Marine Weather and Tide Forecast for South Portland, ME

NEAR TERM /THROUGH TONIGHT/
NW breeze continues this afternoon as pressure gradient slackens and high pressure nudges northeast. The same can be said for remaining light snow NW of the mountains and near the US/CAN border. By the evening, both should subside.

High pressure will quickly cross overnight with increasing cirrus. It will take most of the night for this to thicken and lower. Depending on when winds lighten and daytime clouds dissipate late this afternoon, temps this evening will easily cool along the coast and interior. Overnight lows may be realized shortly after midnight while cirrus overtakes the forecast area.

SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/
Low pressure over the Great Lakes Sunday morning will accelerate NE through Ontario and Quebec. Onshore flow, channeled behind the exiting high pres and incoming low will bring warm, moist air inland as the day progresses. While cold ahead of this system, winds won't be conducive for CAD, bringing 40s across the interior into the afternoon and evening.

That said, snow should be the supported precip type across the forecast area through the morning. Moisture profiles support spots of light snow or snow showers through noon. After, the rain/snow line marches inland as precip rates increase. The system will be moving quick enough to keep above freezing temps from working NW of the mountains most of the event, so all snow is expected here. Accumulations of 1 to 3 inches will be possible, with the greater amts outside of the valleys and at elevation. Snow rates may be impressive, with healthy omega within the snow growth region. HRRR 1hr snowfall highlights the higher terrain with likely probabilities of around 1 in/hr. This is less likely in the valleys with more mild temps and lower ratios.

Precip rates for the CWA peak during the mid to late afternoon.
Again, at this time, much of the population corridor will be rain, with points north of the Whites snow.

Steady precip pulls NE after 8pm as RH aloft dries and brings about lighter and spotty precip. W winds again keep mtn snow showers going as CAA begins. Overnight lows quickly retrace cooler, back into the 20s to around 30.

LONG TERM /MONDAY THROUGH SATURDAY/
Key Message: First widespread accumulating snowfall possible for the coast and southern NH. Generally cooler than normal temps thru the extended.

Impacts: Accumulating snowfall is becoming more likely for at least parts of the area Tue into Wed. Hazardous travel conditions are possible...especially for the evening commute Tue.

Forecast Details: High pressure will be building across the region Mon. This will bring a quick end to any lingering snow showers in the mtns as the ridge axis centers over the area by the evening. That could set up a very cold overnight into Tue morning if high cloudiness can hold off. Widespread temps in the teens and some single digits would be the result. Even if cloud cover were faster than modeled...low temps would be more affected in the northern valleys where readings would be closer to 20.

However most of the attention in this period will be on the evolution of the shortwave trof diving into the Pacific Northwest this evening. Current model guidance has a fast moving but cold storm passing south of the forecast area. And so confidence in our first accumulating snowfall is starting to grow.

The thermodynamic evolution of the storm is one that at this time favors a hybrid banding structure. There are aspects of both laterally quasi-stationary banding and pivoting banding.
The upper level jet
advection
and mid level streamlines tend to favor the quasi-stationary banding. While the Q vector forecast would suggest some potential for a pivoting band. With a little more synoptic support for the quasi-stationary band...that may mean a significant snowfall for this time of year under the band but a rather sharp cutoff to the northwest of that
NBM PoP and temps look reasonable at this time
with likely PoP across the southern half of the forecast area and at least a hint that the coast may be warm enough to flip ptype to rain for a time.

Given how far in the future this event is there are not NBM forecasts for QPF for the entire event in the public forecast.
However there are some potential caution flags to be aware of as we move closer in time. Median QPF internally continues to be a fair bit lower than mean...and suggests that several big hits from a few ensemble members are dragging the mean up. That will probably not be the QPF we want to base the eventual snowfall forecast off of. This is backed up by DESI forecasts showing a fairly typical EFI value...just over half the ensemble members having a high than normal snowfall for this time of year...but a shift of tails around 1. This is another indicator that most of the guidance is not extreme...but a few of the higher end solutions are. Finally perusing the cluster analysis in DESI and there is a strong contingent of members at or below mean QPF across the major ensemble suites. There are just about a third of members wetter than mean QPF...owing to a deeper and sharper trof as it swings thru the Ohio Valley. So that will be something to keep an eye on later Sun.

Otherwise the region will remain near to below normal temps and embedded in the westerly flow. This will lead to upslope snow showers activity thru the remainder of the period...with passing shortwaves increasing the coverage of shower activity.

AVIATION /18Z SATURDAY THROUGH THURSDAY/
Short Term...VFR this evening and overnight. Cirrus will slowly thicken and lower into Sunday morning. MVFR ceilings become more likely by mid morning as SHSN begin. Warming temperatures will bring RA mixing to coast and then inland towards LEW/AUG/CON/MHT by around noon. Precip rates increase in the afternoon, with IFR ceilings spreading inland. Expect lowered vis where snow rates increase, most consistent in the mountains. Passing frontal feature in the afternoon will lower coastal sites to IFR before beginning an improving trend after 00z, gradually to VFR by 12z Mon.

Long Term...VFR conditions thru Tue morning will give way quickly to areas of IFR in snowfall and maybe low CIGs in rain near the immediate coastline. There are some questions about how far inland snowfall will make it and thus how far IFR conditions extend...so this places the lowest confidence at TAF sites like LEB and HIE. Once northwest winds arrive late Tue into Wed...VFR conditions return to the coast and southern NH but MVFR CIGs will develop in the mtns. LEB and HIE will see those thru Wed...but periods of MVFR are possible into Fri as well as upslope snow showers activity waxes and wanes.

MARINE
Short Term...Gusty winds continue this afternoon, but subside this evening. Next system on Sunday will bring a period of Gales to most of the coastal waters. Peak gusts may be short lived with a passing cold front, but low level jet supports Gales overnight and into early Mon morning. Expect a W wind shift Sunday evening with this front, becoming NW Monday morning. With the added wind, wave heights increase quickly Sunday afternoon, building 8 to 10 ft towards the Midcoast coastal waters.

Long Term...Gusty winds will diminish rather quickly as high pressure builds into the area. Northeast winds will increase on Tue as the next storm passes outside of the Gulf of Maine...but as it departs into the northwest Atlantic northwest winds will increase and gale force gusts are possible outside of the bays on Wed. Gusty winds will generally linger thru the rest of the work week and at least SCA conditions are anticipated for most of that time.

GYX WATCHES/WARNINGS/ADVISORIES
ME...None.
NH...None.
MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ150>154.
Gale Warning from 4 PM Sunday to 10 AM EST Monday for ANZ150- 152.
Gale Warning from 4 PM to 10 PM EST Sunday for ANZ151.