textproduct: Boston / Norton

WHAT HAS CHANGED

Winter Weather Advisories posted from 10 AM Friday to 7 AM Saturday due to accumulating snow and ice leading to slippery road conditions. There continues to be uncertainty with regard to the coastal storm late Sunday into Monday, but we are favoring a less impactful offshore track at this time.

KEY MESSAGES

- Mix of accumulating snow, sleet, freezing rain and plain rain Friday, changing over and ending as a period of light- accumulating snow in many areas Friday evening. Slippery road conditions are expected, with Winter Weather Advisories for interior Southern New England to just east of the I-95 corridor Friday into Friday night.

- Cloudy and chilly Saturday with periods of flurries or light snow showers, but very limited if any accumulation.

- Coastal storm late Sunday-Monday more than likely to miss or just brush southern New England but too soon to rule out a more high impact outcome.

- Another round of unsettled weather mid week.

DISCUSSION

Key Message 1...Mix of accumulating snow, sleet, freezing rain and plain rain Friday, changing over and ending as a period of light- accumulating snow in many areas Friday evening. Slippery road conditions expected. Winter Weather Advisories for interior Southern New England to just east of the I-95 corridor Friday into Friday night.

Surface low pressure located as of Thursday afternoon in vicinity of northeast MO is expected to lift northward into the Great Lakes region on Friday, spreading a shield of warm-frontal precipitation into the Northeast states on Friday. A secondary area of low pressure also looks poised to develop off the NJ coast on Friday, which will maintain a "milder" marine- influenced airmass for much of the eastern coast of Massachusetts. Meanwhile, warmer air aloft will at least make an attempt to lift northward, but the majority of model forecasts suggests that nose of warmer air aloft won't make it much further northward than the Mass Pike; it becomes stonewalled and eventually shifts southward into Friday night as the secondary low pulls away and cold advection/cold air damming in NH/ME shifts southward.

This combination of a warm nose aloft struggling to make much northward inroads and above-freezing temps on easterly winds makes for a challenging precipitation-type/precipitation-type changeover forecast. The whole potpourri of precipitation types (snow, sleet/freezing rain, and plain rain) are anticipated, eventually transitioning to a period of light snow in all areas Friday night as colder air returns southward. Winter Weather Advisories have been posted for most of interior Southern New England to just west of the Boston-Providence I-95 corridor from mid-morning Friday to the early Saturday morning period.

Felt the 12z HREF precipitation types looked reasonable and were used for weather types/changeovers. Precipitation breaks out in far western New England after the Friday morning commute, and gradually spreads east-northeast into the afternoon. Areas near and north of the Mass Pike and into the Merrimack Valley/North Shore should remain as mostly snow for the duration of the event, where the greatest accumulations of wet snow are forecast (3-5" with spot 6" amounts along the NH border). Areas along and south of the Mass Pike southward into Northern Connecticut and northwest Rhode Island are more likely to see a mix of precip types, with accreting freezing rain (up to two- tenths of an inch), sleet and minor wet snow (C-3").

For the Greater Boston/South Shore area, the persistent onshore flow should lead to rain mixed with ice pellets, with any wintry precip that falls from the sky struggling to accumulate during the daytime; as colder northerly flow drives southward Friday evening (more likely after the PM commute), a transition over from wintry mix of rain/sleet over to a minor-accumulating but greasy light snow with a couple inches of snow accumulation forecast. Subsequent forecast updates may need to consider expanding the Advisory into Boston and eastern RI/southeast MA mainly for Friday night, due to deteriorating weather conditions affecting travel. Even in interior Southern New England, expect any wintry mix to change/end as light snow as the colder air returns back southward.

Plain rain is expected for most of southeast New England, ending as a coating to an inch of wet snow Friday evening.

Key Message 2...Cloudy and chilly Saturday with periods of flurries or light snow showers, but very limited to no accumulation.

Developing secondary low takes hold and races east early Saturday, but what is left behind for us is this cloudy, raw/chilly airmass with a lot of shallow RH with drier air aloft for much of Saturday. Models show really light QPF ( < 0.05), and forecast temp/moisture profiles reflect that any light snows that develop would have really poor snowgrowth. Because of that, accumulations if any would be limited to an inch or less. Highs may only range around 30-32 for northeast MA, and in the mid to upper 30s for RI, northern CT and southeast MA.

Key Message 3...Coastal storm late Sunday-Monday more than likely to miss or just brush southern New England but too soon to rule out a more high impact outcome.

We continue to monitor a deep mid level trough that digs into the eastern seaboard on Sunday and Monday generating a deepening surface low off the mid Atlantic coast that then tracks north and east. While all possibilities remain on the table, the latest GFS runs continue to be a NW outlier to other global guidance and ensemble guidance (which shows a much less impactful track missing or brushing SNE). Some of the closer tracks are a result of modeling depicting a deeper trough with a stronger downstream ridge. However, the bulk of the guidance including the mean of the EPS, GEFS, and Canadian are more progressive with a positively tilted trough and a flatter downstream ridge, allowing a track further offshore. For that reason, we continue to expect a better chance of a low impact storm. Even with a further offshore pass, it will likely lead to gusty winds and increased waves on the waters, as well as potential for ocean enhanced snow showers over southeast MA. As we get into the range of high resolution guidance we should have a better idea over the next 24 to 36 hours.

Key Message 4... Another round of unsettled weather mid week.

Zonal flow and a surface ridge of high pressure then move overhead briefly on Tuesday bringing a period of quiet weather into mid week, but another frontal system approaches late Wednesday bringing once again the return of wet weather. However, with the trend toward warmer low/mid level temperatures (925 mb temps approaching 0C) much of the warm frontal precipitation likely falls as rain outside of the high terrain.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

Through Tonight...High Confidence.

VFR. Light winds gradually shift from ENE to SE this afternoon.

Friday and Friday Night... High Confidence in trends. Moderate Confidence in timing.

VFR conditions gradually lower to MVFR, primarily with the CIGS, by late morning. By midday, -RA showers begin to overspread the region west to east, but some uncertainty on the exact timing. Light snow will mix into the rain generally by mid-afternoon for western terminals, later in the evening for eastern terminals. It's also possibly some -FZRA may mix into showers later Friday afternoon across interior terminals as temperatures gradually drop below freezing, but uncertainty remains high on when that would occur.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: MVFR/IFR conditions possible. Breezy. Chance SN.

Saturday Night: VFR. Slight chance SN.

Sunday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SN.

Sunday Night: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance SN.

Monday: Mainly VFR, with local IFR possible. Strong winds with gusts up to 50 kt. Chance SN.

Monday Night: Mainly VFR, with local MVFR possible. Windy with local gusts up to 35 kt. Slight chance SN.

Tuesday: VFR. Breezy.

Outlook /Saturday through Tuesday/

Saturday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow. Local visibility 1 to 3 nm.

Saturday Night through Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of snow.

Sunday Night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 14 ft. Chance of snow.

Monday: gale force winds with gusts up to 45 kt. Rough seas up to 22 ft. Chance of snow.

Monday Night: Low risk for gale force winds with gusts up to 45 kt. Rough seas up to 22 ft. Chance of snow.

Tuesday: Winds less than 25 kt. Rough seas up to 15 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 10 AM Friday to 7 AM EST Saturday for CTZ002>004. MA...Winter Weather Advisory from 10 AM Friday to 7 AM EST Saturday for MAZ002>014-026. RI...Winter Weather Advisory from 10 AM Friday to 7 AM EST Saturday for RIZ001-003. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ254>256.