textproduct: Gray - Portland

WHAT HAS CHANGED

With more sunshine that forecast I have adjusted the near term temps upward, though backdoor front will begin to send those back down towards freezing thru the evening. Mixed precip is also expected to be more short duration/less widespread across southern NH. So snowfall totals have been adjusted up slightly, but no change to the current headlines.

KEY MESSAGES

1. Snow and mixed precipitation expected tonight into Friday morning.

2. Prolonged period of dreary weather anticipated Friday evening through Saturday.

3. The warming trend continues through mid week next week. While mostly dry weather is expected early week, the warm conditions will lead to a substantial reduction to the snowpack, especially south of the mountains. Due to the warm environment, rain appears likely as a system approaches from the west on Wednesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Precip continues to gradually lift towards the forecast area this afternoon. At the same time a backdoor cold front is crashing thru eastern Maine/Gulf of Maine. So while temps today have been able to beat guidance due to all the sunshine across the middle of the forecast area, those readings will begin falling quickly thru the late afternoon. Once precip moves in it will be fighting a rather mild antecedent air mass. For this reason I anticipate that precip starts as sleet or freezing rain across southern NH while temps remain warm in the mid levels. However model guidance is forecasting a fairly robust band of frontogenesis due to warm advection sliding across Mass overnight. The increasing lift and latent cooling due to melting precip will both act to cool those mid level temps. Within a few hours I am forecasting mixed precip to flip to snow. I am maintaining a slight chance of sleet or freezing rain, but I expect the predominant ptype to be snow into Fri morning. As a result snowfall totals have ticked up again for southern NH. Generally looking at 3 to 5 inches from the Monadnocks to the Seacoast, with the best chance for mid level warmth to hang on and mixed precip to linger across southwestern NH. I do not see a reason to expand the Winter Weather Advisory at this time, mainly given the limited extent of mixed precip and the fact that most of the snow will end prior to the morning commute.

KEY MESSAGE 2 DESCRIPTION...

Once precip moves out Fri the forecast area will remain under southwest flow aloft. I still anticipate that cold air damming will hang tough across most of the forecast area. There is plenty of snow on the ground, so any warmth trying to push in will give way to latent cooling due to melting helping to reinforce the dam. So this is a recipe for plenty of low cloud cover and likely some fog developing. By Fri evening the boundary layer moisture will be increasing along with the lingering weak ascent due to warm advection. So I am anticipating drizzle developing overnight. There will likely be portions of the forecast area that remain below freezing, so freezing drizzle is a concern. Once confidence is sub-freezing area goes up, it is possible that winter weather advisories may be needed, though thankfully this will not be occurring on a weekday.

KEY MESSAGE 3 DESCRIPTION...

High pressure and a highly amplified ridge pushes into the region behind an exiting cold front Sunday. Cooling from the front itself is muted, and will mainly serve as a pause. Warming regains momentum through midweek as daytime highs look to push well above normal.

850mb temperatures begin to push into the 90th percentile climo early this week, and by the time Wednesday arrives, values closer to the 99th percentile are possible. How this exactly impacts surface temperatures will depend on cloud cover and surface features that sometimes evade representation in longer range guidance.

The going forecast does support temperatures rising into the 50s daily Mon - Wed. Southern portions of the forecast area could even see some lower 60s Tuesday and Wednesday when those 99th percentile 850mb temps near. Overnight lows cool into the 30s, and this will keep the warm up from being a full on torch for snowpack. That said, gradual melt is expected through this warm period and it will likely do a number to existing snow. Stream rises are possible, and ice movement on rivers and streams is anticipated south of the mountains. However, given the low streamflow conditions (historically low in some areas, including Oxford, ME and Windham, ME), there should be more leeway than average before minor flooding conditions are met.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Widespread VFR conditions across the area at this hour. The combination of moist, onshore flow and approaching precip will mean a sharp deterioration to IFR or lower conditions along and south of a LEB to PWM line this evening and overnight. Eventually low CIGs are anticipated as far north as AUG and RKD, but that likely will hold off until Fri morning. Initial ptype is forecast to be freezing rain or sleet at MHT and PSM, but within a few hours should transition to snow. Wintry precip moves out of the area Fri morning, but low CIGs linger thru the day.

Outlook:

Friday Night: Areas of IFR in low CIGs and potential drizzle/freezing drizzle.

Saturday: Areas of IFR in low CIGs and potential drizzle/freezing drizzle.

Saturday Night: Reduced cig/vis favors the ME/NH coast with some improvement for interior NH and far western ME. Drizzle and some SHRA continues. LLWS possible into Sun AM.

Sunday: Widespread improvement to VFR as a cold front pushes east. Vis may still be limited just offshore from coastal terminals.

Sunday Night: Warm temperatures over melting snowpack may produce ground fog that results in lowered visibility. Cigs should be unlimited unless ground fog thickens.

Monday and Monday Night: VFR expected with no sig wx.

MARINE

Backdoor cold front is driving thru the Gulf of Maine at this time. Northeast winds are gusting to near gale force behind the front. SCA remains in effect for the outer waters into Fri. Seas will continue to build thru Sat morning, and become more chaotic as winds shift from northeast to south at that time. An extension of the SCA for seas will likely be needed at some point Fri.

Winds become west Sunday behind exiting cold front. SCA threshold winds may spread over the waters Monday into Tuesday, but there may be a surface inversion that limits how many of these gusts reach the sea surface.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Winter Weather Advisory from 6 PM this evening to 1 PM EST Friday for NHZ011>015. MARINE...Small Craft Advisory until 7 PM EST Friday for ANZ150-152-154.