textproduct: Boston / Norton

WHAT HAS CHANGED

Winter Weather Advisory expanded to include the north shore, Boston, and western RI for freezing rain. Confidence in the potential for strong gusty winds on Tuesday continues to increase. Wind Advisories may be needed for at least the high terrain and Cape/Islands.

KEY MESSAGES

- Freezing rain is expected tonight after 8 pm and ending by 4 am. The highest ice accumulation will be in the northern CT river valley, where up to a quarter inch of ice is possible. Otherwise, expecting between a trace and a tenth of an inch of ice accretion.

- Strong gusty winds expected to start Tuesday morning and peak in the early afternoon.

- Chilly, below normal temperatures continue into the end of the week, with periods of light snow off and on.

DISCUSSION

KEY MESSAGE 1...Freezing rain is expected tonight after 8 pm and ending by 4 am. The highest ice accumulation will be in the northern CT river valley, where up to a quarter inch of ice is possible. Otherwise, expecting between a trace and a tenth of an inch of ice accretion.

Potent shortwave exiting the northern Great Plains will rapidly intensify a surface low over the Great Lakes. The strong pressure gradient from the surface low will allow a warm front to stretch all the way to the northeast today. With snow cover across the region, high temperatures may be stubborn to recover above freezing this afternoon. Precipitation from the warm front wont arrive until closer to 7-10 pm, moving from SW to NE. With surface temperatures falling below freezing shortly after sunset and bufkit soundings showing a deep warm layer aloft, precipitation will initially fall as freezing rain. The 00z HREF showed very little change from the 12z run, showing the highest ice accretion north of the Mass Pike, especially in the northern CT river valley. Flat ice accretion totals will generally range from a trace to a tenth of an inch except in the northern CT river valley, where ice totals will likely range from a tenth to a quarter inch. Further south in SE MA and southern RI, temperatures may stay warm enough to keep precipitation as rain at onset. Reminder: Any amount of freezing rain can be hazardous, making untreated surfaces slippery. Any ice accretion on trees and power lines already holding onto leftover snow could lead to power outages.

Freezing rain transitions to rain from south to north between 11 pm and 4 am as surface temperatures rise above freezing. Dry slot begins to work in behind the warm front, allowing a brief period of drying before more rain and snow showers arrive with the cold front on Monday afternoon. With the region in the warm sector of this system for much of the day Monday, the high will top out well above normal, reaching into the low 50s in SE MA and the low 40s in NW MA. Between the rain and warm temps, the snow from Friday night likely wont last long.

KEY MESSAGE 2...Strong gusty winds expected to start Tuesday morning and peak in the early afternoon.

Once the cold front goes through on Tuesday, afternoon high temperatures will struggle to lift out of the 20s. High wind gusts will also allow for wind chills to plummet into the single digits to mid teens during the day.

Mid-level moisture will be present on Tuesday with steep lapse rates/marginal instability. Combined with enhanced westerly flow, this should be enough to bring a streamer/showers from the Great Lakes into western MA. Isolated snow showers or flurries are also possible for southern New England within the cold advective regime.

There is good agreement among ensemble guidance for continued mid- level troughing across the northeast through late week. Accompanied by cold temperatures aloft, this will support below normal temperatures in the 20s to low 30s through the rest of the week. There is a signal for another shot of colder air for Thursday and especially Friday with high potentially struggling to hit 30 Friday. There may also be a few systems that move through the flow later in the week; however, confidence is low for precipitation chances/timing. This may bring a period or two of light snow fall late Wednesday through Friday.

KEY MESSAGE 3...Chilly, below normal temperatures continue into the end of the week, with periods of light snow off and on.

Once the cold front goes through on Tuesday, afternoon high temperatures will struggle to lift out of the 20s. High wind gusts will also allow for wind chills to plummet into the single digits to mid teens during the day.

Mid-level moisture will be present on Tuesday with steep lapse rates/marginal instability. Combined with enhanced westerly flow, this should be enough to bring a streamer/showers from the Great Lakes into western MA. Isolated snow showers or flurries are also possible for southern New England within the cold advective regime.

