textproduct: Boston / Norton

WHAT HAS CHANGED

The potential for snow to impact areas in the vicinity of I-90 during the Monday morning commute has increased...but impacts to roadways are still uncertain.

KEY MESSAGES

- Periods of rain tonight that will change to a brief period of wet snow in many locales between 4-8 am with a coating to 2" possible before ending by late morning. Snow accumulations favored on grassy surfaces...but can not rule out roads briefly becoming snow covered in heavier bands. Any snow accumulations that occur will melt quickly.

- Scattered snow showers and a few snow squalls develop later Mon into Mon night especially across eastern MA. Some neighborhoods may pickup a coating to 1" of snow with localized 2" amounts possible.

- Dry and seasonable Tuesday becoming unsettled again after mid-week with a few rounds of showers. No significant precipitation expected.

DISCUSSION

KEY MESSAGE 1...Periods of rain tonight that will change to a brief period of wet snow in many locales between 4-8 am with a coating to 2" possible before ending by late morning. Snow accumulations favored on grassy surfaces...but can not rule out roads briefly becoming snow covered in heavier bands. Any snow accumulations that occur will melt quickly.

A cold front will drop south across the region this evening as a wave of low pressure approaches from the west. This is a classic ana- frontal setup...where a narrow but strong band of mid-level frontogenesis sets up north of the surface boundary. Periods of rain should continue through tonight...especially south of I-90 where we expect the heaviest rain to occur on the order of 0.75 to 1.50 inches.

The biggest uncertainty is the potential for the rain to change to a brief period of wet snow from north to south between 4 and 8 am. There may also be a bit of sleet mixed in during the transition. The primary area of concern is 20 miles north or south of I-90...where soundings become isothermal for a time while moisture remains long enough for a brief band of potentially accumulating wet snow. While this will be short-lived...the NAM soundings are showing a brief period of 30-40+ units of omega in the snow growth region in the KORH-KBOS areas between 08z to 14z. Meanwhile...the GFS/RGEM has already pushed the best forcing and moisture well south of the region. The ECMWF seems to be similar to the NAM.

The tricky thing with these ana-frontal waves are is if you can realize the narrow band of very strong mid level frontogenesis before the moisture dries out. If the HRRR/NAM/EC are correct there certainly is the potential for a quick 1-2" of snow with even a low risk of 3" in the vicinity of I-90 right around the morning rush hour. While the bulk of the snow accumulations would occur on grassy surfaces...if we are able to generate that very strong lift in the snow growth region roads would briefly become snow covered too. However...if the stronger forcing has already slid south such as the GFS/RGEM just some light rain/snow showers would occur with no accumulations. This will be more of a nowcast situation...but something we will need to watch for the morning commute. Regardless...any steady precipitation should wind down by late Mon morning. It will remain cloudy/raw with a few rain/snow showers still possible. Any snow that did fall should begin to melt with temps above freezing rising into the middle 30s to the lower 40s.

KEY MESSAGE 2...Scattered snow showers and a few snow squalls develop later Mon into Mon night especially across eastern MA. Some neighborhoods may pickup a coating to 1" of snow with localized 2" amounts possible.

The second part of this event will be later Monday afternoon and night. This in response to a strong northern stream shortwave/cold pool aloft dropping into southern New England. This will combined with plenty of left over low level moisture along with some MUCape and negative 0-2 lapse rates. This should allow scattered snow showers and a few snow squalls to develop later Mon into Mon evening. While the activity will be possible anywhere...the guidance seems to highlight eastern MA as the greatest/most widespread area especially Mon evening. This is likely the result of a subtle inverted trough and land/sea interface. Therefore...scattered snow showers and a few heavy snow squalls may result in some neighborhoods seeing briefly poor visibilities and snow covered roadways. While this will not impact every neighborhood...the potential increases Mon evening given sunset and colder temps working into the region. So some neighborhoods may see a coating to 1" of snow with even localized 2" amounts not out of the question. Again...not everyone will see this activity but some brief snow squalls may impact the Mon evening commute especially across eastern MA.

