textproduct: Boston / Norton

SYNOPSIS

High pressure will bring cold and dry conditions tonight with light winds. The high moves offshore Saturday, then a fast moving low pressure system will bring accumulating snow Saturday night into Sunday morning, especially south of the Mass Pike. Ocean effect snow showers will linger Sunday afternoon and night, otherwise a shot of arctic air follows Sunday night into Monday. It remains cold Tuesday, but a pattern change with a significant warming trend is likely Wednesday into Thursday. The milder air may be accompanied by some showers sometime late Thursday into Friday as a frontal system approaches from the west.

NEAR TERM /UNTIL 6 AM SATURDAY MORNING/

Key Messages...

* Wind gusts 25 to 35 mph linger through the afternoon before diminishing through the evening.

* Low temperatures tonight ranging from the low teens to mid 20s across SNE.

Expecting 25 to 35 mph wind gusts to continue through the afternoon before diminishing this evening as the pressure gradient relaxes. Highs will range from the upper 20s in the high terrain to the lower to middle 30s elsewhere.

Another chilly night as clear skies and calm winds under a ridge allow for radiational cooling across SNE. Temperatures expected in the low to mid teens across the interior, low 20s along the coast, and mid 20s for the Cape and Islands.

SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/

Daytime hours on Saturday should remain dry as a high pressure systems moves just to our south. Expecting to see briefly warmer temperatures Saturday afternoon with a surge of slightly warmer air... upper 20s to upper 30 across the interior and upper 30s to low 40s across the south coast, Cape and Islands.

LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/

Key Messages:

* A period of accumulating snow likely Sat night into Sun morning, especially south of I-90. Highest accumulation near the south coast and Cape Cod

* Ocean effect snow showers linger on the outer Cape into Sun night with additional minor accum possible

* A short lived period of arctic air follows Sun night and Mon

* Pattern change with significantly milder temps Wed and especially by Thu

Details...

Still some model spread regarding the northern extent of higher QPF assocd with developing wave south of New Eng late Sat night and Sun morning. CMC is the furthest north of the guidance while GFS is furthest south. GFS did nudge northward and is a bit closer to the EC which trended south a bit. Given this is a flat and open progressive wave we discounted the CMC heavier QPF for this forecast and leaned toward a GFS/EC blend.

Shortwave energy rotating around upper low north of the Gt Lakes will back the flow enough to bring deep moisture plume northward into SNE but there is drier air to the north that we will have to contend with which will impact accumulations on the northern edge of the precip shield. The main forcing mechanism for this snow event will be the right entrance region of a 140-150 kt upper jet later Sat night into Sun morning which combined with deep moisture plume will bring a period of accumulating snow to SNE, especially south of the MA Pike. The period of accumulating snow is expected to be after midnight Sat night into Sun morning. The precip may start as rain on the Islands but will flip to snow fairly quickly. Temps will be falling into the 20s late Sat night so the snow will be on the dry side.

Tricky forecast as best mid level frontogenesis is forecast to remain to the south while omega in the DGZ is not very impressive. This may keep limit SLRs somewhat so we are thinking generally 1-3 inches south of the MA Pike, with highest amounts along the south coast. Up to 4 inches is possible on Cape Cod. Meanwhile, just a coating to an inch for areas north of the MA Pike. There is still time for some adjustments, and a reasonable worst case scenario if there is a northern shift would be 3-6 inches across RI and SE MA and Cape Cod but this is a low probability (less than 25%) outcome. If the system trends south it would be closer to 1-2 inches near the south coast with decreasing amounts northward.

This is a fast moving system and snow should be ending by mid morning Sunday but ocean effect snow showers will linger on the outer Cape Sun afternoon and night as arctic air pours into the region on NNW-NW flow. Ocean induced CAPES expected increase to 500- 600 J/kg with SST differential around 25C. An additional inch or 2 is possible on the outer Cape but NW flow should keep heaviest snow showers offshore. Otherwise a short duration shot of arctic air will pour into the region Sun night and Mon. Lows will drop into the single numbers and teens with wind chills bottoming out between -5F and +5F late Sun night and Mon morning, with -10F possible in the Berkshires. Highs Mon will be in the 20s with abundant sunshine with very dry air in the column.

Another fast moving shortwave may bring a few snow showers Mon night, otherwise cold and dry Tue with below normal temps with subsidence behind this shortwave. Then a notable pattern change for the middle and end of the week as a strong -PNA pattern develops with strong Pacific jet flooding the CONUS which will result in a warming trend to above normal temps, especially by Thu. GEFS, EMC and CMC all show low to moderate probs of temps reaching 50+ across RI and eastern MA. It may become unsettled toward the end of the week as a frontal system approaches but timing and amplitude of approaching shortwave energy is uncertain.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Tonight... High Confidence.

VFR. Gusty W winds diminish this evening and become southwest less than 10 knots late tonight.

Saturday... Moderate Confidence

Mostly VFR, possibly borderline MVFR for the Cape and Islands after 21z. Southwest winds between 5 and 10 knots with gusts up to 15 knots. Slight chance for areas of -SHRA/-SHSN across the Cape and Islands near 00z.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Wednesday/...

Saturday Night through Sunday: Mainly VFR, with local IFR possible. Breezy. Chance SN.

Sunday Night through Monday: VFR. Windy with local gusts up to 30 kt.

Monday Night: VFR. Breezy. Slight chance SHSN.

Tuesday: VFR.

Tuesday Night through Wednesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Today Through Saturday..High confidence.

Winds drop below gale threshold by 00z over outer waters but seas between 5 and 7 feet into tonight will likely mean small craft advisories into the overnight. Improving conditions later tonight before a weak coastal low approaches later Saturday afternoon into the overnight.

Outlook /Saturday Night through Wednesday/...

Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Snow, chance of rain. Local visibility 1 to 3 nm.

Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of snow. Visibility 1 nm or less.

Sunday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of snow showers. Local visibility 1 to 3 nm.

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

Monday Night: Winds less than 25 kt. Local rough seas. Slight chance of snow showers.

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

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

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 AM EST Saturday for ANZ230>235- 237-251. Small Craft Advisory until 7 PM EST this evening for ANZ236. Gale Warning until 7 PM EST this evening for ANZ250-254>256.