textproduct: Boston / Norton

WHAT HAS CHANGED

Winter Weather Advisories have been expanded north and east as confidence for 3-6 inches of snow increases. Confidence in 6+ inches outside of SW CT remains low. Gaining confidence in the risk for a period of freezing rain Sunday night into Monday.

KEY MESSAGES

- Clipper system brings snow accumulations of 3 to 6 inches across western MA, through central and eastern CT, and into southern RI. The highest totals are still expected in SW CT, where 6-9 inches are possible.

- Storm system around Sunday night into Monday could bring accumulating wintry mix (sleet/freezing rain) to rain.

DISCUSSION

KEY MESSAGE 1...

Upper-level shortwave exits the Great Lakes with a trailing surface low tracking through eastern PA into western NY. Guidance has come into better agreement on the axis of heaviest snowfall across SE NY into SW CT, with lighter totals reaching into NW MA and southern RI. The HREF PMM displays a reasonable forecast with a swath of 6-9 inches in SW CT with 3-6 inches in a line from the northern Berkshires all the way to Block Island. The winter weather advisory was expanded to include the areas mentioned above. There was consideration of expanding the Winter Storm Warning to Hartford and Middlesex counties, as there is a chance the 700 mb FGEN and heaviest snow band pivots a little further north. However, confidence was still too low for widespread 6+ inches of snow reaching well into Hartford County. The other area of highest uncertainty is across SE Mass, where ocean enhancement may boost snow totals into the 3-6 inch range as the system pulls away and another reinforcing shot of cold air pushes south.

Snow will move into the region from west to east tonight between 7 and 10 pm, except in NE MA, where snow wont arrive until after midnight. Snow exits the region between 2-5 am with snow showers possibly lingering until 7 am. The heaviest snow rates will be from 8 pm to midnight and will most likely range from 1-2 inches per hour, but could exceed 2-3 inches per hour in the heaviest snow band.

Today will be cold with highs only in the low to mid 20s as the cold Canadian air mass remains in place. Winds will gradually decrease through the day, becoming calm before the snow starts to fall.

KEY MESSAGE 2...Storm system around Sunday night into Monday could bring accumulating wintry mix (sleet/freezing rain) to rain.

We continue to monitor trends with a storm expected to impact southern New England some time Sunday night into Monday night. The track of this low pressure still appears to be well to our north and west, favoring a more wet-than-white forecast. Model soundings continue to forecast a significant warm nose on the order of +4-6C. The question then becomes what the surface temperatures will be below this warm layer. Such strong warm air advection is likely to lead to rising overnight temperatures Sunday night. It remains uncertain what the depth and extent of freezing air will be by the time the atmospheric column saturates enough to result in precipitation.

At this time, favoring freezing rain over sleet, but there could be a brief period of that as well at the onset. The duration of freezing rain before fully transitioning to rain is also a source of uncertainty. Probabilities favor less than one tenth of flat ice accumulation. Still too early to pinpoint ice accumulations beyond that statement. The areas most at risk for freezing rain will be across western MA and along and north of Route 2 in MA.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

Today...High Confidence

VFR. Gusty NNW winds to 25 knots early, easing late morning with a shift to the ENE. Winds speeds lowering to less than 10 knots.

Tonight...Moderate Confidence.

VFR early, but falling to MVFR/IFR with areas of -SN to SN. Areas with the greatest chance of impactful snow accumulation are western MA and central/western CT. Snow moves in from west to east between 00z and 04z. Light E/NE winds.

Saturday...Moderate Confidence

Snow showers may linger through about 18z for eastern MA into the Cape and Islands. Otherwise turning VFR with NNE winds at 10-15 knots.

KBOS Terminal...Moderate confidence

VFR today with decreasing NNE winds. Light snow moves in between 03-05z tonight. Confidence in CIGS is on the lower side as guidance wants to keep CIGS MVFR, but with snow and light NNE winds, CIGS could easily go IFR.

KBDL Terminal...High confidence in TAF.

VFR. Decreasing N winds today. Snow moves in by 00z tonight and could become moderate to heavy at times between 01-04z. Steady light snow should continue though about 09-12z before moving off to the east. MVFR CIGS tonight in light snow, IFR/LIFR CIGS in moderate to heavy snow.

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR.

Sunday: Mainly VFR, with local MVFR possible. Slight chance FZRA.

Sunday Night: MVFR/IFR conditions possible. Breezy. RA, chance FZRA.

Monday: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA.

Monday Night: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance RA, slight chance SN.

Tuesday: VFR. Windy with gusts up to 35 kt.

MARINE

Forecaster Confidence Levels...

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

Friday & Saturday

Decreasing northwesterly winds today below 20 knots. Seas will gradually diminish today down to 2-5 feet. Winds turn NE around 20 knots as a clipper system brings moderate snow to the southern waters, with light snow in the northern waters tonight into Saturday morning.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.

Sunday Night: Winds less than 25 kt. Rain, chance of freezing rain.

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain.

Monday Night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain likely.

Tuesday: gale force winds with local gusts up to 45 kt. Rough seas up to 15 ft. Local visibility 1 to 3 nm.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 7 PM this evening to 10 AM EST Saturday for CTZ002>004. MA...Winter Weather Advisory from 7 PM this evening to 10 AM EST Saturday for MAZ002-008>011. RI...Winter Weather Advisory from 7 PM this evening to 10 AM EST Saturday for RIZ003-006-008. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ230- 236. Small Craft Advisory until 4 PM EST this afternoon for ANZ231- 251-256. Small Craft Advisory until 10 AM EST this morning for ANZ232>235-237. Small Craft Advisory until 10 PM EST this evening for ANZ250. Small Craft Advisory until 7 AM EST Saturday for ANZ254-255.