textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes to the forecast with this update.

KEY MESSAGES

1. Temperatures turn cooler behind a passing cold front through the end of the week.

2. Well below normal temperatures return this weekend. Bitterly cold temps will combine with gusty winds to produce dangerous wind chills. A cold weather watch is possible in subsequent shifts.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front passes through during the afternoon hours today, with some scattered snow showers and flurries. The front weakens on its way through, so we don't see a major shot of Arctic air with this front; that holds off until the end of the week. With this front, cooler air filters in through tomorrow night.

Radiational cooling conditions improve late tonight as clouds clear and winds ease. Lows are expected to bottom a few degrees either side of 0 tonight, with the coolest readings in the valleys south of the mountains. More clouds linger north of the mountains in weak upslope flow.

Highs range from the mid teens across the north, to the upper 20s along the coast tomorrow. Mainly sunny skies are expected south of the mountains, with more clouds lingering north. A steady northwesterly breeze persists through the day.

More favorable radiational cooling conditions are expected across the north Thursday night as skies clear and winds ease through the evening. High clouds begin to filter in from the south through the overnight hours ahead of the next front, but temps fall into the single digits along the coast, to below zero across the interior before these clouds begin to have any influence.

KEY MESSAGE 2 DESCRIPTION...

A strong cold front will cross the region at some point on Sat. While timing may shift several hours from current forecasts, what is high confidence is that temps will plummet once more. Ensemble guidance is showing the greatest temp anomalies centered southwest of the forecast area, but guidance is still on the order of a once every 2 to 5 year air mass at 850 mb. NBM temps seems reasonable at this range, but Bufkit forecast soundings show mixed layer tapping 30 to 40 kt winds aloft. That should be more than enough for wind gusts up to 30 kt. I blended in some 90th percentile winds/gusts to the forecast to account for this. The result will be dangerously cold wind chills beginning Sat night and possibly last thru Sun night for some northern zones. Currently large portions of the forecast area have wind chills in the 20 to 30 below range Sat night and slightly warmer Sun night, so it seems reasonable that headlines will be necessary at some point. It is possible that conditions will be cold enough that a cold weather watch may be issued prior to the weekend to ensure awareness.

Otherwise the passage the of the front will bring some light snow or snow showers. There are some hints of an inverted trof developing and that could provide the focus for more prolonged and steady snowfall, so I am not ready to rule out some light accumulations yet. NBM forecast is already at high chance to likely PoP, so I did not see any reason to adjust that.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Mainly VFR persists at most terminals through Thursday night. The exception will be HIE, with MVFR conditions likely lingering through tomorrow night. Marginal MVFR ceilings are possible at LEB at times, but mainly VFR prevails.

Outlook:

Friday: VFR conditions expected.

Friday Night: VFR conditions expected.

Saturday: Areas of IFR or lower are possible in snow/snow showers especially near the coast. Second half of day westerly wind gusts of 25 kt or more possible.

Saturday Night: VFR conditions expected south of the mtns, local IFR conditions in the mtns. Westerly wind gusts of 25 kt or more possible.

Sunday: VFR conditions expected. Westerly wind gusts of 25 kt or more possible.

Sunday Night: VFR conditions expected.

Monday: VFR conditions expected.

MARINE

A cold front crosses the waters this evening. Marginal SCA conditions are possible around the time of the frontal passage, but otherwise mainly fair conditions are expected as broad high pressure builds across the waters through Thursday night. Winds and seas below SCA thresholds Fri but a strong cold front will cross the waters Sat. Westerly winds will increase sharply and become gusty. Gale force conditions are possible, especially outside of the bays. Temps will also fall into the single digits and moderate to heavy freezing spray will develop beginning Sat night. The gusty winds and at least areas of moderate freezing spray continue into Mon.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.