Area Forecast Discussion
Issued by NWS Portland, ME

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810 FXUS61 KGYX 132030 AFDGYX Area Forecast Discussion National Weather Service Gray ME 330 PM EST Fri Dec 13 2019 .SYNOPSIS... Low pressure developing along the East Coast will move north through New England on Saturday. Expect warm conditions and rain for most areas, though some light freezing rain is possible at the onset tonight. Once the low moves away a cold front will blow through and bring back colder conditions on a strong westerly wind on Sunday. The next low pressure system tracks near southern New England on Tuesday with a chance of wintry weather for New Hampshire and Maine. && .NEAR TERM /TONIGHT/... ** Storm will move through the region tonight and Saturday bringing heavy rain south with mixed precip north *** Cloud cover has steadily increased through the day as low pressure begins to develop off the southeast coast. Pressure falls were occuring through the Appalachians with good agreement for the low to track northwards and up the eastern seaboard and west of us through the Hudson and Champlain valleys. Tonight expect light precip to develop well ahead of the low center as a jet max moves in aloft paired with a strong theta-E ridge at the surface, forcing moisture into our area. Isentropic lift begins in the low levels with the result being a few hours of drizzle or very light snow expected this afternoon before the whole depth of the atmosphere is able to saturate. Some light snow has been reported and temperatures remain below freezing in areas of western New Hampshire. While the major ASOS across SW NH are now above freezing, the dewpoint remains in the 20s which gives a chance for some cooling and the wet bulb remains below freezing. Any icing will be quite light, but still impactful and so for this reason will go ahead and keep the Winter Weather Advisory in effect until the rain moves in later tonight. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... Saturday the rain will become heavy at times through the morning as the low intensifies and moves northwards. Good agreement on the timing of the heaviest precip between 12Z and 18Z. Trends with the short term guidance has been to increase precip amounts along the coast. Have trended this direction bringing the midcoast to around 2" without going as high as some of the 3"-4" high outliers. Strong frontogenesis oriented east west as the warm front lifts northwards through the morning may help to focus this precip along this front. If flash flooding occurs it will likely be at the intersection of this with the coast, which is already nicely contained within the Flash Flood watch. Temperatures will be warm with 50 degrees possible along the coast. Have kept the temperatures a bit on the cold side of the model spread, but still puts the entire area well above freezing through the day. The rain will continue through the day on Saturday as the low deepens to the west and we remain in the warm sector. Overnight Saturday the cold front will push through the region. Expect a brief changeover back to snow, possibly mixing with sleet through the mountains, however temperatures at the surface and aloft will drop at roughly the same time so limited sleet and ice is expected. Accumulations will be limited so no winter headlines are expected to be needed for this. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Expect cold advection on Sunday behind the cold front which will be surging eastward beneath the departing low. A tight pressure gradient between this departing low and the approaching high will lead to strong westerly winds. Winds at 850MB of near 50 KT suggest the potential for strong wind gusts especially considering the considerable cold advection through the day. In fact, temperatures may be falling as the day goes on. This top down cold advection leads to strong mixing within the lowest layer of the atmosphere which will allow much of these strong winds aloft to reach ground level. Current forecasts keep wind gusts just a bit below Wind Advisory levels (46 mph gusts), but would not be surprised to get a few gusts to that level. Winds will likely continue into the night Sunday night before the pressure gradient finally relaxes with high pressure arriving on Monday. The next wave tracks out of the Ohio Valley on Monday and near southern New England on Tuesday. With our area remaining on the northern side of this low and a cold air mass in place, expect this wave to bring mostly snow to our area. However, there are some indications that some warmer air may lift north into the southern part of the area, at least aloft. This may make precipitation type a bit more dicey, but for now will keep the forecast tilted toward mostly snow until greater confidence exists in any warm layer aloft. The consensus on precipitation amounts at this point suggest up to 0.5 inch liquid equivalent in southern New Hampshire with lesser amounts further north into the cold air. It seems likely this will be a widespread advisory level snow event Monday night into Tuesday. Seasonably cold weather continues behind this wave. Another much stronger shot of Arctic air arrives on Wednesday as a dry clipper system scoots by to our north and opens the door for this cold air to pour in behind it for the end of the week. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... Short Term...Cloud ceilings are already beginning to decrease to MVFR across southern New Hampshire as low pressure begins to develop off the southeast coast. Expect patchy freezing drizzle tonight becoming steadier rain after midnight. Fog combined with heavy rain will keep all terminals at IFR throughout the day on Saturday. Long Term...Sunday will be windy but VFR at least on the coastal plain. MVFR conditions could linger in the mountains into Sunday night or early Monday. Wind gusts to 30 or 35 KT are likely mainly during the day Sunday, gradually diminishing into Monday. The next wave brings mostly snow to the region Monday night into Tuesday with southern New Hampshire and coastal Maine most likely affected. && .MARINE... Short Term... Increasing southeasterly flow ahead of this system will peak at Gale force on Saturday. There will be a brief lull on Saturday afternoon before winds once again return to stronger Gales on Saturday night. Long Term...Westerly gales are expected behind the front lasting at least through Sunday night. Winds gradually diminish as high pressure moves in on Monday. && .HYDROLOGY... QPF/Flash flooding: QPF amounts have increased through the coastal plain with over 2" possible. With the frozen ground and small creeks and streams already running high from the last event expect this will result in some minor flooding through the coastal plain. With the heaviest rainfall in the 12-18Z time frame this would be the time for any Flash flooding if the very high rain rates are able to remain in one spot. Rivers: Greatest threat for river flooding is across southern New Hampshire. Here around 2" of QPF is expected in addition to the melting of the remaining 1-2" of SWE still on the ground from the prior big snow event. The combination will lead to minor flooding on several rivers including the Suncook, Contoocook, Warner, and Piscataquog. This area remains in a Flood Watch. Across the White mountains the rainfall totals will be lower - 1 to 1.5 inches, which will be able to be at least partially absorbed by the snowpack. Thus while there will be rises on the rivers significant flooding is not expected. && .TIDES/COASTAL FLOODING... The combination of increasing onshore winds and building wave action will coincide with relatively high astronomical tides Saturday afternoon. The astronomical tide Saturday afternoon in Portland is 10.6ft. At this point the addition of storm surge look to push water levels right to the minor flooding threshold, with building wave action likely to cause minor splashover and beach erosion in vulnerable areas. Another aspect of this tide cycle will be that most of the rain will have already fallen by the time of high tide around noon. The timing of the high tide will cause runoff to drain more slowly from the marshes and tidal waterways, which could be enough to cause some minor flooding issues on roads and drainage areas within the tidal zone around the time of high tide. Winds will turn offshore and waves will be subsiding in time for the Sunday high tide cycle, and coastal flooding issues are not expected at this time. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Flood Watch from 1 AM EST Saturday through Saturday evening for MEZ018>028. Coastal Flood Advisory from 11 AM to 3 PM EST Saturday for MEZ023-024. NH...Winter Weather Advisory until 10 PM EST this evening for NHZ001>009-011-015. Flood Watch from 1 AM EST Saturday through Saturday evening for NHZ008>010-012>015. Coastal Flood Advisory from 11 AM to 3 PM EST Saturday for NHZ014. MARINE...Gale Warning from 7 AM Saturday to 6 AM EST Monday for ANZ150>154. && $$ NEAR TERM...Curtis SHORT TERM...Curtis LONG TERM...Kimble HYDROLOGY...Curtis COASTAL..CLAIR/TUBBS is the U.S. government's official web portal to all federal, state and local government web resources and services.