Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 241322 AFDGYX Area Forecast Discussion National Weather Service Gray ME 922 AM EDT Mon Apr 24 2017 .SYNOPSIS... A cold front will drop south into northern New England today and stall. Low pressure will slowly move up the Eastern Seaboard Tuesday and Wednesday with rain. A cold front will slowly approach from the west Thursday into Friday, before finally crossing the region on Saturday. && .NEAR TERM /TODAY/...
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9AM UPDATE... After looking at the GYX sounding this morning, have updated the forecast with a few finer details. Strong low level inversion should provide for quick mixing this morning, up into the low to mid 60s. A sea breeze will develop fairly early, possibly before noon, and then spread pretty far inland this afternoon due to a light wind flow as well as high pressure shifting offshore adding a synoptic scale easterly component. Also adjusted dewpoint forecasts based on dry air observed within the inverted layer of the sounding. This was primarily to assist with ongoing fire weather forecasts. Otherwise no significant changes were made. 7AM UPDATE... Have updated the grids to account for current conditions. Some patchy fog continues over southwest New Hampshire with patchy fog being reported at times in Rockland as well. This fog will mix out quickly this morning. Have adjusted temperatures per current observations and raised today`s forecast max readings by a couple degrees as plenty of sunshine will allow for some readings in the lower 70s across southern New Hampshire today. A sea breeze will lead to lowering temperatures along the coast this afternoon. ORIGINAL DISCUSSION... Patchy fog early this morning will dissipate around sunrise in the valleys of south-central New Hampshire. A weak cold front will continue to slowly press southward this morning across the northern and central portions of our forecast area. This front remains relatively moisture starved with little if any precipitation expected and will likely dissipate later today over the region. With plenty of sunshine, especially over southern areas, temperatures will climb through the 60s. A couple locations over southern New Hampshire will nudge 70 degrees. Along the coast, winds will become onshore leading to temperatures falling the through 50s during the afternoon.
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&& .SHORT TERM /TONIGHT AND TUESDAY/...
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A coastal low pressure system will reach the North Carolina tonight. A large area of high pressure will remain over the northern Maine acting as a blocking pattern and keeping precipitation from advecting up the coast and into our area. By Tuesday, this surface high will retreat to the Canadian Maritimes as low pressure slowly moves towards the Mid Atlantic region. 00Z model suite is in relatively good agreement with the positioning of this system. Model ensembles are in very good agreement as well with the upper level low pressure system gradually opening up and moving northeast with time. An onshore flow will allow for rain and perhaps some drizzle to enter the region with the gradient being created by the exiting surface high as much as the approaching weak area of low pressure.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... A coastal low moving slowly northeast up the east coast of the U.S. will bring rain Tue night into early Thu. All models agree on the track of this system moving over eastern New England by Thursday and spreading heavy rain at times with qpf totals to range between 1-2 inches. An persistent onshore flow will keep temps cool and damp through Thursday. See Tides/Coastal flooding section below regarding coastal effects. The system finally moves off to the east by late Thursday slowly bringing an end to the rain from west to east. With lack of any dry ventilation following this system, the low level moisture will likely linger and not much in the way of clearing expected into early Friday. Some partial clearing should take place during the day Friday with warmer temperatures as the flow aloft and in the lower levels becomes southwest ahead of an approaching upper short wave helping clear out much of the moisture. The short wave moves through during the afternoon so some scattered convection should develop. A broad bermuda high setup develops over the weekend but along its northern periphery over northern New England, models are showing a broad area of overrunning rain to develop. && .AVIATION /13Z MONDAY THROUGH FRIDAY/... Short Term...VFR conditions are expected today and tonight. We may start to see some MVFR cigs develop across southern NH and coastal ME late Monday night. Long Term...IFR to LIFR conditions expected Tue night into early Thursday. Conditions improve by late Thu to MVFR from west to east and then mainly VFR by Fri. && .MARINE... Short Term...A brief period of marginal SCA conditions will be possible tonight in advance of a cold front. However, probability is pretty low, so will hold off an advisory at this time. Otherwise sub-sca conditions are expected through Monday night. Long Term...A prolonged weak onshore easterly flow will generate wave heights 5 to 9 FT by Tuesday night into Wednesday and potentially staying above 5 FT into Friday. && .FIRE WEATHER...
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Expect good mixing late this morning along the coastal plain and in southern New Hampshire. Temperatures warm into the upper 60s with relative humidity falling to 30 percent or below in spots. Expect an early sea breeze along the coast which will make good progress inland during the afternoon, helping to bring cooler air and higher humidity.
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&& .HYDROLOGY... We are expecting 1-2 inches of rain with locally higher amounts from Tuesday into early Thursday. With snowmelt and the additional rainfall, river rises will be possible with the chance of some localized minor flooding. && .TIDES/COASTAL FLOODING... We are entering a period of high astronomical tides which will coincide in part with the weak storm system moving up the coast. There are two primary tides we are monitoring for minor coastal flooding, Tuesday evening and Wednesday evening, though high water conditions may continue during the high tides into the weekend. For Tuesday night the astronomical tide at Portland is 11 FT. Winds over the waters will be out of the east at about 20 KT with nearshore waves building to 5 or 8 FT. This would lead to about a 1 FT surge along coastal New Hampshire and southwest Maine and create the potential for minor coastal flooding, splash over, and beach erosion. The tide Wednesday night is 11.5 FT at Portland. At this time winds will be out of the southeast at about 15 to 20 KT which will not promote a strong surge on its own but will likely keep a residual surge in the Gulf of Maine of up to 1 FT with waves of 5 to 9 FT. Minor coastal flooding, splash over, and beach erosion is possible with this tide as well. The next two tidal cycles will see tides of 11.7 FT at Portland Thursday night and Friday night, but atmospheric conditions contributing to storm surge will be on the decrease and we are not expecting significant issues with these tides. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 7 PM Tuesday to 6 PM EDT Wednesday for ANZ151-153. Small Craft Advisory from 2 PM Tuesday to 6 PM EDT Wednesday for ANZ150-152-154. && $$ NEAR TERM UPDATE...Kimble

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