Area Forecast Discussion
Issued by NWS Portland, ME

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016 FXUS61 KGYX 182217 CCA AFDGYX Area Forecast Discussion...CORRECTED National Weather Service Gray ME 617 PM EDT Thu Apr 18 2019 .SYNOPSIS... A warm front will lift north through the region this evening. While this front stalls in southern Canada, a moist southerly flow will persist over New England through the weekend. Several rounds of showers will be possible as low pressure tracks northward to the west of New England. A cold front on Sunday will finally bring the wet weather to an end. Another cold front drops in from the northwest Monday night and Tuesday. High pressure will briefly build over the region Wednesday. A weak weather disturbance will cross the region Thursday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 615 PM Update...Minor adjustments to temps and PoPs at this time based on latest trends in observational data as well as latest mesoscale model output. Drizzle is setting in across the coastal plain and should continue overnight along with the development of fog. Otherwise, shower activity should move in from the west shortly, before moving off to the northeast after midnight. Previously... High Impact Weather Potential: Areas of fog possible overnight. Pattern: Longwave trough is marching east across the central portion of the continent with an impressive moisture plume lifting from the Gulf north and east across the eastern half of the United States. The leading edge of increasing moisture has manifested itself as a band of showers /along an elevated warm front/ that has moved across the region today. This is the very beginning of the transition to a warm and unsettled period of weather. Forecast concerns for tonight include continued shower chances in the broad warm/moist advection well as the potential for drizzle and fog. Through this evening: Earlier showers/sprinkles are pulling east of the region with surface warm front still well south of the region. As this lifts north...expect weak shower activity over southern New England will continue to move north and east...with southeastern areas likely to see renewed sprinkle/shower activity through 8pm. Temperatures will not move much through this evening...with most locations in the 40s at 8pm. Tonight: Deep southwest flow continues through the night with surface warm front lifting north of the region and allowing both temperatures and dewpoints to climb. As the aformentioned shower activity south of our region continues to lift north and east... moist isentropic ascent will likely allow it to continue to expand...with increasing coverage/intensity to these showers as they reach our eastern zones. Otherwise...with impressive moisture advection resulting in a profile with moisture increasing with height suggests fog and drizzle development and have included that potential in the forecast overnight. Behind the initial batch of showers along the warm front...could see a few other isolated showers...but no significant precipitation is expected overnight. With the strong warm advection...expect temperatures to slowly climb overnight...reaching near 50 in the mountains...with temps climbing through the 50s to near 60 in the south. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... High Impact Weather Potential: * Locally heavy rain possible near the international border Friday night. * Areas of fog possible again Friday night. Pattern: Upstream longwave trough cuts off to our west and slows as it becomes removed from the stronger westerlies over Canada. At the area of low pressure over the Ohio Valley 12 UTC Friday morning will occlude and fill as it moves slowly north and east to a position over western Pennsylvania by 12 UTC Saturday. To the north of this low...connected by a slow moving cold front...will be another surface wave north of New England. This feature will push east with the aforementioned front sinking towards the international border during the day Friday and into our northern zones Friday night. This feature will be the first focus for potentially heavier rainfall activity and will be the focus of this forecast period. A secondary focus will be how much warmth we achieve on Friday in the warm sector as well as the potential for a renewed round of fog Friday night. Friday: With the warm sector overhead...Friday has the potential to be rather warm as T9s warm into the lower teens south of the now stalled frontal boundary to our north that will begin to sink back south behind a departing wave of low pressure. There is little in the way of forcing for ascent through the day...with the mid levels remaining rather dry. Thus...will continue to focus PoPs on our far northern zones...closest to the southward sinking cold front. Even here...the best chances for significant precipitation will likely hold off until Friday night. South of the mountains...would expect that we/ll slowly erode the morning stratus and may in fact be able to see a few sunny breaks. With the aforementioned warm temperatures aloft...expect that southern areas will make a run at /or even above/ 70...with 75 in reach given enough sun. Closer to the mountains...low to middle 60s are expected given more cloud cover. Friday Night: As a surface low occludes over the Ohio Valley/eastern Great Lakes...associated triple point will push east with cold/stationary front east of this feature draped over our northern zones. Therefore...with forcing for ascent increasing from both the north and west...expect shower activity to increase in coverage and intensity overnight. PWATs increase to around 1.5"/+3-4 sigma with warm cloud depths making a run at 10kft. This...and model sounding suggesting some potential for a bit of elevated instability suggest some heavier showers and even an embedded thunderstorm are possible. an