textproduct: Boston / Norton

WHAT HAS CHANGED

Winter Weather Advisory expanded across southern New England due to the potential for orographic enhancement along the western hills. Snowfall trending lower across the islands, potential for rain to mix with snow on Nantucket has lowered totals, which lead to the cancellation of the Winter Weather Advisory on the island.

KEY MESSAGES

- Light to moderate snowfall this evening through early tomorrow morning, possibly ending with a snow squall due to an incoming Arctic cold front.

- Vessel icing due to accretion of light freezing spray over the waters is possible Thursday night into this weekend.

- Increasing chance for snow showers on Sunday, otherwise no changes.

DISCUSSION

KEY MESSAGE 1...Light to moderate snowfall this evening through early tomorrow morning, possibly ending with a snow squall due to an incoming Arctic cold front.

Bottom Line Up Front: Snow overspreads the region between 10pm to midnight west, and midnight to 2am east, heaviest snowfall overnight 2am to 6am Thursday. If your plans for celebrating the New Year include travel by car, please take it slowly when you return home. Roadways will become snow covered quickly given the antecedent conditions.

What's New: Guidance over the last 24-hours has shown a subtle shift in wind direction to the southwest from the west-southwest. As for Nantucket, consensus has grown in a primary wind direction out of the south-southwest, lending a "warmer" outcome, leading to rain mixing in with snow overnight, thus lowering totals. Opt'd to cancel the Winter Weather Advisory here - though, those traveling tonight on the island should expect minor snow accumulation around an inch or less. With the rain, some of the accumulations could wash away by morning and falling afternoon temperatures could lead to icy roads if untreated. With a shift in the prevailing wind to the southwest, snowfall axis has shifted further north. Will discuss further below, but expanded the Winter Weather Advisory to include; western zones of Franklin and Hampshire Counties, Worcester County, Tolland and Windham Counties, most of Rhode Island, and all of southeast Massachusetts. Here we feel there is the best potential for 2-4" with the higher of the totals on the western facing hills, due to upsloping from the southwest flow.

Forecast Details: Quiet through the rest of the daylight hours, then a robust 850mb shortwave races through southern New England this evening through early tomorrow morning. This feature has near normal moisture to work with, PWATs of 0.3" to 0.4" doesn't sound overly too impressive, but is enough to work with. Looking through BUFKIT forecast soundings, there is good saturation and OMEGA/lift in the dendritic growth zone (DGZ), with light to moderate snow for several hours 2am to 6am. HREF supportive of this idea, with probabilities between 30% and 60% for snowfall rates 1+" an hour during this time across Connecticut, Rhode Island, and southeast Massachusetts. As mentioned above, those traveling late tonight or tomorrow morning will want to plan accordingly as slick travel is expected. There is a level of uncertainty around how far north the warm air reaches, for the time being confidence for the northern extent is Nantucket. It is there the best chance for rain to mix with the snow. Elsewhere, remains cold enough to stay snow. Additionally, southwest flow enhances snow potential due to orographic lift/upslope along western facing hills of northeastern Connecticut, central Massachusetts... even western hills of Rhode Island benefit from an upslope component. Finally, the shortwave moves off shore early Thursday morning, followed with an Arctic front midmorning, with additional snow showers or possibly snow squalls.

Post: CAA brings falling temperatures into the afternoon, lowering into the 20s and teens during the afternoon. Southwest wind shifts to the northwest with gusts reaching between 30 to 40 mph. Dry and cold weather expected Thursday afternoon and overnight. Nighttime lows dip further, reaching down into the teens and upper single digits.

KEY MESSAGE 2...Vessel icing due to accretion of light freezing spray over the waters is possible Thursday night into this weekend.

Mostly for marine interests, but with a prolonged period of cold air over the relatively milder ocean waters, periods of freezing spray could accrete on vessels through the first week of January. There is potential for moderate freezing spray late Thursday night into early Friday, therefore a Freezing Spray Advisory has been issued for the waters off of Cape Ann, Buzzards Bay, Vineyard Sound, and Nantucket Sound.

Threat for freezing spray continues this weekend, with a lower sea state the threat should be minimal. However, we did want to give it at least a mention, since the risk for light vessel icing due to a prolonged period given the spell of colder temperatures.

KEY MESSAGE 3...Cold but dry through early next week. Increasing chance for snow showers on Sunday.

