textproduct: Boston / Norton

WHAT HAS CHANGED

- Trended snowfall totals lower for the south coast of Rhode Island and south coast of Massachusetts. This does include the Cape and Islands, due to the threat of snow turning to sleet, which will lower totals.

- Freezing Spray Advisory issued for lingering moderate freezing spray on eastern MA waters through this evening.

- Gale Warnings issued for increasing NW winds to 40kt on coastal waters behind storm Sunday night into Monday.

KEY MESSAGES

- Coastal storm will bring heavy snow to Southern New England Sunday into Monday. Heavy snow with 1-2"+ inch per hour snow rates Sunday afternoon and evening, making travel treacherous. Waning snow Monday followed by colder temperatures.

- Freezing spray lingers into the evening followed by another round of Gale force winds Sunday night into Monday.

- Cold and mainly dry this week but watching the possibility of another coastal low sometime later in the week.

DISCUSSION

Key Message 1...

Quiet, very cold temperatures, and gusty conditions this afternoon as final preparations are made ahead of an impactful widespread winter storm, set to begin in less than 24-hours.

A major winter storm will bring widespread snowfall across all of southern New England Sunday into Sunday night, leading to dangerous travel by Sunday afternoon and through the night. Intense snowfall rates of 1-2" per hour will accumulate widespread 12-18" across most of southern New England, with 8-12" across the immediate south coast, Cape Cod, and Islands due to the transition from snow to sleet.

A dynamic winter storm moving out of the southern states will tap into rich moisture from the Gulf and transport it north on Sunday. Model guidance over the past 12-24 hours shows less than ideal conditions for a full out high-end snow event. Nevertheless there is plenty of snow to go around. The upper- level pattern shows the 700mb wave never closes off and the 850mb low tracking either over southern New England or just north (depending on your flavor of guidance). The surface low pressure tracks inside of the benchmark and remains somewhat weak, with pressure ~1000mb. What we have working in our favor, the Arctic air mass, moisture plume with PWATs 2-3 standard deviations above climo, and strong frontogenesis. In addition, a coastal front develops, this mesoscale features may lead to isolated heavier bursts and subsequent totals of snow. High-res guidance is picking up on the feature Sunday afternoon into the evening. Essentially this front is a dividing line between Arctic air and "seasonable" cold air. Along and west of this front, there is potential for enhanced snow amounts. With this setup, a front end thump is likely, with snowfall rates on the order of 1-2" per hour, though cannot rule out instances of 3" per hour between 3 PM and 9 PM. These intense rates arrive to SW CT between 3-5 PM then reaching NE MA by 7-9 PM. During this time, it is not advisable to travel, unless absolutely necessary as whiteout conditions are likely given the intensity of the snowfall rates.

And given how dynamic the system is, thundersnow is a possibility, cue the excitement! It's also worth a mention, the HREF shows low probabilites (less than a 30/40 percent chance) of coastal area to reach blizzard criteria. While winds are expected to become gusty, feel that gusts will fall shy of criteria from the east-northeast around 30 mph. Don't feel it will become the dominate feature, thus have held off on an upgrades to the winter weather headlines. Either way, travel during this time will be treacherous.

As the storm wanes, warm air becomes introduced aloft, transition from snow to sleet across southeast MA and southern RI. However, there is lower confidence in how far north/inland the sleet reaches. Current thought are areas to the south of a line from Willimantic, to Warwick, to Taunton have the greatest potential for sleet, which has lowered expected snow totals, as mentioned above. There is even potential for sleet to change to rain on the island of Nantucket, which would compress the snowpack, leading to a dense snow and make it look like "less" snow fell. Snow continues overnight and wanes on Monday, this is associated with a weak surface low pressure off shore, flipping the wind from onshore to the north-northwest with additional minor snow accumulation. And the current snowfall map issued encompasses all snowfall from Sunday through Monday afternoon. Snow wraps up from west to east, starting first in western areas late Monday morning, eastern areas by the coast are later in the day, later afternoon to early evening.

