Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 260221 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1021 PM EDT Tue Jul 25 2017 .SYNOPSIS... High pressure will continue to build into the region tonight and will bring drier weather and clearing skies. It will shift offshore on Wednesday...allowing some warmer air back into the region. A cold front pushes toward the area on Thursday. High pressure will attempt to build into the region Friday and Saturday as low pressure forms south of New England. && .NEAR TERM /TONIGHT/...
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1015PM UPDATE... Have updated the forecast to adjust hourly temperature and dewpoint forecasts toward latest observed values. Already seeing many automated observing sites with temperatures at their dewpoints, so each additional degree of cooling from here on out will contribute to deposition (dew) and condensation (fog). Still think the most likely fog areas will be the valleys, with temperatures broadly on track to reach forecast lows. No significant changes were needed. 7PM UPDATE... Have updated the forecast based on latest observations and expected conditions overnight. High pressure nestling down into our area tonight will bring calm winds with sky cover increasingly becoming clear. This has already been observed in the late afternoon visible satellite as earlier cloud cover over New Hampshire is rapidly dispersing. This will set the stage for an excellent radiational cooling night, with low temperatures reaching a base of about 50 degrees, with typical cold spots in valleys getting significantly colder, and urban warm spots staying slightly warmer. Expect a good deal of radiation fog to develop overnight, with much of this accumulating in the valley locations, becoming dense in the sharper valleys by daybreak. Given the spotty and localized nature of the dense fog, will not issue a Dense Fog Advisory, however, we do expect many valley locations to have very low visibility by morning. ORIGINAL DISCUSSION... Clouds will continue to dissipate as drier air builds into the region from the northeast. High pressure will continue to allow for clearing during the overnight period with southwestern New Hampshire being the last to clear. With high pressure overhead and light winds, some radiational cooling will take hold. Overnight low temperatures will drop into the 40s in many areas. As temperatures fall towards the dew points, patchy fog will form. Inland valleys will have the lowest visibilities.
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&& .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... Patchy fog will gradually mix out Wednesday morning across Maine and New Hampshire. There will be ample sunshine when compared to today. With modest warming aloft, surface temperatures will climb through the 70s with many communities topping out at 80 degrees away from the shoreline. As usual, the Midcoast region will be the coolest location as a weak gradient will allow for onshore sea breezes to form. A cold front will approach from the west Wednesday night. A southwest gradient will develop, allowing for a breeze to develop late at night. Increasing cloud cover may prevent as much fog compared to what is expected tonight. && .LONG TERM /THURSDAY THROUGH TUESDAY/... The extended forecast will be characterized by large scale troughing over the eastern CONUS which will move southeast from northern Canada Thursday into Friday. The system will then stall over the region due to downstream blocking through at least the weekend. Several spokes of short wave energy rotating around the upper low as well as a significant southern stream short wave will bring clouds and at least one shot of showers and thunderstorms to northern New England. Pacific/NA teleconnection has recently been in a negative phase corresponding to negative temperature anomalies at both 500mb and and the surface. The good news is temperatures will be just a few degrees above normal for highs Thursday and Friday with a warming trend thereafter. The main concern for the Thursday through early next week time frame will be the short wave and surface boundary affecting the region Thursday. The boundary should enter the northern zones Thursday morning triggering numerous showers and thunderstorms. This airmass does not look particularly moist so not expecting any particular hydro issues with this boundary, but localized heavy rain is possible. The front should clear the coast late Thursday night. Instability will be mostly concentrated in southern NH where we may get more SBCAPE depending on antecedent clouds and showers. Have included thunderstorms but not any particular enhanced wording at this time. It is noted a portion of NH is under a marginal risk of thunderstorms while the remainder of the area is in general thunder per SPC. Baroclinic zone will shift south on Friday as secondary cyclogenesis occurs over the Mid Atlantic region. Some rain showers could brush southern sections early but dense Canadian high pressure building in from the northwest will suppress precipitation quickly during the day. Sunday through early next week look tranquil with a weak system possible for Tuesday/Wednesday. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Short Term...Patchy fog tonight, mainly across the inland valleys. A few locations will drop all the way down to LIFR conditions, namely LEB and possibly HIE and CON. Conditions improving shortly after sunrise on Wednesday. Long Term...Areas of MVFR conditions are possible at all terminals in areas of heavy rain/TSRA Thursday into early Friday morning. Becoming VFR Friday midday into Saturday. && .MARINE... Short Term...Winds becoming calm overnight as high pressure builds in. Wave heights will continue to diminish as well. Long Term...Seas and winds remain below small craft advisory levels Wednesday through the extended. Heavy rain/fog/poor visibilities are likely over the waters Thursday into Saturday. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM UPDATE...Kimble SHORT TERM...Cannon LONG TERM...Hanes

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