textproduct: Boston / Norton

WHAT HAS CHANGED

Increasing confidence in a period of snow showers with light accumulations late Friday night into Saturday ahead of the arctic front, then additional ocean effect snow showers for Cape Cod and Nantucket through Saturday night.

KEY MESSAGES

- Dry through Fri with very cold nights. Daytime temperatures slightly below normal.

- More active Friday into the weekend, and bitterly cold into early next week. Chance for dangerously cold wind chills this weekend along with strong winds and heavy freezing spray over the waters.

DISCUSSION

KEY MESSAGE 1...Dry through Fri with very cold nights. Daytime temperatures slightly below normal.

A quiet weather pattern will continue through the end of the week. A weak cold front passes this morning, otherwise high pressure will be in control with dry conditions and mostly sunny days and mainly clear nights. The weak front will bring slightly colder daytime temps through the end of the week as 925 mb temps are a bit colder, with highs mostly upper 20s to lower 30s. Nighttime temps will be quite cold due to favorable radiational cooling. Expect overnight lows generally in the single numbers with teens along the immediate eastern MA coast, but below zero in the colder locations of the CT valley.

KEY MESSAGE 2...More active Friday into the weekend, and bitterly cold into early next week. Chance for dangerously cold wind chills this weekend along with strong winds and heavy freezing spray over the waters.

A high pressure-influenced week ends with a low moving across northern New England and its strong arctic front passing through southern New England sometime Friday night into Saturday morning. This is still expected to be a moisture-starved frontal passage. The upper level trough associated with this system may continue to dig south through the region and an inverted trough forms with the development of an offshore surface low. As the front moves through, some light, fluffy snow showers are possible across southern New England, which could be more enhanced towards the eastern coastline by any lingering ocean effect showers. Some snow squalls are a possibility as well with this front. However, as this system is still a couple days away, it's a bit too early to nail down any totals.

Following the frontal passage, dangerously cold wind chills will settle over the region. Confidence is increasing in the possibility for cold weather headlines, especially when the core of this arctic air moves in sometime Saturday night into Sunday morning. As mentioned above, an offshore low is expected to deepen, which will lead to southern New England being caught in the tightening pressure gradient as high pressure remains towards the Great Lakes. Gale- force winds are a possibility over the waters, and we may end up needing wind headlines on land as well, particularly towards the outer Cape and Nantucket. Temperatures aloft sink to around -25C at 925 mb Saturday night, which when combined with the winds, it could end up feeling like -30F in parts of the Berkshires and between -20F and -10F elsewhere. Ensemble guidance continues to center the higher probabilities for wind chills below -25F over the Berkshires and the higher elevations of central MA. Worth noting, though, that probabilities for wind chills at or below -20F for the rest of the region are between 30-50 percent across GEFS and ECMWF ensemble guidance. These bitterly cold conditions are expected to last into the start of next week before the arctic airmass aloft exits as surface high pressure builds in along with ridging aloft. Temperatures aloft recover a bit going into midweek, increasing from -20C to around -10C at 925 mb, which should allow for daytime surface temperatures to get closer to the freezing mark.

With the elevated winds and very cold temperatures, confidence is increasing in the chance for moderate to heavy freezing spray over the waters this weekend.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through Thursday...High confidence.

VFR through the period. NW gusts to 20-25 kt developing by mid- late morning, then becoming light NW to calm tonight. NW wind 7-12 kt Thu.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High Confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Chance SN.

Saturday: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance SN.

Saturday Night: VFR. Strong winds with gusts up to 45 kt. Slight chance SN.

Sunday: 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.

Through Thursday...High confidence.

Quiet weather across the coastal waters through Thu with winds and seas below SCA thresholds. NW winds will persist through Thu with gusts around 20 kt developing today.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: Winds less than 25 kt.

Friday Night: Winds less than 25 kt. Chance of snow. Local visibility 1 to 3 nm.

Saturday: Moderate risk for gale force winds with gusts up to 45 kt. Areas of rough seas. Snow likely, freezing spray. Visibility 1 to 3 nm.

Saturday Night: gale force winds with gusts up to 45 kt. Rough seas up to 17 ft. Freezing spray, chance of snow. Local visibility 1 nm or less.

Sunday: Strong winds with gusts up to 35 kt. Rough seas up to 15 ft. Freezing spray.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.