Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 181423 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1023 AM EDT Thu May 18 2017 .SYNOPSIS... Much warmer air is spilling into New England from the west as a ridge of high pressure moves over the New England. Expect hot conditions today with many areas reaching 90 degrees. A cold front will drop through the area on Friday with some gusty northwest winds and cooler temperatures in the afternoon. A shower or thunderstorm is possible as the front arrives. High pressure builds across the region on Saturday with cooler weather. Low pressure arrives from the west on Monday bringing the next chance of widespread significant rainfall. High pressure will briefly arrive on Tuesday with another system approaching from the west Wednesday. && .NEAR TERM /TODAY/...
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1030AM UPDATE... Temperatures have popped up into the mid 80s already, just as noted by the earlier update. Have updated the forecast primarily to place these observations in the database and get a more solid hourly temperature trend through the rest of the day. However, we did notice that Rockland was able to sneak up to 84 degrees already this morning, much warmer than the forecast high. Expect this area to be the most likely to be affected by a sea breeze, so will not forecast much additional warming there. 845AM UPDATE... After looking at the morning sounding and analyzing the results of yesterday`s air mass, have decided to raise afternoon high temperatures a few degrees. Subsidence inversion around 700MB noted on last night`s sounding has lowered to below 700MB this morning. This will serve as a cap to the instability generated by the warm, humid low levels today but will also serve as the maximum mixing potential for this afternoon. Temperatures at this level extrapolated to the surface via heating/mixing would result in afternoon temperatures peaking around 97F. As a result, areas that are well mixed particularly across southeast New Hampshire and southwest Maine can expect to reach the mid 90s today. Extreme low level inversion near ground level also suggests that temperatures will pop up into the mid 80s within just a couple of hours from now, with the rest of the warming today happening more slowly. Have attempted to show this in the hourly temperature forecast as well. With the extreme heat generated today over land areas, there will be strong thermal forcing for a sea breeze, but this will be counteracted by westerly to southwesterly winds within the mixed layer of around 20 KTS. Areas most likely to get a sea breeze will be along the Midcoast of Maine east of Portland. Portland could get a sea breeze late as well, but will likely not prevent them from hitting 90. 6AM UPDATE... Latest package was to update current temperature readings which range significantly from south to north and town to town. Have also added patchy fog over portions of New Hampshire for the next few hours, before the low level moisture mixes out. ORIGINAL DISCUSSION... The well advertised heat is upon us, with still some surface temperature readings at or above 70 degrees late into the night last night. Expect a deep mixed layer today with plenty of warmth throughout the column and minimal amount of cloudiness today to allow for record shattering high temperatures in some areas. Temperatures of +18C at H8 with good mixing will yield temperatures in the lower to mid 90s for much of the area. This will likely beat the record of 82 degrees for this date in the Forest City set back in 1949. Of note, the warmest day for any period during the month of May in Portland is 94 degrees set back on a very hot day in 1987. Augusta and Concord likely to beat records as well today. Went higher than most guidance with respect to winds today with a southwest gradient aligned through a deep layer. Winds will be very gusty over our higher terrain. The opposite will be true over our larger cold lakes with poor mixing as water temperatures will only in the upper 40s and 50s. Showers and a few scattered thunderstorms will attempt to approach our northern border with Canada late in the day today. However, a large cap will be in place keeping the region basically dry.
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&& .SHORT TERM /TONIGHT AND FRIDAY/... Scattered showers and thunderstorms will enter northern areas along and ahead of an approaching cold front tonight. It is possible that a couple storms may produce gusty winds and hail with conditions destabilizing over the north, however confidence levels are not high enough to include in the forecast at this time as we head into the overnight hours. Friday will be a challenging forecast. If surface cold front gets hung up for a couple hours over southern areas, temperatures will quickly rebound during the morning hours. This is oftentimes the case with cold air delayed over coastal Maine and New Hampshire and have forecasted max temperatures on the warm side of most available guidance. If the gradient briefly drops off with the front nearly overhead, this may allow for sunshine, warm, temperatures, relatively light winds for a short period and diminishing seas. This would be followed by the passage of a cold front during the afternoon with gusty winds and a windshift to the northwest. They may produce a greater risk for paddle craft across the region on Friday with area water temperatures being so cold and conditions changing from benign to hazardous for very small craft. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... A large Canadian ridge of high pressure with cooler and very dry air builds southeast into the region Friday night and Saturday and becomes centered over the region by Sunday morning. Afternoon high temperatures will be near or slightly below seasonal levels. Radiational cooling can be expected Sat night due to clear skies, calm winds and very dry air over the region so temps will be rather cool bottoming out in the mid 30s to lower 40s. On Sunday, mid/high level moisture in advance of the approaching upper trof across the western Great Lakes spills across the upper ridge axis over the area so increasing clouds can be expected. Temps will be seasonable. Sunday night and especially Monday, models show widespread showers moving across the region associated with an upper trof that rotates through the area. Timing of onset/ending times differs some on models so used the superblend average. By late Monday night and Tuesday the system exits and a nice s/wv ridge and dry slot moves across the area. Temps will continue to be close to seasonable. Wednesday a broad southerly flow develops in advance of another upper trof bringing more wet damp weather. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... Short Term...Expect areas of LLWS through sunrise with light winds at the surface and strong winds aloft continuing. Expect a west, southwest wind today with VFR conditions continuing. There could be a few thunderstorms in the northern part of the area this evening into tonight as a cold front drops toward the area. LLWS will redevelop as well. Long Term...VFR through Sunday. MVFR conditions develop in showers Sunday night and continue Monday. && .MARINE... Short Term...Very warm air over icy, cold Atlantic waters will allowing for a dampening to the wind speeds today and tonight. The prevailing direction will be southwest. Winds switch to the northwest behind a passing cold front on Friday. Long Term...Light winds and seas over the weekend. A developing southerly flow may allow some gusts to 25 kts over the outer waters Monday. A light offshore west flow returns Tue. && .FIRE WEATHER... Hot weather spreads across the rest of the area on Thursday. Although dewpoints will be in the upper 50s and 60s, the heat will allow relative humidity to fall to as low as 30 percent during the afternoon with gusty west southwest winds. Expect cooler weather Friday through the weekend resulting in higher humidity levels. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM UPDATE...Kimble

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