Marine Weather and Tide Forecast for North Plymouth, MA

KEY MESSAGES

- Widespread 2-4 inches of snow likely later tonight through Sat, with localized bands of heavier snow across northeast MA, possibly up to 8 inches in Essex county, including Cape Ann. Less snow westward into CT and western MA. Ocean effect snow showers over Cape Cod Sat night.

- Dangerous cold Saturday night into Sunday, with gusty winds.
Hazardous marine conditions expected as well, including Gales & heavy freezing spray.

DISCUSSION

KEY MESSAGE 1...Widespread 2-4 inches of snow later tonight through Sat, but localized bands of heavier snow over northeast MA, possibly yielding amounts up to 8 inches. Ocean effect snow showers over Cape Cod Sat night.

-AO and -NAO teleconnections yielding a very suppressed polar jet this period, with storm track well south of New England. However, this supports a potent, high latitude closed low to descend from James Bay into New England this weekend. This feature will be accompanied by an arctic front Sat, which will yield a complex snow event for SNE. This vigorous closed low induces cyclogenesis well southeast of the 40N/70W benchmark Sat. However, such strong forcing for ascent results in an inverted/Norlun trough from the offshore low extending northwest into SNE Sat. This will result in narrow bands of snow Sat moving west to east across CT/MA/RI.

Model data reveals a combination of steep low level lapse rates (up to 9C/km in 0-2 km layer), deep saturated layer, strong convergence and at least modest omega in the snow growth region, will yield a 2- 4 hr period of moderate to perhaps heavy snow at times, from late Sat morning into the afternoon. The snow squall parameter ramps up rapidly between 18-22z across eastern MA Sat afternoon. 12z NAM bufkit for KBVY, indicates up to 20 u/bars of lift in the -10C to - 15C layer 15z-18z. This should yield favorable SLR and low vsbys.
This is also supported by HRRR offering 30-50% probs of hourly snowfall rates exceeding 1" and 20-30% for greater than 2" per hour, with these high rates focused across eastern MA westward into RI and the Worcester Hills. Duration of strong lift, deep saturation and steep low level lapse rates is brief (2-4 hr window) and this will limit accumulations to mainly 2 to 4 inches, with highest amounts across RI and central-eastern MA.

This boundary becomes more enhanced as it enters eastern MA, with multiple models generating over 0.50 inches of qpf across Essex county, with some ocean effect enhancement contributing to the higher totals. In addition, the 13z NBM (v5.0) snow probs have 20- 30% probs of 8+ inches across Essex county, esp the Cape Ann area, and 50% prob of 6+ inches of snow. Given multiple models have strong low level convergence, steep low level lapse rates, a deep saturated layer and at least modest forcing for ascent in the snow growth region, will hoist a small winter storm watch area for Essex county.
Snowfall projections here will be for 4-8", with even a low prob of isolated higher amounts pending the evolution of this mesoscale snow band associated with the inverted/Norlun trough. Keep in mind, these mesoscale snow bands are very difficult to forecast and this is a situation where snowfall amounts can vary greatly over short distance. Thus, continue to check back and monitor our updated forecast. For the remainder of coastal eastern MA, given the OES component, isolated amounts of up to 6 inches is possible.

For the remainder of the region, the short duration/progressive nature of the snow bands, thinking most locations across the remainder of eastern and central MA into RI will experience 2-4".
Thus, have issued a winter weather advisory for this area. Given the boundary is not as robust as it enters western MA into CT, thinking less snowfall here, 1-3". Hence, did not issue an advisory for this region. Not a blockbuster snow event, but roads will become snow covered and travel will become difficult, especially in northeast MA from late morning into the afternoon.

The snow should taper off in the early afternoon in CT/western MA behind the arctic front, then exiting offshore 4-7 PM across eastern MA.

KEY MESSAGE 2...Dangerous cold Saturday night into Sunday with gusty winds. Hazardous marine conditions expected as well.

The Arctic front passing through the region will usher in a bitterly cold Arctic airmass aloft in its wake. An offshore low to our southeast is also expected to deepen, which will lead to southern New England being settled underneath a tightening pressure gradient as high pressure sits to our north and west. Forecast soundings continue to limit mixing to around 925 mb where winds are around 40 kt. Gusts at the surface could easily reach the 35-45 mph mark even with this shallow mixing as strong cold air advection takes hold. These 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. As a result, we've hoisted a Heavy Freezing Spray Warning across all Massachusetts and Rhode Island coastal waters, including Narragansett Bay. Winds over land are expected to fall below Wind Advisory criteria, so we did not issue one at this time.

