textproduct: Gray - Portland

WHAT HAS CHANGED

The Winter Storm Watch has been upgraded to a Winter Storm Warning for all of western ME as well as all of NH.

A Cold Weather Advisory has been issued for the foothills and points northward for tonight.

A Gale Watch has been issued for the coastal waters outside of the bays for Sunday night through Monday.

KEY MESSAGES

1. A bitterly cold air mass will remain in place through tonight. Dangerous wind chills are anticipated for the region.

2. A significant snow storm will bring widespread accumulating snowfall to the region Sunday afternoon through Monday night into early Tuesday morning. All snow is expected, which will bring hazardous travel conditions through the Monday morning and evening commutes across the area.

3. Below normal temperatures continue Tuesday through the end of the work week. After the winter storm moves out Monday night into Tuesday morning, low chances for snow return by the end of the week outside of some periodic mountain snow showers.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION: An Arctic air mass will remain in place through Sunday as a ~1040 hpa sfc high crests over the region before departing to our north late Sunday. Skies will remain mainly clear through the first half of the night as winds gradually diminish. This combined with very low dew point values will set the stage for several hours of strong radiational cooling before clouds begin to increase after midnight. Forecast lows range from around 20 below across the north and some interior valleys with 0 to 10 below zero in most other locations. Some lingering wind combined with these very cold temperatures will allow windchill values to fall to around 20-25 below zero from the foothills and points northward and therefore a Cold Weather Advisory has been issued for these locations.

KEY MESSAGE 2 DESCRIPTION: The forecast is still on track for a significant snowstorm to impact the region beginning tomorrow afternoon across NH and then into western ME by tomorrow evening before gradually ending from west to east Monday night into early Tuesday morning. The primary impact will be for extremely difficult travel conditions Sunday evening into Monday night with a growing potential for snowfall rates to approach 2"/hr Sunday evening and night. In addition, increasing winds will result in blowing and drifting snow, further reducing visibility and poor road conditions.

Forecast guidance continues to be in strong agreement that low pressure will exit the Mid-Atlantic coastline Sunday afternoon and evening before passing near the 40N/70W benchmark Monday morning and then arriving over Nova Scotia by early Tuesday morning. During this same time period, a sharp trof axis will be arriving from the west, placing us in the favorable jet entrance region, further enhancing the precipitation shield and prolonging the event across the region. Mesoscale guidance is also beginning to show hints of a few mesoscale features as well including some possible ocean enhancement as well as perhaps an inverted trof. These features are notoriously difficult to forecast but broadly speaking, SE NH and perhaps portions of the SW ME coastline could be most likely to be these "hot spots."

Latest ensemble and deterministic based guidance shows generally between 0.75-1.00" of QPF for areas south of the mountains with <=0.50" towards the Canadian Border. Snow-liquid ratios are expected to be within the 15-20:1 range as although we have strong lift within the DGZ and very cold temperatures in place, forecast soundings show around 35 kts of wind at H850, which will help to break the dendrites and reduce ratios. In addition, there is a general lack of drier air above the DGZ zone, which should reduce dendritic growth/stacking some. That being said, this will be a dry/fluffy snow character and is not expected to result in power outage issues.

A "front end thump" of snow is likely to lift northward Sunday evening/night as strong H7 FGEN moves northward of the H850 low track. This will likely result in snowfall rates up to around 2"/hr, which will result in rapidly deteriorating and dangerous travel conditions. Following this, guidance shows a brief window of drier air to move in within the -12C to -18C layer on Monday morning, which will likely lower snowfall rates before they increase again by late Monday morning and afternoon as added moisture and lift arrives from the approaching trof axis. Snowfall amounts of between 12-18" is expected before the snow ends by early Tuesday morning. Locally higher amounts are possible but for now this seems to be a reasonable compromise based on latest ensemble probabilities. Closer to 8-12" is expected north of the mountains.

KEY MESSAGE 3 DESCRIPTION: The winter storm will end by early Tuesday morning, but will be slowly losing it's punch as the upper trough starts to pivot and move out to the northeast. Progressive ridging will then approach. Highs will be chilly on Tuesday, ranging from single digits across the north country of New Hampshire and the western Maine mountains, to the upper teens and lower 20s over southern New Hampshire and coastal/interior Maine. Another upper low then quickly crosses the region early Wednesday with little impact other than a few isolated light snow showers.

Towards the end of the week, model agreement breaks down and high uncertainty prevails with regards to the pattern. Models/ensembles will likely continue to struggle with the late week pattern until the early week winter storm is out of the way. That being said, NBM temperature spread remains fairly low until Friday so we may have a bit more confidence in the temperature guidance which suggests highs similar to Tuesday on Wed/Thurs. The coldest period of the week looks like it could be Wednesday night into Thursday morning when lows are forecast to range from the double digits below zero north, to around zero south.

Precipitation chances then may increase Friday and into the weekend as another potential storm moves into the region. While this potential is reflected in some widespread low precipitation chances across New Hampshire and western Maine, it will still be several days to determine whether this storm will impact the region or move out to sea.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Expect VFR conditions at all sites through at least Sunday morning. Ceilings will start to lower late tonight early Sunday afternoon as snow chances also start to increase. Conditions will gradually deteriorate from southwest to northeast through the day on Sunday, with widespread IFR to MVFR conditions likely by the afternoon hours.

Outlook:

Sunday Night and Monday: IFR in widespread moderate to heavy snow. LIFR vis possible in passing snow bands Sunday night. Snow intensity drops Monday, perhaps some improvement in vis. N winds gusting 15 to 20 kts for most terminals S and E of the ME/NH mtns.

Monday Night: Trending VFR. Winds shift NW 15 to 20 kts. HIE may keep MVFR ceilings with light SN.

Tuesday, Wednesday, and Thursday: Mostly VFR with no sig wx. W winds 15 to 20 kts. MVFR ceilings may remain NW of the Whites.

MARINE

Moderate freezing spray is expected through Sun morning. Gale force NE winds are possible outside of the bays Sun night- Mon with SCA winds to near 30 kts across the bays. Seas will build to 6-12 ft outside of the bays with 2-6 ft in the bays.

Gusts to 35 kts may continue over the outer coastal zones into Tuesday morning. Waves in the 6 to 12 foot range will also remain possible through at least late Tuesday morning. Conditions subside to SCA later in the day on Tuesday. Finally, expect periods of light to moderate freezing spray through Tuesday given cold air mass in place.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Cold Weather Advisory until 7 AM EST Sunday for MEZ007>009- 012>014-033. Winter Storm Warning from 7 PM Sunday to 1 AM EST Tuesday for MEZ007>009-012>014-019>022-024>028-033. Winter Storm Warning from 1 PM Sunday to 7 PM EST Monday for MEZ018-023. NH...Cold Weather Advisory until 7 AM EST Sunday for NHZ001>004. Winter Storm Warning from 1 PM Sunday to 7 PM EST Monday for NHZ001>015. MARINE...Freezing Spray Advisory until 10 AM EST Sunday for ANZ150>154. Gale Watch from late Sunday night through Monday evening for ANZ150-152-154. Small Craft Advisory from 1 AM to 7 PM EST Monday for ANZ151- 153.