Old Res Weather

Lexington, Massachusetts, USA

35°F

2/16/2025 7:17pm

Lexington Conditions: Light snow, Overcast
Temperature: 35.1°F / 1.7°C
Dew Point: 32.4°F
Relative Humidity: 90%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 29.00 in Falling Slowly
Visibility: 1½ miles
Rain Today: 0.01 in
UV Index: $VPuv None

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 212009
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
309 PM EST Wed Jan 21 2026

.WHAT HAS CHANGED...
Increased confidence for a dusting to an inch of snow across 
interior MA overnight. Confidence is increasing in a 
significant winter storm Sunday into Monday but specific 
details remain uncertain.

&&

.KEY MESSAGES...

- A weak clipper system brings some light snow to the higher terrain 
  areas and light rain for the coastal plain tonight.

- Temperatures surge above normal Thursday with gusty winds 
  25-35 mph.

- Arctic front moves through late Fri into Fri evening, 
  bringing dangerously cold wind chills Fri night into Sat 
  night. Low risk for a few snow showers or squalls with the 
  passage of the front.

- A significant winter storm may bring heavy snow to southern
  New England Sunday into Monday, but specific details are
  uncertain.

&&

.DISCUSSION...
KEY MESSAGE 1...A weak clipper system brings some light snow to the 
higher terrain areas and light rain for the coastal plain tonight.

Latest suite of model guidance has come into better agreement with 
respect to precipitation amounts associated with a weak Clipper 
system moving over The Northeast this evening. QPF amounts currently 
consistently coming in between 0.05 to 0.15 " of liquid across most 
of southern New England. Interior MA, RI, and CT will be cold enough 
to support snow. Accumulations will be minimal with only a coating 
to an inch forecast. Some locations over the higher terrain of The 
Worcester Hills may see as much as 2 inches. Further south and east 
across the coastal plain, temperatures will be more marginal, so 
expect rain mixing in after midnight. This will result in little to 
no accumulations for these areas. South coast, southern RI, and 
Cape/Islands should only expect rainfall given overnight 
temperatures in the mid to upper 30s. 

KEY MESSAGE 2...Temperatures surge above normal Thursday with gusty 
winds 25-35 mph.

Moderate south/southwest winds develop Thursday morning and persist 
through Thursday evening. This will support low-level WAA which will 
allow 925 hPa temps to warm to between -3 and 1 degree C. This will 
be an increase of 5 to 10 degrees C from Wednesday afternoon. Thus, 
expect warmer afternoon temps on Thursday in the upper 30s to low 
40s under partly cloudy skies. Sunshine should support enough 
surface heating for efficient diurnal mixing. As a result we could 
see gusty winds at times with gusts between 20 and 30 mph
possible.

KEY MESSAGE 3... Arctic front moves through late Fri into Fri 
evening, bringing dangerously cold wind chills Fri night into 
Sat night. Low risk for a few snow showers or squalls with the 
passage of the front.

Arctic front will be dropping south into SNE Fri afternoon and 
reaching the south coast by evening. While mod els are not 
showing much if any QPF, the signals in the guidance suggest 
there could be a few snow showers or squalls with the arctic 
front passage. There is an area of enhanced moisture and lift 
along the boundary as well as steep low level lapse rates. This 
suggests a few snow showers or squalls will be possible 
although areal coverage will be limited. Something to watch as 
we approach Friday.

The bigger issue will be the arctic air that pours into the region 
Fri night into Sat on gusty NW winds. 850 mb temps bottom out 
around -25C late Fri night and early Sat before slowly moderating
during Sat. Lows Fri night will drop to zero to -5F over the 
higher terrain and single numbers elsewhere. Highs Sat will only
reach the single numbers higher terrain and low to mid teens in
the coastal plain. The strong cold advection will be accompanied
by strong wind gusts Fri night as soundings show potential for 
a few gusts to 35-45 mph. The gusty winds and arctic cold will 
result in wind chills of -10 to -20F across much of SNE late 
Fri night and early Sat, recovering to only -10 to +5F during 
Sat afternoon. Another very cold night Sat night with lows zero 
to 10 above but winds will be less of a factor. 

Additionally, the combination of gusty winds and very cold air 
will produce widespread moderate freezing spray on the waters 
FrI night into Sat night with low risk for heavy freezing 
spray.

KEY MESSAGE 4... A significant winter storm may bring heavy 
snow to southern New England Sunday into Monday, but specific 
details are uncertain.

The northward trend in the guidance continues resulting in 
increasing confidence for a significant winter storm. Despite 
the northward trend, there is still a fair amount of track 
spread in the ensemble guidance with the EC members furthest 
north and GFS members on the southern edge of the guidance 
envelope. In fact, there are almost as many EC members north of 
the benchmark as there are to the south while most GFS members 
are south of the benchmark. As a result it is best that we 
remain probabilistic as this is still a day 4-5 event and it 
will likely change going forward. 