area-wide sense...QPF does not look tremendously heavy overnight... especially over coastal areas which may not see much more than an isolated showers and drizzle overnight. A very mild night is in store...which will cause significant melting of remaining snowpack over the foothills and mountains. While areas along and north of the stalled front will be able to sneak back down into the the south of this 50s...and even lower 60s are expected with dewpoints in the 50s. Sounding profiles are less suggestive of fog...but would not be surprised to see some...especially in areas of remaining snowpack. && .LONG TERM /SATURDAY THROUGH THURSDAY/... A warm front will become relatively stationary over northern portions of the forecast area Saturday. This will allow for warm, moist air and increasing precipitable water values to ride quickly poleward on an increasing southerly flow. Rain will be likely during the day with warm temperatures and snowmelt continuing in the mountains. H8 temperatures reach +12C to +14C fueling that warm air. This will allow for very warm surface temperatures across portions of the interior. Dew points will continue to be on the rise as well during this period adding to the snowmelt. The resulting runoff will be compounded with over an inch of rainfall over much of the region. There is the potential for higher rainfall totals in north-central areas such as the Presidentials in New Hampshire and the Foothills in Maine. Periods of rain and patchy fog will continue through Sunday as the front becomes washed out during the period. There may be a break in the precipitation over far western areas Sunday as best dynamics and moisture shift towards eastern Maine. 12Z guidance in relatively good agreement bringing a focus for precipitation back to the area Sunday night into Monday. Showers will lift northward across our region in association with the upper level low pressure system. Models are then in disagreement as to whether a cold front will cross the region late Tuesday and Tuesday night, potentially allowing dryer air to enter the region from the north. The European models is somewhat more pessimistic, triggering weak cyclogenesis as additional upper level support enters the region from the west. In fact, the Euro is marginally cold enough to bring snow to some areas on Wednesday and Wednesday night. && .AVIATION /22Z THURSDAY THROUGH TUESDAY/... Short Term... Summary: A warm front will lift north of the region through tonight with deteriorating flying conditions. Low pressure will pass north of the region overnight with a cold front then settling into northern areas late Friday and Friday night. Restrictions: Expect conditions to deteriorate this evening to MVFR/IFR due to CIGS and potentially VSBYS throughout the terminals. On Saturday...VSBYS will improve to VFR...but CIGS will be slow to respond and may remain MVFR through much of the day. Deterioration is likely again Friday night to IFR/LIFR in fog/drizzle and an increasing threat for rain...particularly at LEB-HIE-AUG. Winds: Southeasterly winds 8-15kts will continue into this evening before slowly shifting southerly overnight around 10kts with south or southwesterly winds 12g20kts expected on Friday...continuing through the night Friday night. LLWS: Tonight...southwesterly winds at 1.5-2kft increase to over 40kts with LLWS likely throughout the terminals. It is likely that LLWS will again occur under similar conditions Friday night. Long Term...Sat right through early Tuesday...MVFR with areas of IFR in shra, and fog. An isolated thunderstorm is possible. Southerly wind gusting up to 25 kt. Conditions gradually improve during the midweek period. Mon...Sct MVFR psb in -shra. && .MARINE... Short Term... Southerly winds will strengthen ahead of an approaching low pressure system with SCA conditions all waters and a few gusts to gale force over the outer waters tonight. SCAs will likely continue for all waters through the end of the short term period Friday night with occasional gusts to around 35kts in the outer waters. Long Term...SCAs likely over the weekend with gale force gusts possible as a southerly gradient continues over the region. && .HYDROLOGY... High river flows already in place will set the stage for another possible flooding scenario this weekend. Warm, moist air will move north into the region with several rounds of rain possible. Through the weekend it is expected that much of the remaining snow cover in northern New Hampshire and western Maine will melt and run off into the rivers. Although the locations and amounts of the heavier rainfall are still quite uncertain, there is the potential for significant flooding to occur mainly in the headwaters. This would most likely occur on Saturday into Sunday, with any flooding possibly lasting into early next week. The flood watch will remain unchanged this afternoon. Minor flooding appears likely at many locations...with moderate flooding possible...particularly in the Connecticut... Pemigewasset...and Saco basins. && .TIDES/COASTAL FLOODING... High astronomical tides this weekend will combine with a prolonged onshore southerly flow to bring a threat for beach erosion, splashover, and coastal flooding. Tides during the overnight hours on Saturday and Sunday reach 11 feet at Portland and a modest increase in storm surge values may bring water levels to near or above flood stage over the weekend. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Flood Watch from Saturday morning through Sunday afternoon for MEZ007>009-012>014-019>022-024>028. NH...Flood Watch from Saturday morning through Sunday afternoon for NHZ001>009-011. MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ150>154. && $$ NEAR TERM...Arnott/Ekster LONG TERM...Cannon is the U.S. government's official web portal to all federal, state and local government web resources and services.