Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 211747
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
147 PM EDT Sat Mar 21 2026

.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 /18Z SATURDAY THROUGH THURSDAY/...

Forecaster Confidence Levels...

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

18z TAF Update... 

This afternoon into Sunday night...Moderate confidence.

VFR conditions persist through most of tonight. An approaching
warm front should allow for some precipitation to overspread
portions of mainly western and northern MA after midnight. 
Ptype will probably be initially in the form of some wet snow in
the highest terrain of northern MA and areas north of route 2. 
Otherwise...Ptype should change to rain in all areas by mid-
late Sun morning and gradually sink south and impact the rest of
the region Sun afternoon and night. Conditions will lower to 
mainly MVFR levels but some localized IFR conditions possible 
too Sun into Sun night. NW wind gusts of 20-25 knots diminish 
by early evening and become light NE tonight. S-SW winds 5-10 
knots Sun become N Sun night. 

Monday and Monday night...Moderate confidence.

A few left over rain/snow showers possible Mon morning across
the coastal plain. Otherwise...mainly MVFR ceilings should 
improve to VFR from west to east as the day wears along.
However...a period of snow showers is possible late Mon into Mon
night across central MA/RI and especially eastern MA. This may 
lower conditions back to MVFR levels with perhaps even localized
brief IFR conditions. NNW 5-15 knots with some 20-25 knot gusts
expected at times at least along the coastal plain.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday through Thursday/...

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 Thursday/...

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 8 PM EDT this evening for ANZ250-
     254>256.

&&

$$

DISCUSSION...Loconto/Mensch
AVIATION...Frank
MARINE...Loconto/Mensch