Lexington Weather

Lexington, Massachusetts, USA

9/22/2021 1:40pm 
  • Lexington Conditions: Overcast
  • Temperature: 73.2°F / 22.9°CColder 2.4°F than last hour.
  • Dew Point: 68.2°FDecreased 0.7°Fsince last hour.
  • Relative Humidity: 84%Increased 4.0% since last hour.
  • Wind: Wind from W W 4 mph, 10-min avg: 4 mph, gust: 12 mph
  • Barometer: 30.10 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

FXUS61 KBOX 171749

Area Forecast Discussion
National Weather Service Boston/Norton MA
149 PM EDT Fri Sep 17 2021

Low pressure south of New England brings lots of clouds, spotty
light drizzle and an isolated shower to region today. The low 
will also generate building waves and surf. As the low moves out
to sea Saturday, breaks of sunshine develop along with milder 
temperatures. High pressure then establishes itself over 
Southern New England Sunday into midweek, with above normal 
temperatures and dry weather. A cold frontal passage around 
Wednesday night or Thursday ushers in a cooler autumn-like air 
mass with below normal temperatures to end the week.


145 pm update...

As expected earlier this morning, those showers that affected
portions of the Cape and south coastal areas have pretty much
faded. Otherwise, generally cloudy conditions across most of the
region, though in a rare situation a good amount of sun has
broken out across the Cape and Islands. As a result, those 
areas are also the warmest with temperatures well into the 70s.
60s to near 70 elsewhere. There is a semi coastal front that 
has set up across SE MA. Winds north of there are primarily from
the NNE, while a more east wind south of there. Along that front
we've seen spotty drizzle and some light fog, although that is
coming to an end.  

On the larger picture, the developing weak low pressure off the
VA/NC coasts is slowly lifting northeastward. Whether it gets
formally declared anything by NHC doesn't really affect the
forecast. Models all show the system will continue
northeastward, but stay well south of our region. No significant
impact, however some bands of rain are expected well north of
the center, and late tonight some of those showers may come
close to clipping the outer Cape and Islands. I leaned on the
HREF for PoPs and have values in the 15-35% range just prior to
daybreak. Not expecting any heavy rain. Winds may gust upward of
25mph across the Cape and Islands late tonight as well. 

Otherwise the forecast challenge revolves around the potential
for fog. Lots of low level moisture is in place and not going
anywhere. Lots of the guidance suggests fairly widespread
visibilities down to 1/2 mile and lower later tonight,
especially Ri and the eastern half of MA. Confidence is not high
at all, given that the guidance also saying it's quite foggy
right now. Obviously that is not the case. That said, have
maintained the mention of patchy fog across much of the area,
but suspect most of the night will feature very low stratus
floating just a few hundred feet above the ground. With all the
clouds and semi-muggy conditions, lows will be in the 60s area
wide. A good 10+F above normal. 


4 AM Update...


As offshore low exits out to sea, mid level flow becomes 
anticyclonic across New England, supporting large scale subsidence 
and dry weather, after spotty showers/drizzle departs southeast MA 
in the morning. As winds shift from NE in the morning to NNW in the 
afternoon, combined with clouds giving way to sunshine and 925 mb 
temps from +17C to +19C, supporting highs in the upper 70s to low 
80s, cooler eastern MA coastline. Thus, a warm afternoon and a bit 
humid with dew pts in the 60s.



* Cold front moves through Sat nite, with spotty showers and breezy 
  post-frontal north winds. 

* High and dry Sunday thru at least Wed. Above normal temps and dry 
  weather thru midweek, which precede a cold front whose passage 
  occurs as early as late Wed but more likely on Thurs.
* Indications for a pattern change toward below-normal, autumn-like 
  temperatures late next week into early next weekend.


Offshore low pressure/possible tropical cyclone will continue to 
pull away into the Maritimes early Sat nite. A shortwave disturbance 
aloft which looks to pass well to our north will spread a cold front 
eastward into Sat nite. Convergence looks weak and moisture also 
looks rather shallow and have kept PoPs fairly low with this cold 
frontal feature. Not looking at a washout and most areas should stay 
dry. Fairly strong cold advection should bring about a period of 
breezy north winds/gusts post-frontal. Lows mid 50s to lower 60s. 

