textproduct: Boston / Norton

WHAT HAS CHANGED

Little change to the forecast. Winter Storm Warnings have been issued for all of southern New England since confidence is high for heavy snow with snowfall rates as high as 1 to 2 inches per hour during the peak of the storm Sunday night. There is still some uncertainty if snow mixes with or changes to sleet near the South Coast which could limit snowfall totals somewhat there.

KEY MESSAGES

- Dangerously cold wind chills tonight into Saturday.

- Significant icing from freezing spray on the coastal waters tonight into Saturday.

- A winter storm looks to bring a substantial accumulation of dry, powdery snowfall Sunday into Monday. Heavy snow with 1"+ inch per hour snow rates Sunday early evening to early Monday morning will make travel very difficult. Increasing easterly winds could produce areas of blowing snow, and near-blizzard conditions could develop in short intervals.

DISCUSSION

Key Message 1...Dangerously cold wind chills tonight into Saturday.

Arctic cold front remains on schedule to cross southern New England later this afternoon and early this evening. Strong cold advection will produce deep mixing up to 850 mb resulting in 30-40 mph gusts for much of the night. Temperatures dropping below zero across the interior and into single numbers closer to the coast will result in dangerous wind chills down to -25F along east slopes of Berkshires and to -10F to -15F elsewhere, including Cape Cod and the Islands where we have expanded the Cold Weather Advisory. Some recovery in wind chills Saturday as winds slowly diminish but actual highs will only reach the teens, with single numbers in the higher elevations.

Key Message 2...Significant icing possible from freezing spray on the coastal waters.

NW Gale force winds up to 40kt on the coastal waters combined with the arctic cold is expected to lead to moderate to heavy freezing spray tonight into Saturday. Vessels encountering heavy freezing spray could see dangerous ice accretion rates of 0.8"-1.6" per hour. The more favored areas for heavy freezing spray include the eastern MA waters and waters around Cape Cod and the Islands, especially Nantucket Sound. Light freezing spray should persist much of the day Saturday.

KEY MESSAGE 3...A winter storm looks to bring a substantial accumulation of dry, powdery snowfall Sunday into Monday. Heavy snow with 1"+ inch per hour snow rates Sunday early evening to early Monday morning will make travel very difficult. Increasing easterly winds could produce areas of blowing snow, and near- blizzard conditions could develop in short intervals.

A large winter storm impacting the Southern Plains, Mid-South and the Appalachians/Mid-Altc region tonight into Saturday remains poised to affect Southern New England Sunday, continuing into Monday before tapering off Monday evening. An anomalous amount of deep moisture falling into an antecedent frigid, Arctic airmass is likely to produce a substantial accumulation of dry, powdery snow for much of Southern New England. With the 700 mb trough positioned to our northwest into Upstate NY allowing for some intrusion of warmer air aloft from the mid- Atlantic/waters, the potential exists for sleet to still mix in along the immediate south coast and the Islands. Although that could cut into accumulations for those locations, it's anticipated that warning-criteria snowfall should be met before that transition potentially happens and won't significantly affect messaging. Weighing that, Winter Storm Warnings have now been posted for all of Southern New England, segmented out in such a way that Block Island, Martha's Vineyard and Nantucket which have the best shot at periods of sleet with lower (7-10") accumulation, with all snow indicated for the large remainder of Southern New England, with snow totals in the 12" to 17" range. The potential exists for localized, isolated totals higher than that could develop pending placement of heavier snowbands, which is still uncertain at this day- 3 timeframe.

