textproduct: Boston / Norton

WHAT HAS CHANGED

Increased cloudiness into this morning. Sunday and Monday now look more unsettled with periods of rain, mixed with or ending as wet snow late Sunday night and Monday but still uncertain on the details.

KEY MESSAGES

- Overcast skies continue into at least part of the morning, with decreasing cloudiness around noontime and turning breezy through sundown.

- Unsettled Sunday through Monday. Rain mixed with wet snow possible north of Route 2 early Sunday, but better chances for rain (mixing with/ending as wet snow) Sunday night into early Monday.

- Below average temperatures to start the work week then moderating temperatures and a few rounds of showers possible mid to late week.

DISCUSSION

KEY MESSAGE 1...Overcast skies continue into at least part of the morning, with decreasing cloudiness around noontime and turning breezy through sundown.

The cold front is moving offshore early this morning, with lingering rain showers coming to an end in short order over southeast MA and southern RI. However the post-frontal environment depicts extensive low clouds and intervals of mist/fog in a weak surface wind regime have governed, low clouds which trail all the way northward into central Quebec. Most guidance shows these clouds scattering out by sunrise, but that seems too soon based on upstream obs. Think it will take stronger northwest flow developing around mid to late this morning before decreasing cloudiness takes hold. Today starts off overcast, with gradual improvement but that improvement will bring with it some northwesterly breezes up to around 25 mph.

KEY MESSAGE 2...Unsettled Sunday through Monday. Rain mixed with wet snow possible north of Route 2 early Sunday, but better chances for rain (mixing with/ending as wet snow) Sunday night into early Monday.

A challenging, low-confidence forecast spanning overnight tonight through Sunday night/early Monday, as a frontal system in a fast midlevel WNW flow traverses from the Gt Lakes through Southern New England. Model guidance is really struggling with a number of factors here, even in this short forecast horizon. We can say it looks unsettled with periods of rain mixing with/ending as wet snow. Timing will be critical and any significant change could greatly influence the timing of precip and in what form that precip could take.

For Sunday morning, initial period of warm-advection-driven precip moves in around the pre-dawn hours. Overcast skies in most areas, with the best chance for precip north of the Mass Pike and especially along/north of Route 2. Temperatures could be marginal enough for wet snow and minor accumulation along/north of Route 2 but shouldn't be impactful. There are some solutions (ECMWF and Canadian, and the 00z NAM is trending that way) which are further north with the precip shield into NH/VT which could limit how much precip we see from this early morning activity. After this early- morning activity lifts north, warm air aloft (to +4 to +10C at 850 mb) builds into Southern New England. Wouldn't rule out spotty showers in this warm sector but it could be a pretty dreary Sunday with a lot of clouds around. We reduced temps somewhat, offering highs in the mid 40s to the mid 50s for Sunday as a middle-ground, but some solutions (the NAM) are quite a bit cooler as it holds onto the shallow moisture enough underneath the warm layer of air aloft to restrict diurnal heating.

The west-east oriented cold front then sags southward late Sunday and Sunday night. This period most likely has the strongest chance for precipitation, and most of it looks to fall as rain with a quarter to half inch of liquid precip possible. However shallow colder air seeping southward from NH/VT will allow for falling temperatures into the 30s, which could force rain to mix over to brief snow with coatings to an inch of snow possible. It's uncertain at this time but a bigger possible concern is if and where the sagging front stalls out, and if a secondary low pressure can develop on the boundary and send frontogenetically-forced precip back northward into the cold/cooling air late Sunday night into part of Monday. This is hinted at more by the ECMWF and Canadian guidance, which also have a much stronger 500 mb shortwave coming out of the Gt Lakes than the GFS/NAM show. Were that to happen, the potential exists for accumulating wet snow to fall during the Monday morning commute for areas south of the Mass Pike. ECMWF positive snow depth change shows a ribbon of 2-3 inches of snow in eastern CT, RI and southeast MA Monday morning, which illustrates the potential. On the other hand, the GFS/NAM are far enough south to lead to a chilly, cloudy but mostly dry Monday.

KEY MESSAGE 3...Below average temperatures to start the work week then moderating temperatures and a few rounds of showers possible mid to late week.

High pressure builds in Monday night into Tuesday promoting a brief period of dry conditions. High temperatures Tuesday will be around normal for late-March. Mid to late week, the upper level pattern trends more "active" with ensembles showing a series of shortwave troughs moving through the flow aloft. There is lacking agreement still on the exact timing of these waves which leads to uncertainty in the highest chances for seeing precipitation. Bottom line, we may see a few rounds of showers from Wednesday through Friday. In this same timeframe, temperature moderate back to normal temperatures in the 40s. Overnight temperatures range in the 20s to around freezing, so if a shower were to pass through overnight or early morning this may mean a light snow shower (higher chances for the interior).

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Today: Moderate confidence.

Remaining showers across the Cape/Islands exits 06-08z. Winds shift to WSW then WNW at 06-12 kts. MVFR- IFR ceilings this AM. Confidence is lower with regards to the timing of the improvement in the ceilings this morning. Should begin to trend toward VFR mid to late morning, though lingering on a little longer over the Cape and Islands through as late as 18Z.

NW winds increase 15-18Z with gusts 18-22 kts, decreasing after 22Z.

Tonight: High confidence. Moderate for shower chances early Sun AM.

Mainly VFR, though cloud cover will be gradually lowering. A few showers are possible after 06Z which may bring MVFR ceilings. Confidence not high enough to include in TAF at this time. NW winds shift to NE and then to SE after 06z Sun around 5-10 kt.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Sunday through Wednesday/...

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

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

Monday: Mainly VFR, with areas MVFR possible. Breezy. Chance RA.

Monday Night: Mainly VFR, with local MVFR possible. Breezy.

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Breezy. Slight chance SN.

Wednesday: VFR. Breezy. Chance RA, chance SN.

MARINE

Forecaster Confidence Levels...

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

SCAs continue on most waters except Narragansett Bay and Boston Harbor. These will be gradually coming down through today, though will take longer out on the outer waters as seas begin to slowly decrease. West winds around 10-15 kt shift to NW by mid to late morning with gusts around 20 kt through sundown, then decreasing to around 10-15 kt early tonight, trending light and shifting to east/southeast toward Sunday morning.

Winds Sunday then shift to S/SW and increase to around 10-15 kt with gusts nearing SCA criterion. Marine headlines could be needed for Sunday, but a better chance of SCA-level wind gusts and seas by Sunday night as winds turn to NE and increase to 25-30 kt.

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. 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. Areas of rough seas. Slight chance of snow.

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 5 AM EDT early this morning for ANZ231>235-237-251. Small Craft Advisory until 8 PM EDT this evening for ANZ250- 254>256.