Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 161907 CCA AFDGYX Area Forecast Discussion...CORRECTED National Weather Service Gray ME 307 PM EDT Wed Aug 16 2017 .SYNOPSIS...
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High pressure will continue to build into New England through Thursday bringing dry conditions and seasonable temperatures. On Friday...a warm front associated with low pressure over the Great Lakes will lift through the region followed by a cold front Saturday. These features will bring the threat of showers throughout the region. High pressure will build back into the region this weekend with warming temperatures.
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&& .NEAR TERM /THROUGH TONIGHT/... High Impact Weather Potential: Dangerous surf conditions along the midcoast. See marine section for details. Current Pattern: Early afternoon water vapor imagery reveals an impressive vortex north of northern New England over Quebec... gradually moving south and east with time. Brisk westerly flow aloft is the result over much of New England...with yesterday/s cold front well south and east of the region and satellite-based total precipitable water products showing a significant pool of drier air gradually sinking south into northern New England. Drilling down to the surface...high pressure is located over the eastern Great Lakes...making gradual progress to the east. Thus...a dry period is foreseen through the near term forecast period as high pressure gradually arrives from the west. Our only forecast concerns center around valley fog development...and the coastal impacts of waves originating from Hurricane Gert...now well south and east of New England...but making it/s closest approach before racing north and east into the northern Atlantic. Through this evening...Dewpoints continue to fall under robust dry advection with cloud cover thinning as a result. Expect the clearing trend to continue with the dry airmass promoting a pretty rapid temperature fall this evening with temperatures likely at or below 60F in the mountains by 8pm...with 60s elsewhere outside of the MHT/ASH corridor where temps will remain at or a bit above 70. Tonight...PWATs fall to around one half inch...representing a -1-2 sigma departure from climatology. This...and building heights will ensure a clear and quiet night. A modest surface pressure gradient will remain in place behind surface low associated with mid level low over Quebec and arriving high pressure. 1000 mb geostrophic winds fall to 15-20kts...suggesting that only the deeper valleys will fully decouple. This will likely limit low temperatures in some spots...and also reduce the amount of fog development. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... High Impact Weather Potential: Dangerous surf conditions along the midcoast. See marine section for details. Pattern Summary: Flow aloft increasingly becomes northwesterly through Thursday as mid level low over Quebec settles south and east into the Canadian maritimes. Surface high crests over the region Thursday before shortwave ridge arrives Thursday night...with warm advection pattern developing overnight. Thus...very quiet weather will continue through the short term forecast period. First potential of precipitation associated with developing warm advection regime will be at our doorstep as this forecast period ends. Thursday: Surface high pressure crests overhead with dry wedge remaining centered along the eastern edge of the forecast area. Surface pressure gradient looks at least as impressive as today through the morning...with wind gusts 20-25 mph again likely before diminishing some as the high arrives in the afternoon. Ample sun is expected throughout the forecast area. Mixing to T8s around +10C suggests mid 70s as the average high for much of the area...with southern NH/southwestern ME likely to reach right around 80...and the mountains likely closer to 70. Thursday Night: Surface ridge axis begins to pull east of the region with shortwave ridge axis arriving around daybreak Friday. Top down saturation will commence with westerly flow at H2 likely bringing in some cirrus clouds in the evening before warm advection pattern and continued advection brings thickening/ lowering high and then mid level cloudiness especially after midnight. Mesoscale guidance /NAM...RGEM and SREF/ all support the idea of shower activity making a run at southwestern NH towards daybreak Friday...with the global guidance a bit slower on it/s arrival. Will hold off on introducing QPF before 12Z...but there is enough model support for a chance PoP around daybreak. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Models in fairly good agreement for the end of the week and the weekend. Low pressure lifting northeast out of the Great lakes will swing a warm front toward the region on Friday. Expect a mostly cloudy day with occasional showers. Increasing onshore flow will bring a fair amount of ocean stratus into the mix by late in the day as the front remains south of the region. Highs on Friday will top out in the lower to mid 70s. Friday night will feature a fair amount of low clouds...areas of fog and drizzle along with a few stray showers. Lows will range from the mid 50s north to the mid 60s south. Warm front will slowly push east through the region on Saturday. As boundary layer winds switch into the southwest expect low clouds to gradually shift out of New Hampshire and far western Maine during the day. With a slow moving cold front approaching from the west will likely see some afternoon convection in western zones depending how much heating is achieved by early to mid afternoon. Midcoast and interior central portions of Maine likely remain stuck in marine layer and ocean stratus for much of the day. Highs on Saturday will be in the lower to mid 70s in midcoast and interior central sections of Maine. New Hampshire and western Maine will see highs in the upper 70s to mid 80s. Showers and some thunderstorms Saturday evening will gradually end as the front pushes east of the region. Expect variable clouds with areas of valley fog overnight. Lows will range from the mid 50s north to the mid 60s south. High pressure will build in from the west on Sunday delivering cooler and less humid air on a brisk northwest flow. Upper trough swinging in from the west will result in variable cloudiness through the day with a few stray showers in the mountains. Highs will range through the 70s to near 80. Fair weather will continue Sunday night and Monday as high pressure builds south of the region. High pressure will slide offshore south of New England Monday night and Tuesday bringing warmer and more humid air back into the northeast. A cold front approaching from the west will bring the next chance of rain to the region on Wednesday. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... Short Term... Summary: High pressure will build into the region through Thursday...before moving east of the area Thursday night. This will allow for generally good flying conditions through the period. Restrictions: VFR conditions will dominate the TAF period outside of overnight fog potential HIE/LEB tonight...with somewhat lesser potential again on Thursday night. Solid agreement from the guidance and climatology for a period of LIFR/VLIFR fog in the 8-13Z timeframe at HIE/LEB early Thursday. Otherwise...expect mostly clear skies to dominate through Thursday before increasing high and mid level cloudiness arrives Thursday night. This cloudiness...if it arrives fast enough...may help preclude additional fog development Thursday night. Winds: Northwest winds 10g18kts will continue through the afternoon before diminishing to 5kts or less tonight. Northwest winds will again strengthen to 10g18kts /strongest AUG-RKD/ for the day on Thursday before likely going calm/light-variable Thursday night. LLWS: No LLWS expected through Thursday night. Lightning: No thunderstorms expected through Thursday night. Long Term...IFR/LIFR ceilings developing Friday and persisting through Friday night. Conditions gradually improve to VFR in New Hampshire and western Maine on Saturday with areas of MVFR/IFR ceilings in midcoast and central interior sections of Maine. Becoming VFR with areas of Valley fog Saturday night. VFR Sunday and Monday. && .MARINE... Short Term... Forecast generally remains unchanged for tonight through Thursday night as long period swell originating from Hurricane Gert reaches the coast of Maine. 4-6 ft swell with a long period /12-15 seconds/ will bring high surf conditions to south facing aspects primarily along the midcoast through downeast. Will continue high surf advisory as a result. A small craft advisory due to seas will continue over the adjacent outer coastal waters for the same period. Westerly winds will near 25kts for a time Thursday afternoon. Long Term...SCA`s may be needed Friday through Friday night and again on Sunday afternoon. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...High Surf Advisory from 8 PM this evening to 8 PM EDT Thursday for MEZ025>027. NH...None. MARINE...Small Craft Advisory for hazardous seas from 8 PM this evening to 8 PM EDT Thursday for ANZ150-152. && $$ SYNOPSIS...Arnott NEAR TERM...Arnott SHORT TERM...Arnott LONG TERM...Sinsabaugh AVIATION...Arnott/Sinsabaugh MARINE...Arnott/Sinsabaugh

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