textproduct: Gray - Portland

WHAT HAS CHANGED

Precipitation amounts have backed off a little bit and have shifted farther south. Still think we will see some light accumulations over the higher elevations.

KEY MESSAGES

A coastal storm will continue to bring rain and snow to portions of the forecast area today into tomorrow. Winter conditions remain possible tonight, with the highest confidence of accumulating snow at elevations over 1000 feet.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An broad upper-level trough is currently located over the eastern CONUS as a surface low is deepening over the Mid- Atlantic. A mid-level shortwave is starting to spread over the southern parts of the region, leading to developing precipitation along an around a west to east oriented stationary front. Rain will continue to increase in coverage and intensity through the rest of today across the southern half of New Hampshire and portions of western Maine (mainly the southwest) as the surface low starts to lift into southern New England and eventually out into the Atlantic. Some of the guidance in previous days had this low moving up into the Gulf of Maine but we seem to have converged on a farther south solution which will likely keep eventual QPF totals suppressed. This makes sense as the system is obviously fighting a very dry airmass to the north.

Still, as the low advances north and the shortwave influence spreads across southern New Hampshire and southwest Maine, we will see precipitation coverage and intensity increase in during the evening and overnight hours before moving out quickly on Friday. Looking at potential QPF totals, NBM 24-hour probabilities suggest that the best chances for a half inch of liquid will be over southwest and south central New Hampshire. Here, the latest NBM expects a 40 to 80 percent chance of at least a half inch (probabilities decrease quickly moving east and north). Decreasing the threshold to a quarter inch, the moderate to high probabilities expand into southeast New Hampshire and extreme southwest Maine. So, for most locations across the south, expect a broad quarter to half inch of liquid with amounts decreasing rapidly moving north.

As far as frozen precipitation goes, we have see a few reports of light ice pellets early this afternoon so wouldn't be surprised if some of that activity continues through the rest of the afternoon as the atmosphere moistens. We will see snow starting to mix in some location through the afternoon and some locations may change to all snow tonight, especially at elevations over 1000 feet across southern New Hampshire (the Monadnocks sill appear to be the best chances for a couple inches of accumulation). Upslope precipitation could lead to some minor accumulations along and south of the White Mountains as well. NBM probabilities have decreased markedly for accumulations outside of the higher terrain, but many locations across the south could still see their first flakes of the season and/or a brief dusting.

After the bulk of steady precipitation moves out early on Friday, we could still see a few rain/snow showers hanging around over the mountains. Saturday looks mainly dry with a few more hit or miss shower across the higher terrain but an upper low dives down from east central Quebec Saturday night into Sunday morning, which could lead to better coverage of showery activity. High pressure then moves in at the end of the weekend and into next week, which will keep the forecast dry with moderating temperatures.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday: Widespread MVFR to IFR conditions will continue to develop from southwest to northeast through the rest of today. MVFR to IFR conditions then prevail through Friday morning as rain changes over to or mixes with snow across the interior. Brief LIFR conditions will be possible at times.

Outlook...

Friday: MVFR/IFR to start, but precipitation tapers off through the morning. Ceilings may remain MVFR. Moderate confidence.

Saturday-Sunday: Improving to VFR at most terminals. HIE may stay MVFR or near MVFR. Moderate confidence.

Monday-Tuesday: VFR conditions expected. Moderate confidence.

MARINE

Northeasterly gales are likely tonight into Friday over the outer waters as low pressure passes south and east of the Gulf of Maine. SCA conditions are likely over the coastal waters and possibly in the bays. 25 to 30+ knot winds likely continue through Friday night along with 5-7 foot seas. Sub-SCA conditions are then anticipated through the weekend and into at least the middle of next week as high pressure builds into the region.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 7 PM this evening to 7 PM EST Friday for ANZ150>154. Gale Warning from 1 AM to 1 PM EST Friday for ANZ180-182-184.