Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 180028 AFDGYX Area Forecast Discussion National Weather Service Gray ME 828 PM EDT Sun Jun 17 2018 .SYNOPSIS... Southerly flow ahead of an approaching cold front will bring very warm and humid conditions to the area tomorrow. Some areas on Monday will see heat index values break 95 degrees. The atmosphere destabilizes significantly during the afternoon tomorrow with widespread thunderstorms expected. Once the front moves though tomorrow night, temperatures will moderate a little bit. Another cold front moving through on Wednesday will push temperatures down to near normal before they start rising again toward next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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800 PM Update... High clouds continue to spill into the region from the northwest as a warm front moves through southern Quebec and Northern Maine early this evening. Radar showing associated rain shower activity mainly confined to northern Maine but a few showers are dying out as they drift into northern Somerset County so have increased pops in this area for the next 3 to 4 hours. Elsewhere....strong marine layer has spread well inland in southern Maine and and southwest New Hampshire dropping temps into the 50s and 60s. Have tweaked temps and dew points based on current obs and adjusted RH and Apprnt temp grids....but no major changes planned for current forecast. Prev Disc... It should be a fairly quiet night across most of the area. Increasing low level moisture could result in some patchy areas of fog, but shouldn`t result in anything widespread at all. Temperatures will be a bit on the warm side, with most places only seeing lows in the 60s to 50s over the mountains.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
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Tomorrow will definitely be the focus of the forecast as it looks to be a very busy day. Increasing dewpoints and temperatures ahead of an approaching cold front will set the stage for convection in the afternoon and evening. Heat...strong southwesterly flow will allow temperatures to climb to around 15 degrees above normal. The warmer than normal temperatures will combine with rising dewpoints to generate some very high heat index values. With the new criteria of two hours or more of 95F degree heat index now in place, we will be hoisting a heat advisory for tomorrow afternoon over southern New Hampshire and York county Maine. Convection...The entire forecast area is now in either a slight risk or marginal risk on the day two convective outlook. Although the cold front won`t arrive until sometime during the overnight, models are consistent in developing convection well ahead of the actual frontal boundary. Expect convection to develop first in New Hampshire and the mountains of Maine, then move southeast toward the coast. SBCape values over most of the area approach or exceed 2000 j/kg by early afternoon. There are also some very respectable shear values that develop during the afternoon as well. PWAT values are expected to exceed 2", which is at/above SPC daily climatology max values. Once convection fires, there should be plenty of moisture and instability to work with. What is less certain, is how much if any convective debris will affect storms later into the evening. Expect convection to continue into the overnight hours, but the severe threat should diminish dramatically as the evening progresses. Due to the abundant low level moisture in place ahead of the cold front, there could be some fog development Monday night. It may be short lived though as the cold front moves through and scours it out.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/... Looking at the big picture across North America, we see a large upper level ridge of high pressure building all the way north along the West Coast even up into the Yukon Territory of Canada. Beneath this ridge exists a cut off low over the northern Rocky Mountains of Idaho and Montana, helping to sustain the high amplitude ridge above it while also keeping the hottest air baking over the Sonoran Desert and southwestern United States. Meanwhile to the south and east, the subtropical ridge is a bit squashed to the south by the downstream trough extending from that western Canada ridge. This trough meanders largely in place across Labrador and northern Quebec, with occasional spokes of energy rotating around this trough sending cold fronts through New England. Since the broader flow pattern features northwesterly flow extending from deep in the Canadian Arctic and traversing the still-frozen Hudson Bay, the overall pattern across New England will stay largely as it has been for some time: Mostly dry with only brief periods of more warm/humid air arriving just in time for another cold front to usher in the cooler/drier air again. In order to get any relief from the developing drought especially across coastal areas, we will need to get this weather pattern to shift and allow a coastal storm or two to bring a good dose of rain. By Tuesday morning the cold front will have moved offshore, ushering in a cooler and drier air mass on a northerly breeze. With cool advection and downsloping winds, expect the warmest temperatures to be along the coastal plain extending all the way to the beaches where another day in the 80s is forecast with noticeably less humidity. High pressure building out of the Great Lakes and into southern New England will allow for a clear and mostly calm night Tuesday night, with low temperatures dropping into the 40s and 50s. It will be a good night to open windows and let out the heat and humidity of the previous few days! High pressure cresting to our south will bring a brief shift to southwesterly flow on Wednesday. This brings a bit warmer air and a slight increase in humidity. However, the rich Gulf of Mexico moisture remains locked in over the Great Plains and Mississippi/Ohio River Valleys. Thus even as another cold front pushes into the area on Wednesday, there won`t be a lot of convective fireworks with it and thus no drought relief expected. Behind this cold front exists a cooler and much drier air mass originating from the Arctic coast of Alaska and traversing the still frozen Hudson Bay on its way here. While the cold temperature of this air mass will modify with each day of June sun on its route, it will still have those dry Arctic dewpoints. Raw data from both the GFS and the ECMWF indicate dewpoints dropping down into the 30s Thursday afternoon for much of the area. This was significantly lower than the SuperBlend which we typically initialize the forecast with, so I deliberately blended in some of these drier models to get lower dewpoints into the forecast. This is a significant factor for fire weather concerns, but also for the temperature forecast Thursday night as high pressure slides out of the Great Lakes and across New England. This sets up ideal radiational cooling conditions on the shortest night of the year. Although the short nighttime period does limit how cold it can get, we saw just last week how cold it can get in a dry air mass under the right conditions even well into June. Thus we cannot rule out the possibility of frost in the northern valleys on the first day of summer. High pressure slides offshore on Friday with temperatures mainly in the 70s and dewpoints slowly recovering. Meanwhile, that cut off low over the northern Rocky Mountains will have slowly drifted east across the Great Lakes this week. What form the resulting surface pressure features take is still not well agreed upon by the models, but generally we should see a southerly flow begin again on Saturday with warming temperatures and increasing moisture. Then we could see some showers on Sunday. It`s worth noting, though, that although most models paint precipitation over the entire forecast area on Sunday, they don`t have a great handle on the finer details of it yet and thus are likely too broad with the precipitation. It is likely that rain will be showery in nature and focused along the accompanying frontal boundaries, so it may not be a washout for everyone or the drought relief that we need. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... Short Term...Mainly VFR tonight. Showers and thunderstorms tomorrow will produce some aviation issues. Expect to see IFR conditions as storm move through. Long Term...VFR conditions expected as several dry air masses move through the region this week. && .MARINE... Short Term...Will hoist a small craft advisory for increasing seas and winds tomorrow morning throughout the day. Advisory will only be for the outer waters as the bays should remain protected for this event. Long Term...After the cold front moves through early Tuesday morning, expect a shift to northwest winds gusting to 20 or possibly 25 KT. High pressure slides by Tuesday night with light winds becoming southwesterly on Wednesday ahead of the next front Wednesday night. && .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...Heat Advisory from noon to 7 PM EDT Monday for MEZ018-023. NH...Heat Advisory from noon to 7 PM EDT Monday for NHZ005>015. MARINE...Small Craft Advisory from 8 AM Monday to 8 AM EDT Tuesday for ANZ150-152-154.
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