textproduct: Boston / Norton

WHAT HAS CHANGED

Extreme cold watch issued for the Berkshires Saturday night into Sunday morning, with increasing confidence that cold weather headlines will be needed for much of the rest of southern New England.

KEY MESSAGES

- Dry weather through Friday, but with cold temperatures especially during the overnight and early morning hours.

- More active Friday into the weekend. Bitter cold continues into early next week. Dangerously cold wind chills this weekend along with strong winds and heavy freezing spray over the waters all increasingly likely.

DISCUSSION

KEY MESSAGE 1...Dry weather through Friday, but with cold temperatures especially during the overnight and early morning hours.

High pressure will be in control through Fri providing dry but cold weather. Expect lots of sunshine today but more cloud cover later tonight into Fri as developing NE flow leads to lower clouds from the ocean overspreading the region from east to west, while mid-high clouds move in from the west. Below normal temps will continue with highs today ranging from mid 20s to lower 30s and possibly slightly cooler on Friday due to more cloud cover. Tonight's low temp forecast will be tricky because of aforementioned cloud cover but may have several hours of mainly clear skies for the first half of the night providing good radiational cooling. We blended the colder GFS MOS with NBM guidance which yielded lows mostly in the single numbers, with teens along the immediate coast. Some below normal temps are possible in the colder locations of the CT valley.

KEY MESSAGE 2...More active Friday into the weekend. Bitter cold continues into early next week. Dangerously cold wind chills this weekend along with strong winds and heavy freezing spray over the waters all increasingly likely.

High pressure continues to dominate the pattern over southern New England ahead of an arctic frontal passage expected Friday night into Saturday. This is still expected to be a moisture-starved frontal passage. Light, fluffy snow showers are expected with this front still, along with the risk for snow squalls as it moves through. In terms of totals, most could expect to see around an inch of snow, except towards the eastern coastline and the outer Cape in particular. Totals towards the eastern coastline may end up higher due to lingering ocean effect processes, which guidance has captured pretty well. Accumulations there could be closer to 2-3 inches, with spots on the outer Cape possibly reaching borderline Winter Weather Advisory criteria with totals possibly reaching 4 inches. Will continue to monitor how this trends.

Confidence has continued to increase in dangerously cold wind chills developing across southern New England Saturday night into Sunday morning. The development of an offshore low continues to be highly supported across guidance suites, but the low expected to cross over northern New England has started to trend south with the latest runs. The 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. The latest forecast soundings do limit mixing to around 925 mb where winds are around 40 kt, which increases confidence in gale-force conditions over the waters, but would be a more borderline Wind Advisory situation over land for the Cape and Islands. Elevated winds and bitterly cold conditions will also favor the risk for moderate to heavy freezing spray over the waters, particularly Saturday night into Sunday, and even into Monday.

Temperatures aloft sink to around -20C at 925 mb Saturday night with pockets in the higher elevations reaching -25C, which when combined with the winds, it could end up feeling like -30F or colder in parts of the Berkshires and between -20F and -10F elsewhere. Ensemble guidance has continued 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 have increased to between 30 and 60 percent across GEFS and ECMWF ensemble guidance (GEPS guidance has continued to be less keen on this for the last few runs). As confidence has continued to increase for the higher elevations, we went ahead and issued an Extreme Cold Watch for the eastern slopes of the Berkshires. The rest of the region will likely see Cold Weather Advisory criteria wind chills and the Worcester Hills could reach Extreme Cold Warning criteria, but we will continue to monitor this in upcoming forecast cycles.

These bitterly cold conditions are expected to last into the start of next week before the arctic airmass overhead exits and surface high pressure builds in along with ridging aloft. Temperatures aloft recover a bit going into midweek as well, increasing from -20C to around -10C and -5C at 925 mb, which should allow for daytime surface temperatures to get closer to above freezing.

AVIATION /07Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

06z TAF Update...

Through Friday...High confidence.

Mainly VFR through the period, but cigs around 3-5k ft developing from E to W late tonight into Fri which may result in localized MVFR. NW winds 10-15 kt today, becoming light N tonight and light and variable Fri.

Outlook /Friday Night through Monday/...

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

Saturday: MVFR/IFR conditions possible. Windy with gusts up to 35 kt. SN likely.

Saturday Night: VFR. Strong winds with local gusts up to 40 kt. Slight chance SN.

Sunday: VFR. Windy with gusts up to 30 kt.

Sunday Night through Monday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Friday...High confidence.

Quiet boating weather through Fri with winds and seas below SCA thresholds. NW winds today will become N tonight then NE by Friday.

Outlook /Friday Night through Monday/...

Friday Night: Winds less than 25 kt. Chance of snow.

Saturday: Low risk for gale force winds with gusts up to 40 kt. Local 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 12 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 12 ft. Freezing spray, slight chance of snow.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Freezing spray.

Monday: Winds less than 25 kt. Areas of rough seas. Freezing spray.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Extreme Cold Watch from Saturday evening through Sunday morning for MAZ002-008-009. RI...None. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ231>235-237-250-251-254>256. Heavy Freezing Spray Watch from Saturday afternoon through Sunday afternoon for ANZ231>237-250-251-254>256.