Area Forecast Discussion
Issued by NWS Portland, ME

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041 FXUS61 KGYX 240455 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1255 AM EDT Mon Apr 24 2017 .SYNOPSIS... A cold front will drop south into northern New England tonight into Monday and stall. Low pressure will slowly move up the Eastern Seaboard Tuesday and Wednesday with drizzle and periods of rain. A cold front will slowly approach from the west Thursday into Friday, before finally crossing the region on Saturday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1255 AM Update... Have updated the grids to account for current conditions. Mainly clear skies continue over much of the forecast area except the far north. Temperatures remain very mild over southern New Hampshire with 30s elsewhere. Clouds from cold front will attempt to slip southward over central portions of the forecast area late tonight. 1048 PM Update... Forecast was updated to include latest temperatures and dew point information. Lowered overnight lows in a couple spots that should be favored for radiational cooling outside of the cloud covered areas. Winds have diminished at the surface quite a bit but latest KGYX VAD profile is showing ~45 kts at 1 kft. This should only impact aviation at this time with an inversion having set up...but this as well as increasing cloud cover over the south indicates the speed of the incoming system may be faster than the models have indicated of late. 820 PM Update... Only minor changes to the forecast at this time. Quiet weather persists this evening with a weak cold front drifting towards our northern borders. Some associated cloud cover will arrive for northern sections later tonight, with a little blow off reaching southern NH as well from warm air advection over the mid Atlantic states. Temperatures should drop quickly tonight with dew point depressions in the 10-20 degree range. Previous discussion... A fine Sunday afternoon will translate into a fine Sunday evening across the region with little in the way of clouds. A weak cold front will drop into ME and NH overnight, but won`t have too much of an affect on our sensible weather other than a couple of light showers, if any, across the western ME mountains. There will be an increase of clouds later tonight elsewhere but that`s about it.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/... A weak cold front will be draped across or just south of the CWA on Monday. However, this front will remain moisture starved and offer little adverse affects to our sensible weather. There will be more clouds around than what we saw on Sunday but temperatures still should make it into the 60s most everywhere, with temps around 70 likely across southernmost NH. Cooler temperatures will push into our region Monday night as high pressure ridging builds southwestward out of the Maritimes. Clouds will be on the increase also, but overall it should be a nice night for most. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... The continental view of the forecast shows plenty of warmth building in the oven of the continent over the southwestern deserts where the temperature is warming well into the 90s each day. Meanwhile cold lingers in the ice box over Hudson Bay where subzero readings are still observed. The heat from the deserts gets advected eastward by the westerlies into the Great Plains in the form of an elevated mixed layer. As moisture from the Gulf of Mexico occasionally streams northward beneath this mixed layer, thunderstorms form, often violent, and serve the purpose of translating this heat into moisture at all levels of the atmosphere and releasing the excess heat as precipitation. This then gets absorbed into synoptic scale pressure systems which mix the warm, moist air with the remaining Arctic air. For this coming week there will be a prevailing upper trough over the western United States which will advect the warmth from the southwestern ovens into the Great Plains and Great Lakes where an upper ridge will develop in response. Meanwhile well to the southeast of this developing ridge, a remnant pocket of cooler air over the southeastern United States will slowly make its way northward along the East Coast, cut off from the westerlies by the developing ridge. So while our area will be broadly beneath the developing northern stream ridge and protected from the Arctic cold to the north, for most of this week we will not see much of this warmth due to the cut off low slipping through the region. While the cut off upper level low remains well to the south over the Outer Banks on Tuesday, there will be a persistent onshore easterly wind developing between high pressure near the Canadian Maritimes and the low over the Outer Banks. This will bring increasing clouds and an increasing chance of drizzle and rain. Forecast low level moisture profiles east of the mountains favor drizzle from Tuesday evening right through Wednesday night and even into Thursday for some areas. With Gulf of Maine temperatures in the low to mid 40s and low level moisture feed from the Western Atlantic arriving in the form of 50s