textproduct: Gray - Portland

WHAT HAS CHANGED

Forecast for snow continues to shift south for the disturbance Wednesday into Wednesday night. Accumulating snow is looking most likely in the forecast area for southern New Hampshire and far southern Maine.

KEY MESSAGES

1. Light snow will move across the forecast area late tonight and through the morning hours Tuesday. While light, generally less than an inch, any accumulation could cause surfaces to become slick during the morning commute.

2. A narrow and quick moving wave of low pressure will pass through New England Wednesday into Wednesday night. A swath of accumulating snow may impact the Wednesday evening commute, especially across southern New Hampshire and far southern Maine. Exact amounts and location remain uncertain at this time.

3. Keeping our eye on the track of a complex weather system Friday into Saturday with the potential for plowable snow for parts of the area

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Not much change for expected light snow arriving late tonight through the first half of Tuesday.

The most uncertainty lies with the inconsistent RH profile in the low levels. While there should be enough saturation for snow to fall, areas where the moisture depth isn't as continuous could see a brief window of freezing drizzle. Ideally would like to see a more saturated surface layer for that to be a bigger threat, so held off on any headlines relating to this. Will continue to monitor this as dry, cold conditions could allow for untreated surfaces to slicken quickly.

Precip will lighten west to east as Tuesday progresses with temperatures climbing into the mid to upper 30s. Upslope snow could continue on the NW sides of the mountains through the evening.

KEY MESSAGE 2 DESCRIPTION...

Next chance for precip accumulation arrives Wednesday afternoon through the overnight hours. There remains substantial spread in areal coverage of precipitation, thus making for a tricky forecast of potential travel impacts Wednesday afternoon and Thursday morning.

Guidance trends seem to be meeting in the middle of the two camps, ECMWF/Canadian vs. GFS/NAM, with a band of snow passing through southern NH and far southern ME. Northerly flow that is dry will limit northward extent of snow, especially when it comes to measurable amounts. Opted to go below the NBM mean here to limit QPF extent and amounts, focusing on the area outlined above. Should this hold, there may be one or two periods of moderate snow come Wed evening. While modeled surface temperatures ride up into the mid to upper 30s across southern NH and far southern ME, think the extensive cloud cover combined with any dynamic cooling due to better precip rates within the band will contribute to values closer to the low to mid 30s and provide mostly snow. Northern portions of the forecast area may end up warmer, close to 40, as the cloud shield may also be limited with only thin cirrus towards the north.

All in all, current forecast supports a glancing blow Wed afternoon into Wed night. Accumulating snow chances remain the greatest across southern NH, which may impact the evening commute.

KEY MESSAGE 3 DESCRIPTION...

Thursday is trending toward a drier and quieter day high pressure returns to the region. The forecast becomes more complex heading into the end of the week as a system from the Great Lakes comes up against the blocking high-pressure over East Central Canada. Trends have favored the system tracking south forced by the block, however sufficient overrunning moisture could be sufficient to accumulate advisory level snowfall particularly across NH and SW ME. The latest forecast has QPF or liquid around 0.15-0.5", which could translate into plowable snowfall. Because the warm front is expected to remain pinned to our south, temperatures should stay cold enough to favor all snow. Given this season's trend of systems generally tracking further south as we get closer, it is worth watching to see if this does the same. As we move further into the weekend, a modest reinforcing shot of cold air will keep scattered snow showers in the forecast through Sunday, though the dry airmass will limit accumulations. Seasonable temperatures will linger into next week as the broader pattern lingers.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18z Tuesday...Patchy MVFR deck will continue to invade along coastal terminals and into southern NH TAF sites. Some uncertainty how thick this layer continues overnight, but should see continued lowering into the morning hours Tuesday. Light SN will accompany this invasion from the west, reducing vis to MVFR/IFR between 12 and 18z.

Outlook...

Tuesday Night and Wednesday: Uncertain how quickly improvement comes overnight, but terminals like RKD/AUG will trend VFR Wed AM. Terminals to south will see additional thickening/lowering as SN arrives again. IFR vis possible for CON/MHT/PSM.

Wednesday Night: VFR north with gradual improvement for southern terminals.

Thursday and Thursday night: VFR. No sig wx.

MARINE

Conditions remain below SCA through Wednesday. There will be a weak wave passing over the waters Tuesday with southerly winds shifting west Tues night. Northerly winds then build Wed night as a low passes south of the waters.

Next disturbance arriving late week may increase winds towards Gale force Friday night into Saturday morning.

GYX WATCHES/WARNINGS/ADVISORIES

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