Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 090935 AFDGYX Area Forecast Discussion National Weather Service Gray ME 435 AM EST Mon Dec 9 2019 .SYNOPSIS... A milder and increasingly moist southerly flow this morning precedes a low pressure system approaching from the Great Lakes today. This system will bring mainly rain to the area Monday and Tuesday along with unseasonably mild air. The mild weather and snow melt combined with the rain could push a few rivers to near bankfull by late Tuesday. Much colder air follows this system Tuesday night and Wednesday perhaps allowing for a light accumulating snow for the coastal plain as well as southern New Hampshire. An active weather pattern looks to continue for late week into the weekend. && .NEAR TERM /THROUGH TONIGHT/... Today, the pattern amplifies in response to a long wave trough axis driving south from the Plains of Canada into the upper Mississippi Valley and eventually into the Great Lakes. The long wave aloft picks up a closed mid-level low and transports it ENEward across the Great Lakes today and tonight, which drives sensible weather locally. Light snow this morning over the mountains lifts northward as southerly flow strengthens ahead of the approaching trough. Dry air in place has prevented much of the precipitation from reaching the ground, except for across the mountains and areas north where moisture depth is more favorable. Further south, an area of precipitation presently over the Mid- Atlantic will expand as it travels NNEward toward New England. Expect precipitation, mainly rain, to enter southern New Hampshire mid to late morning, spreading northeastward into western Maine through mid- day. Chose to err on the slower side of guidance with 00Z sounding suggesting a stout mid-level dry layer to erode before precipitation reaches the ground. At the same time, increasing surface dew points and WAA over snowpack will lead to the development of patchy fog this morning, lasting through much of the day. Mid-level warm advection is well on its way by this point with h850 temperatures reaching above freezing areawide about 12Z- 17Z. Meanwhile at the surface, some degree of a CAD is expected to stunt warming initially, and preserve some cooler pockets, but in general temperatures rise above freezing across lower elevations by noon. A brief period of mixed precipitation may impact interior NH and western ME upon onset as h850 warming outpaces surface warming, but without significant impacts. The core of strong mid-level winds driving rounds of rain today strengthens later today into tonight as a short wave trough axis rotates around the larger mid-level low. This ramps up forcing and moisture transport, yielding moderate rain and increasing winds. Better mixing with the crossing jet will allow temperatures to warm through tonight as WAA continues aloft. Mixing isn`t however strong enough to transport a strong portion of h850 winds in excess of 70kts to the surface. The southerly clip to winds near the surface does however allow strong winds to develop over the waters and to an extent over the MidCoast of Maine including the Penobscot Bay area. BUFKIT soundings generally paint sustained winds 20-25 kts along the coast and higher terrain of the MidCoast region with occasional gusts 35-45kts. A wind advisory has been posted for this evening into tonight as a result. For rain, expecting widespread amounts exceeding an inch across southern NH and coastal Maine along with in the south-facing upslopes of the Whites where rainfall totals may approach two inches. More details on this in the hydrology section below. Expect a substantial erosion of snowpack as a result tonight thanks to breezy conditions, high dew points, and rainfall. The broad area of stratiform rain exits to the northeast late tonight. && .SHORT TERM /TUESDAY/... For the better part of Tuesday, a lull in precipitation is expected behind the departing shortwave trough axis with dry air intruding in the upper levels, coinciding with a strengthening jet aloft. New England is placed in the right/exit region of this screaming jet in excess of 170kts at h300, leading to with synoptic descent. The exception would be light rain in southwest facing upslopes. Later in the afternoon into the evening, a lagging cold front crosses, with some uptick in precipitation activity but likely without much in the way of significant QPF. Cold advection does however spill in aloft behind the cold front and begins an airmass change for the region. Expect remaining light rain to switch to snow Tuesday evening. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... The long term periods starts Tuesday evening with a cold front just offshore and cold air bleeding into all of our zones from the northwest. At the same time a short wave trough will approach from the west, with some influence from the southern stream. The various members of the 00z model suite are in good agreement with some light overrunning precipitation moving northeastward in the wake the SFC cold front Tuesday night into Wednesday. With the column becoming colder with time, any light rain will turn to light snow from west to east as Tuesday night progresses. An ensemble approach was employed with the QPF and temperature forecast, allowing for 1-3" of snow on the coastal plain as well as the southern portion of NH. There will be a tight moisture/precipitation gradient with northwestern sections of the forecast are not seeing anything except cirrus come Wednesday morning. Thereafter fair weather follows for a few days, but the pattern continues busy with a few more potential fairly high amplitude weather systems on tap, one for the weekend and another for early next week. Some mixed precipitation may be involved for both, but at this time modeled amplification of the mid and upper level pattern suggest a couple of chances for heavy rain. Will continue to monitor. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... Short Term /Through Tuesday/...A transition to MVFR ceilings from south to north is well underway this morning amid strengthening southerly to southwesterly flow. IFR in -ra and fg arrives across the south and coastal terminals by noon today, early afternoon further north. Additionally patchy fog may impact sites at times today with warm air over snowpack. This afternoon, rainfall intensifies with increasingly gusty southerly winds and widespread IFR or worse ceilings and visibility. These conditions persist into tonight, with rain and associated VIS restrictions pulling out early Tuesday morning with winds turning southwesterly, although patchy may allow VIS restrictions to linger through the morning. CIG improvements are possible Tuesday in a lull between rounds of precipitation. Of note, LLWS is a concern throughout the short term, primarily this evening with southerly to southwesterly LLWS on the order of 45-55 kts developing. Long Term...Rain will change to light snow across the forecast area Tuesday night and last through a good portion of Wednesday. This will allow for IFR conditions, especially on the coastal plain of Maine and southern New Hampshire where 1-3" of snow will be possible later Tuesday night into Wednesday. VFR conditions are then expected Wednesday night through Friday night. && .MARINE... Short Term /Through Tuesday/...Southerly flow strengthens through today, ahead of an approaching frontal system. This evening and tonight, southerly gale force winds develop area wide with storm force winds impacting the eastern two-thirds of the waters including western Penobscot Bay. Winds decrease gradually Tuesday with SCAs persisting under gusty southwest flow, turning northwest Tuesday night behind a cold front. Long Term...SCA conditions will linger through Tuesday night and Wednesday morning before diminishing. However, winds will quickly ramp up again to SCA levels Wednesday night and Thursday, with gales not out of the question. && .HYDROLOGY... Mild temperatures, high dew points, and periods of rain will impact the area Monday and Tuesday and erode snow cover across the region, particularly over southern New Hampshire and coastal western Maine. While the fresh snow will initially absorb rainfall, the snow will likely ripen and begin melting. This will lead to an influx of water into local waterways. At this time, southern New Hampshire has over a foot of snow in places with liquid equivalent likely exceeding 1", with some areas possibly closer to 2". Moderate rainfall in excess of an inch is expected over this area and may lead to instances of localized flooding in poorly drained areas. River forecasts also suggests a flood threat for waterways, particularly across flashy tributaries in the Whites and southern New Hampshire. Although no flooding is forecast, action stage is expected to be reached on the Swift (Roxbury), Pemi (Woodstock), Suncook (N. Chichester), and on the Saco (Conway). && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Wind Advisory from 6 PM this evening to 4 AM EST Tuesday for MEZ022-027-028. NH...None. MARINE...Gale Warning from 6 PM this evening to 4 AM EST Tuesday for ANZ153-154. Storm Warning from 9 PM this evening to 4 AM EST Tuesday for ANZ150>152. && $$ NEAR TERM...Casey SHORT TERM...Casey LONG TERM...Ekster

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