textproduct: Gray - Portland

WHAT HAS CHANGED

With the expectation that low clouds, fog, and freezing drizzle form tonight and showers forecast to move in towards dawn, a Winter Weather Advisory has been issued for the overnight into mid morning Saturday.

KEY MESSAGES

1. Prolonged period of near-freezing, dreary weather expected through Saturday.

2. Warm temperatures next week will bring about a spring thaw, greatly reducing the snowpack and causing some ice movement on area rivers. The flood risk remains low, but with expansive river ice across the area levels will have to be monitored closely.

3. A chance for widespread precipitation starts the the second half of next week with temperatures returning closer to seasonal normals.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The morning snow has moved out and warm advection is beginning in its wake. Visible satellite is showing clear northward movement of cloud cover at this hour and gradual upglide over the cold surface air is expected thru tonight. This should establish a nice low cloud deck that at least banks up against the mtns. There are two main concerns as this happens this evening. One is that weak ascent and shallow moisture will promote drizzle formation tonight. With temps largely remain below freezing this will mean freezing drizzle. Added to this threat is that some warm advection showers are forecast to arrive around daybreak Sat where temps are most likely to remain below freezing. With the light ice accumulation forecast, I have issued a Winter Weather Advisory. It is possible that if temps are cooler than forecast this may need to be expanded. The second concern is for fog development. Hi-res guidance is fairly bullish on this forming around or after midnight. If we can get those dewpoints to near freezing over the snow this should result in at least some areas of fog. I cannot rule out the chance that fog is dense in places and we may have to hoist a Dense Fog Advisory.

KEY MESSAGE 2 DESCRIPTION... The overall pattern to start next week favors mean troughing with a strong Atlantic ridge in place, but a couple days of southwest flow looks to warm 850 mb temperatures to around +3-6C which is nearly 20 degrees above normal for this time of year. This should be good for widespread temperatures topping out in the 50s on Monday and Tuesday. At this point the confidence in a couple warm days is high, however there are some factors that may limit temperatures both days from going quite as high at what the NBM and other models have been advertising. Low pressure slides just north of the area Monday which could add some clouds into the mix. Whether it's enough to actually stunt warming will have to be fleshed out as we get closer, but we usually seem to end up with more clouds than models show when we are in the middle ground with low pressure to the north. Clouds won't be an issue Tuesday as high pressure begins to push in, but we will have a cold front on approach as well. This would most likely impact temperatures in the north as the front likely doesn't push much past that point during the day, and the difference would be 40s versus 50s.

Bottom line: It's going to feel warm and we are going to see significant reductions in snow pack and some movement in river ice as a result. A key takeaway here is that dew points stay in the mid 30s so melt is going to be controlled and therefore the flood risk remains low (but it is never zero). We do have expansive river ice across the area which will need to be monitored, but latest observations suggest that it is still around a foot thick on the larger mainstems. Ice of that magnitude would require rapid river rises to break that up and that is not anticipated with the little QPF and melt that is anticipated through the forecast period. KEY MESSAGE 3 DESCRIPTION... Low pressure rides along the aforementioned front into New England late Tuesday into Wednesday. Fairly large model spread on 850 mb temperatures during this time is leading to some uncertainty in precipitation types along models. The GFS and Canadian quickly usher in strong high pressure to the north and expect it to start cooling the column leading to more mixed precipitation while the Euro keeps warm air locked in at the surface, and thusly keeps the dominant precipitation mostly rain.

Regardless of how quickly models think it occurs, confidence is high that we return back to more seasonable temperatures for the second part of the week with temperatures looking to top out in the the mid- to upper 30s during the day, and bottom out in the 20s and teens at night.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Widespread MVFR CIGs will begin to lower this evening from south to north as onshore flow and warm advection increase moisture over cold snow pack. Widespread IFR or lower conditions are expected south of the mtns. Freezing drizzle will also be possible at all TAF sites until temps begin to warm Sat morning. HIE may remain VFR or MVFR as low cloud cover struggles to make it past the mtns.

Outlook:

Sunday: Becoming VFR, except in the mtns where MVFR CIGs may linger. Westerly surface gusts to 25 kt possible in the afternoon.

Sunday night-Tuesday: VFR conditions expected.

Tuesday night: Conditions becoming MVFR/IFR as rain and snow move into the area overnight.

Wednesday-Wednesday night: MVFR/IFR conditions continue. There is uncertainty in precipitation type for this time period, so widespread IFR is not out of the question if the forecast trends colder and terminals see more of a mix or snow.

MARINE

Northeast winds will gradually turn east and then southerly thru Sat. Seas are expected to remain above 5 ft even while winds diminish, however eventually increasing southerly winds ahead of a cold front will also begin gusting above 25 kt again Sat. The front will cross the waters Sun with winds and seas gradually diminishing. SCA has been extended thru Sat for all of the outer waters.

Wave heights will remain around 5-6ft into Tuesday morning. A brief break in SCA conditions occurs Tuesday afternoon and night before winds and seas ramp up Wednesday as a disturbance moves through the Gulf of Maine. There is even some suggestion that gale force gusts may be possible at times through Thursday. Seas fall below 5ft Thursday afternoon, but winds gusts remain 25-30kts through Friday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 7 PM this evening to 10 AM EST Saturday for MEZ007>009-012>014-018>022-033. NH...Winter Weather Advisory from 7 PM this evening to 10 AM EST Saturday for NHZ001>006-009. MARINE...Small Craft Advisory until 7 PM EST Saturday for ANZ150-152- 154.