textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Windy with showers tonight but rainfall will not be significant.

- Rain arrives Sunday which could start or end as some wet snow. Minor accumulations possible especially in northern MA and in higher elevations.

- Below average temperatures to start the work week then moderating temperatures and some showers possible Wednesday and Friday.

DISCUSSION

KEY MESSAGE 1...Windy with showers tonight but rainfall will not be significant.

Approaching cold front will bring a period of showers to much of southern New England tonight, mainly in 6 pm to midnight time frame, but a little later on Cape Cod and Islands. Not much in way of dynamics or deep moisture present ahead of front, so rainfall is expected to be light, mainly 0.25 to perhaps 0.75 inch.

Ahead of front, tightening pressure gradient and increasing low level jet (as high as 45-55kt) will bring windy conditions to much of region. Forecast soundings show shallow mixing layer which will prevent strongest winds aloft from reaching surface, but we should still see gusts on order of 25-35 mph and perhaps up to 40 mph in a few locations. Looks like gusts will fall short of Wind Advisory criteria. Winds diminish overnight as front pushes through and low level jet heads offshore.

High pressure builds into New England Saturday. Expecting a breezy day with gusts of 20-30 mph due to cold advection and deep mixing aloft, but March sunshine should offset cold advection and help highs get well into 40s and lower 50s.

KEY MESSAGE 2...Rain arrives Sunday which could start or end as some wet snow. Minor accumulations possible especially in northern MA and in higher elevations.

Timing is everything and in this case will determine whether or not we see some wet snow Sunday morning (if precipitation begins early) or as things wind down later Sunday night and Monday morning (if enough cold air can sink southward sooner).

We're getting into window for high-res guidance and there are hints that some scattered snow showers could sneak into southern New England Sunday morning due to weak warm advection and enough cold air still in place, especially north of Mass Pike (as seen on 3km NAM and RRFS). That's something that will be resolved as we get closer in time but at this point does not look to be impactful.

Overall, this looks to be a rain event for southern New England with most of rain falling Sunday evening into early Monday morning as frontal boundary lies just to our north, keeping us in warm sector. Models do sag front southward later Sunday night, allowing colder air to seep southward, but by then all of the appreciable lift and deeper moisture should be exiting region.

Should front drop south sooner, however, we would see an earlier changeover to snow and would see some minor snow accumulations, especially north of Mass Pike and in higher elevations. Latest ensembles continue to show low chances (20-30%) of 1" or more of snow for those locations, but it would not take much of a shift southward to bring a little more snow. Elsewhere, even if it snows, little or no accumulation is expected. Model snow depth change maps will give best depiction in this case but we've seen 12z NAM which is definitely an outlier right now. It closes off 850 low near Cape Cod which brings stronger lift and deepens colder air quickly Monday morning, something not supported by majority of guidance which favors a more progressive system. Something to watch though.

One other thing to mention is we are in a period of high astronomical tides through the weekend. The early Monday morning high tide (2-4 am) along E MA coast is one we are keeping an eye on since the astronomical tide is 11.0 ft at Boston and 3.8 ft at Nantucket. Right now the higher-end scenario with a surge of 1.0 feet keeps both below flood stage but with increasing N/NE winds around the time of high tide (depending on how fast the front drops south), the surge could end up being a little higher. Worst case right now looks to be splashover or lower-end minor coastal flooding.

KEY MESSAGE 3...Below average temperatures to start the work week then moderating temperatures and some showers possible Wednesday and Friday.

High pressure brings dry weather Monday and Tuesday along with seasonable temperatures for late March, although it looks like we're in for a couple of cold mornings both days. Upper pattern shows fairly rapid west to east flow aloft which makes it difficult to time short waves moving through, but it does appear we could see some showers Wednesday and again Friday as a series of weak lows cross the region. We should also see a moderating trend in temperatures but it may be cold enough initially both days for some wet snow at onset across interior.

AVIATION /16Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

18z TAF Update: High confidence (60%).

VFR conditions this afternoon give way to MVFR and eventually IFR as a band of showers moves through southern New England, starting around 21z in western MA, reaching BDL-ORH-BED around 23z, BOS-PVD around 00z, and FMH/HYA/ACK 02-03z. Showers last about 6 hours and leave IFR cigs/vsbys in their wake, before improvement to VFR Sat morning as winds shift to NW.

Main concern is wind, especially through early tonight with S/SW winds gusting to 20-30kt and LLWS with 020 winds 190/40-50kt. Winds subside later tonight and become W, then NW Sat morning, as front moves offshore. A few 20kt gusts expected into early Sat afternoon.

KBOS Terminal...High confidence in TAF. Peak gusts expected just before 00z Sat with brief 35kt gusts possible.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday through Tuesday/...

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

Sunday Night: Mainly MVFR, with areas IFR possible. Breezy. RA likely, chance SN.

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

Monday Night through 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 weekend.

SCAs remain posted through tonight for increasing S/SW winds gusting as high as 30kt. It's possible a few gusts reach marginal Gale force (34kt) but we don't think that will be widespread or last more than an hour or two at most. Showers move through waters tonight and should exit waters east of Cape Cod overnight.

NW winds behind front Saturday should remain below SCA but it will remain in place for outer waters where seas will take awhile longer to fall below 5 ft.

Cold front dropping south across waters Monday may be accompanied by period of 20-25kt N/NE winds.

Outlook /Sunday through Tuesday/...

Sunday: Winds less than 25 kt. Chance of rain.

Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely.

Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain.

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft.

Tuesday: Winds less than 25 kt. Areas of rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ230-236. Small Craft Advisory until 5 AM EDT Saturday for ANZ231>235- 237-251. Small Craft Advisory until 8 PM EDT Saturday for ANZ250- 254>256.