textproduct: Boston / Norton

WHAT HAS CHANGED

Extreme Cold Watches have been upgraded to Extreme Cold Warnings for northern Connecticut and for western and central Massachusetts. Cold weather advisories have been issued for the rest of southern New England, except Nantucket. Snowfall amounts for Saturday were increased and winter weather advisories will likely be issued this afternoon, especially for portions of eastern MA.

KEY MESSAGES

- Widespread 1-4 inches of snow likely later tonight through Sat, with localized bands of heavier snow possible. Ocean effect snow showers over Cape Cod Sat night.

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

DISCUSSION

KEY MESSAGE 1...Widespread 1-4 inches of snow likely later tonight through Sat, with localized bands of heavier snow possible. Ocean effect snow showers over Cape Cod Sat night.

Rather dynamic pattern for Fri night into Sat as deep mid level trough amplifies across New Eng and eventually takes on negative tilt, with 700 mb low eventually developing south of New Eng Sat afternoon. Strong height falls developing off the coast will lead to cyclogenesis well offshore on Saturday, but given the strong upstream trough there will be an inverted trough that develops and extends across New Eng. Increasing forcing for ascent ahead of the trough and low level convergence with the inverted trough combined with deepening moisture plume with near saturation through the mid levels will result in widespread snow developing from late evening through Saturday. The snow should taper off in the afternoon in western New Eng behind the arctic front, while becoming more focused in eastern MA. Interesting set up as snow will be moving in from the west ahead of the trough, but hi-res guidance shows also bands of snow moving into eastern MA from the ocean later tonight through Sat from convergent NE flow ahead of the arctic front.

The bands moving in from the ocean and the inverted trough are wildcards to the snow forecast. These inverted troughs are often associated with narrow mesoscale bands of enhanced snowfall where the low level convergence sets up. We also have potential for localized snow squalls along the arctic front. There are decent model signals given strong low level frontogenesis, relatively steep 0-3km lapse rates and some low level CAPE. Snow squall parameter is highlighted across eastern New Eng Sat afternoon. In addition, model soundings show very favorable snow growth for a few hours in eastern New Eng with 20-30 units of omega in the DGZ suggesting locally brief heavy snowfall rates of 1"+/hr within any enhanced snow bands. However, forecasting the exact location and axis of where these heavier snow bands is very challenging often leading to low confidence forecasts in location of potentially higher impact snowfall. This is a situation where snowfall amounts can vary greatly over short distances.

A general snowfall of 1-4 inches is expected, heaviest central and eastern New Eng, but there will likely be heavier amounts from mesoscale factors which makes this a very challenging forecast. We leaned on hi-res guidance which favors eastern MA for the heavier snow where locally up to 4-6 inches is possible in the heaviest bands. However, we are going to wait until this afternoon before deciding on where to issue advisories as the heavier snow will be the result of mesoscale processes and want to leverage the 12z hi-res guidance to hopefully gain higher confidence on placement.

Behind the arctic front as winds turn northerly expect ocean effect snow showers to develop over Cape Cod and Nantucket. Additional light accumulations are possible but the shallow boundary layer and marginal ocean induced CAPES will probably limit the intensity of the snow showers somewhat.

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 /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Friday...High confidence.

Mainly VFR with cigs 3-5k ft developing from E to W, but areas of MVFR cigs possible this afternoon. Light N-NE winds.

Friday night 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.

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 /Sunday through Tuesday/...

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

Sunday Night through Monday Night: VFR. Breezy.

Tuesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Headlines:

* Gale Watches were upgraded to Gale Warnings from Sat afternoon into Sunday.

* Heavy Freezing Spray Watches were upgraded to 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 /Sunday through Tuesday/...

Sunday: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft. Freezing spray, slight chance of snow.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Freezing spray.

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

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

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Freezing spray.

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. Cold Weather Advisory from 6 PM Saturday to 1 PM EST Sunday for MAZ005>007-013>023-026. RI...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. 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.