Overall no major changes on the synoptic scale with some subtle wobbles in the storm track. Besides the still-credible potential for sleet to mix in along the immediate south coast/the Islands, there has been a few other changes to take note of. Period of heaviest snowfall still looks to develop Sunday early evening through early Monday morning, where the combination of strong frontogenetic forcing and heaviest liquid-equivalent QPF produces bands of heavy snow potentially with snow rates in the 1-2" per hour neighborhood. Travel is likely to be very difficult to nigh-impossible during this period of time. However as illustrated across several model outputs, it now looks like a dryslot aloft sapping RH in the snow growth layer races ENE across the southern roughly two-thirds of Southern New England Monday morning - the implication being that snow accumulations for Monday mid-morning to Monday afternoon could be lighter. Models are also now showing stronger ENE wind gusts in the 25-30 to 35-40 mph range Sunday night to early Monday with the strongest gusts over the Cape and Islands. That could open the door for areas of blowing snow to develop and this was indicated in the weather grids moreso along the immediate coastlines. Brief periods of near-blizzard conditions could be possible if stronger gusts materialize. The other uncertainty is the exact placement of mesoscale snowbands; we probably won't have a solid grasp on where these localized bands may develop for another day or two, but one area which could be favored is just west of a modeled intense coastal front roughly along the I-495 to I-95 corridors. Further adjustments to the snow forecast could be possible pending the extent of sleet mixing in and the location/placement of heavier snowbands.

Snow develops Sunday morning to early afternoon from SW to NE, becoming steady into Sunday afternoon. In most areas, snow will be falling with temperatures in the teens, which will favor a drier snow. As mentioned, the period of heaviest snowfall and most treacherous travel conditions looks to be from Sunday early evening to early Monday morning, with snow rates in the 1 to 2 inch per hour range possible. As sleet mixes in along the South Coast and the dryslot moves in aloft into Monday morning, we should then see lighter-accumulating snow for Monday/Monday afternoon before tapering off into Monday evening.

Even though easterly winds are a little stronger, the risk for coastal flooding is minimal as astro tides are low.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

18Z Update: High confidence.

VFR. Arctic cold front crosses southern New England 21z-00z with wind shift to NW and increasing winds of 15-25kt with gusts to 30-35kt which persists through most of the night. NW winds gradually subside Sat but 20-25kt gusts should continue through midday.

KBOS Terminal...High Confidence in TAF.

KBDL Terminal...High Confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday: Mainly VFR, with areas IFR possible. Windy with local gusts up to 30 kt. SN.

Sunday Night: Mainly IFR, with areas MVFR possible. Windy with gusts up to 35 kt. SN, FZRA.

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

Monday Night: VFR. Windy with gusts up to 35 kt. Chance SN.

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

Tuesday Night through Wednesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

High confidence through the weekend.

Arctic air surges into region tonight and Saturday on Gale force NW winds, resulting in moderate to heavy freezing spray, which becomes lighter Saturday but should persist all day. Winds diminish Saturday night.

Winter storm expected Sunday into Monday with poor visibility in heavy snow and perhaps sleet on south coastal waters. NE winds may reach SCA Sunday, but of more concern is for a period of Gale force W/NW winds behind storm later Sunday night into Monday.

Outlook /Sunday through Wednesday/...

Sunday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Snow. Visibility 1 nm or less.

Sunday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Snow, rain, sleet. Visibility 1 nm or less.

Monday: Strong winds with gusts up to 35 kt. Rough seas up to 13 ft. Snow, rain likely. Visibility 1 to 3 nm.

Monday Night: Moderate risk for gale force winds with local gusts up to 45 kt. Rough seas up to 16 ft. Chance of snow, chance of freezing spray. Local visibility 1 to 3 nm.

Tuesday: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of freezing spray, slight chance of snow.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Chance of freezing spray, slight chance of snow.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of freezing spray.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Cold Weather Advisory from 10 PM this evening to 10 AM EST Saturday for CTZ002>004. Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for CTZ002>004. MA...Cold Weather Advisory from 10 PM this evening to 10 AM EST Saturday for MAZ003>007-010>024-026. Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for MAZ002>024-026. Extreme Cold Warning from 7 PM this evening to 1 PM EST Saturday for MAZ002-008-009. RI...Cold Weather Advisory from 10 PM this evening to 10 AM EST Saturday for RIZ001>008. Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for RIZ001>008. MARINE...Gale Warning until 7 AM EST Saturday for ANZ230>237-250-251- 254>256. Freezing Spray Advisory from 10 PM this evening to 4 PM EST Saturday for ANZ230-233>237-256. Heavy Freezing Spray Warning from 10 PM this evening to 1 PM EST Saturday for ANZ231-232-250-251-254-255.