KEY MESSAGE 3...Dry and seasonable Tuesday becoming unsettled again after mid- week with a few rounds of showers. No significant precipitation expected.

A very different day on Tuesday as high pressure brings the return of sun and much warmer temperatures, in the mid to upper 40s. This will be 5 to 10 degrees warmer than Monday so after a cold and raw day on Monday with snow, it will feel like a switch to early Spring.

Beyond Tuesday the synoptic pattern becomes quasi-zonal with overall low impact weather expected as a result. A series of shortwave disturbances moves through the steering flow which will bring a few rounds of unsettled weather. The first and weaker of the two arrives Wednesday night into Thursday with a weak surface low moving from the Great Lakes into the maritimes. Moisture is lacking (PWATs 0.4- 0.7") as is strong forcing so only expecting widely scattered light showers. The more robust shortwave and deeper surface low moves overhead or to our north around Thursday night into Friday and this comes with better moisture (PWATs >1") and forcing (surface cold front and stronger LLJ). For this reason we expect more widespread rain with higher rainfall totals and breezy wind gusts. Even so, not expecting it to be a significant storm. High pressure then moves in for at least the first half of the weekend keeping a dry forecast.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

00z TAF Update:

Tonight through Monday Night: Moderate confidence.

IFR to MVFR conditions prevail as a cold front is ever- so- slowly sagging southward. Rains are beginning to lighten up for ORH-BED-BOS and could become more intermittent by 02z Mon, with steadier rains becoming mainly focused at BDL-PVD-Cape airports. SW winds to prevail south of the cold frontal boundary and winds becoming N to NE around 5-10 kt north of it.

After 07z Monday thru 13-15z Monday...a renewed burst of precipitation lifting slightly northward falling into colder air will facilitate a changeover to sleet and wet snow along or within 20 miles or so of the Mass Pike by the pre-dawn hrs. Coatings to up to 2 inches of snow/sleet are possible mainly on grassy surfaces with mostly wet runways, but TAFs that see heavier busts of accumulation could see minor, slushy accumulation on runways. This activity should continue thru 13-15z Monday from west to east, mainly for BDL- ORH-BED-BOS but could slip as far south as PVD. Mainly rain expected for the Cape and Islands. Most of the TAFs should then see precip come to an end with MVFR-IFR conditions continuing. NE increase markedly in this period to around 10-13 kt with gusts 20-25 kt.

Ceilings could improve into the MVFR range into Mon afternoon, but will have to watch for possible scattered snow showers or even some snow squalls as soon as 20z Monday, which linger and/or move eastward thru 04z Tue. Best chance in eastern MA but could be possible anywhere from BDL to PVD to PYM north and west, with visbys locally dropping into the IFR range in snow showers. Improvement in ceilings toward all VFR more likely by daybreak Tue. NE to N winds around 10-15 kt thru midnight, then becoming NW and decreasing to around 5-10 kt after 06z Tuesday.

Tuesday: High confidence.

VFR. NW winds become light then shift to SW around 5-10 kt late in the day.

KBOS Terminal...Moderate confidence in TAF. Greatest concern for a period of snow/sleet is after 8z/9z until about 15z. However...scattered snow showers and a few snow squalls will be in the vicinity of the terminal later Monday and especially Monday night.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night through Thursday: Breezy. Slight chance RA.

Thursday Night: Windy with local gusts up to 30 kt. RA likely.

Friday: Breezy. Chance RA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Monday night...High confidence.

A cold front crosses the region this evening. This will be followed by northerly wind gusts of 25 to 30 knots overnight into Monday. A few brief gusts near 35 knots are possible...but continued with the small craft headlines as any gusts near 35 knots will be rather short-lived. Northerly wind gusts diminish a bit Mon night...but gusts of 20-25 knots still possible along with rough seas persisting.

Outlook /Tuesday through Friday/...

Tuesday: Winds less than 25 kt. Local rough seas.

Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

Wednesday Night through Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain.

Thursday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of rain.

Friday: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Areas of rough seas. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Monday for ANZ230>237. Small Craft Advisory until 2 PM EDT Tuesday for ANZ250-251- 254>256.