textproduct: Boston / Norton

WHAT HAS CHANGED

Lingering snow showers depart from west to east between 8 am and 11 am this morning with an additional inch or two of snow possible some locations. Otherwise...no significant changes to the forecast. We are watching the potential for an arctic outbreak for the upcoming weekend and also the track of a storm system to our south.

KEY MESSAGES

- Period of lingering light snow ends from west to east between 8 and 11 am. Arctic front delivers cold conditions Tuesday, with some moderation Wednesday and Thursday. Colder again next weekend.

- Dry but very cold Tue & Tue night. Highs Tue in the upper teens to the middle 20s with lows Tue night mainly 0 to 10 above

- Temps moderate Wed and particularly Thu when highs may break 40 in spots...dry outside a few brief rain/snow showers possible

- Long range model guidance indicates an arctic outbreak with potentially near record cold & dangerously cold wind chills for the weekend. Meanwhile...much of the AI versions of the model guidance are not quite as cold but have a potential winter storm with a low pressure system much closer to the coast.

DISCUSSION

KEY MESSAGE 1...Period of lingering light snow ends from west to east between 8 and 11 am. Arctic front delivers cold conditions Tuesday, with some moderation Wednesday and Thursday. Colder again next weekend.

One more shot of snow works across the region this morning. Primary forcing for snow shower activity will be low level convergence with low level ENE flow near the coast and broad westerly flow further inland. This convergence helps to wring out lingering low level moisture. Will also see some low level instability as the mid levels cool from cold advection. Model soundings even have non- zero levels of CAPE so snow showers that form may bring brief pulses of moderate snow. Some locations may see an additional inch or two of snow. The last of the snow showers should exit the region from west to east between 8 and 11 am.

Otherwise...partial sunshine emerges this afternoon and it will become a bit breezy. High temperatures will be in the lower to middle 30s. A strong Arctic cold front crosses the region tonight. Gusty west winds will send overnight low temps down into the teens. Wind chills by daybreak will drop between 5 below and 10 above. Outside a brief flurry/spot snow shower across the interior...dry but very cold weather is on tap.

KEY MESSAGE 2...Dry but very cold Tue & Tue night. Highs Tue in the upper teens to the middle 20s with lows Tue night mainly 0 to 10 above.

A ridge of high pressure builds to our south Tue into Tue night. This will result in dry but very cold weather. Strong cold advection with 850T near -20C will hold high temps Tue in the upper teens to the middle 20s. Westerly wind gusts of 20 to 30 mph on Tue will put wind chill values between 5 below and 10 above Tue morning. Winds will gradually diminish Tue night setting the stage for a very good night of radiational cooling. It may take until the latter part of Tue night for winds to decouple in the typical spots...but given the fresh snowpack and the very cold airmass overhead temps will likely drop lower than guidance. Thinking overnight lows will be in the 0 to 10 above range with some below zero readings in the typical coldest outlying locations. The urban heat island of Boston will probably only drop into the middle teens as temps begin to warm aloft.

KEY MESSAGE 3...Temps moderate Wed and particularly Thu when highs should break 40 in spots...dry outside a few brief rain/snow showers possible. Much colder temperatures works back into the region by Fri.

High pressure to our south early Wed moves east and away from the region. At the same time...low pressure will be tracking well to our north across Quebec Wed night into Thu. The result will be a southwest flow of significantly milder temperatures. We should see high temps Wed moderate into the upper 20s to the middle 30s which is still below normal for this time of year.

As a cold front approaches from the west Wed night into Thu...a modest southwest LLJ of 45 to 55 knots will develop. This will result in slowly rising temps Wed night especially along the coastal plain where they should be above freezing by daybreak Thu. In fact...high temps on Thu may break 40 in spots with gusty southwest winds. The warm air advection pattern may result in a few brief snow showers and perhaps even rain showers near the coast Wed night into early Thu... but duration of any activity will be short with rather light amounts. Much colder air begins to return to the region on Fri behind a cold frontal passage.

KEY MESSAGE 4...Long range model guidance indicates an arctic outbreak with potentially near record cold & dangerously cold wind chills for the weekend. Meanwhile...much of the AI versions of the model guidance are not quite as cold but have a potential winter storm with a low pressure system much closer to the coast.

The upcoming weekend is an eternity in the model guidance world...but it looks quite interesting. Much of the long range guidance indicates a piece of the polar vortex breaking off and sending us a shot of arctic air that would potentially challenge records and result in dangerously cold wind chills. Meanwhile...the AI model suite tends to not penetrate this arctic cold as far south resulting in low pressure tracking much closer to the coast. This would limit the deep southward penetration of the bitter cold...but also allow low pressure to track closer to the coast and bring us a potential winter storm.

It is way too early to say much more at this time. The AI guidance tends to favor a potential coastal storm with the regular model guidance favoring more bitterly cold. However...it is not one to one as the individual members of the regular and AI guidance do show both possibilities. The AI guidance has shown skill in recent events including the current snow event that will be wrapping up shortly. That being said...even its individual ensembles are showing large spread in possible outcomes. And given this is 6-7 days in the future...will be at least a few more days until we have a better idea.

AVIATION /08Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

06z TAF Update...

Moderate confidence.

Snow will gradually taper off from west to east in the 10-14z period. Chance for continued snow showers through 15z especially for the far eastern terminals on the Cape and Islands. Expect IFR/MVFR ceilings and visibilities to continue thru 15z until drier air starts moving in. Conditions will trend toward VFR this afternoon, with west winds gusting 20kt or higher in many areas, especially after 18z.

Monday Night is expected to feature VFR conditions with increasing West winds. Gusts may increase to 30kts after 00z and persist through the night. KBOS Terminal... Heaviest snow begins to wind down in the next 1-2 hours. Additional snow showers develop thru 14-15z this morning.

KBDL Terminal...Moderate confidence in TAF. Bulk of the snow winds down in the next 1-3 hours with lingering snow showers through 10z.

Outlook /Tonight through Friday/...

Monday Night: VFR. Windy with gusts up to 35 kt.

Tuesday through Wednesday: VFR. Breezy.

Wednesday Night: VFR. Windy with local gusts up to 30 kt. Slight chance SHSN.

Thursday: VFR. Breezy. Slight chance SHRA, slight chance SHSN.

Thursday Night through Friday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Today and Tonight...

Departing low pressure brings a period of increasing west winds with seas building 5-8 feet today, with up to 10 feet across the southern outer waters. Westerly winds increase during the late afternoon into early evening. A strong Arctic cold front crosses the waters overnight into early Tuesday, bringing westerly gales. Increasing cold air advection may also result in freezing spray across all waters late tonight into Tuesday morning. The theme Tuesday will be gradually decreasing seas as broad high pressure approaches from the southwest. Seas will likely remain between 5 and 7 feet through much of the day.

Outlook /Tuesday through Friday/...

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

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft.

Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of rain showers.

Thursday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Slight chance of rain showers.

Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory until 7 AM EST this morning for CTZ002>004. MA...Winter Weather Advisory until 7 AM EST this morning for MAZ002>023-026. RI...Winter Weather Advisory until 7 AM EST this morning for RIZ001>007. MARINE...Small Craft Advisory from 11 AM this morning to 7 PM EST Tuesday for ANZ230-236. Gale Warning from 4 PM this afternoon to 9 AM EST Tuesday for ANZ231>235-237-250-251-254>256.