Sprawling area of sfc high pressure with deep-layer anticyclone then 
settles over New England Sunday into at least Wednesday. This 
supports several days of sizable diurnal temperature ranges with 
above-normal temperatures, dry weather and comfortable humidity 
levels. Nudged highs up a bit compared to NBM given mid-teens 850 mb 
temps and the expectation of several days of dry weather. Highs 
should range in the upper 70s to low 80s and lows in the 50s to near 
60 most days thru midweek.

A rather potent sfc cold front associated with a strong shortwave 
disturbance aloft then translates across New England later Wed or 
Thurs. GFS is the most progressive (FROPA late Wed) but given the 
amplified mid-level regime, sided toward the slower international 
guidance with higher PoP later Thurs into Thurs nite. Fairly strong 
dynamics with the frontal system which suggests the front would 
likely be a decent precip maker, though too early to pinpoint 
thermodynamic/instability details for thunder. Despite the timing 
differences, what is fairly clear across the models, though, is that 
a significant pattern change toward below-normal temperatures more 
typical of autumn looks to be in the offing in the post-frontal air 
mass late next week as broad 500 mb troughing sets up. 925 mb temps 
in the upper single digits Celsius by at least Friday, which would 
support highs only in the 60s and lows in the 40s to lower 50s! 
However in the extended range, often these values tend to be 
overdone and the air masses tend to modify, so will have to see if 
these values transpire to the extent shown. However there's fairly 
high confidence in below-normal temps moving into late next week 
into next weekend.


Forecaster Confidence Levels.

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

18z TAF Update:

Overall moderate confidence in the forecast.

Currently quite a bit of IFR ceilings across the region.
Exception is across the Cape and Islands. In a nutshell, those
ceilings will continue all evening, all night and finally start
to break and lift between 12z-16z Saturday. Expecting the IFR
ceilings to come back in this evening across the Cape and
Islands. The uncertainty is around how low the visibilities will
drop. Much of the guidance suggests widespread LIFR 
visibilities developing this evening and continuing overnight. 
Think the models are a little overdone, so rather than showing
1/4SM everywhere, went with lowest visibilities generally in the
3/4SM to 2SM range. 

Could be a spot or two with a little drizzle, but expect dry
conditions through 06z everywhere. Could be a few light rain
showers across the Cape and Islands between 08z-15z. 

Saturday - ceilings will be lifting with VFR conditions expected
in most areas after 16z. Outside chance of showers late in the
day, primarily across the interior. Probabilities are so low
that they don't warrant even a "VCSH" in the TAFs at this point.

KBOS Terminal...moderate confidence. high probability for IFR
ceilings, but uncertain as to how low visibilities will drop
during the overnight. 2SM is shown in the TAF, but some model 
guidance suggests 1/2SM visibilities could occur. VFR conditions
expected to develop after 15z.

KBDL Terminal...Moderate confidence.

Outlook /Saturday night through Sunday/

Saturday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Sunday: VFR. Breezy. 

Sunday Night through Tuesday: VFR.


Forecaster Confidence Levels.

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

4 am update...

Today...offshore low generates ENE winds 10-15 kt gusting up to 20 
kt at times, yielding rough sea conditions. Hence SCA remains posted 
south coastal waters of MA/RI. Vsby limited at times in patchy 
drizzle and fog this morning, improving this afternoon. 

Tonight...offshore low tracks just southeast to the 40N/70W 
benchmark. NE winds 15-20 kt and rough seas across the ocean waters 
of MA/RI. Spotty light rain/drizzle and fog will limit vsby at 

Saturday...improving conditions as offshore low exits out to sea, 
with NE winds becoming NNW.  Any morning spotty drizzle and fog, 
improves by midday with vsbys to the horizon. 

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight 
chance of rain showers. A small craft advisory will need to be 
considered to cover this period. 

Sunday through Monday: Winds less than 25 kt. Seas up to 5 ft. A 
small craft may need to be considered for this period.  

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

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


MARINE...Small Craft Advisory until 8 PM EDT Saturday for ANZ235-237-


LONG TERM...Loconto

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