textproduct: Boston / Norton

WHAT HAS CHANGED

Winter Weather Advisories remain in effect from 10 AM today 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 still favoring a less impactful offshore track at this time.

KEY MESSAGES

- Messy day on tap with rain, snow, sleet and freezing rain expected. Mostly a rain event for areas southeast of I-95. All locations will see a change back to snow tonight with light accumulations possible. Slippery road conditions expected.

- Light snow and snow showers continue Saturday with limited additional accumulations.

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

- Another round of unsettled weather mid week.

DISCUSSION

KEY MESSAGE 1...Messy day on tap with rain, snow, sleet and freezing rain expected. Mostly a rain event for areas southeast of I-95. All locations will see a change back to snow tonight with light accumulations possible. Slippery road conditions expected.

Winter Weather Advisories for interior Southern New England to just east of the I-95 corridor today into tonight.

Low pressure continues moving northeast towards the Great Lakes spreading an area of warm front-driven precipitation over much of the northeast CONUS England by the afternoon. Next, a secondary area of low pressure develops somewhere S of Long Island and moves east overnight. The trend in guidance has been to keep the column cooler with each run. Whereas last night's 00z output showed the mid-level warm nose making it well north of the Mass Pike, the vast majority of tonight high- resolution guidance shows the warm nose struggling to make it to the Mass border. This trend appears to be driven by stronger cold advection/cold air damming in NH/ME being moved further south by the departing secondary low pressure.

Still a somewhat challenging p-type forecast under the warm nose, but the trend has been for colder temps in this layer. In terms of sensible weather, a colder above 0C layer means that areas along and north of the Mass Pike can expect a mostly snow and sleet event while areas further south see more in the way of sleet and freezing rain. As the last shift aptly described it, "a potpourri of precipitation types". The higher confidence part of the forecast comes after the heaviest precip Friday night as the mid levels cool. Strengthening northerly flow means that the entire CWA transitions to a period of light snow in all areas Friday night. Winter Weather Advisories continue for most of interior Southern New England.

Still feel the HREF has a good handle on precipitation types and was used for weather types/changeovers. Forecast remains largely on track since the previous discussion. 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 this 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").

Lower snowfall totals expected in the Boston metro area due to stiff onshore flow helping to warm highs into the upper 30s. Any wintry precip that falls during the day likely struggles to accumulate. Things change somewhat quickly Friday night as colder northerly flow drives southward Friday evening. Expect 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. While totals will be light (1-2 inches) the timing of the transition will not be ideal for the evening commute and will likely result in slick travel. Opted to not go with an advisory as confidence in snowfall and sleet totals over 2 inches is still somewhat low.

Elsewhere, this is a mostly rain event SE of I-95 with a period of light snow Friday night. In these areas, a coating to a half inch of snow is possible mainly on grassy surfaces or side streets.

KEY MESSAGE 2...Light snow and snow showers continue Saturday with limited additional accumulations.

A leftover inverted trough continues light snow/snow shower activity through mid morning Saturday. Moisture profiles will become increasingly unfavorable for snow growth as the DGZ dries out. 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 widespread high impact outcome.

Latest guidance suite has trended the consensus track of an expected powerful low pressure closer to, but still southeast of, the 40N/70W benchmark Sunday night into Monday. That said, there remains significant spread in the details. Much can change with the forecast for Sunday into Monday over the next 48 hours.

At present, thinking most impacts from snowfall and wind are confined to RI and southeastern MA. Strong winds also possible elsewhere along the immediate coasts. This is mainly due to the expected size of this low pressure, even with the more offshore track. A closer track would mean more snow and wind impacts for more of southern New England.

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 another chance for wet weather. However, with the trend toward warmer conditions, much of the warm frontal precipitation likely falls as rain outside of the high terrain areas at night.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

Today and Tonight...High Confidence in trends. Moderate Confidence in timing.

VFR conditions gradually lower to MVFR, primarily with the CIGS, by early afternoon. 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 this evening for eastern terminals. It's also possibly some -FZRA may mix into showers later this afternoon across interior terminals as temperatures gradually drop below freezing, but uncertainty remains high on when exactly that would occur.

IFR tonight as precipitation changes back over to -SN. Changeover should occur from NW-SE between 02-06z. Some areas of -PL possible as the change transition starts.

Saturday and Saturday Night...Moderate confidence.

MVFR Saturday, increasing to VFR from E to W Saturday Night.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly VFR, with local MVFR possible. Chance SN.

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

Sunday Night: MVFR/IFR conditions possible. Windy with areas of gusts up to 35 kt. Chance SN.

Monday: Mainly VFR, with local IFR possible. Strong winds with areas of gusts up to 40 kt. Chance SN.

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

Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

High confidence through the end of this week.

Increasing winds and seas this evening into Saturday as a low pressure passes by. Winds diminish Saturday, but lingering rough seas expected over the outer coastal waters.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of freezing rain, slight chance of snow.

Sunday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow.

Sunday Night: Strong winds with gusts up to 40 kt. Rough seas up to 16 ft. Chance of snow. Local visibility 1 nm or less.

Monday: gale force winds with gusts up to 45 kt. Rough seas up to 23 ft. Chance of snow. Areas of visibility 1 nm or less.

Monday Night: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 22 ft. Chance of snow.

Tuesday: Winds less than 25 kt. Rough seas up to 13 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 10 AM this morning to 7 AM EST Saturday for CTZ002>004. MA...Winter Weather Advisory from 10 AM this morning to 7 AM EST Saturday for MAZ002>014-026. RI...Winter Weather Advisory from 10 AM this morning to 7 AM EST Saturday for RIZ001-003. MARINE...Small Craft Advisory from 7 PM this evening to 1 PM EST Saturday for ANZ250-254>256.