Lexington Weather

Lexington, Massachusetts, USA

55°F
12/4/2020 12:55pm 
  • Lexington Conditions: Partly cloudy
  • Temperature: 54.7°F / 12.6°CWarmer 0.2°F than last hour.
  • Dew Point: 36.0°FIncreased 2.4°Fsince last hour.
  • Relative Humidity: 49%Increased 4.0% since last hour.
  • Wind: Wind from WSW WSW 2 mph, 10-min avg: 4 mph, gust: 9 mph
  • Barometer: 29.90 inFalling 0.03  inHg/hr Falling Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   
Southeast Middlesex County - Winter Storm Watch
 - SEE ALL NEARBY ALERTS -

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 041439
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
939 AM EST Fri Dec 4 2020

.SYNOPSIS...
Areas of light rain move in late today into tonight ahead of the 
next storm. A strong storm system will impact the region Saturday 
into Saturday night bringing heavy rain and strong winds along the 
coast, with potential for significant snow accumulation in the 
interior. The storm exits Sunday, then mainly dry and colder weather 
Monday and Tuesday with a moderating trend Wednesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

935 AM Update...

Previous forecast is on track today with above normal
temperatures for early December. While we expect an abundance 
of clouds, should see peeks of sun at times at least through 
early afternoon. Good mixing on southwest flow with 850T above 
0C will result in afternoon high temperatures reaching well into
the 50s across much of the region. Mildest temps should be 
across southeast New England, where a few locations may approach
60. While dry weather should dominate for a good chunk of the 
day, moisture plume lifting northward will result in some light 
rain impacting parts of CT/RI and SE MA by late afternoon or 
early this evening. 

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
* A rapidly intensifying storm system will produce gusty winds
  and heavy rain, changing to a period of accumulating snow,
  heavy at times by Saturday afternoon into evening. 

A higher than usual amount of uncertainty still exists with how 
exactly the potential weekend storm will unfold despite being 24 to 
36 hours out. This is due to the models struggling to handle the 
complex interaction of northern and southern stream energy and 
potential phasing in a rather chaotic negative NAO regime. In 
addition, there is a lack of cold air injection beforehand, which 
means that the system has to generate its own cold air from its 
sheer intensity and dynamic cooling of the atmospheric column. 

We are increasingly confident of the following, however. Firstly, 
the low pressure will likely track inside of the 70W/40N benchmark, 
which is typically used as a reference for accumulating snow along 
the I-95 corridor in RI and MA. Secondly, this is going to be a 
highly dynamic system, which means despite marginal temperatures, 
there will likely be a period of heavy snowfall rates from mesoscale 
banding. The challenge is, however, where the axis of heavy snow 
will fall and consequently, the possibility for power outages
with the loading effect of the heavy, wet snow. 

With a rapidly deepening cyclone, the latent heat release from the 
convection will likely lead to downstream ridging and if anything, 
reinforces the possibility of a more inland track (inside the 
70W/40N benchmark). The dynamics are very impressive. The guidance 
is pretty much unanimous in showing 30-40 units of omega in the 
dendritic snow growth zone. So we have high confidence there is 
going to be a mesoscale band of 2 inches per hour snowfall rate. Of 
course, the big question is where will this band set up. 00z HREF 
Probability Matched Mean shows a 6-12 inches zone in Worcester 
county and significantly cut down on QPF amounts in Western MA. Based
on the model trends, we boosted the snowfall amounts between 
Worcester and metrowest Boston and consequently, expand the 
Winter Storm Watches to along the MA/CT border and into 
metrowest Boston. Nonetheless, we cannot discount the
possibility that the forecast storm track shifts west again so
we kept the Winter Storm Watches across the Western zones. 

Finally, high wind potential exists, especially near the coast as 
the surface cyclone rapidly deepens. This is why we hoisted a High 
Wind Watch for the MA South Shore into the Cape and the Islands.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...

No changes to long term portion of the forecast as main focus was 
placed on upcoming storm.

Looks quiet for much of the extended period with blustery and cold 
conditions Sunday into early next week, then a moderating trend 
around midweek. Another amplifying shortwave will result in a storm 
offshore around the Tue time frame that will have to be monitored. A 
westward shift would bring the risk of accumulating snow to SNE.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

High confidence through today. Then moderate confidence. 

Today...VFR cigs today with gradually lowering cigs. Some light
rain may develop later today across CT/RI and SE MA. SW gusts 
to 20 kt at times, especially in the coastal plain. 

After 00z...Lowering to widespread MVFR and IFR conditions as 
rain overspreads the area. Conditions deteriorate to LIFR as 
rain changes over to snow, heavy at times from 18z Sat onwards.
The axis of heavy snow is still uncertain at this time. Winds 
could also gust over 40 kts along the coast between Saturday 21z
and Sunday 06z as a low pressure system rapidly deepens off 
Eastern MA. 

KBOS TAF...High confidence in the TAF through 00z Saturday then
moderate confidence. 

KBDL TAF...High confidence in the TAF through 00z Saturday then
moderate confidence. 

Outlook /Sunday through Tuesday/...

Sunday: Mainly VFR, with local IFR possible. Windy with gusts
up to 30 kt. 

Sunday Night through Monday: VFR. Breezy. 

Monday Night through Tuesday: VFR.

&&

.MARINE...

* Gale Warnings for all waters Saturday into early Sunday. 

A strong storm system will likely affect Southern New England
Saturday into early Sunday. Winds will be WSW to SW on Friday 
with speeds of 15-20 kts and gusts of 20-25 kts. Wave heights 
generally 4-6 feet with the highest waves across the outer 
waters.

Winds diminish to 5-15 kts Friday night ahead of the incoming system 
and the direction gradually shifts to the NE with gusts of 10-15 
kts. Seas falling to 3-4 feet across the eastern waters and remain 
around 4-6 feet across the southern waters.

By Saturday afternoon, seas build to 10-12 feet over the
southern waters then 11-16 feet across all waters as winds turn
northeast with a rapidly strengthening low pressure center that
is expected to track just inside the 70W/40N benchmark. Winds
will gust to over 40 kts. Storm-force gusts are not out of the
question on Saturday evening across the open waters but
confidence is not high enough for Storm headlines. 

Outlook /Sunday through Tuesday/...

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft. 

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. 

Monday: Winds less than 25 kt. Seas up to 5 ft. 

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. 

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Wind Watch from Saturday afternoon through Saturday 
     evening for MAZ019-022>024.
     Winter Storm Watch from Saturday morning through Sunday 
     morning for MAZ002>006-008>012-014-026.
RI...None.
MARINE...Gale Warning from 7 AM Saturday to 7 AM EST Sunday for 
     ANZ230>237-250-251-254>256.

&&

$$
SYNOPSIS...KJC/Chai
NEAR TERM...Frank
SHORT TERM...Chai
LONG TERM...KJC
AVIATION...KJC/Chai
MARINE...KJC/Chai
      

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