Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 170421 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1221 AM EDT Tue Apr 17 2018 .SYNOPSIS... Our wintry mix of precipitation will gradually transition to rain across all but the far northern areas by this evening. Strong gusty winds will continue into tonight as well, especially on the coast. A broad area of weakening low pressure will persist over the region Tuesday and will shift east on Wednesday. Another area of low pressure will approach from the west Wednesday night and will move east through the region on Thursday. This could bring a round of rain or snow. Low pressure will shift east into the maritimes Friday, followed by high pressure building in for the weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1200 AM Update... Quick update to drop Winter Weather Advisory headlines which expired at midnight. Precipitation continues to lift off to the northeast and will likely clear the remainder of eastern zones by 300 AM. Some patchy dense fog forming in brief clearing across southern New Hampshire and southwest Maine but expect this to mix out as more clouds swing in from the southwest over the next several hours. Have adjusted pop and wx grids based on current radar trends. Aside from a few temp/td tweaks no other changes planned attm. 11PM UPDATE... Area of heavy rain has moved east into the eastern part of the forecast area, primarily from Portland eastward extending north to the Canadian border. Low level temperatures remain near freezing in spots up north, but are on the rise, thus will likely be able to let the Winter Weather Advisory expire at midnight. Wind Advisory for the Maine coastline has been trimmed a bit as well as the strongest winds will remain ahead of and within the heavier rain areas. Winds diminish quickly behind the trough axis, thus this will likely be able to expire at midnight as well. 8PM UPDATE... Temperatures across the coastal plain of Maine have warmed to freezing or above, so believe the freezing rain threat has ended there and have let the advisory expire. Further north, the threat remains as temperatures are still below freezing with more substantial rain moving in. Expect temperatures here to warm as well as heavier rain begins, though only to near freezing until the low moves across tonight. Meanwhile, the band of heavy rain moving through southwest Maine and northern New Hampshire at this time has yielded storm total rainfall amounts of 2.5 inches or so, and thus will need to watch the rivers for potential flooding as this drains away. Until then, some nuisance flooding of poor drainage areas could occur. 5PM UPDATE... Have updated the forecast primarily to extend the Winter Weather Advisory for Androscoggin, Kennebec, and Waldo Counties as temperatures there remain near freezing or below, suggesting icing is still occuring in spots. We are seeing the temperature gradually warm closer to the coast as onshore flow increases in response to the approaching surface low. To the north of the surface low track, expect temperatures to hold near freezing until the broader trough axis crosses the area overnight, at which point the temperature will finally warm. ORIGINAL DISCUSSION... A very late season winter storm continues across the forecast area. Temps are slowly continuing their climb thru the 30s...but large areas of the interior remain at or below freezing. Additionally very cold lower levels of the atmosphere are allowing for continued snow grains and sleet...despite some surface temps above freezing. I have left remaining winter wx advisories in place...and extended the mtns and foothills until 8 PM. Untreated roads and elevated surfaces will remain icy into this evening. As temps rise above freezing precip will turn to heavy the cold front sweeps NEwd tonight. Strong ESE LLJ will lift up over the cold dome...producing strong isentropic lift across the region. While there is some instability trying to work N...the main surface front remains stuck across Srn New England and so it will be a struggle to maintain that Nwd. A rumble or two of thunder would not surprise me...but it probably is not worth adding to an already complicated forecast. Precip rates will be moderate to heavy at times...for about a 6 hour period and then things taper off behind the front. Additional QPF around 1.5 to 1.75 inches is possible. At this time I am not too worried about widespread flooding...but localized poor drainage flooding is possible. Downsloping winds and colder temps will also produce lighter rain and not much melting of snow in the headwaters. As a result the forecasts for mainstem rivers is generally action stage but not flooding. Wind advisories remain in effect as well. Observations are a little