textproduct: Boston / Norton

WHAT HAS CHANGED

Confidence in a period of freezing rain continues to increase for Sunday night into early Monday morning. The highest chances of ice accretion exceeding a trace will be in low-lying areas north of the Mass Pike. Freezing rain will transition to a light to moderate rain Monday morning. Growing concern for strong post-frontal winds on Tuesday.

KEY MESSAGES

- Lingering flurries over the eastern coast give way to sunny but cold day.

- Storm system likely brings a period of freezing rain Sunday night into early Monday before transitioning to plain rain later Monday morning. The best chance for Ice accumulation of greater than a trace is north of the MA pike, especially in the northern CT river valley.

- Growing concern for strong post-frontal winds on Tuesday. High probabilities for 46mph gusts in the high terrain.

DISCUSSION

KEY MESSAGE 1...Lingering flurries over the eastern coast give way to sunny but cold day.

A few last rounds of heavier snow pass through before sunrise, bringing an additional coating to 2" across southern New England. A deformation zone setting up offshore brings northeasterly winds today, allowing for ocean effect flurries to linger through the morning along the south shore and coastal plain. This will likely bring storm totals to 2"-5", with highest totals across western CT/MA.

A dry and chilly airmass moves in behind the storm today into Sunday. Temperatures this afternoon peak in the mid 20s to low 30s, but low winds will allow for good radiational cooling and drop temperatures into the single digits across the interior and mid teens across the south shore and coastal plain.

KEY MESSAGE 2...Storm system likely brings a period of freezing rain Sunday night into early Monday before transitioning to plain rain later Monday morning. The best chance for Ice accumulation of greater than a trace is north of the MA pike, especially in the northern CT river valley.

A negatively tilted trough becomes positively tilted as it enters the Great Lakes and the Ohio River Valley. This sends a strong warm front into the northeast well ahead of the parent surface low back over Michigan. Bufkit soundings are a bit concerning heading into Sunday night, with precipitation arriving as temperatures below 2kft are well below freezing and a deep warm nose extending from 2-6kft. This kind of profile is very conducive to freezing rain, as precip will not have enough time to refreeze before reaching the surface. Guidance has been relatively consistent over the last couple of days in calling for a period of freezing rain from Sunday evening into the early hours of Monday, before transitioning to regular rain by sunrise Monday. The highest confidence for ice accretion greater than a trace is in low-lying areas north of the Mass Pike, especially in the northern CT river valley. We should get a much better sense of how much ice to expect after the 12z HREF run, which uses the Freezing Rain Accumulation Model (FRAM), a far more reliable method than standard model ice accretion.

Surface temperatures begin to catch up with the warmer temperatures aloft and warm above freezing near or before sunrise on Monday, allowing freezing rain to transition to plain rain. The dry slot works in behind the warm front, allowing rain to taper off for the afternoon. The cold front is expected to arrive sometime late Monday and could bring another quick round of rain.

KEY MESSAGE 3...Growing concern for strong post-frontal winds on Tuesday. High probabilities for 46mph gusts in the high terrain.

There is a growing concern for a period of strong winds behind the cold front on Tuesday as the pressure gradient becomes very tight, squeezed by the occluding 975 mb low over Quebec and a building surface high near the Gulf. The GFS currently has the most robust 850mb jet with winds nearing 55 knots, while the EURO and Canadian show a more modest 35-40 knot jet. The NBM is already showing high probabilities of 60-80% for 46 mph winds (wind advisory criteria) across the high terrain and near the Cape and Islands. The probability of 58mph gusts (high wind warning) is still low at this time, at 10-30%, mainly near the high terrain.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

Today: Moderate Confidence

Snow is wrapping up and should be offshore between 13-15z this morning. CIGS around the region are already improving to VFR with the exception of coastal terminals where MVFR CIGS will linger into the afternoon. There may be some gusty NNW winds this morning that could blow snow around, but winds this afternoon become light at 5-10 knots.

Tonight: High Confidence

VFR. Light NW winds under 5 knots.

Sunday: High Confidence

VFR. Light WSW winds at 5-10 knots

KBOS Terminal...Moderate Confidence

Light snow could linger until about 13z with MVFR CIGS lingering until 17z. Otherwise VFR with NNW winds around 10 knots for the afternoon

KBDL Terminal...High Confidence

VFR. Light NNW winds at 5-10 knots.

Outlook /Sunday Night through Wednesday/...

Sunday Night: MVFR/IFR conditions possible. Breezy. RA, chance FZRA.

Monday: MVFR/IFR conditions possible. Breezy. RA.

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

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

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

Wednesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Sunday

Winds turn NE around 20 knots as a clipper system brings moderate snow to the southern waters, with light snow in the northern waters tonight into Saturday morning. Seas between 2-5 ft. Winds turn light SW late in the day Sunday and seas drop below 5ft.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain, chance of freezing rain.

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

Monday Night: Moderate risk for gale force winds with local gusts up to 45 kt. Rough seas up to 17 ft. Chance of rain.

Tuesday: gale force winds with gusts up to 45 kt. Rough seas up to 17 ft. Slight chance of snow.

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

Wednesday: Strong winds with gusts up to 30 kt. Rough seas up to 12 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Winter Weather Advisory until 10 AM EST this morning for MAZ002>004-008-010>012-016>021. Winter Storm Warning until 10 AM EST this morning for MAZ009. RI...Winter Weather Advisory until 10 AM EST this morning for RIZ001>008. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ250- 256. Small Craft Advisory until 1 AM EST Sunday for ANZ254-255.