Scientific Forecaster Discussion
000 FXUS61 KGYX 130446 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1146 PM EST Thu Dec 12 2019 .SYNOPSIS... High pressure moves east tonight with a warming southerly wind beginning on Friday. Low pressure tracking up the East Coast will spread rain across the region Friday night and Saturday, although this could begin as some light icing especially in interior areas. Low pressure moves off to the northeast Saturday night with a cold front ushering colder air back into the area from the west. Some showers could linger into early Sunday with the temperature falling into Monday. The next low pressure system will bring some snow on Tuesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Update... Temperatures have steadied off now after dropping quickly with radiational cooling during the evening hours. Clouds will continue to gradually thicken during the overnight hours as warm air advection begins over the region. Sufficient warm air advection to allow for a couple snow showers across our northern border in Canada which the mesoscale models are depicting late this evening. Most of the remainder of the forecast will be unchanged. However, 00Z guidance is upping the QPF for the tomorrow night into Saturday period. Prev Disc... At 3pm a 1040 high was centered over New Jersey. Ahead of this subsidence was keeping skies clear over northern New England and into the Maritimes. A few high clouds creeping into Vermont foreshadow the next day. Warm advection cloud shield stretches all the way up Lakes Erie and Ontario with southerly flow pushing temperatures into the 40s. As this begins to approach our area overnight look for cloud cover to increase as low pressure begins to develop over the Ohio River Valley. With increasing cloud cover expect overnight temperatures to be a bit warmer, and yet with the falling temperatures aloft the overall effect will be low similar to last night with single digits in the north. Because of this have strayed away from the colder MOS guidance and instead gone with a consensus of the raw models. This will be largely dictated by the cloud cover and the eastern portion of the forecast area, where clouds may be slower to arrive is more likely to have the coldest spots. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/... Friday will see a warming trend and increasing clouds as the precursor to the next storm. Initially high pressure retreats to our east, allowing southerly flow to develop. Expect a steady increase in temperatures as the high pressure remains on a similar latitude to us and will not provide as much help to hold the cold air. Despite this, with the cold initial temperatures things will be slow to warm and many locations may stay below freezing tomorrow. With temperatures below freezing in the valleys through midday the concern becomes whether any precipitation will develop prior to the temperatures warming. While the upper levels remain dry, the lower levels moisten giving a chance for drizzle to develop on Friday afternoon. With surface temperatures remaining marginal expect pockets of freezing drizzle across southern New Hampshire. It will be a race between warming temperatures and the onset of precipitation to determine whether it will be freezing drizzle or drizzle initially and with this uncertainty still there at 24hrs out will leave it for the next shift to determine if a Winter Weather advisory is appropriate for a portion of the area. Friday night our focus shifts to the developing low. Consensus is now clear for a track to the west of us, allowing our area to reach into the warm sector, at least aloft. Steady precipitation will develop after midnight on Friday night and continue through the day on Saturday. Expect precipitation to be rain for all but the most northern zones where an initial period of snow my change over to sleet at elevations. Saturday the evolution of the temperatures may be a bit tricky. Some colder air is likely to linger through the coastal plain, although this will still be above freezing, just not as warm as southern New England. Will likely see temps hold in the mid 30s across the coastal plain while would not be surprised to see 50s on the water as a coastal front sets up. May even manage to get KMWN as a warm point in the area for a while as the depth of the warm layer increases beyond 6kft. With a direct connection from the warm ocean to our south expect precipitation amounts to be impressive for this time of year and 1-2 inches is expected. A focus for that will likely be along the coastal front. This highest risk for flash or areal flooding would be along this precip axis along the coast. However without any snow on the ground in this area there will be nothing to block drains or contribute to runoff, which limits the flooding. On the other hand, frozen ground may make what falls runoff that much more. This will be something to monitor over the next day or two but right now do not feel there are enough red flags to warrant a flood watch on the basis of rainfall. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Low pressure moves NNE through New England and into the Saint Lawrence Valley Saturday night with a cold front moving west to east behind it. This front will be mostly through our area by Saturday evening, bringing an end to the steady rainfall. However, behind the system some showers could linger especially in the mountains. The temperature will be falling as cold air pours in from the west. Expect precipitation to shift to a rain or snow pattern depending on the surface layer temperature. The cold continues to pour in on Sunday. Although high temperatures may reach the 40s on the coastal plain due to good downsloping, this could actually occur in the morning with temperatures falling during the afternoon. In this sort of flow we have to start thinking about wind speeds as a cold advection wind would be especially favorable for mixing down stronger winds from aloft. 850MB winds of 50KT suggest there is a good deal of stronger gusts to mix down, so have adjusted the forecast a bit higher on wind speeds than what the multi-model blend suggests. At this point widespread gusts to 35 MPH or so are expected, though it is possible that we eventually need a wind advisory here. High pressure nudges its way eastward on Monday with temperatures likely topping out near freezing in the south and near 20 in the north with winds decreasing through the day. Monday night would typically be the coldest night behind the front, and we will get in on some decent radiational cooling in northern and eastern areas. However, expect increasing clouds as the night goes on as our next low pressure system begins affecting the area. This will likely limit cooling for most of the area with lows in the teens and 20s. The next low tracks to our south on Tuesday and at this point it looks like this will be a mostly snow event for our area. Although it's too far out for official snowfall amount forecasts, it looks like a light to moderate snowfall event centering on southern New Hampshire and coastal Maine. Even colder air pours in behind this low as it departs on Wednesday. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... Short Term...VFR conditions will hold through the next 24 hours as high pressure moves to the east. Increasing clouds will remain above 10kt initially before lowering steadily tomorrow afternoon approaching MVFR by 00Z Saturday when light precip will develop across the western portion of the area. Long Term...Should see conditions improving for most areas Saturday night as the steadier rain moves to the east. However, lingering MVFR conditions are likely in the mountains through the day Sunday. It will also be rather windy on Sunday as west winds gust to 30 or 35 KT at times. These winds gradually diminish into Monday with conditions becoming VFR even for the mountains. && .MARINE... Short Term...Gale watches or warnings may be needed per current forecast model solutions for next upcoming event in some areas. High pressure will cross the waters tonight into tomorrow. Another low will move up the St Lawrence valley on Friday night into Saturday. Increasing southerly flow may bring winds to near Gale force, but with some uncertainty in the low track will hold off on a Gale watch for the moment. Long Term...West winds behind the departing low will likely gust to gale force over the Gulf of Maine on Sunday with these winds diminishing on Monday. The next low pressure passes to our south on Tuesday with winds likely remaining below advisory levels with this one. && .HYDROLOGY... Most rivers have long since peaked after our recent rainfall event, although river levels remain high. Most of the snow south of the mountains has been eradicated except for some areas of southern New Hampshire. We expect another round of rain beginning Friday night and lasting through Saturday. A general 1 to 2 inches of rain is expected, however 00Z guidance continues to suggest there could be locally higher amounts. Areas most likely to see rises closer to flood stage would be in southern and possibly central New Hampshire where some continued snow melt. && .TIDES/COASTAL FLOODING... We are entering a period of higher astronomical tides, and this will coincide with the next storm system moving through the area on Saturday. The highest tides are on Saturday and Sunday afternoons, each at 10.6 FT at Portland. The tide on Saturday will have the greatest potential for an added storm surge as onshore flow may increase water levels by about a foot. Waves on top of this could cause splash over and beach erosion in vulnerable areas. By Sunday winds will be offshore and expect the surge to no longer be a factor. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None.