Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 191715 AAC AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 1215 PM EST Fri Jan 19 2018 .SYNOPSIS... A weak ridge of high pressure will build into the region today with temperatures a bit warmer than yesterday. A milder flow and above normal temperatures follow for the upcoming weekend. A storm system moving into the Great Lakes will bring wintry precipitation Monday into Tuesday with temperatures warming. A cold front Tuesday night brings colder air back into the region. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1215 PM Update... Minor changes to the forecast for temperatures and dew points. No other changes at this time. 925 AM Update: Only minor changes this hour to match radar trends and early morning temperature observations. With snow showers exiting to the east and anticyclonic vorticity advection overspreading the area behind compact shortwave...expect dry conditions for the remainder of the day with a mix of sun and clouds. Update... Forecast update was to continue to increase the chance of snow showers for the line of precipitation. This area of snow showers will exit New Hampshire and enter western Maine early this morning with the precipitation exiting the Midcoast region around 8 am. Made minor adjustments to the current temperature, dew point and wind forecasts as well. Prev Disc... A short wave will cross the region this morning. This system is relatively moisture starved, however there could be a brief period of scattered snow showers across central and northern portions of the region this morning. The chance for snow showers will continue into the afternoon across the far north as a broad area of warm air advection develops in the Northeast. We will be beginning a gradual warming trend. Expect high temperatures to mainly be in the 30s in the central and southern portions of the region this afternoon, with 20s in the far north. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Low pressure will pass to our north tonight, limiting any scattered snow showers to the far north. Warm air advection during the overnight hours will lead to a non-diurnal temperature trend with temperatures warming late at night. The morning will be followed by a weak cold front and modest cold air advection. Nevertheless, expect some melting to occur with readings in the 30s in the north to the 40s in the south. There will be a continued threat for scattered snow showers in the mountains. This will mainly be across the northwest facing upslope regions of the higher terrain. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Well, if it hasn`t been advertised here yet, the persistent western ridge / eastern trough pattern that we`ve been stuck in for about the past 5 weeks is in the process of breaking down. December was 4.5 degrees above normal in Las Vegas and 5.5 below normal in Portland, illustrating the pattern that has been in place with few brief exceptions for much of the winter now. This, however, is changing. A trough is moving onshore into southern California and will track generally east northeastward across the country and arrive in New England early next week. Behind it a more zonal, or at least much less amplified, flow is expected for much of the rest of the month, transporting Pacific air into North America with more quickly moving ridges and troughs. Meanwhile a cold pool of air lingers over Hudson Bay and northeast Canada, close enough that occasional colder surges into New England are still possible. Ahead of the Southwest trough, a broad ridge will build over the central and eastern parts of North America, with warmth from the desert southwest being transported northeastward into the Midwest. Temperatures in the southwest deserts have been in the 70s, even warmer than tropical Florida! As the trough moves out onto the Great Plains this weekend it will pull in rich moisture from the Gulf of Mexico. That warm, moist air will condense into precipitation over the Midwest as it moves into the colder northern latitudes. The trough moves into the Great Lakes early next week with the downstream warmth moving into the Northeast. By the time it arrives here it will be significantly modified. In fact, just how much of this warm air we eventually feel at ground level in eastern New England is highly in doubt as it may just advect in above a colder layer of air stuck near the ground leaving the low levels inverted. This could be a problem as precipitation moves in. After the trough axis eventually moves across New England we will see some of that cold air over eastern Canada pulled south behind the trough, lingering with us through the end of the week. As for the details of the daily forecast, we start here with Sunday. The trough that moves through Quebec Saturday will slide a cold front southward into northern New England Saturday night into Sunday. Ahead of the front, a westerly flow will keep mild air and downsloping conditions over the coastal plain, leading to temperatures in the 40s. But as the day goes on the colder air will be bleeding in from the north with the front settling into southern New England by Monday morning. The core of the cold air will be over Quebec and northern Maine with a northeast flow keeping this locked into eastern New England for Monday in spite of the ridge building aloft ahead of the approaching trough. Expect temperatures near or below freezing for highs with the low level flow becoming southeasterly as the surface high moves into eastern Maine and New Brunswick. It looks like quite a complex forecast Monday night into Tuesday. Models not in full agreement on the timing of the trough moving through the great lakes. The GFS is about 6 to 12 hours faster than the ECMWF, with the GFS lifting a warm front into central New Hampshire and the foothills of Maine with a newly forming secondary surface low forming and tracking eastward along it. The ECMWF however holds the trough back a little bit later, with the previously mentioned cold front pushing further south into southern New England before lifting north to a stationary position over southern New Hampshire merging with a coastal front along the Maine coastline. The Great Lakes low then pinches off with the new low forming along this warm front right along the Maine coastline. With this track the developing surface low along the coast is deep enough to keep enough cold air in for a longer duration of snowfall at more locations. The ECMWF has some support from the CMC and also follows a more climatologically favored path, thus I am inclined to believe the ECMWF a bit more than the GFS, though the current forecast does represent at least somewhat of a blend between the two. Even the GFS has some kinks in the isobars suggesting cold air damming will play more of a role than the model itself suggests. Current thinking is that there will be a period of light snow Monday night into Tuesday morning followed by another round of precipitation during the day on Tuesday. As the second round of precipitation arrives, it may fall as freezing rain for a good portion of central New Hampshire and the coastal plain of Maine, while temperatures along the immediate coast and southern New Hampshire warm into the 40s with rainfall. Further inland in the mountainous areas there is a better chance that precipitation stays as snow. Seeing as this is Day 5 of the current forecast, the gridded forecast is necessarily broad and includes multiple precipitation types possible, though not all may be observed. The best chance of icing is within our typical icing corridor from about Fryeburg to Augusta. As the low departs Tuesday night a cold front will push across the area. It may take until Wednesday for the colder air to really be felt as we will not be all that warm in the "warm sector" ahead of the front Tuesday in most areas. Expect some lingering snow showers in the mountains, but have also included some low chances for light snow even toward the coastal plain as the remnant occluded low and upper trough move near the area. Have been surprised by enough of these light snows in the wake of deeper passing lows this season to avoid going full nil for PoP on the coastal plain in this situation. Cold high pressure builds in for Thursday and especially Thursday night. Expect highs in the 20s, with lows falling into the single digits and teens (near zero in colder northern spots), a few degrees below normal. && .AVIATION /17Z FRIDAY THROUGH TUESDAY/... Short Term...Mainly VFR conditions across the region, however northernmost regions such as HIE will likely have a period of MVFR conditions. Long Term...May see some MVFR or IFR ceilings moving in Saturday night into Sunday north of the mountains as a cold front drops in from the north. Expect increasing clouds area wide on Monday with ceilings lowering to IFR Monday night into Tuesday with light snow arriving. Snow changes to a wintry mix for much of the area on Tuesday. && .MARINE... Short Term...Winds and seas will increase late tonight and Saturday. There will be a period of SCAs, especially out over the outer waters. Winds may approach gales, but confidence is not high enough to run with that headline at this time. Long Term...As the next trough/low move through on Tuesday, winds ahead of it could reach gale force. Colder air moves in behind a cold front Tuesday night into Wednesday with winds likely at least warranting a Small Craft Advisory if not another period of gales. && .HYDROLOGY... Have lowered the warning for the Kennebec River at Augusta this morning. River continues to slowly recede. People should continue to monitor river levels for any significant changes in river levels. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 6 AM to 6 PM EST Saturday for ANZ153. Small Craft Advisory from 1 AM to 6 PM EST Saturday for ANZ150>152-154. && $$ NEAR TERM...Hanes

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