By Monday night much drier air return and snow ceases. Temperatures Monday night falls to the single digits.

Key Message 2...Freezing spray lingers into the evening followed by another round of Gale force winds Sunday night into Monday.

Still seeing buoy reports of moderate freezing spray on E MA waters including Nantucket Sound so we maintained the Freezing Spray Advisory until this evening for these waters.

Issued Gale Warnings for increasing NW winds to 40kt as the storm departs later Sunday night into Monday. May also see some additional light freezing spray during this time as another surge of colder air works into region.

Key Message 3...Cold and mainly dry this week but watching the possibility of another coastal low sometime later in the week.

Mainly dry but cold NW flow is expected to persist much of next week and some weak disturbances aloft could bring some flurries from time to time, as well as at least some light freezing spray on the coastal waters. Longer range ensembles show the potential for another southern stream low pressure system to move off the East Coast later in the week or perhaps even next weekend, but for now, it's equally possible that any system tracks well offshore.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

18Z Update: High confidence.

VFR through tonight as NW winds diminish by late afternoon and veer to N/NE overnight. Light snow begins in western New England 13z-15z and 15z-17z farther east which brings a rapid drop to IFR conditions. Expecting heavy snow to arrive within 1-3 hours of onset with widespread LIFR conditions later Sunday afternoon and the first part of Sunday night. Snowfall rates of 1-3" per hour are likely. Snow lessens in intensity overnight into Monday but there should be a change to rain for a time on the Islands and possibly Cape Cod, with some sleet possible along the South Coast.

Snowfall totals by Monday morning should range from 6-12" along the South Coast, Cape Cod, and Islands to 12-18" elsewhere. Much of this will fall from later Sunday afternoon through midnight Monday.

KBOS Terminal...High Confidence in TAF. Forecast is for 16" of snow at Logan Airport.

KBDL Terminal...High Confidence in TAF. Forecast is for 15" of snow at Bradley Airport.

Outlook /Monday through Thursday/...

Monday: Mainly IFR, with areas MVFR possible. Windy with local gusts up to 30 kt. SN likely, FZRA.

Monday Night: Mainly VFR, with local MVFR possible. Windy with gusts up to 30 kt. Chance SN.

Tuesday: VFR. Windy with local gusts up to 30 kt.

Tuesday Night through Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday: VFR. Breezy. Slight chance SN.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

High confidence through early next week.

NW winds diminish quickly tonight and turn NE Sunday ahead of the coastal low pressure system heading up the coast. Snow arrives by early afternoon and quickly becomes heavy at times, before changing to rain or sleet on most of south coastal waters Sunday night.

Outlook /Monday through Thursday/...

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 13 ft. Rain likely, freezing spray, chance of snow, freezing rain likely. Local visibility 1 nm or less.

Monday Night: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of snow, chance of freezing spray, chance of freezing rain. Local visibility 1 to 3 nm.

Tuesday: Strong winds with gusts up to 35 kt. Rough seas up to 13 ft. Chance of freezing spray, slight chance of snow.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of freezing spray.

Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of freezing spray.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of freezing spray, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for CTZ002>004. MA...Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for MAZ002>024-026. RI...Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for RIZ001>008. MARINE...Gale Warning from 10 PM Sunday to 5 AM EST Monday for ANZ230- 236. Freezing Spray Advisory until 7 PM EST this evening for ANZ231- 232-250-251-254-255. Gale Warning from 10 PM Sunday to 6 AM EST Monday for ANZ231. Gale Warning from 7 PM Sunday to 6 AM EST Monday for ANZ232>234. Gale Warning from 6 PM Sunday to 6 AM EST Monday for ANZ235- 237. Gale Warning from 10 PM Sunday to 7 AM EST Monday for ANZ250- 251. Gale Warning from 7 PM Sunday to 7 AM EST Monday for ANZ254. Gale Warning from 4 PM Sunday to 7 AM EST Monday for ANZ255- 256.