Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 260401 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1101 PM EST Sat Jan 25 2020 .SYNOPSIS... Low pressure will cross the region this evening bringing a mix of snow north and rain in the south. Northwesterly flow will set up with clouds and snow showers lingering in the mountains through Tuesday. A few flurries remain possible through Wednesday, but then a strong high will build in for the second half of the week with clear skies and cold overnight temperatures. && .NEAR TERM /OVERNIGHT/... 11PM UPDATE... Updated the forecast to remove the Flood Watch for New Hampshire where the heavy rain has ended. Otherwise no current changes to the forecast. 930PM UPDATE... Back edge of the heavier rainfall has already arrived in southern NH and will move east rather quickly this evening. Have adjusted hourly precipitation timing to try to get a better grasp on the ending time. An initial batch of showers has given much of western Maine a pre- show with the precipitation, and thus I have upped the precipitation amounts here just a bit. It also seems likely that more of this precipitation will fall as snow in the north with the freezing rain transition happening around the time that precipitation is ending, so snowfall totals have been bumped up a bit, though still covered by the Winter Weather Advisory. 6PM UPDATE... Made some slight adjustments to the forecast based on latest GYX sounding data. Should see a preceding area of drizzle ahead of the heavier precipitation. Once the heavier precipitation arrives, it could begin as snow for a broader part of the foothills or interior coastal plain as the lowest level inversion tops out right around freezing and the one above it should wet bulb below freezing. However, do expect continued warming of this warm layer and a shift to all rain across the area rather quickly. The fire hose will be pointed at our area from offshore of southern New England, pivoting around through the Gulf of Maine tonight. Expect a brief period of heavy rain especially near the coast, while interior areas especially in northwest Maine will see snow and areas of freezing rain. No significant changes in previous forecast thinking regarding precipitation amounts, though it`s possible that the low level moisture stream squeezes out locally more rainfall right along the coastal front. Primary flooding concerns center around urban areas that might have poor drainage issues especially if snow banks block efficient drainage. ORIGINAL DISCUSSION... ***SHORT DURATION HEAVY RAINFALL EXPECTED TONIGHT*** A weaker atmospheric river of moisture ahead of an occluded front will cross the region late this afternoon through late this evening. 25.12z HREF is forecasting 1 to 2 inches of rain...with the majority of that falling in a 3 hour window. This is supported by upstream observations of 1 inch per hour in parts of NY...PA...and NJ. Given the fresh snowpack I expect that it will absorb much of that rainfall despite the heavy rates. The bigger concern is that snowbanks are covering storm drains in many areas and the faster runoff on streets in the urban areas may lead to excessive ponding and minor flooding. Flood watch remains in effect for this reason...more details in hydrology section below. Still seeing pockets of sub-freezing air across Nrn zones and into the foothills. Increasing onshore flow and increasing winds on top of that layer are leading to more widespread drizzle and freezing drizzle. It looks like the winter wx advisory covers the freezing drizzle areas well and no changes were made to headlines. The Srn advisory areas will eventually change to rain. The Nrn advisory areas will see some areas remain below freezing and a slight longer period of sleet or freezing rain after initial snow. Overall warming aloft and short duration are expected to keep amounts on the light side. Will have to keep an eye on freezing levels...as forecast soundings indicate the highest summits may hang on to all snow. If that lowers a bit that could bring the Wrn ME summits like Sugarloaf into play for higher snow totals. Finally SE LLJ will increase overnight and peak around midnight give or take an hour based on location. NAM is probably the most aggressive of any model...with 60 knots below 900 mb. Even in that scenario the expectation would be for gusts in the 35 to 40 mph range. So I have kept wind gusts overnight generally below 30 kt outside the Midcoast. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Forecast area remain in cyclonic flow and trapped low level moisture should keep plenty of cloud cover in Wly upslope zones into Mon morning. I expect snow showers as well...with at least light accumulations in the higher terrain. Temps will be mild Sun as colder air delays until upper trof axis passes. Given that occurs overnight and CAA should keep boundary layer somewhat mixed I blended in raw 2 m temps to trend down to lows Mon morning. