textproduct: Boston / Norton

WHAT HAS CHANGED

Increased confidence in patchy fog across the region tonight, especially in low-lying areas. Less snowfall expected Sunday with a drier cold front. Looking warmer and wetter next week with a chance for precipitation during the mid to late week timeframe.

KEY MESSAGES

- Patchy fog tonight and warmer Saturday

- Cold front brings a period of accumulating snow on Sunday. A very cold Arctic airmass follows in its wake.

- Warmer and unsettled next week with multiple chances for precipitation starting Tuesday into Wednesday.

DISCUSSION

Key Message 1...Patchy fog tonight and warmer Saturday.

High pressure overhead will continue shifting east heading into the start of the weekend. Southerly to southwesterly winds will persist across southern New England with sea breezes remaining possible through the rest of this afternoon. Low level temperatures climb above 0C tonight in the midst of WAA from light southerly to southwesterly flow. Higher dewpoints will be advected in as well, shrinking dew point depressions and increasing the chance for patchy fog across the region, especially amidst all the snow on the ground. Guidance has continued to trend up the chance for patchy fog across southern New England tonight, with the highest chances in low-lying areas. Lows tonight will be mostly in the 20s and upper teens.

The warming trend overnight will continue through Saturday as southwesterly flow continues and 925 mb temperatures remain around 2- 4C. 850 mb temperatures will remain above 0C through the afternoon as well. A front passing through Saturday will shift winds more NW heading into Saturday night. Latest guidance does not indicate much in the way of a spot shower or two for this frontal passage as moisture is lacking and forcing is weak (notably, lapse rates are quite low). Lows for Saturday night will be similar to tonight's, falling mostly in the 20s across the region ahead of a more significant cold front Sunday.

Key Message 2...Cold front brings a period of accumulating snow on Sunday. A very cold Arctic airmass follows in its wake.

A strong cold front moves through on Sunday and brings a period of snow from W-E late Sunday afternoon into the evening. Trend in guidance has been to lower overall moisture and QPF amounts. Notably, models have shown a sharp decrease in 700-800mb RH and PWAT values along and ahead of the front. These changes result in a largely unsaturated DGZ with poor snowflake growth and lower overall totals Sunday afternoon. The NBM has followed suit with probs for 1" or greater dropping from 40-50% areawide to around 30-45%. Thus, have gone ahead and lowered forecast snowfall totals with 1-3 inches possible mainly along and north of the Mass Pike and lesser amounts of up to an inch elsewhere. Still think a reasonable worst-case scenario is a 2-4 inch event if QPF trends higher. Higher-end snow totals should remain confined to higher terrain as temperatures become more marginal closer to the coast.

Bigger story with the frontal passage will be a sharp drop in temperatures Sunday night. Highs start in the 30s early in the day, then drop into the upper 20s and upper teens by 00z Monday. Core of the Arctic airmass moves through Sunday night with 850mb temps falling into the mid teens below zero Celsius. No surprise that it will be a cold night with lows falling into the single digits and lower teens for most of the region. Well mixed boundary layer will keep most spots from realizing the coldest possible temps. Exception will be in interior valleys where the boundary layer is able to decouple and increase radiational cooling. In these spots, lows may even fall to near or just below zero! Temps will be slow to rebound Monday even under mostly clear skies. Highs range from the lower 20s at elevations in the interior to the upper 20s to near 30 across the coastal plain. Low temperatures Monday night bottom out in the single digits once again. Winds will be elevated, but do not look as strong as previous cold air advection events as the LLJ will likely only be 20-25 kts. Still, could see impactful wind chills Monday morning. Most locations will see wind chill values around or just below zero, with the lowest values across elevated portions of the interior.

Key Message 3... Warmer and unsettled next week with multiple chances for precipitation starting Tuesday into Wednesday.

Upper air pattern over the CONUS becomes more zonal through the middle of next week with a strengthening Pacific Jet. Embedded disturbances bring multiple chances for precipitation mid week. Pattern becomes more amplified late next week with a strengthening southeast ridge. At the surface, these changes result in temperatures warming back to seasonal averages Tuesday through the rest of next week. Precip type will be a bit less straightforward than with the last few events as highs warm into the upper 30s and low 40s.

Chances of precipitation arrive with several weak waves of shortwave energy moving in from the Great Lakes and Ohio River Valley during the middle to end of next week. Given the cold air in place ahead of the first wave on Wednesday, any WAA will lead to a messy mixed precipitation setup with anything from snow to sleet to freezing rain. The active pattern may continue for much of the week as a warm front stalls near Southern New England on its northward journey. NBM has a 30-50% chance of an inch of QPF for this timeframe so we could see hydrologic impacts but confidence is low at this time.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

18z TAF update...

Rest of Today...High Confidence.

VFR. Some variable winds this afternoon near the eastern coastal areas, but all should become more SW by around 21z today. Winds remain light under 10 kt.

Tonight...Moderate Confidence.

VFR with areas of MVFR possible in BR over the CT Valley, RI, and into NE MA. Light SW winds, becoming calm in some locations.

Saturday...High Confidence.

VFR. Light SW-ly winds 5-10 knots. Increasing mid-level cloudiness into the afternoon.

Saturday Night...Moderate Confidence.

VFR with areas of MVFR and some IFR possible. Light winds shift more NW. Chance for some precipitation increases heading into Sunday morning.

KBOS Terminal...High confidence in TAF.

Sea breeze continuing this afternoon before shifting more S-ly around 00z this evening. Low chance for MVFR BR later tonight, so not included in TAF at this time.

KBDL Terminal...Moderate confidence in TAF.

VFR outside of a chance for MVFR visbys in BR early Sat morning. Moderate confidence in lower ceilings at that same time.

Outlook /Sunday through Wednesday/...

Sunday: Mainly VFR, with local IFR possible. Chance SN, chance RA.

Sunday Night: VFR. Breezy.

Monday: VFR.

Monday Night: VFR. Slight chance SN.

Tuesday: VFR. Breezy. Chance RA, slight chance SN.

Tuesday Night: Mainly MVFR, with areas IFR possible. Breezy. RA likely.

Wednesday: Mainly VFR, with local MVFR possible. Chance RA.

MARINE

Forecaster Confidence Levels...

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

Through Saturday Night...High confidence.

Small Craft Advisory dropped over the outer waters as seas continue to diminish tonight below 5 ft. Winds generally below 15 knots, with the exception of the occasional gust up to 20 knots over the far northeast waters later tonight into Saturday morning. S to SE winds shift more N Saturday night into Sunday morning.

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt. Chance of rain, chance of snow.

Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 20 kt. Areas of seas approaching 5 ft.

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

Monday Night: Winds less than 25 kt.

Tuesday: Winds less than 25 kt. Chance of rain, slight chance of snow.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Rain likely.

Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.