sparse...but what mesonet and spotter reports we have received show that winds have been gusting in the 40 to 50 mph range from EEN all the way N thru Pittsburg. That advisory will remain in effect for downslope winds thru 8 PM...when LLJ moving Ewd will weaken flow over the mtns. Along the coast of Wrn ME...the LLJ sweeping N will continue to strengthen over the next couple of hours. There is some uncertainty above how far inland those stronger gusts will make it...but near the coast will certainly see some 40 to 50 mph gusts. After midnight winds along the coast will weaken as the core of the LLJ moves into Downeast ME. Finally astronomical tides will be high tonight...see the coastal flooding section below for more information.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Large upper low gradually pinwheels across the region Tue. Plenty of clouds and showers will linger as a result. With gradual cooling aloft...mtns should see snow showers...while rain showers persistent in the valleys and S of the high terrain. Clouds will be slow to clear Tue night and so overnight lows are on the mild side. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Pattern changes at 500 MB across North America and surrounding areas, show a trend toward a less amplified flow later in the period, which should allow temps to eventually moderate to around normal, but not before a series of closing troughs moves through late in the week, which will keep things unsettled. Wednesday will see 500 mb closed low situated over the area move east toward the maritimes. Will see a lot of clouds, and likely some SHRASN, mainly in the mountains. Highs will mostly be in the 40s, with some warmer spots on the coast and in srn NH topping out around 50. After that, confidence in the details of the forecast drops off as two 500 MB waves move S_SE out of Canada, and interact Thu- Fri, perhaps closing off into one low. Should see some precip in this timeframe, probably centered around late Thu or Thu night, but not convinced that the models are handling this interaction well. I`d lean toward rain, or possibly non- accumulating snow, outside the mountains, with snow in the mountains, although any accums look to be at higher elevations. Highs will once again be mostly mostly in the 40s, but mid to upper 30s in the mountains. Friday, may see some lingering showers associated with the upper level low, but some downsloping should help temps warm a bit outside the mtns. At this point the weekend looks dry as sfc ridging holds beneath NW flow aloft. It`ll be a gradual warming trend and temps could reach close to normal by Sunday, but temps over the weekend will run in the mid to upper 40s N, to the mid to upper 50s south. && .AVIATION /04Z TUESDAY THROUGH SATURDAY/... Short Term...Areas of IFR and LLWS will continue thru the early evening as the heaviest precip moves NEwd across the forecast area. Behind the cold front LLWS will diminish as winds shift to SWly and weaken aloft. There are some indications that IFR CIGs may linger...and that fog may develop underneath the upper low pressure. I was not confident enough on fog to include in the TAFs at this time however...but did keep CIGs on the low side. Gradually moisture mixes out and CIGs lift to MVFR by Tue. Long Term...Variable MVFR/VFR conditions expected on Wednesday, with VFR Wed night. Flight restrictions possible in RA or SN late Thu into Thu night. VFR Fri and Sat. && .MARINE... Short Term...Gale warning remains up for the eastern waters where the strongest winds continue. Winds diminish quickly behind the trough axis overnight. Long Term...There is a possibility of SCA late Thu and Thu night as low passes to our southeast. && .HYDROLOGY... With the rain this evening across New Hampshire and western Maine, the rivers are on the rise. It is possible that one or two hit minor flood stage before cresting. The areal flood threat is diminishing as well. && .TIDES/COASTAL FLOODING... High tide is forecast for 10.6 feet just after midnight tonight at Portland Harbor. Strong Ely flow will help to build a storm surge around 1.5 ft. This will put water levels right around flood stage late this evening. Minor flooding is possible, but beach erosion and splash over are likely. A coastal flood advisory is in effect for all of the coast. && .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...Wind Advisory until 2 AM EDT early this morning for MEZ025>028. Coastal Flood Advisory until 2 AM EDT early this morning for MEZ023>028. NH...Coastal Flood Advisory until 2 AM EDT early this morning for NHZ014. MARINE...Small Craft Advisory until 4 AM EDT early this morning for ANZ153-154. Gale Warning until 4 AM EDT early this morning for ANZ150>152.
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