textproduct: Boston / Norton

WHAT HAS CHANGED

A Wind Advisory has been put into effect for the east slopes of The Berkshires and Cape Cod and The Islands starting tomorrow morning and continuing through early tomorrow afternoon. Strong northwest wind gusts up to 45 mph will be possible during the advisory period.

Another brief period of snow will be possible late tomorrow morning/early tomorrow afternoon over southeast MA.

KEY MESSAGES

- Cold front brings gusty winds for Christmas Eve with northwest gusts of 35-45mph.

- Ocean effect snow showers may bring accumulating snow to portions of southeast MA late tomorrow morning/early tomorrow afternoon

- Few rain and/or snow showers on Christmas Day, turning breezy northwest winds during the evening and overnight, with wind chills on either side of 0F.

- Lower-confidence forecast featuring snow, ice, and rain mix beginning late Friday through the weekend.

DISCUSSION

Key Message 1...A cold front brings gusty winds for Christmas Eve with northwest gusts of 35-45 mph.

Some light/rain snow showers continue this afternoon and evening across southern New England as a frontal wave traverses over The Northeast. An associated cold front will push through the region after midnight bringing a cooler/drier air mass into the region. Additionally, this front will be associated with a robust low-level jet defined by winds ranging from 35 to 55 knots in the 925-850 hPa layer. Strong CAA should support steep lapse rates and efficient mixing that will allow gusts from 35-45 mph to mix to the surface early tomorrow morning through early tomorrow afternoon.

For most of southern New England, gusts will be strong, but likely below Wind Advisory criteria as peak 925/850 hPa winds will be moving overhead during the first half of the morning when mixing may not be as efficient. Expect gusts to pick up by the mid-late morning hours as lapse rates steepen. BUFKIT soundings reveal momentum transfers between 30-40 knots across the interior and 35-45 knots over the high terrain of The Berkshires, Cape Cod, and The Islands. For this reason, the latter locations have been put into a Wind Advisory lasting from early tomorrow morning through about 3PM tomorrow afternoon.

Key Message 2...Minor snow accumulations possible across portions of southeast MA tomorrow afternoon.

Surface low-pressure digs southeast from Ontario over The Gulf of Maine tomorrow morning into tomorrow afternoon. Hi-Res model guidance supports the development of an inverted trough that may provide sufficient forcing for some rain/snow showers along the east coast and The South Shore in particular. Temperatures will be marginal for snow-fall in the mid 30s, but some heavy wet flakes or rain will be a possibility between about 10am and 2pm tomorrow. If surface temperatures are cool enough for snow, we would expect minimal accumulations <1". Should temperatures trend cooler and meso-scale forcing trend stronger, we could see more substantial accumulations. The latest HREF model supports a 10-20 percent chance of accumulations greater than 1" along and east of the I-95 corridor in MA tomorrow.

KEY MESSAGE 3...Few rain and/or snow showers on Christmas Day, turning breezy northwest winds during the evening and overnight, with wind chills on either side of 0F.

Do not anticipate any significant issues for Christmas Day, cannot rule out isolated to scattered rain and/or snow showers with the passage of an 850mb trough and shortwave. With PWATs around 0.5" and the progressive nature, snow and/or rain showers will be short-lived between the hours of 6am and noontime. Seasonable temperatures in the upper 30s and lower 40s, from the southwest flow. Wind direction becomes northwest early to midafternoon for the evening/overnight hours. BUFKIT shows mixing up to 850mb, with modest CAA, gusts of 30 to 40 mph are possible. Will need to monitor this as we are nearing criteria for a Wind Advisory. Otherwise, a chilly night with lows in the teens/single digits and wind chill temperatures on either side of 0F. Brief high pressure builds in for Friday with below normal afternoon temperatures.

KEY MESSAGE 4...Lower-confidence forecast featuring snow, ice, and rain mix beginning late Friday through the weekend.

There are two time frames to monitor for the potential of impactful weather conditions; Friday night into Saturday and then late Sunday into Monday. But, when it comes to exact details, this is where the uncertainty comes into play. First, a low-pressure system moves out of the northern Great Plains and through the Midwest, and ejects off the Mid Atlantic Coast. As the previous forecaster mentioned, there is varying guidance on the overall track and that remains the case as of this update. Because of this, there is a vast spread in the amount of QPF, with the 25th percentile of a few hundredths of an inch and the 75th percentile of more than half to two-thirds of an inch. Given the cold air in place, PTypes favor snow, but still time to watch track development. At this time highest probabilities for accumulation are across southwest CT and the NYC metro - where there are 60 to 80 percent probabilites for more than 2" of snow.

Sunday afternoon through early Monday could bring another chance for wintry weather, but there is much more uncertainty for this timeframe. These are days 5 and 6, and felt it's important to mention because of the amount of people traveling for the holidays. At this time exact details aren't available because of the varying timing and spread of the guidance, but there is potential for mixed wintry PTypes; snow, ice, and rain mixed.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through 00Z...Moderate Confidence

Patchy -RASN continues this afternoon with ceilings continuing to fall to MVFR/LIFR levels. Light south/southeast winds.

Tonight... High Confidence in Trends

Mainly IFR through about 06Z. Winds increase in speed and shift to west/northwest after 06Z with sustained winds from 10 to 15 knots and gusts up to 25 knots by 12Z. Drier air in northwest flow will help improve ceiling to MVFR and eventually VFR from west to east .

Tomorrow... High Confidence

Mainly VFR. There will be a chance for some -RASN over The Cape/Islands and possibly BOS. These may be accompanied by periods of MVFR ceilings, mainly between 15-21Z. Very gusty northwest winds tomorrow. 15-25 knot northwest winds sustained with gusts ranging from 30-40 knots. Gusty winds are largely expected during the first half of the day and should settle down between 18-00Z.

Tomorrow Night...High Confidence

VFR. Winds becoming light out of the west/southwest.

KBOS Terminal...Moderate Confidence

Precip more off and on through mid-afternoon before becoming more steady rain/snow mix in the evening. CIGS will be stubborn to drop below VFR until more steady precip can work in, then low end MVFR to IFR CIGS can be expected.

KBDL Terminal...High confidence

Outlook /Thursday through Sunday/...

Christmas Day: VFR. Breezy. Slight chance SN.

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

Friday: VFR. Breezy.

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

Saturday: Mainly VFR, with local MVFR possible. Chance SN.

Saturday Night: VFR. Slight chance SN.

Sunday: Mainly VFR, with local IFR possible. Breezy. RA likely, chance SN.

MARINE

Forecaster Confidence Levels...

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

GALE WARNING IN EFFECT FOR WEDNESDAY

A cold front will bring gale-force winds to the waters starting early Wednesday morning through Wednesday afternoon. Winds shift to the NNW, gusting 30-45 knots beginning before sunrise through the early afternoon. High winds bring rough seas with 6-10 foot waves away from the coasts and sounds.

Winds gradually diminish on Wednesday night with seas of 3-6 feet.

Outlook /Thursday through Sunday/...

Christmas Day: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain, chance of snow.

Thursday Night: Strong winds with gusts up to 30 kt. Rough seas up to 10 ft. Chance of freezing spray.

Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of freezing spray.

Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow. Local visibility 1 nm or less.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow, slight chance of rain.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain, slight chance of snow.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory from 3 AM to noon EST Wednesday for MAZ002-008- 009. Wind Advisory from 7 AM to 3 PM EST Wednesday for MAZ022>024. RI...None. MARINE...Gale Warning from 5 AM to 3 PM EST Wednesday for ANZ230>237- 250-251-254>256.