds shift to the southeast on Wednesday and decrease in intensity Wednesday night. The onshore fetch will allow wave heights to built up to 5 to 9 FT through Wednesday night. Long Term...While winds will subside...lingering wave activity will necessitate SCAs through at least Friday...with quiet conditions for the coming weekend. && .FIRE WEATHER... Moist onshore flow will bring high humidity and a wetting rain to all areas Tuesday through Wednesday. && .HYDROLOGY... Continued snow melt in the mountains will keep river levels high. Rain expected Tuesday night and Wednesday will add run off to area rivers already at high levels. While widespread flooding is not expected, it is not out of the question that a few rivers could reach minor flood stage. && .TIDES/COASTAL FLOODING... High astronomical tides begin Tuesday night and last through the weekend. The combination of high tides and an onshore flow with the coastal low will bring the chance of beach erosion, splash over, and minor coastal flooding during the time of high tide Tuesday night and Wednesday night. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 7 PM Tuesday to 6 PM EDT Wednesday for ANZ151-153. Small Craft Advisory from 2 PM Tuesday to 6 PM EDT Wednesday for ANZ150-152-154. && $$

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