Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 281849 AFDGYX Area Forecast Discussion National Weather Service Gray ME 249 PM EDT Fri Apr 28 2017 .SYNOPSIS... A cold front will move offshore by early evening. Another cold front will push across New England on Saturday before stalling to our south. High pressure will briefly build in from the northwest late Saturday then shift offshore Sunday as the stalled frontal boundary lifts north as a warm front. The warm front will stall over the region Sunday night and Monday. Low pressure lifting northeast out of the Great Lakes will drive an occluded front toward the region Monday night and will cross the region on Tuesday. Low pressure will slowly move east through southern Canada Tuesday night through Wednesday. && .NEAR TERM /THROUGH TONIGHT/... At 18z...a surface cold front extended from Quebec Province southward through western New Hampshire. Along and ahead of this boundary...NWS Doppler Radar mosaic showed widely scattered convection developing. Expect this activity to exit the coast by early evening along with the boundary and associated weak shortwave impulse. We`ll see a brief lull early tonight with patchy stratus and fog developing before a shortwave impulse over the upper Great Lakes races eastward along the U.S. - Canadian border. This impulse and associated cold front will approach northern New England towards dawn accompanied by increasing clouds and scattered showers. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... The shortwave impulse and associated cold front will race east and exit the region by afternoon. Clouds and scattered showers should give way to clearing skies by afternoon as the system exits the region and drier air pours in from the northwest. Highs will range from 60s along the international border to the 70s elsewhere. A few 80 degree readings will be possible over far southeast New Hampshire. The cold front will stall to our south by Saturday evening as surface high pressure builds by to our north across eastern Canada. The boundary will try to inch back north after midnight as a warm front marked by increasing middle and high clouds across southern and western sections of the forecast area. && .LONG TERM /SUNDAY THROUGH THURSDAY/... High Impact Weather Potential: The pattern supports heavy rain by the end of next week...but still way to far out for any confidence that this will occur. No other significant impacts expected. Pattern: A somewhat messy five wave pattern exists across the northern hemisphere early this afternoon with a broad longwave trough centered across the western half of North America...where it has been stationed throughout this week. It is the season of cutoff lows...with two meandering over the western Atlantic with another one upstream over western Canada. Through the long term forecast period...there is good deterministic/ensemble agreement in a realignment of this pattern...with the eastward progression of the western North American trough to the eastern half of the continent with large scale ridging building in from the west. As we reach the end of the forecast period...a -AO/-NAO signal becomes rather robust in the GEFS/EC-EPS...which would anchor this trough over the eastern states. Pattern Implications: Precipitation likely at or above normal with some threat for heavy amounts late in the week depending on the details of nearing longwave trough. Temperatures below normal to start the period under the influence of backdoor cold front...returning to near or above normal under deep southwesterly flow through midweek before trending back to or below normal as longwave trough arrives to end the week. Model trends/confidence: Deterministic/ensemble guidance agreement is rather strong through about Wednesday of next week. Beyond this...the ejection of the last remaining western trough becomes much less certain in the guidance with substantial model to model and run to run disagreements. Thus...confidence falls below average for Thursday and Friday. The Details.... Sunday: High pressure noses in from the north with high PWAT plume shunted south of the region with temps aloft seasonal. Active seabreeze under light gradient with a mix of sun and clouds with warm front nearby to the south. Highs near or just below seasonal norms. Sunday Night - Monday: Deep southerly flow between Bermuda high and eastward moving longwave trough transports deep moisture plume north with open Gulf of Mexico. Focus for precipitation along northward lifting warm front with PWATS nearing 1.25"/99th percentile. Strongest signal for precipitation is over the northern half of the area given best LLJ support. Greatest threat window is 06Z-18Z Monday. Ensembles support 0.5-1.0" basin-wide averages over the northern half of the forecast area with some locally higher amounts along the southeastern slopes of the mountains likely. Temperatures likely steady or warming overnight Sunday night in the south. Location of the front portends large temperature differences southwest /near 70/ to northeast /near 50/ on Monday with confidence lower than average on high temps. Monday Night-Early Tuesday: Cold front crosses the forecast area with narrow PWAT plume reaching about an inch and a third /over the 99th percentile vs climo/ but front will be fast moving with forcing in question. Speed of the front should limit precipitation amounts to less than one half inch basin-wide...although given deep southerly flow...the southern mountain slopes would be most likely to see heavier amounts. Temperatures relatively mild in the deep southerly flow. Tuesday Afternoon-Wednesday: Gradually drying cyclonic flow behind departing low pressure system. PWATS as well as T8s +1-2 sigma on Tuesday decrease to seasonal norms on Wednesday. Expect scattered to numerous rain showers in the mountains Tuesday with partly/mostly cloudy skies to the south and east...with shower coverage decreasing through Wednesday and a similar decrease in cloud cover. GFS significant colder than GEFS mean and the EC/EC-EPS and will lean toward the ECMWF solution which suggests less of a stark temperature difference between the days with temperatures above normal Tuesday given temps aloft...and closer to normal on Wednesday with some downsloping assistance. Robust gradient suggests breezy conditions both days. Thursday-Friday: Longwave trough sharpens/deepens as it pushes east for the end of the forecast period. Orientation of trough suggests an increase in available moisture along the east coast...with broad agreement that low pressure will form downstream of this feature...but very poor agreement on the location and evolution of this low. There is general agreement on increasing precipitation potential for Thursday Night/Friday vs. Thursday with deep southerly flow suggesting the potential of a heavy rainfall threat...but by no means is this yet likely or expected. Will also refrain from moving temperatures too far from climatological norms with temperatures aloft near normal and a trend for increasing clouds/precipitation. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... Short Term /through Saturday night/...Lingering MVFR/IFR in stratus and fog VCNTY of KRKD otw mainly VFR through tonight. SCT MVFR btw 12 and 16z Sat in -shra with a cold frontal passage. NW sfc wind gusting to 25 kt at times behind the cold front Sat afternoon. VFR Sat night with lcl IFR psb in valley stratus and fog. Long Term... VFR Sunday with MVFR/IFR restrictions possible Sunday night/Monday especially north as warm front lifts through the region with SHRA and stratus/fog. Cold front bring wind shift and continued restrictions in SHRA on Monday night. Improvement on Tuesday with residual SHRA bringing potential MVFR restrictions away from the coast. VFR likely Wednesday. && .MARINE... Short Term /through Saturday night/...SCA for seas remains in effect through early evening for swells of 4-6 ft outside the bays. Building winds and seas on Saturday afternoon with a cold frontal passage. We could see brief marginal SCA winds and seas outside the bays late Saturday into Saturday night. Long Term... SCAs likely Tuesday-Wednesday in the southwesterly flow ahead of...and westerly flow behind a potent cold front. There is some potential for an east coast low pressure system by the end of next week...but confidence in occurrence is low at this time. && .FIRE WEATHER... Good relative humidity recovery is expected tonight. A cold front will move across the region Saturday morning with scattered showers. Otherwise warm dry weather is expected both Saturday and Sunday punctuated by good relative humidity recovery at night. && .TIDES/COASTAL FLOODING... We`ll see an 11.7 ft mllw tide at Portland once again tonight (1:32 AM Saturday). However, swells are expected to be lower tonight and winds offshore. Therefore...we are not expecting any flooding at this time and will forgo any sort of coastal flood advisory. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory for hazardous seas until 8 PM EDT this evening for ANZ150-152-154. && $$ Schwibs/Arnott

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