textproduct: Boston / Norton

WHAT HAS CHANGED

Light snow remains possible Sunday, but confidence in total accumulation has decreased. Confidence is increasing that snow will change to a wintry mix and rain late Tuesday.

KEY MESSAGES

- Mild today, then turning very cold behind an arctic front Sunday

- Light snowfall remains possible Sunday with lower than normal confidence

- More seasonable temps return mid next week, along with more chances for wintry precipitation.

DISCUSSION

Key Message 1...Mild today, then turning very cold behind an arctic front Sunday

A warm front brings mild temperatures today, with 850 mb temps warming from -2C to +2C. Surface temperatures today will top out in the low 50s with southerly winds gusting 15-20mph. Guidance has trended skies more cloudy today, with mid- to high-level clouds streaming in with a weak shortwave late in the day. Moisture with this weak shortwave is extremely limited, so not anticipating any shower activity as it passes through the region late in the afternoon. The warm weather will be short-lived as an arctic front moves through Sunday, bringing high temperatures back into the 30s. The core of the arctic air arrives Sunday night, with low temperatures dropping into the low single digits. Winds will be diminishing overnight Sunday; however, the wind chill index will likely be below zero across interior Southern New England. High temperatures on Monday will be quite cold for March standards, only topping out in the low to mid-20s. Arctic air remains in place Monday night with low temperatures once again bottoming out in the single digits. Little to no wind is expected Monday night, limiting the wind chill factor.

Key Message 2...Light snowfall remains possible Sunday with lower than normal confidence

The cold front on Sunday will also be accompanied by snow; however, an unusually wide spread in the guidance has reduced forecast confidence. Deterministic QPF ranges from a hundredth in the EURO to up to four tenths in the HRRR. Ensembles show similar spread, with the NBM 25th-75th percentile ranging from 0.01 to 0.25. The front will not have much moisture to work with, as guidance is generally in good agreement, with less than a half inch of precipitable water. Forcing also seems to be limited to the cold front itself, with the parent shortwave staying in northern New England. However, 850 mb FGEN may make up for the lack of upper- level forcing. Given these uncertainties, the snowfall forecast remains nearly unchanged, with a widespread trace to 1 inch expected across much of Southern New England, with a worst-case scenario of 2- 4 inches. The most likely spots to see the higher totals will be the high terrain across the northern Berkshires and Worcester hills. Snow will move from west to east on Sunday, starting in the mid to late morning and moving offshore around sunset. Given the lack of stronger forcing, hourly snow rates will be light, under a half inch per hour.

Key Message 3... More seasonable temps return mid next week, along with more chances for wintry precipitation.

Temperatures begin to moderate to more seasonable levels on Tuesday, and possibly even back into the mid-40s by Wednesday and Thursday. Long-range global guidance continues to advertise zonal and active patterns starting Tuesday and lasting through the end of the week. The first system looks to arrive late Tuesday as a low-pressure system tracking through the Great Lakes pushes a warm front into the region. Given the cold air still in place on Tuesday, precipitation type is likely to start as snow, then switch to a wintry mix, and eventually rain. Precipitation chances are expected to continue into late next week as several weak shortwave impulses traverse the zonal flow.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

Rest of Today...High Confidence.

VFR. Light SW-ly winds 5-10 knots. Increasing mid-level cloudiness this evening.

Tonight...Moderate Confidence.

VFR with areas of MVFR and some IFR possible across S RI and SE MA with fog development. Light winds shift more NW-ly.

Sunday...Moderate Confidence.

Generally MVFR cigs as the chance for some light snow showers increases late Sunday morning into the early afternoon. Light snow moves from W to E between 15 and 18z then gradually comes to an end between 19 23z from W-E. Not expecting much accumulation with this system. Possibly up to 1-1.5 inches, but confidence is low given the wide spread in model guidance at this time. Northerly winds at 10-15 knots.

KBOS Terminal...High confidence in TAF.

VFR. SW-ly winds 5-10 knots today veer NW-ly late tonight.

KBDL Terminal...Moderate confidence in TAF.

MVFR cloud deck slowly expanding across western CT should gradually lift through the morning. Otherwise VFR with light southerly winds today.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Breezy. Slight chance SN.

Monday: VFR. Breezy.

Monday Night: VFR.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance RA, chance SN.

Tuesday Night: Mainly IFR, with areas MVFR possible. Windy with local gusts up to 30 kt. RA.

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

MARINE

Forecaster Confidence Levels...

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

Through Saturday Night...High confidence.

Winds generally below 15 knots, with the exception of the occasional gust up to 20 knots over the far northeast waters this afternoon. S to SE winds shift more N Saturday night into Sunday morning. Winds begin to increase to near 25 knots Sunday night into Monday morning, but seas remain below 5 feet.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Slight chance of snow.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.

Monday Night: Winds less than 25 kt.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Chance of rain.

Tuesday Night: Strong winds with gusts up to 30 kt. Seas locally approaching 5 ft. Rain.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.