Area Forecast Discussion
Issued by NWS Portland, ME

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
236 FXUS61 KGYX 191911 AFDGYX Area Forecast Discussion National Weather Service Gray ME 311 PM EDT Mon Mar 19 2018 .SYNOPSIS... High pressure will continue to settle over northern New England tonight and Tuesday bringing continued colder than normal temperatures. A storm system will pass to our south Wednesday and Thursday likely bringing accumulating snow to much of the forecast area. High pressure then gradually builds in through the weekend. && .NEAR TERM /THROUGH TONIGHT/... No significant weather to speak of tonight as cold high pressure gradually builds in across the forecast area. Clear to mostly clear skies are expected overnight with diminishing winds. This should mean temperatures falling to below guidance levels at the good radiators and took that approach with mins. && .SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/... Mostly sunny skies Tuesday with high pressure ridging in place. Colder than normal temperatures will continue but with lighter winds than Monday. Increasing high clouds likely later Tuesday night as low pressure gathers well to our south. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... High Impact Weather Potential: Potential for plowable snowfall Wednesday afternoon and Wednesday Night. Potential for coastal flooding particularly south of Portland late Wednesday through Thursday. Pattern: Early afternoon water vapor imagery reveals blocked flow downstream of North America over Greenland /-NAO/ with longwave trough axis east of our longitude...allowing a significant amount of arctic air to pour down into the northeastern United States as a result. Over the eastern Pacific...there is a cutoff low spinning off the western North America coast. Between these features...there are a series of weaker disturbances moving across the country in the west northwest flow aloft. There are indications that the downstream block will give way this week...allowing the upstream flow across North America to become increasingly progressive. As the trough now east of us begins to push will re- load as energy over the Midwest carves arrives. The evolution of this energy is the primary focus of this forecast package. As we reach the weekend...ridging over central North America will begin to arrive from the west. The Details: Wednesday - Thursday: Complex evolution to the mid and upper level pattern portends reduced confidence through what will likely be the most impactful portion of the long term forecast. Energy now moving through the Mid Mississippi Valley will be joined by rapidly moving shortwave energy now coming ashore along the coast of California. This will help force a digging trough over the southeastern United States Tuesday night into early Wednesday. At the same time... another mid level wave representing the remnants of the departing eastern trough will cutoff north of New England. The relative magnitude and interplay of these two features will largely determine the sensible weather impacts in our region. A fairly strong consensus has developed over the past 12-24 hours which favors an initial wave of low pressure passing south of the region on Wednesday with a stronger low taking shape off the Mid Atlantic coast...and slowly tracking northeast late Wednesday and Wednesday night...with a consensus track just south and east of the benchmark. There is also broad agreement on the H7 low track moving south and east of Cape Cod late Wednesday night. Pattern recognition with this setup would favor accumulating snow for about the southeastern half of the forecast area with a plowable snowfall possible...especially the closer one moves to the coast. Wednesday evening through Thursday morning would currently be the most favored time for travel impacts with any snow tapering to snow showers on Thursday. EC-EPS has a strong signal for 0.25" liquid...with a modest signal for 0.5"...with both of these signals highest along the coast. advisory event from the mountains south and east...with some potential for warning level snows along the coast would be a most likely solution at this time. Winds are also a potential concern for the coastal plain given impressive pressure gradient between ~1030mb high over the Great Lakes and ~985 low southeast of New England. Given what becomes a persistent northeasterly wind...coastal flooding will become a potential hazard. See that section of the AFD below. Friday: Differences in the evolution of the Wednesday-Thursday storm persist into Friday. While there is agreement that Canadian high pressure will begin to build in from the north and west...the orientation of the low level flow and amount of residual moisture are in question. Expect that we/ll at least see residual snow shower activity in the mountains...with some potential for snow showers to the south and east given the position of the mid level trough overhead. Temperatures aloft look to be close to...or just below seasonal norms...and thus...expect temperatures just below normal for this time of year. Saturday: Mid level trough pushes east of the region with backing flow overhead as closed low south of James Bay interacts with energy moving across the mid and upper Mississippi Valley. Residual snow showers are again possible...although with the loss of cyclonic flow aloft and some decrease in deep moisture...expect that they will be less widespread than on Friday. Despite the backing flow aloft...temperatures at H8 don/t move...remaining at about -1 sigma...with highs in the lower 30s north of the mountains to just above 40 in the foothills and along the coast being about all we/ll be able to muster...about 5F below seasonal norms. Sunday-Monday: Run to run changes in the guidance have been pretty substantial at this range...with previous model simulations advertising another fast moving wave in the increasingly progressive flow. However...the multi-day trend is favoring a stronger area of high pressure building north of New England...favoring a drier and cooler scenario. Best threat of precipitation /likely in the form of snow if it falls/ would be on Sunday with temperatures continuing to be below seasonal norms with the polar front most likely south of the region. && .AVIATION /19Z MONDAY THROUGH SATURDAY/... Short Term /through Tuesday Night/...VFR conditions are expected tonight through Tuesday night with high pressure in place. Wind gusts diminish this evening. Long Term...Increasing potential for VLIFR/LIFR particularly MHT-PSM-PWM-RKD beginning late Wednesday and continuing through Thursday morning. VFR by mid morning Thursday and continuing through Friday. && .MARINE... Short Term /through Tuesday Night/...SCA conditions continue on the ocean waters as well as outer Penobscot Bay through this evening before diminishing. Small craft conditions will likely redevelop late Tuesday night as a storm system develops to our south. Long Term...Given the northward trend in surface low position in the Wednesday-Thursday timeframe, gale force winds are a strong possibility for Wednesday into Thursday...and have issued a gale watch with this package for this likelihood. Given the forecast low level jet strength...storm force winds are possible in the outer waters and will continue to watch this potential. Winds and waves will subside beyond this through the end of the week. && .TIDES/COASTAL FLOODING... Northward trend in coastal cyclone Wednesday-Thursday portends another round of potential coastal impacts. Surge guidance suggests that water levels will increase Wednesday evening and overnight Wednesday night...with 2` total tide departures possible. Given the forecast tides this week /see below/ this will likely bring us near flood stage for at least the early Thursday morning high tide. Compared to recent events...a somewhat weaker low pressure system well offshore suggests 10-15` waves with an increasingly long period. Certainly will be watching over the next 24 hours and considering potential headlines. Tides - Portland Wed 2:10 AM - 10.2` Wed 2:38 PM - 9.6` Thu 2:55AM - 10.2` Thu 3:28PM - 9.3` Fort Point Wed 2:20 AM - 9.7` Wed 2:49 PM - 9.1` Thu 3:05AM - 9.7` Thu 3:40PM - 8.9` && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Gale Watch from Wednesday morning through Thursday afternoon for ANZ150>154. Small Craft Advisory until midnight EDT tonight for ANZ150>152- 154. && $$ NEAR TERM...Ekster SHORT TERM...Ekster LONG TERM...Arnott is the U.S. government's official web portal to all federal, state and local government web resources and services.