Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 231620 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1220 PM EDT Mon Oct 23 2017 .SYNOPSIS... High pressure will hold over the maritimes tonight through Monday. A slow moving cold front will approach from the west Monday night and Tuesday and will slowly cross the region Wednesday through Thursday. A ridge of high pressure will build over the region on Friday and will shift offshore on Saturday before the next low pressure system approaches from the southwest. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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1215 PM...Initially batch of low stratus from early this morning has mixed out well, but secondary batch is advecting in from south and may struggle initially to move onshore where its over water, but will likely begin to move onshore along coastal ME by mid-late afternoon. Across interior srn NH, the clouds will show some breaks but it`ll stay partly to mostly sunny. This area may see the most sun later in the day. 930 AM...Just a quick update, mostly to tweak sky cover a bit to better reflect areas that stay mainly clear N and W of the mountains, and to show some brief partial clearing around midday closer to the coast. Otherwise, forecast mainly unchanged. 710 AM Update...Little change to the going forecast. Stratus and fog are well represented in the forecast and will likely hold tough on the coastal plain and southernmost nh today. Previously... Low stratus has overspread a good portion of the forecast area this early Monday morning except for the northern mountains. Most locations with stratus and some fog are in the 50s, while temperatures in the 30s to mid are common northwest of the cloud shield. The main question today is how much of the low clouds burn off. There isn`t much of a scouring wind today with low level inversion likely to hold firm for much of the day. The northwest/west edges of the stratus deck should erode with daytime heating, but the coastal plain may be stuck with it much of the day. Even there, it should go BKN for awhile this afternoon with some heating, but then quickly fill in again by early evening. Therefore, we think mostly cloudy should do it for the coastal plain, with sunnier skies expected the more inland one is. Despite some early morning drizzle, it should be dry today all areas with highs well above normal values.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Low clouds will once again push westward this evening and overnight along with some fog and drizzle. Temperatures will remain much warmer than normal overnight due to the cloud cover and increasing low level jet and associated moisture advection. The pressure gradient tightens up on Tuesday as a cold front approaches from the west with strong high pressure centered south of the Maritimes. This will result in increasing southerly winds during the day, with gusts around 25 mph likely by afternoon. Cloudy skies will be the norm with low level moisture continuing to stream on in on a steadily strengthening low level jet. Any heavy rain will hold off until Tuesday night but there will be showers around during the day. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... A long awaited pattern shift finally arrives later this week as a deep trough develops just to our west. The trough will dig south out of the Canadian prairies and form a cut off low over the great Lakes by Wednesday. This trough will then moderate and shift eastwards on Thursday. A second reinforcing cold air mass dives south for next weekend. Tuesday night will see a strong cold front on our door step to the west. Ahead of it we`ll have strong southerly flow bringing warm moist air into the region. Despite the calendar saying late Fall, the moisture will run up to 4 standard deviations above normal which should be enough to provide around 500 J/kg of CAPE. Along with the cold front comes a very strong low level jet. Still some variation with the strength of this jet in the forecast models but expect at least 50kts within 3000ft of the surface to provide plenty of wind shear. See SPC day 2 outlook for marginal risk of severe thunderstorms. As the front moves through we`ll see two main effects, wind and rain. Wind gusts to 30kts are possible along the coast, especially just ahead of the front itself. Still some uncertainty in the timing and extent of these stronger gusts and so have not issued a wind advisory for the moment, however one may be needed. With the front taking over 24hours to clear our area this looks like an excellent producer of much needed rain. Storm total rainfall will reach 2-3 inches across the region and higher amounts are possible with any heavier more convective lines. The highest amounts of rainfall will be in the upper kennebec basin in Maine where the southerly flow runs into the moutnains as well as along the immediate coastline where we may see some coastal front effects. Both these regions could use the rain and the coast remains in D1 drought. With rivers low the only chance for flooding would be in the flashier basins where the rainfall rates could briefly exceed the ability of the water to be absorbed. The Pemi at Woodstock and Swift river at Roxbury are the only points near the headwaters flash flood guidance. After the front finally clear the region on Thursday a brief ridge of high pressure will build back in before the next system begins to move in for the weekend. && .AVIATION /16Z MONDAY THROUGH FRIDAY/... Short Term...IFR conditions across much of the region early this morning should gradually improve a bit as the morning progresses. However, MVFR cigs are likely today on the coastal plain. Cigs and vsbys are expected to come down to IFR or lower tonight and remain through Tuesday. Long Term...Slow moving cold front will move through the region on Wednesday into Thursday. This will bring widespread rain with a rumble of thunder possible to all terminals. Expect a decrease to IFR conditions in heavy rain as the front moves through. Morning heavy rain, combined with a strong LLJ, as high as 60kts at 3,000ft will impact the region ahead of the line with conditions at or near airport minimums possible through Wednesday morning. Despite fairly gusty surface winds overnight, this strong jet will still result in low level wind shear through the coast early Wednesday morning. && .MARINE... Short Term...SCA conditions are expected to develop Tuesday in response to increasing southerly winds. Until then winds and seas should remain below SCA thresholds, but marine fog will likely limit vsbys. Long Term... Southerly flow will continue to increase into Tuesday night out ahead of the cold front. With high pressure parked off shore there will be a long southerly fetch which will help to increase the winds and waves. A low level jet of over 50kts will move in just in front of the front and wind gusts to gale force are expected across the outer waters. A gale watch has been issued for Tuesday night. With the southerly flow, expect a large portion of penobscot bay to also be exposed to the winds and have issued a gale watch for them as well. Casco bay will remain more sheltered and thus will likely only see small craft conditions. && .TIDES/COASTAL FLOODING... A slow moving cold front will provide a prolonged period of onshore flow. Seas will build to around 12ft in onshore flow. While this is a descent wind wave, all other factors for coastal flooding remain low. We are at an astronomical low point in the tidal cycle with high tide at Portland in the 9ft range. Additionally all of the wave is in the wind wave at around 8 seconds with no long period swell. The low tide and short period means that even though there may be around a foot of storm surge it will not be enough to cause problems. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Tuesday for ANZ150>154. Gale Watch from Tuesday evening through Wednesday evening for ANZ150>152-154. && $$ SYNOPSIS... NEAR TERM...Cempa/Ekster SHORT TERM...Ekster LONG TERM...Curtis AVIATION... MARINE... TIDES/COASTAL FLOODING...

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