textproduct: Boston / Norton

WHAT HAS CHANGED

The forecast remains on track, with minimal changes to Sunday's snowstorm.

KEY MESSAGES

- Dangerously cold wind chills linger through Saturday.

- Significant icing possible from freezing spray on the coastal waters.

- A coastal storm will bring heavy snow to Southern New England 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.

- Cold and mostly dry mid week then there is potential for some unsettled weather the latter half of the week.

DISCUSSION

Key message 1...Dangerously cold wind chills linger through Saturday.

The arctic front has now passed through southern New England, bringing gusty winds and plummeting temperatures. 00z soundings from OKX and ALY show 850mb temperatures of -11C and -21C, and expecting temperatures aloft to continue falling to -25C to -27C throughout the night and into the morning. Deep mixing to near 850mb will bring gusts of 30-40 mph to the surface through the night. Prolonged gusty winds in combination with already frigid surface temperatures in the negatives to low single digits will bring wind chills as low as -25F in the eastern slopes of the Berkshires and -10F to -15F everywhere else. Temperatures Saturday will struggle to improve, with highs expected in the low to mid teens... except single digits in the high elevations.

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

Northwesterly gale force winds of up to 40kt over the coastal waters combined with the arctic cold is expected to lead to moderate, possibly even heavy, freezing spray through Saturday. Vessels encountering heavy freezing spray could see ice accretion rates of 0.8"-1.6" per hour. The possibility for heaviest freezing spray is expected along the southern and easter waters, particularly within Nantucket Sound. Light freezing spray should persist much of the day Saturday.

Key Message 3...A coastal storm will bring heavy snow to Southern New England 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.

Guidance continued to hold steady overnight with no significant changes to thinking with regard to our upcoming snowstorm. This major storm, the largest we've experienced in several years, originates as a multifaceted winter storm over the southern Plains and southeast U.S. later today. The plume of subtropical moisture will then move up through the mid-Atlantic and into New England on Sunday. As it does, strong warm advection will encounter the dome of arctic air currently overhead and be lifted, squeezing out copious amounts of moisture in the form of snow. All the ingredients for heavy snow are there, with a very cold antecedent airmass, plume of subtropical moisture, and strong warm advection/frontogenetical forcing in the low/mid levels. Models indicate a very strong band of frontogenesis at 700mb and 850 mb lifting north Sunday afternoon while BUFKIT soundings show a period of strong omega within the -12 to -18F dendritic growth zone. Together with the anomalous moisture present this will result in a period of snowfall rates 1-2 inches Sunday afternoon through around midnight for various parts of SNE. After midnight a dry slot moves overhead desaturating the DGZ so snow is expected to become lighter (with smaller dendrites), continuing through the day on Monday as moisture wraps around the exiting low. Given the strong forcing in the DGZ and the cold temperatures, expecting very fluffy snow during the bulk of the event. This will lead to very hazardous driving conditions centered around Sunday evening/early overnight. Generally expecting 13-18 inches of snow for most, with some locations especially in eastern MA and the Berkshires seeing localized amounts closer to 20 inches thanks to ocean and orographic enhancement respectively. We also have to watch for some mesoscale banding which is where we'd more likely see these higher totals. These bands tend to set up NW of the frontogenesis maxima, so this would suggest somewhere in the central MA region. Also a coastal front over southeast MA could also make that area favorable. It is next to impossible to nail the exact placement of these with much lead time.

We continue to see signs of a 700 mb warm nose moving up into the south coast during the early overnight hours, which may cause some sleet to mix in and bring down totals in RI and southeast MA. Regardless, though, highly impactful snow is expected everywhere.

Finally, while we're not expecting a big power outage risk from this system thanks to the fluffy nature of the snow and the lack of damaging wind gusts, winds will still be quite gusty. 30-40 mph gusts Sunday evening could lead to some localized blowing snow and reduced visibility for a time.

Key Message 4...Cold and mostly dry mid week then there is potential for some unsettled weather the latter half of the week.

The snow isn't going anywhere fast as the subfreezing airmass sits overhead well into next week thanks to a lingering mid level trough. Expecting dry NW flow through at least Wednesday before our next disturbance rounds the base of the trough bringing precipitation chances back to the forecast around Thursday.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

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. Northwesterly winds 15-25kt with gusts 25-35kt persist into the morning. Winds gradually subside today, but 20-25kt gusts should continue through midday. Winds diminish to 5-10kt this evening.

KBOS Terminal...High Confidence in TAF.

KBDL Terminal...High Confidence in TAF.

Outlook /Sunday Night through Wednesday/...

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 local gusts up to 30 kt. SN.

Monday Night: Mainly VFR, with local MVFR possible. Windy with areas of gusts up to 30 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 arrives into Saturday morning along with northwesterly Gale force winds, resulting in moderate to heavy freezing spray. Freezing spray will become lighter throughout Saturday afternoon, but should still persist all day. Winds diminish Saturday night.

A winter storm arrives Sunday into Monday with poor visibility during heavy snow. Sleet may mix into the snow at some point in the later half of the storm over south coastal waters. Northeasterly winds may reach SCA Sunday, but of more concern is for a period of Gale force west-northwesterly winds behind storm later Sunday night into Monday.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 12 ft. Snow, rain, sleet. Visibility 1 nm or less.

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 13 ft. Snow, rain likely. Visibility 1 to 3 nm.

Monday Night: Strong winds with gusts up to 40 kt. Rough seas up to 15 ft. Chance of snow, chance of freezing spray. Areas of visibility 1 to 3 nm.

Tuesday: Strong winds with gusts up to 35 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. 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. Local rough seas. Chance of freezing spray.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Cold Weather Advisory until 10 AM EST this morning for CTZ002>004. Winter Storm Warning from 7 AM Sunday to 8 PM EST Monday for CTZ002>004. MA...Cold Weather Advisory until 10 AM EST this morning 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 until 1 PM EST this afternoon for MAZ002- 008-009. RI...Cold Weather Advisory until 10 AM EST this morning 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 this morning for ANZ230>237-250- 251-254>256. Freezing Spray Advisory until 4 PM EST this afternoon for ANZ230-233>237-256. Gale Watch from Sunday afternoon through Monday afternoon for ANZ230>237-250-251-254>256. Heavy Freezing Spray Warning until 1 PM EST this afternoon for ANZ231-232-250-251-254-255.