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Upslope flow continues through the first part of the week as an upper level low slowly pulls northeastwards into the Maritimes. Northwesterly flow will result in downsloping into the coastal plain with clear skies across the south and mostly overcast with snow flurries in the north. Through the middle part of the week split flow develops over the central part of North America. While some weak waves pass both north and south of us we remain largely average with just a chance for precip on Wednesday when the northern stream wave approaches. Have increased PoP for the mountains on Wednesday as soundings show enough lift to eek out some flurries despite the dry air mass. Strong high pressure will build into the region for the end of the week. With clear, dry, and calm conditions expect overnight lows to drop quite well in the moutnains and have leaned towards the colder statistical guidance for this. Global ensembles would point towards a breakdown of this pattern for the end of the week as the block in the Pacific breaks down and we see a wave train propagate all the way to the east coast by Saturday. With this type of scenario the most suspect part is the timing. The split flow and blocks can`t last forever, and it`s likely a trough will develop over the eastern CONUS returning us to the storm track, but the time of return could be as late as the end of the weekend or early next week. With low confidence have kept PoPs to under 30 through the weekend time frame. && .AVIATION /04Z SUNDAY THROUGH THURSDAY/... Short Term...Widespread MVFR CIGs continue to gradually lower with some areas of IFR now filling in. That trend will continue into the early evening where widespread IFR or lower CIGs are expected S of the mtns. Additionally VIS will lower in pockets of DZ...especially where slightly upsloping SE flow occurs. RA/+RA moves in from W to E tonight generally between 00z and 06z. LLWS is also expected beginning late this afternoon thru late tonight. LLWS and +RA come to a quick end behind the occlusion between 06z to 09z. VFR conditions expected S of the mtns thru Sun night...upslope SHSN and MVFR CIGs expected N. HIE may see local IFR in any SHSN. Long Term... MVFR clouds and snow showers will linger in the mountains through the first part of the week. Elsewhere VFR will prevail in downslope northwesterly flow. By midweek a strong high will being to build into the region and hold through the end of the week keeping conditions VFR. && .MARINE... Short Term...Gales warnings remain in effect for all waters with strong SEly LLJ expected tonight. Gales should be relatively short duration...with a roughly 6 hour window of strongest wind gusts. Winds and seas gradually diminish as offshore SCA gusts Sun keep seas confused. Long Term... Northwesterly flow will develop in the wake of the departing low for the start of the week, with a few gusts to SCA on the outer waters possible. High pressure will build in for the end of the week with calm seas and winds. && .HYDROLOGY... Quick hitting heavy rainfall will move through the southern portion of the region tonight with widespread 1-2 inches expected in just over 6 hours. Water vapor imagery shows a deep moisture plume extending well south into the Cape Hatteras region ahead of a cold front. This moisture plume will meet both the topography of the mountains, as well as the convergence already seen in the surface observations along the coastal front and the combination will focus precipitation just inland of the coast. With frozen ground and drains blocked by snow the impact will be more than normal for this rainfall amount and a flood watch has been issued for the coastal areas. For the rivers expect most points to stay below action as the snowpack across the region remains at around 20-25% density and will be able to absorb quite a bit of the rain. Points to watch in southern NH include the Lamprey which may reach action and Suncook at North Chichester where both the heavy rain and ice affects may combine to reach action. Across southern Maine, the Presumpscot at Westbrook is expected to peak around 10ft, but with the focus of the mesoscale rainfall within the basin it has the potential to reach action as well. A final note is that the ice jam remains in place on the Kennebec at Augusta. Although water levels have continued to drop and not a lot of runoff is expected to reach the mainstem Kennebec the ice jams could move at any time. && .TIDES/COASTAL FLOODING... East flow increasing today brought storm surge above a half foot this morning. With increasing winds expected tonight I expected at least a foot of surge on top of the tide. Given this is the lower of the daily tides I do not anticipate any flooding...but splashover is likely with building nearshore waves. The 10.0 ft noon tide Sunday may also see additional splashover...but offshore winds should again mitigate the flooding threat. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Winter Weather Advisory until 7 AM EST Sunday for MEZ007>009- 012>014. Flood Watch until 3 AM EST Sunday for MEZ018-019-023>026. NH...Winter Weather Advisory until 7 AM EST Sunday for NHZ001>005. MARINE...Gale Warning until 7 AM EST Sunday for ANZ150>154. && $$ NEAR TERM UPDATE...Kimble SHORT TERM...Legro LONG TERM...Curtis

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