Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 210624
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
224 AM 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 /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.

&&

$$

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