textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Cold front brings gusty winds from the NNW at 35-45mph this morning

- Ocean effect snow showers may bring light accumulating snow to portions of the eastern coast into Cape Cod today

- Breezy with a few rain/snow showers Christmas Day. Temperatures fall quickly below freezing on Christmas Night with wind chill values in the single digits.

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

DISCUSSION

KEY MESSAGE 1...Cold front brings gusty winds from the NNW at 35-45mph this morning

A shortwave trough and weak cold front drop down from northern New England this morning. While this cold front is not bringing a large temperature swing with highs remaining in the mid-30s to low-40s, the main impact will be strong NNW winds this morning. Guidance still indicates a 45-50 knot LLJ moving through the region this morning. With the light cold air advection, it will take time for mixing to Be able to tap into the higher winds at 850mb. Bufkit sounds show the highest potential for gusts reaching above 35-45mph between 9 am and 1 pm before the LLJ begins to move offshore to the south, and the pressure gradient weakens. The best chance for 45- 50mph gusts remains in the high terrain and across the Cape and Islands, where wind advisories remain in effect.

KEY MESSAGE 2...Ocean effect snow showers may bring light accumulating snow to portions of the eastern coast into Cape Cod today

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...Breezy with a few rain/snow showers Christmas Day. Temperatures fall quickly below freezing on Christmas Night with wind chill values in the single digits.

Christmas morning starts off quite with mostly cloudy skies. Skies will begin to clear throughout the day as a stronger cold front and shortwave approach from the northwest once again. There could be a few snow showers associated with this cold front, but not anticipating any accumulating snow at this time. Winds will be breezy from the NW during the day at 20-30mph, with the potential for stronger winds to arrive on Christmas night with 35-45mph gusts mainly in the high terrain and near the waters. High temperatures top out in the upper 30s to low 40s, but will drop like a rock after sunset, possibly below freezing across the entire region by 7 pm. Wind chill values fall to the single digits by midnight with the continued gusty winds.

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 /07Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

06z TAF Update..

Rest of Tonight...Moderate confidence.

Low end MVFR to mainly IFR conditions this evening. Areas of snow this evening across central and interior northeast MA with a mixture of rain/snow near and southeast of I-95. Bulk of precipitation exits the coast by 06z with improvement in cigs/vsbys from west to east. Most locations should be VFR by 12z except areas near the immediate coast. Light S winds early this evening shift to the WNW toward daybreak and begin to gust between 20 and 30 knots.

Wednesday...High Confidence

VFR outside the Cape and Islands where stratocumulus and ocean effect rain/snow showers will bring MVFR to possibly IFR conditions in the afternoon. Any snow that falls will be light, under a half inch. Winds aloft increase early this morning which will bring wind shear concerns until surface winds can catch up later in the morning. Gusts this morning will range from 30-35 knots from the NNW with 40 knots possible over the Cape and Islands. Winds gradually diminish after 18z with winds going calm after 00z.

Tomorrow Night...High Confidence

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

KBOS Terminal...Moderate confidence in TAF.

Rain/snow will end by 08z with drying conditions this morning. CIGS will gradually lift to VFR by sunrise with gusty NNW winds up to 35 knots today.

KBDL Terminal...Moderate confidence in TAF.

Gradually improving CIGS this morning with VFR and gusty winds expected by sunrise. NNW gusts could reach up to 35 knots.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Windy with 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 MVFR possible. Chance RA, slight chance SN.

MARINE

Forecaster Confidence Levels...

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

Today through Christmas Day

A gale warning remains in effect today as a cold front brings gusty NNW winds of 30-40 knots. Rough seas are anticipated today with 7- 10 foot waves offshore, and 4-6 foot waves near shore and in the sounds. Brief reprieve from the gusty winds tonight before stronger winds arrive again for Christmas Day and night. Gale force winds from the NNW once again look possible starting in the late afternoon to early evening.

Outlook /Thursday Night through Sunday/...

Thursday Night: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of snow.

Friday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow. Local visibility 1 to 3 nm.

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

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory until noon EST today for MAZ002-008-009. Wind Advisory from 7 AM this morning to 3 PM EST this afternoon for MAZ022>024. RI...None. MARINE...Gale Warning until 3 PM EST this afternoon for ANZ230>237-251. Gale Warning until 5 PM EST this afternoon for ANZ250-254>256.