textproduct: Boston / Norton

WHAT HAS CHANGED

- Trended snowfall totals higher for northeast MA, central MA high terrain, and the Berkshires.

KEY MESSAGES

- Major winter storm will bring intense, heavy snow to Southern New England Sunday into Monday. Heavy snowfall with 1-2"+ inch per hour snow rates Sunday afternoon and evening, making for dangerous travel conditions.

- Abnormally cold and mostly dry with a low probability of another storm off the coast next weekend.

DISCUSSION

KEY MESSAGE...Major winter storm will bring intense, heavy snow to Southern New England Sunday into Monday. Heavy snowfall with 1-2"+ inch per hour snow rates is expected Sunday afternoon and evening, making for dangerous travel conditions.

A major winter storm arrives in western southern New England 9AM- 11AM and 11AM-1PM in the east and lasts into Monday. Expecting snowfall rates of 1"-2+" per hour to start soon after storm onset, leading to dangerous travel conditions in the afternoon/evening and into the overnight period. Storm total snowfall expected to be in the 12"-18" range across the majority of southern New England, with 8"-12" expected on the Cape, Islands, and immediate southern coast. The higher elevations and northeastern MA will likely see 18"-20", including Boston.

A widespread winter storm moves out of the Deep South/Midwest and into the Mid-Atlantic/New England region Sunday. Guidance has narrowed in on an 850mb closed low tracking over southern New England, bringing PWATs 0.75"-1.0" (200%-300% of normal) and very strong frontogenesis. Model guidance is indicating large, negative omega values within a deep dendritic growth zone leading to the development of large snowflakes. Also expecting a coastal front to push into the north and eastern portions of southern New England Sunday evening leading to a sharper gradient dividing the arctic airmass from Canada and slightly warmer air being funneled in off the ocean. There will likely be development of heavier snow bands on the northwest side of the strongest frontogenesis. Given this is a meso-/micro- scale feature, there is high uncertainty in where exactly the banding will set up, but thinking somewhere between Plymouth and Worcester at this time.

The high PWATs, strong frontogenesis, and coastal front combined with the arctic airmass indicates this event will be quite the efficient snow maker, with intense snowfall rates of 1"-2+" per hour likely over several hours into late Sunday night. Guidance is also indicating gusty winds arriving Sunday night, especially over the southern waters, Cape and Islands. HREF blizzard probs remain around 20%-30%, so low confidence in long- duration blizzard conditions. However, can't rule out periods of short-lived white-out conditions.

As the storm advances into Sunday evening, expecting a shot of warm air to mix in aloft and lead to temperatures warming closer to 32F. This will likely allow sleet to mix into the snow, lowering snow ratios and creating a wetter snow that will reduce totals. The GFS and NAM bring the warmer temperatures further inland into the south shore, but currently favoring a mix of the ECMWF, Canadian, and NBM which keep the warm layer confined to the immediate southern coastline, Cape, and Islands.

By early Monday morning, a dry slot pushes into southern New England, cutting the available moisture for snow growth and leading to the development of smaller snowflakes. This will dramatically lower snowfall rates through the morning, eventually tapering off to light flurries. Prolonged northeasterly flow on Monday will allow for ocean enhanced snow flurries to linger across the northeastern coastal plain, including Boston, into Monday afternoon.

Key Message 2...Abnormally cold and mostly dry with a low probability of another storm off the coast next weekend.

Overall benign weather is expected to follow this weekend's snowstorm. Dry, cold NW flow persists through at least the start of next weekend keeping precipitation out of the forecast for the most part while temperatures remain well below normal each day. Normal highs/lows for late January are in the mid 30s and low 20s respectively; we are forecasting highs in the teens and 20s with lows in the single digits thanks to an anomalously cold airmass overhead. A few shortwaves rounding the broader trough could bring some flurries off and on but on the whole things look dry. We'll continue to monitor a potential storm off the coast toward next weekend but odds are still favorable that it would pass harmlessly offshore.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

12Z update...

Today and Tonight: High Confidence.

VFR through this morning. NNE winds 5-10kts through the morning, with gusts picking up 25-35kts this evening. Light snow begins in western New England 13z-15z and 15z-17z farther east, bringing 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 8-12" along the South Coast, Cape Cod, and Islands to 12-18" elsewhere, with 18-20" possible in the higher elevations and northeastern coast, including BOS. Much of this will fall from later Sunday afternoon through midnight Monday.

Monday: High Confidence in treads. Moderate Confidence in timing.

Snow begins to lighten up Monday morning. Prolonged northeasterly flow brings ocean enhanced snow to the northeastern coastal plain, including Boston, through midday. Winds shift to north westerlies Monday afternoon and flurries finally move offshore.

KBOS Terminal...High Confidence in TAF.

KBDL Terminal...High Confidence in TAF.

Outlook /Monday Night through Thursday/...

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

Tuesday through Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday: VFR. Breezy.

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.

NE winds later today 20-35 kt gusting 30-40 kt by tonight 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. Seas increase to 8-10 ft tonight, increasing to as high as 13 ft on the outer waters Monday.

Outlook /Monday Night through Thursday/...

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

Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 14 ft.

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

Wednesday: Winds less than 25 kt. Seas up to 5 ft. Chance of freezing spray.

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

Thursday: Winds less than 25 kt. Chance of freezing spray, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

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