There is good agreement among ensemble guidance for continued mid- level troughing across the northeast through late week. Accompanied by cold temperatures aloft, this will support below normal temperatures in the 20s to low 30s through the rest of the week. There is a signal for another shot of colder air for Thursday and especially Friday with high potentially struggling to hit 30 Friday. There may also be a few systems that move through the flow later in the week; however, confidence is low for precipitation chances/timing. This may bring a period or two of light snow fall late Wednesday through Friday.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

Today: High Confidence

VFR. Light SSW at 5-10 knots

Tonight: Moderate Confidence

A period of freezing rain is expected tonight across the interior and possibly and BOS and PVD, although confidence there is not as high being so close to body's of water. Freezing rain will begin from west to east tonight from 00z-03z and turn to rain from south to north between 04z-08z. CIGS will drop with precipitation tonight down to IFR levels, LIFR near the south coast and Islands with light southerly winds. LLWS will be a factor after 06-07z tonight as 2kft winds increase to 35-45 knots from the SW.

Monday: Low Confidence

Guidance wants to keep CIGS IFR/LIFR for much of Monday before the cold front arrives, however, being in the warm sector and with the dry slot aloft, this could lead to some off and on clearing. Rain moves back in late morning to early afternoon with the arrival of the cold front. CIGS rapidly improve to VFR behind the cold front Monday evening. LLWS will remain a concern until the low level inversion can erode and surface winds catch up with winds aloft.

KBOS Terminal...Moderate Confidence

VFR today. Confidence in freezing rain at the terminal is still moderate, but if its gonna happen, it will be between 04-07z tonight. Otherwise expecting rain with IFR CIGS overnight. LLWS will move in after 07z as 2kft winds increase to 40-45 knots while surface winds say light out of the south around 10 knots.

KBDL Terminal...High Confidence

VFR today. Confidence in freezing rain is high and should start between 01-02z tonight and last through 06-07z. After 07z, surface temps should warm above freezing turning pcpn to rain. CIGS gradually drop to low end MVFR to IFR tonight with pcpn. LLWS expected after 06z tonight as 2kft winds increase to 35-40 knots from the SW while surface winds remain light out of the south.

Outlook /Monday Night through Thursday/...

Monday Night: VFR. Windy with gusts up to 35 kt. Chance RA, slight chance FZRA.

Tuesday: Mainly VFR, with local MVFR possible. Strong winds with local gusts up to 40 kt.

Tuesday Night: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt.

Wednesday: Mainly VFR, with local MVFR possible. Breezy.

Wednesday Night: VFR. Breezy. Slight chance SN.

New Years Day: VFR. Breezy. Chance SN.

MARINE

Forecaster Confidence Levels...

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

Through Monday:

N-NE winds up to 20 kt across the eastern MA waters yielding SCA seas up to 7 ft. Otherwise, high pressure advects from west to east across SNE tonight and Sunday, yielding tranquil marine conditions by late Dec standards. Powerful low pressure (sub 980 mb) enters the Great Lakes Sunday night into Mon, yielding SW winds up to 25 kt. Stronger winds arrive Monday night as winds turn west with gusts up to 45 knots possible

Outlook /Monday Night through Thursday/...

Monday Night: High risk for gale force winds with gusts up to 40 kt. Rough seas up to 16 ft. Chance of rain.

Tuesday: gale force winds with gusts up to 40 kt. Rough seas up to 16 ft. Slight chance of snow.

Tuesday Night: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of freezing spray.

Wednesday: Strong winds with areas of gusts up to 30 kt. Areas of rough seas.

Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain, slight chance of snow.

New Years Day: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 7 PM this evening to 6 AM EST Monday for CTZ002>004. MA...Winter Weather Advisory from 7 PM this evening to 6 AM EST Monday for MAZ002>016-026. RI...Winter Weather Advisory from 7 PM this evening to 6 AM EST Monday for RIZ001-003. MARINE...Gale Watch from Monday evening through Wednesday morning for ANZ230>237-250-251-254>256.