dewpoints, expect this to readily condense into fog and drizzle as it arrives onshore and gets squashed into the terrain. As the upper low gets closer we will get a little better synoptic scale support for rain with the ascending quadrant of the low positioning itself to advect moisture in off the Atlantic and wring it out over New England likely in several batches. The heaviest rain will be Tuesday night into Wednesday as the upper low eventually makes its way toward Cape Cod. Have decided to word the forecast for the Tuesday night through Wednesday time period as "periods of rain and drizzle" as there will likely be several batches of steadier rain interspersed with plenty of drizzle. Total rainfall could exceed two inches along the coastal plain. Models are now suggesting that low level onshore flow could continue right into Thursday for the eastern half of the area especially. Cut off low is not moving as quickly to the north as previously anticipated, which makes sense considering model tendencies to get rid of these lows too quickly along with the intensifying ridge to the north depriving it of the kick it needs to get out of here. Could be a fair amount of bust potential for the temperature forecast on Thursday considering how much warm air is available in the southwesterly flow beneath the ridge and the potential for onshore flow and clouds keeping this warmth aloft only. Have tilted the forecast toward mid 70s in the west on Thursday where onshore influences will be felt the least, and low 60s (or even upper 50s) across Maine where the coastal influences will be the strongest. The wave that develops within the broader trough tracks north through the Great Lakes Thursday and then to the east of Hudson Bay on Friday. This means that while the broader scale southwesterlies ahead of the trough will be over our area, the low level cold front will have difficulty pushing east so far to the south of the parent wave. Models have thus delayed the advance of the cold front again, with its most likely arrival now appearing to be Friday or Friday night. There could be showers and thunderstorms as the front moves through considering the warm, moist, moderately unstable air mass ahead of it. But whether this occurs will be dependent on whether there will be a supporting shortwave trough within this southwesterly flow. At any rate, the southwesterly flow should finally kick out the maritime air mass over coastal Maine and temperatures should warm into the 70s for most of the forecast area on Friday, with 80s possible if we can get some good mixing. While there is some indication that a piece of the remnant Arctic cold could get shoved toward our area behind the front for this weekend, the best cold air will be moving more east than south and will not be strongly noticed in our area. This cold will also be tempered by downsloping flow behind the front and a strong April sun, yielding temperatures in the 60s this weekend which is a few degrees above normal. Ridging begins to develop across the center of the continent again late this weekend and will make another attempt at venting the oven warmth toward our region early next week. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... Short Term...VFR conditions are expected tonight through most of Monday night. A brief period of MVFR conditions will be possible at RKD overnight tonight. We may start to see some MVFR cigs develop across southern NH and coastal ME late Monday night. Added LLWS due to KGYX VAD winds showing about 45 kts at 1 kft associated with a LLJ. This will only be a concern for the next couple of hours. Long Term...Should see increasing clouds and lowering ceilings on Tuesday as an onshore flow moistens up the low levels. IFR conditions will be likely by evening with an increasing chance of drizzle and rain. Conditions to the west of the mountains will fair a little better at least initially. Onshore flow, drizzle, and IFR or worse conditions are likely to continue through Wednesday and Wednesday night. Could see this continue into Thursday for much of Maine while New Hampshire may finally begin to see breaks in the clouds and a return to VFR conditions. VFR expected to return areawide on Friday. && .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...East to southeast flow increases between high pressure over the north Atlantic and low pressure tracking up the East Coast. Winds could gust to 25 KT at times Tuesday night through Wednesday. With the onshore fetch expect wave heights to be on the increase, rising to 5 to 10 FT on Wednesday and potentially staying above 5 FT into Friday. && .FIRE WEATHER... RH values will be generally in the mid 30s to mid 40s on Monday with relatively light winds. The exception looks to be in the western ME mountains where min RH values should be in the 20s. However, winds are still expected to be light. Rain is expected to develop Tue and last into Wed and Thu. && .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 waves building to 5 or 6 FT. This would cause 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...None. && $$ JC

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