Area Forecast Discussion
Issued by NWS Portland, ME

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829 FXUS61 KGYX 150139 AFDGYX Area Forecast Discussion National Weather Service Gray ME 939 PM EDT Mon Aug 14 2017 .SYNOPSIS... A cold front will move south out of Canada Tuesday triggering showers and thunderstorms by late morning. High pressure builds in Wednesday before another low pressure system affects the region Thursday night and Friday with the potential for a period of heavy rain. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 940 pm Update: Quiet night with latest temps from obs on track. No changes needed. 655 pm Update: Another quiet night. The veil of high clouds over southern areas will gradually exit to the east over the next several hours based on extrapolation of h7 trajs. Input mesonet data with only minor tweaks needed to temps. Made some adjustments to sky cover based on satellite to reflect clear conditions across the north and mountains with the solid area of high clouds only in southern areas. Previous Discussion: Water vapor imagery shows moisture streaming in from the southwest in the mid to upper levels of the atmosphere. Temperatures had warmed into the 70s and 80s and this combined with low humidity has resulted in a very pleasant day. A sea breeze has formed and will be pushing inland shortly capping highs near the coast. High pressure dominates the region through tonight before being shunned offshore. Tropical moisture moving into the region from TS Gert from the south will result in higher dew points and more clouds overnight. The increasing low levels will result in patchy fog in the valleys and other sheltered areas with denser areas of fog and very low stratus over the Mid Coast. The latest SREF and HRRR runs support denser fog in this area. Overnight lows will drop into the 50s north to the lower 60s south. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... On Tuesday the upper low over northern Canada will drop south and stall north of the Great Lakes. A lead vorticity maxima currently over northern Minnesota will will eject push a cold front through New England. A the same time Gert will begin to eject into the Atlantic somewhere near the Carolinas, pushing additional moisture northward. Scattered showers and thunderstorms should form generally ahead and along the frontal boundary tomorrow. Precipitation will spread from north to south along with decent frontogenetical forcing. Ample heating (highs should warm into the upper 70s to lower 80s) should take place ahead of the boundary which will result in decent instability. This along with the approaching short wave will allow convection to blossom and contribute to a few strong storms. Small hail and gusty wind terminology was included in the forecast in agreement with SPC`s day two outlook which has much of NH and western Maine in a marginal risk for severe thunderstorms tomorrow. A few stronger rotating cells are likely with about 50 kts of shear in the 0-6km layer. Individual supercells will move ENE with the potential for heavy rain, marginally severe hail, and damaging winds. Expect this activity to die out with the loss of heating and as forcing for ascent moves away. The front itself should move offshore by Wednesday morning bringing an end to precipitation. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... The models have come into better agreement over the past 24 hours with decent agreement on the long wave pattern into next weekend. The omega block centered across the Canadian Prairies...with the eastern anchor trough found across eastern Canada and New England...will break down early in the extended period. We`re left with shortwave ridging for Wednesday and Thursday. The next shortwave impulse and associated surface reflection will then approach from the Great Lakes for Thursday night and the ECMWF has come into much better agreement with the remainder of the model suite on a slower arrival time with this system. Unfortunately...shortwave energy now carves out a trough axis to our west by the start of the weekend. This promises a period of onshore flow and unsettled weather for the weekend as a complex area of low pressure slowly traverses New England. In the dailies...A ridge of high pressure will build into the region for Wednesday and Thursday. Low pressure approaches from the Great Lakes for Thursday night and Friday. This slow-moving system is then expected to linger into the upcoming weekend. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... Short Term...Low clouds and stratus will form tonight, mainly on the Mid Coast of Maine but also for LEB and HIE as moisture increases and forcing for ascent nears. Otherwise RKD and AUG look to be hardest hit. VFR conditions are expected on Tuesday after any stratus and fog burns off, but a few showers and thunderstorms may bring briefly lower ceilings in heavy rain. LEB and HIE will be the first and most likely to experience -TSRA. There will be a wind shift to the west northwest late Tuesday night as the front moves offshore. Long Term... Thu night - Sat...Areas of MVFR in shra/tsra with lcl IFR conditions psbl in fog. && .MARINE... Short Term...Conditions are expected to remain below small craft thresholds through Wednesday morning. Long Term... Wed PM...Winds and seas may briefly approach small craft outside the bays. Fri PM - Sat...Winds and seas may approach small craft on all waters. && .EQUIPMENT... The weather radar at Gray (GYX) will be down for maintenance until approximately Tuesday, August 14th. During that time technicians will be installing important upgrades. This work has been scheduled to minimize any potential impacts to office operations. During the outage, radar coverage is available from adjacent radar sites including Burlington-Vermont (KCXX), Taunton- Massachusetts (KBOX), and Hodgdon-Maine (KCBW). Also, the radar at Albany (KENX) suffered a failure that has been attributed to azimuthal motor/gear issues. It is expected to be down for several days. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.