Significant -NAO block sets up allowing for a cold but dry period of weather from the end of this week through the start of the next week. Wave of low pressure approaches from the S on Sunday but impacts, if any, are uncertain at this time.

High pressure takes control of our weather to close out the week with cold and dry conditions developing. Expecting well below average temps with 925 mb temperatures between -10C and -15C translating to sfc temps the mid 20s to lower 30s. Overnight lows are likely to be slightly more variable, in the mid teens through the first half of this weekend, then dropping to single digits to lower teens by Sunday night.

Beyond the sustained cold, there is some uncertainty in the overall placement/track of an area of low pressure later Sunday into Sunday night. General theme amongst guidance is for a deep trough to dig across the eastern CONUS with two embedded disturbances. Guidance shows a potent northern stream disturbance and a weaker, more diffuse disturbance along the southern jet. General consensus amongst operational and ensemble guidance is to keep these features separate with a weaker and more suppressed low. The 12z suite has shown a bit more amplification with the chance for slightly faster intensification but even in these runs, any snowfall comes purely from the northern stream shortwave with minimal phasing. In short, while light snow is possible, upper level confluent flow will likely suppress the bulk of the precipitation well offshore.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Tonight into Thursday morning...High confidence.

VFR into this evening. A steady shield of snow should overspread the region from west to east in the 03z to 06z time frame. This will result in IFR to briefly LIFR conditions developing. Ptype should be all snow except for ACK where a period of rain or rain and snow mixed will occur. Steady shield of snow departs by 10z/11z...but lingers a few hours longer near the Cape and Islands. In addition...a cold front may trigger brief scattered snow showers and perhaps a localized snow squall or two between 10z and 15z. Total snow accumulations of 1 to 4 inches with the higher amounts near the south coast and western Hills of MA/CT/RI. SW winds 5-12 knots tonight shifting to the WNW with gusts of 25-35 knots developing Thursday morning behind the cold front.

Thursday afternoon and night...High confidence.

VFR conditions. WNW wind gusts of 25 to 35 knots Thu afternoon gradually diminish Thu night and become more westerly.

KBOS Terminal...High confidence in TAF. Snow arrives after 05z/06z and continues until about 10z/11z. A brief round of scattered snow showers and perhaps a localized snow squall will then be possible with the cold frontal passage through about 15z.

KBDL Terminal...High confidence.

High confidence in TAF. Snow arrives after 04z/05z and continues until about 08z/09z. A brief round of scattered snow showers and perhaps a localized snow squall will then be possible with the cold frontal passage through about 13z/14z.

Outlook /Friday through Monday/...

Friday: VFR. Breezy. Slight chance FZRA.

Friday Night through Saturday: VFR. Breezy.

Saturday Night: VFR. Chance SN.

Sunday: VFR. Slight chance SN.

Sunday Night through Monday: VFR.

MARINE

Forecaster Confidence Levels...

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

* Northwest Gale Force Winds late tonight through Friday morning along with the threat of moderate freezing spray for some waters.

This Afternoon through Thursday Night... High Confidence.

Southwest flow through this afternoon ahead of an approaching cold front, this produces gusts in the range of 20-25 knots. Late tonight the wind shifts to the northwest with Gale Force gusts through Thursday night for all waters. Easing wind by Friday morning.

Threat for moderate freezing spray for waters north of Cape Ann to the Merrimack River, as well as Buzzards Bay, Vineyard Sound, and Nantucket Sound between Thursday evening and Friday morning.

Building seas, overnight are 3-5 feet and are increasing 5-9 feet on the southern waters and 4-8 feet on the eastern waters.

Outlook /Friday through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Freezing spray, slight chance of freezing rain, slight chance of snow.

Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Freezing spray.

Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Saturday Night through Sunday: Winds less than 25 kt. Slight chance of snow.

Sunday Night through Monday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory until 1 PM EST Thursday for CTZ003-004. MA...Winter Weather Advisory until 1 PM EST Thursday for MAZ002-004- 008-012-013-016>023-026. RI...Winter Weather Advisory until 1 PM EST Thursday for RIZ001>007. MARINE...Gale Warning from 7 AM Thursday to 4 AM EST Friday for ANZ230>237-251. Freezing Spray Advisory from 10 PM Thursday to 1 PM EST Friday for ANZ232>234-250-251. Gale Warning from 7 AM Thursday to 7 AM EST Friday for ANZ250- 254-255. Gale Warning from 4 AM Thursday to 7 AM EST Friday for ANZ256.