textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes. Minor snow accumulations still expected Tue, especially in the interior. Low confidence temp and precip forecast for Friday and Saturday.

KEY MESSAGES

- Dry and blustery today as temperatures plummet tonight into Monday.

- Minor snow accumulations of 1-3" possible Tue, especially interior.

- Mainly dry and seasonable Wed & Thu. Blustery Wed with lighter winds Thu.

- Low confidence temp and precip forecast for Fri and Sat but trending colder Sat.

DISCUSSION

KEY MESSAGE 1...Blustery today, then temperatures plummet tonight into Monday.

Gusty SW winds have already begun to arrive ahead of the frontal passage, especially along the coast and Islands where better mixing is present. Expecting to see gusts around 25-35 mph. Should see a lull in the winds for a few hours as the front passes through, then another 45kt low level jet pushing in behind the front. This LLJ will bring a secondary round of 25-35 mph gusts later tonight into Monday morning. Winds gradually diminish through Monday as the pressure gradient weakens.

Moderate temperatures this afternoon ahead of the front, then temperatures plummet behind the front later tonight. 850 mb temps bottom out around -12C to -15C. Wind chills by Monday morning will range from single digits in the interior to mid teens across the Cape/Islands... except slightly below zero in the Berkshires. Monday's temperatures will be several degrees below normal with highs in the upper 20s to lower 30s and plentiful sunshine in very dry airmass.

KEY MESSAGE 2...Minor snow accumulations possible Tue.

A shortwave will be moving into the region while a warm front approaches from the SW. Given NW flow aloft, it is likely the warm front will remain to the west of SNE which keep the region in the colder air aloft ensuring snow to start for all SNE. There is some uncertainty with timing of snowfall as GFS has shifted timing more to Tue afternoon and especially Tue night, however global ensemble guidance has Tue as most favorable period of snowfall so expecting snow to develop by Tue morning. But developing onshore flow expected to warm the boundary layer especially south and east of I-95 where snow may transition to a mix or rain for a time by late morning or afternoon. It is possible snow or mixed snow/rain will linger into Tue evening, but this is not a significant QPF event so expecting mostly light accumulations on the order of 1-3 inches in the interior and an inch or less closer to the coast. Confidence is moderate but will have to watch for low pres developing on the front near or south of SNE which could enhance precip with a low risk for somewhat higher accums. This is a low prob scenario as even 90th percentile of snowfall from GEFS, EPS and Canadian ensembles are just 1-2" near the coast and 2-4" further inland.

KEY MESSAGE 3...Mainly dry and seasonable Wed & Thu. Blustery Wed with lighter winds Thu.

Low pres will deepen well east of New Eng on Wed leading to gusty NW winds but dry weather, then lighter winds Thu as high pres moves into New Eng. There is a boundary that sets up to the SW Thu but deeper moisture is expected to remain to the south and west so mainly dry weather. Temps Wed will be a bit below normal with highs mostly in the 30s, with slight moderation Thu in the upper 30s to lower 40s near the south coast.

KEY MESSAGE 4...Low confidence temp and precip forecast for Fri and Sat but trending colder Sat.

Low confidence forecast for Fri and Sat as polar jet will be over New Eng with more shortwave energy approaching and global guidance showing significant spread on where a boundary will set up and track of low pres. As previous forecaster mentioned, there will be quite a thermal gradient from north to south across New Eng and mid Atlc region and location of the boundary will be critical to temps and sensible weather. Potential outcomes for Fri range from temps into the 50s if boundary is to the north, to temps in the 20s and 30s if boundary is to the south. There is also uncertainty in precip coverage and ptype with potential for snow or mixed wintry precip if boundary sets up to the south. There is higher confidence the boundary will be south of New Eng Sat with much colder temps but uncertainty if wintry precip lingers. Bottom line, there is very low confidence for Fri and Sat and would expect significant modifications to the forecast going forward.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

12Z TAF update...no significant changes.

Today through Monday...High Confidence.

VFR through the period. SW winds becoming W this morning with 25-30 kt gusts, then turning NW and gusty this afternoon into tonight post-FROPA, then gradually diminish Monday.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: VFR. Chance SN.

Tuesday: Mainly IFR, with local MVFR possible. Breezy. SN likely, RA.

Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Chance SN.

Wednesday: VFR. Windy with local gusts up to 30 kt.

Wednesday Night: VFR.

Christmas Day: VFR. Breezy. Slight chance RA.

MARINE

Forecaster Confidence Levels...

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

Today through Monday...High Confidence.

Marginal Southwesterly gales this morning ahead of a cold front with gusts 25-35 kts expected. Winds shifting to NW this afternoon with 25-35 kt gusts persisting into Mon morning before diminishing. Seas 7-10 ft over the outer waters will begin to subside during Monday.

Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow.

Tuesday: Winds less than 25 kt. Local rough seas. Rain likely, chance of snow. Areas of visibility 1 to 3 nm.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain.

Wednesday: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of rain.

Wednesday Night: Winds less than 25 kt. Areas of rough seas.

Christmas Day: Winds less than 25 kt. Local rough seas. Slight chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ230-236. Gale Warning until 7 AM EST Monday for ANZ231>235-237-250-251- 254>256.