Temperatures aloft fall to around -20C at 925 mb Saturday night into Sunday morning with pockets in the higher elevations getting colder. When combined with the winds, it could end up feeling like 30 below 0F or colder in parts of the Berkshires.
Wind chills between -20F and -10F are expected elsewhere. As confidence in these dangerous wind chills has grown, we've gone ahead and issued an Extreme Cold Warning for western and central MA, along with northern CT. The rest of southern New England with the exception of Nantucket is under a Cold Weather Advisory for Saturday night into Sunday as well.

These bitterly cold conditions are expected to last into the start of next week before the Arctic airmass overhead weakens a bit. Winds ease up going into Monday and Tuesday as the gradient slackens. A weak shortwave passes the region Monday night which could lead to a spot shower or two, but generally, dry conditions are expected for the start of the week. Temperatures aloft recover a bit going into midweek, climbing to around -10C at 925 mb, which should allow for daytime surface temperatures to get closer to above freezing.
Temperatures into midweek are still expected to be below normal with highs stuck in the low to mid 30s and lows in the teens and low 20s.

Guidance continues to hint at another system that may impact southern New England during the mid to late week timeframe, however, significant disagreements across model suites still exist and this possible system is roughly a week away. So, confidence in any more details is low at this time. Stay tuned for future updates.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/
Forecaster Confidence Levels...

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

18z TAF Update...

Tonight into Saturday...Moderate confidence.

Conditions lowering to MVFR-IFR as areas of snow develop from late evening through Sat. Localized heavier snow bands possible with LIFR conditions, especially Sat in eastern MA. Gusts 20-25 knots develop by mid-morning, increasing to up to 30 knots by the afternoon.

Saturday night...Moderate confidence.

MVFR becoming VFR as the night goes on and snow starts to clear the region. Some snow showers may linger longer towards the Cape.

Outlook /Saturday Night through Wednesday/...

Sunday: VFR. Windy with gusts up to 35 kt.

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

Monday through Monday Night: VFR. Breezy.

Tuesday through Tuesday Night: VFR.

Wednesday: Mainly VFR, with local IFR possible. Chance SN.

MARINE
Forecaster Confidence Levels...

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

2 PM update...

Headlines:

* Gale Warnings from Sat afternoon into Sunday.

* Heavy Freezing Spray warnings for Saturday afternoon through early Mon morning.

Light NE winds through tonight, then increasing NE winds Sat morning turning N-NW later Sat into Sun with gusts increasing to 35-40 kt behind the arctic front. Areas of heavy freezing spray will develop, especially Sat night as gusty winds combine with arctic air moving into the region. Heavy freezing spray produces ice accretion rates up to 1 inch/hr creating very dangerous conditions for vessels.

Outlook /Saturday Night through Wednesday/...

Sunday: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft. Freezing spray, slight chance of snow.

Sunday Night: Strong winds with gusts up to 30 kt. Rough seas up to 9 ft. Freezing spray.

Monday through Monday Night: Winds less than 25 kt. Seas up to 5 ft. Freezing spray.

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

Tuesday Night: Winds less than 25 kt.

Wednesday: Winds less than 25 kt. Slight chance of rain, slight chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES
CT...Extreme Cold Warning from 6 PM Saturday to 1 PM EST Sunday for CTZ002>004.
MA...Extreme Cold Warning from 6 PM Saturday to 1 PM EST Sunday for MAZ002>004-008>012.
Winter Weather Advisory from 4 AM to 7 PM EST Saturday for MAZ004-005-012>021-026.
Cold Weather Advisory from 6 PM Saturday to 1 PM EST Sunday for MAZ005>007-013>023-026.
Winter Storm Watch from late tonight through Saturday evening for MAZ006-007.
Winter Weather Advisory from 4 AM Saturday to 1 AM EST Sunday for MAZ022>024.
RI...Winter Weather Advisory from 4 AM to 7 PM EST Saturday for RIZ001>008.
Cold Weather Advisory from 6 PM Saturday to 1 PM EST Sunday for RIZ001>008.
MARINE...Gale Warning from 4 PM Saturday to 7 AM EST Sunday for ANZ230- 236.
Freezing Spray Advisory from 4 PM Saturday to 7 PM EST Sunday for ANZ230.
Gale Warning from 4 PM Saturday to 1 PM EST Sunday for ANZ231- 232-250-251-254.
Heavy Freezing Spray Warning from 4 PM Saturday to 7 AM EST Monday for ANZ231>237-250-251-254>256.
Gale Warning from 1 PM Saturday to 10 AM EST Sunday for ANZ233>235-237.
Gale Warning from 1 PM Saturday to 1 PM EST Sunday for ANZ255- 256.