textproduct: Boston / Norton

WHAT HAS CHANGED

A winter storm watch has been issued for Essex county, beginning 4 AM Saturday through 7 PM Saturday. Winter weather advisories have been issued for the same time period for the remainder of eastern MA into the Worcester Hills and down into RI.

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, gusty winds will lend to wind chills 10-30 degrees below zero. In addition, freezing spray becomes hazardous for the mariners.

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 HREF 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, gusty winds will lend to wind chills 10-30 degrees below zero. In addition, freezing spray becomes hazardous for the mariners.

Forecast confidence is high for dangerous cold conditions Saturday night through Sunday, in addition, high confidence in heavy freezing spray across most waters, with the exception of Boston Harbor. This is all brought by an Arctic front, arriving late Saturday afternoon as 925mb temperatures fall from 7-10 below zero to 20-25 below zero cross the interior and 10-15 below zero across the coastal plain. In conjunction is a 925mb LLJ with winds 35-45 knots, with is roughly the height of the mixed layer. Strong northwest winds 30-40 mph will make the air temperature, which will be 5-10 below zero across the interior and 0-5 above zero across the coastal plain. Warmest areas are the outer Cape and Nantucket where lows are in the low teens.

There hasn't been changes to the headlines. The Extreme Cold Warning and Cold Weather Advisory go in effect Saturday evening into Sunday afternoon. The only area in southern New England not included is Nantucket. Areas under the Extreme Cold Warning should prepare for wind chill values as low as 20-30 degrees below zero. As for areas under the Cold Weather Advisory, wind chill values are expected to be as low as 15-20 degrees below zero. There isn't much improvement Sunday afternoon, while the 925mb jet diminishes, surface winds remain gusty 20-30 mph. Leading to wind chill values Sunday afternoon in the single digits on either side of zero.

During this time, it's worth reminding boater of heavy freezing spray across all the waters as icing rates could reach 0.5-1.0 inch per hour. This has prompted a Heavy Freezing Spray Warning for all waters with exception of Boston Harbor, where a Freezing Spray Advisory has been issued.

A 1035mb high builds in Sunday night into Monday across the eastern Great Lakes, leading to diminishing winds. Still very cold, it's a possibility there may be a need for Cold Weather Advisory headlines again Sunday night into Monday morning. Opt'd to hold off on any issuances at this time as not to cause confusion with the more dangerous of the cold. Nevertheless, air temperatures are only a degree or three higher Sunday night into Monday morning, but with less wind, the wind chill values are expected to be 5-15 degrees below zero with far interior Massachusetts as low as 20-25 degrees below zero.

The area of high pressure remains Monday and Tuesday, below normal temperatures continue into early next week, daytime highs are in the teens and 20s on Monday, near 30 on Tuesday, and trending towards the middle 30s for Wednesday and Thursday. Outside of a weak clipper system on Tuesday there is fairly quiet conditions early next week. Will need to monitor a mid to late week system, though there is low confidence in this part of the forecast due model disagreement on placement and timing. Stay tuned.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...No significant changes. Lowering cigs/vsbys after midnight as snow develops.

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 4 AM EST Saturday 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.