textproduct: Gray - Portland

WHAT HAS CHANGED

Have allowed the Winter Weather Advisory to expire as scheduled at 8 PM EST.

KEY MESSAGES

1. Snow will taper off through the evening southwest to northeast across the forecast area. Slick travel from fallen snow is expected through the evening.

2. A coastal storm that stays mostly out to sea spreads some snow onto the coastal plain Sunday night. Amounts may be plowable and slick travel is likely. Lighter amounts expand into the foothills and mountains where some accumulation may result in slick travel. Winter Weather Advisories may eventually be needed for the coastal plain and southern New Hampshire.

3. Below normal temperatures are likely from Tuesday onward next week as a series of Arctic air masses descend upon the Northeast. Several nights of sub-zero ambient or wind chill temperatures are possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Decreasing snow rates are expected this afternoon as the core of forcing and moisture moves east. Remaining impacts into the evening will be slick travel conditions from fresh snowfall. Winds are expected to remain light tonight.

Drier air will continue to invade through the evening, ending precipitation. A few light snow showers may remain towards the Kennebec Valley into the overnight hours, but confidence is currently low here.

KEY MESSAGE 2 DESCRIPTION... Unfortunately hi-res model guidance is not bringing about any confidence in the track of the coastal storm Sunday night. The differences between the 12z NAMNest and 12Z HRRR are frankly shocking, with the HRRR showing more of a negative tilt to the trough allowing for snow amounts on the order of 3-5" on the coastal plain. The NAMNest shows a slightly positive tilt, but flattens out the trough as it does. This keeps the precip shield suppressed with snowfall amounts a resounding zero for all but far southern New Hampshire where it thinks up to an inch may be possible.

Hedging with the ensembles looks like the way to continue with this forecast package. The GEFS and Euro Ensemble are holding steady with 20-30% of members bringing greater than 3" of accumulating snow to the coastal plain and 10% for the foothills. Accumulating snowfall does seem likely with amounts greater than 1" in a tight gradient of 50-80% probabilities from the foothills down to the coast. The AI Ensembles remain bullish with a tight 50-70% probability gradient of greater than 3" from the foothills southward. Lastly the HREF, has little to no accumulation in the foothills with around 2-3" along the coast and local smatterings of 4".

There are two main takeaways here among all this uncertainty. The confidence in the system is high, a coastal storm is going to pass on Sunday night and some snow seems likely as a result. The low confidence is in the extent of the precipitation shield, and while all the models have slightly different opinions on totals, it seems clear the gradient is going to be tight. Coastal locations should expect to see the higher end of amounts weather that ends up being 1" or 4". That segues into the next takeaway nicely in that fortunately we aren't caught between a blockbuster and nothing, it was mentioned in a previous AFD, and it continues to look like, this ends up as an advisory scenario at worst (less than 6"). Anyone south of the foothills should prepare to see some snow, but don't be too disappointed if it is not as much (or as little) as you'd hoped. Ultimately, the current forecast calls for 1-3" from the foothills southward (lower amounts to the north, higher near the coast), but any slight shifts in the track of this system will change that.

KEY MESSAGE 3 DESCRIPTION... The eastern U.S. remains under a broad trough through next week. This allows some Arctic air intrusion, plummeting temperatures well below normal. High pressure builds in at the surface midweek which if it aligns correctly could further allow very cold overnight low temperatures. Some shortwaves traverse the broader trough during the week as well, but one looks to align with high pressure at the surface midweek. The better chance for one resulting in snow looks to come more toward the late week time frame.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through 00z Sunday...Cigs will lift and scatter through the rest of tonight. Some fog or BR will linger at CON, AUG, RKD, and HIE for the next couple of hours before some drier air arrives. Mainly VFR then prevails through Sunday morning,

Ceilings thicken and lower through Sunday again, with restrictions possibly working into coastal/southern NH terminals in the afternoon.

Outlook...

Sunday Night: IFR for most terminals in snow. More northern terminals like LEB and HIE may remain MVFR is precipitation stays south.

Monday: Precipitation comes to an end and ceilings lift to VFR. HIE may remain MVFR.

Monday Night-Tuesday Night: VFR prevails at all terminals.

Wednesday: Ceilings lower through the day, with most terminals VFR by the afternoon.

Wednesday Night-Thursday: MVFR prevails with light snow showers possible. IFR restrictions possible at terminals that see these showers move overhead.

MARINE

Weak low pressure crosses the waters this evening with SCA conditions continuing overnight. Wave heights may remain 4 to 6 ft over the coastal waters outside the Midcoast, but general decrease in wave heights is expected along the NH Seacoast and off the York coast.

A window for sub-SCA conditions continues to be Sunday night. After that it looks like, between periods of elevated wind gusts and elevated seas, SCAs are going to potentially be needed for the bulk of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 8 AM EST Sunday for ANZ150-152-154.