Using the 25th percentile snowfall as a low end suggests at 
least advisory snowfall near the south coast, while the 75-90th
percentile would indicate potential for a major snowstorm for 
SNE. These snowfall accums are based on 10:1 ratio. Given deep 
cold air in place, higher ratios are certainly likely but we 
can't just assume it'll be a 15-20:1 ratio as this depends on 
deep saturation and strong omega through the snow growth region 
for getting dendrites and achieving these higher ratios. And 
these are details that can't be determined in a day 4-5 
forecast. I do think it is fair to assume a 12-15:1 ratio as a 
preliminary estimate. So based on all this we have high 
confidence of at least an advisory event in SNE, especially 
near the coast, with a risk of a major snowstorm for SNE if 
this northward trend locks in. Interesting that the ProbWSSI is 
showing high probs (90%) of moderate impacts which aligns 
closely to winter storm watch/warning criteria, and moderate 
probs (50-60%) of major impacts. This is based on a WPC 60 
member ensemble from the latest model cycle so this will change 
with updated guidance.

So what can go wrong? This is a day 4-5 event and snowfall in 
SNE is very track dependent which can't be determined with high 
confidence at this time range. I would expect wobbles in track 
going forward and these wobbles can result in significant 
changes in impacts. Also, there is a lot cold dry air to the 
north which makes us susceptible to snowfall gradients on the 
northern edge if storm wobbles southward aND it does appear we 
could be near that gradient. So probably won't be able to lock 
in to a forecast with confidence until Friday. 

So bottom line, while there is certainly a risk of a major 
snowstorm for SNE this is a day 4-5 forecast and a subtle shift 
back to the south would keep the significant impacts to the 
south. 

Timing wise, it appears snow could develop by Sun morning with 
heaviest snow expected in the afternoon and Sunday night. 
Heaviest snow should be over by Monday, however strong upper 
trough and shortwave approaches from the west with inverted 
trough at the surface and deep moisture in place so expect light
snow to continue for much of Monday with additional minor 
accums. 

Beyond Monday, well below normal temps will continue with 
arctic air remaining in place. 

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

18Z update...

Through 00Z: High Confidence.

VFR. Southwest winds 5-10 knots.

Tonight: High Confidence.

Increasing mid-level cloudiness. Low-end MVFR cloud bases 
around 3000 feet possible with scattered -SN arriving as early 
at 00-03z, from west to east, and -RA for the Cape/Islands.
-RA/SN clears out between 06-09Z from west to east.

Thursday: High Confidence.

Conditions improve to VFR with SW winds gusting up to 25 kts.

Thursday Night: High Confidence.

VFR. Light west winds, 5-10 knots.
 
KBOS Terminal...High Confidence in trends. Moderate Confidence 
in timing.

Expect -SN to move over the terminal around 03Z. -SN persists
for a couple of hours and may changeover to -RA/-RASN by 06-07Z
before tapering off west to east.

KBDL Terminal...High Confidence in trends. Moderate Confidence 
in timing.

Outlook /Friday through Monday/...

Friday: VFR. Windy with local gusts up to 30 kt. 

Friday Night: VFR. Windy with gusts up to 30 kt. 

Saturday: VFR. Windy with local gusts up to 30 kt. 

Saturday Night: VFR. Breezy. Slight chance SN.

Sunday: Mainly VFR, with areas IFR possible. Breezy. Chance SN.

Sunday Night: Mainly IFR, with areas MVFR possible. Windy with
local gusts up to 30 kt. SN likely.

Monday: MVFR/IFR conditions possible. Breezy. SN likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Tonight through Thursday Night

Winds increase out of the west/southwest tonight with sustained
wind between 10 and 20 knots over the coastal waters with gusts
between 20 and 30 knots. These winds will support SCY criteria 
through Friday morning. Seas tonight into tomorrow will be 6-8 
feet over the outer marine zones and 3-5 feet for the near-shore
zones. Expect seas to gradually subside to 2-5 feet over all 
the marine zones between Thursday night and Friday morning.

Outlook /Friday through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. 

Friday Night: Strong winds with gusts up to 35 kt. Areas of
rough seas. Freezing spray, slight chance of snow. 

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Freezing spray. 

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of snow. 

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Seas locally approaching 5 ft. Snow likely, freezing
spray. Visibility 1 nm or less.

Sunday Night: Strong winds with gusts up to 35 kt. Seas up to
5 ft. Snow likely, freezing spray. Visibility 1 nm or less.

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Chance of snow, freezing spray.
Local visibility 1 nm or less.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 7 PM this evening to 7 PM EST 
     Thursday for ANZ232-233-235-237.
     Small Craft Advisory until 7 PM EST Thursday for ANZ250-
     254>256.

&&

$$

DISCUSSION...KJC/RM
AVIATION...RM
MARINE...KJC/RM