Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 210441 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1241 AM EDT SAT MAY 21 2016 .SYNOPSIS...
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High pressure will slide offshore overnight and early Saturday. A cold front will drop south from Canada and into northern New England Saturday night while a coastal low pressure system develops south of the New England coast. This system will drift north over the area Sunday and into early next week likely bringing rain and cooler weather to the area late Saturday night through early next week.
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1241 AM...minor ESTF update to ingest the current mesonet values into near term grids. PREV DISC... 10 pm...Just made some adjustments to T and Td based on current obs...but no real changes to forecast. 655 pm...Made a few minor tweaks to most of the grids based on current obs, but not much change overall, as we look for a clear and seasonably cool night, with lows 40-45 in the north, and maybe a few readings in the upper 30s, ranging to 45-50 in the south. Previously...High pressure will dominate once again tonight as it crests over the area. Skies will be clear and winds near calm. Overnight lows will be in the 40s over most areas except some lower 50s over southeast NH.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 AM SUNDAY/... 655 pm...Have made some adjustments to Sat and Sat night, mainly to scale back pops. GFS is really only model to bring precip into the CWA before 06Z Sunday. The other models, including NAM12, ARW, NMM, GEM, SREF and ECMWF all hold back precip until early Sunday, except maybe for a few isolated showers in the mountains late Sat into Sat evening. Previously...High pressure moves off the coast Saturday as a weak cold front drifts south from Canada. With the cold front dropping south across the region in the afternoon some showers may develop over the mountains and foothills. Meanwhile to the south of New England a developing coastal low off the mid atlantic coast will slowly drift north Sat night spreading rain to develop from south to north. Models differ on timing of onset of rain Sat night so used an equal weight on a series of models. Temperatures will be mild once again Saturday and with clouds and developing rain Sat night overnight temps will remain mild with lows in the upper 40s to lower 50s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Confidence increasing in a Wet and Cool Period... High Impact Weather Potential: Minimal. Pattern: A look at early afternoon water vapor imagery over North America shows significant troughing along the west coast /-PNA/ with somewhat flatter flow to the east...with a smaller scale upper low centered over the Mid-Mississippi valley and slowly translating east. Through early next week...strengthening jet energy over the Gulf of Alaska will help gradually break down the trough over the west /good ensemble agreement in a shift towards a neutral PNA/. While this does open the door for more Pacific flow over the western states, our weather will first be dictated by the interaction of the aforementioned slow moving upper low now crossing into the eastern states with energy now seen in water vapor imagery crossing into the Dakotas from Canada. These two will combine and form a cutoff off the east coast by the end of the weekend...with this initially helping to develop another omega block over the United States. By the second half of next week...with the western trough eroding...we should see the eastern cutoff gradually be pushed east as the flow slowly becomes more progressive. This overall forecast idea suggests that at least the first half of the long term forecast period will be dominated by unsettled weather /numerous precipitation chances/ and cooler than normal temperatures. This idea is well-agreed upon by extended statistical guidance. Model Trends/Confidence: The most significant trend in the guidance over the past 24 hours has been to push the cutoff low over the eastern states further north more quickly. This trend is evident both in run to run changes in the ECMWF as well as a look at the most recent GFS compared to the GEFS ensemble mean. Unfortunately this spells a cooler and wetter solution for our forecast area...but...given that we are talking about cutoff lows /a notorious challenge for operational guidance/ I wouldn/t be surprised to see some additional changes in the guidance envelope before next week. The Details.... The guidance envelope is gradually collapsing around low pressure developing off the Delmarva Saturday evening...moving near the benchmark by Sunday morning before filling over the Gulf of Maine through Monday. Given this increasing consensus...will continue to trend PoPs up for Sunday...with a widespread wetting rainfall looking likely. With this slow evolution into a developing cutoff aloft...the dynamic forcing is not overly impressive...with most guidance scenarios suggesting only modest low pressure development off the coast. This will limit the overall impacts...but still paints some potential for heavy rains /especially along the coast/ and marine impacts with possible gales. The period of greatest impact looks to be from 12Z Sunday - 00Z Monday. GEFS/SREF members paint a 30-50% likelihood of 1"+ QPF over southern New Hampshire and possibly coastal Maine...with a weakening signal further north. Heading into Sunday night and Monday we should see a transition to more showery precipitation as the nearby surface low fills. Temperatures will be largely dependent on clouds and precipitation... with the potential for coastal locations to be stuck in the 50s /lower 50s along the coast?/ especially on Sunday...with some moderation on Monday /especially north and west/ as the surface low fills. The cutoff low remains pretty much overhead for Tuesday with additional potential for surface low development /albeit weaker/ off the east coast. The spread in guidance solutions increases greatly at this range /particularly with the details of a secondary surface low/...but the idea of a cold pool of air aloft and moist low level cyclonic flow portends at least scattered shower activity with some daytime enhancement. The more robust solutions would suggest /yet/ another wetting rain. Will not move pops into the likely category yet given the guidance disparity in a cutoff low scenario. Do expect temperatures to remain depressed...however...likely remaining in the 60s. While guidance can be too fast to break down blocking scenarios will at least cautiously be more optimistic as we reach Wednesday and Thursday...with the cutoff low over the east starting to weaken and reconnect with the westerlies...with temperatures aloft moving back above seasonal norms /+1-2 sigma/. Would not be surprised if we actually realize a dry spell /!/ for 24-36 hours in this period...and given the temperatures aloft...this would suggest highs that could make a run through the 70s /even 80 south?/ by Wednesday and particularly Thursday. Can/t eliminate PoPs at this time for either nearby upper low on Wednesday and developing warm advection pattern by the end of the week...indicated at least some chance for additional showers...and possibly thunderstorms by week`s end. Chance PoPs and mild temperatures will continue to be the rule as we end the forecast period Friday...with broad agreement on a cold frontal passage during this period. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... Short Term /through Saturday/...VFR tonight and saturday. Conditions will become mvfr and then IFR later Saturday night in developing areas of rain from south to north. Long Term...Low pressure moving off the New England coast will likely bring a period of MVFR/IFR restrictions /worst at coastal locations/ for Sunday with some improvement heading into early next week. However...through next Wednesday there will be an elevated potential for occasional MVFR/IFR restrictions along the coast and MVFR inland. && .MARINE... Short Term /through Saturday/...Winds and seas should remain below Small Craft threshold through Sat night. Long Term...Low pressure tracking from the benchmark into the Gulf of Maine on Sunday will bring the potential for gale force wind gusts...especially outside of the bays. Winds will gradually diminish beyond this...but expect residual small craft conditions /due to both winds and waves/ to persist through the first half of the week. && .FIRE WEATHER... High pressure across the area with dry weather, warmer temperatures, and light winds into Saturday. A coastal low will affect the area with wet weather Saturday night through early next week. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ ES is the U.S. government's official web portal to all federal, state and local government web resources and services.