Lexington Weather

Lexington, Massachusetts, USA

75°F
9/22/2021 1:06pm 
  • Lexington Conditions: Overcast
  • Temperature: 74.8°F / 23.8°C 
  • Dew Point: 68.2°FDecreased 0.7°Fsince last hour.
  • Relative Humidity: 80%Decreased 2.0% since last hour.
  • Wind: Wind from WSW WSW 1 mph, 10-min avg: 4 mph, gust: 11 mph
  • Barometer: 30.11 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 161346
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
946 AM EDT Thu Sep 16 2021

.SYNOPSIS...
A cold front slides through the region this morning, then stalls 
along the south coast this afternoon, resulting in scattered showers 
and an isolated thunderstorm. A coastal low off the South Carolina 
coast, slowly tracks north and maintains clouds across southern New 
England into Friday, although very little if any additional
showers. This offshore low will result in building waves and 
surf and a modest increase in northeast winds Friday night into 
Saturday night for coastal New England as it recurves toward 
Atlantic Canada. Thereafter, high pressure remains in control 
with milder than average temperatures and several days of dry 
weather through at least midweek.

&&

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

10 AM...
The strongest storms have moved off the south coast and Cape Cod
as of this writing. A few strong to marginally severe storms
continue to impact our outer waters. Otherwise, a heavier batch
of rain is moving through northern Rhode Island and will move 
northeast over southeastern Massachusetts before finally moving 
out to sea. A lull in showers is expected this morning, but 
spotty pop up showers may redevelop this afternoon. The greatest
risk for showers remains along the South Coast, Cape and 
Islands, but a few inland areas could see a shower.

Breaks of sun look to develop across western MA and CT this 
morning with drier mid level air around 500 mb. However, high 
relative humidity below 500 mb should allow clouds to redevelop 
and sock southern New England in for most of the day. The 
clearing line just to our west will struggle to clear our area 
today. 

Max temperatures for the day were bumped down slightly,
especially across northeast and eastern Massachusetts as a
northeast wind will keep things cool. Pockets of the
Connecticut River Valley that are exposed to more breaks in the
cloud deck will warm into the mid 70s. 



515 AM update...

Line of storms have developed along and just behind the frontal
boundary across eastern CT/RI and eastern MA. High shear (up to
50 kt of effective shear) and fairly steep mid level lapse 
rates supporting nocturnal convection early this morning. 
Synoptic scale lift from RRQ of upper level jet streak also 
enhancing updrafts. 00z HREF max updraft and updraft helocity 
indicates an upward trend this morning, as this line moves into
a more unstable environment across RI and eastern MA, with 
mesoanalysis indicating 0-3 km CAPE up to 75 j/kg along the 
south coast of RI/MA. Thus, line of convection may intensify as 
it traverses eastward into axis of better instability. Satellite
confirms this idea with cooling cloud tops and lightning 
activity on the upswing. Therefore, a few of the stronger storms
may produce small hail, gusty winds and heavy rainfall. Earlier
discussion below.

======================================================================

Cold front has cleared much of the region at 3 am, with boundary now 
lined up across eastern RI into southeast MA. RRQ of upper level jet 
enhancing synoptic scale lift and yielding rain shield in the post 
frontal airmass (Anafrontal) across CT into western-central MA. 
Marginal SB instability ahead of the front from southeast CT into RI 
and southeast MA, per latest SPC mesoanalysis. Along and behind the 
front, marginal elevated instability giving way to isolated thunder. 
This rain shield will continue to slide east this morning, with 
showers becoming more numerous across the entire region. 

By afternoon, deep layer moisture and forcing for ascent exit 
offshore, and replaced with column drying from top down. While a 
drying trend develops this afternoon for much of the region, clouds 
will remain stubborn, thus not expecting much if any sunshine this 
afternoon. As cold front stalls or washes out along the south coast 
this afternoon, scattered showers may continue to fire along this 
boundary. Thus, showers may persist along/near the south coast this 
afternoon, including Cape Cod. 

Post frontal airmass today is seasonably cool with highs in the 70s. 
Clouds and light onshore flow will accentuate the cooler conditions.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
4 AM update...

Tonight...

Offshore ridge begins to build back into SNE with rising heights. 
Large scale subsidence will provide mainly dry weather. Although low 
level moisture will become trapped beneath subsidence inversion, so 
low clouds will be on the increase along with spotty light drizzle 
possible. Mild with lows in the 60s along with a light onshore wind. 
 
Friday...

Deep layer ridge axis remains across SNE, providing mainly dry 
weather. Although clouds will dominate with just partial sunshine 
possible at times. Can't rule out an isolated low top shower, but 
overall most of the day will remain dry. Clouds and light onshore 
flow will limit highs to the 70s, although there will be a humid 
feel to the airmass, with dew pts 65-70.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
4 AM update...

Highlights: 

* Coastal low or possible tropical cyclone expected to pass to our 
  southeast may bring building seas, possible rip currents and an 
  increase in northeast winds over southeast New England and the 
  waters Fri nite thru Sat nite. Impacts appear minimal but will 
  depend on low track and strength. 
  
* Dry weather and milder than normal temperatures likely into next 
  week with high pressure in control.  

Details:

Coastal low or possible tropical cyclone (NHC prescribing a 70 
percent chance of cyclone formation in 48 hrs as of the 200 AM 
Thursday Tropical Weather Outlook) will be the main focus for Friday 
night into Saturday afternoon/night. Most models now appear in good 
agreement in the cyclone passing just south of 40N/70W overnight 
Friday night and into the Maritimes thru Saturday. Storm track and 
intensity will dictate impacts but effects on our area still appear 
to be fairly minimal. Mainly building seas, rip currents/possible 
high surf and an increase in NE to N winds. Variable amounts of 
cloud cover (more east/southeast and less over the interior) with 
periods of off-and-on rain showers too, but the heaviest QPF looks 
to lie east of the cyclone center and over water. Cold frontal 
passage Saturday late-overnight into early Sunday but appears 
moisture starved with limited QPF. 

Aside from the aforementioned coastal low, the main story for Sunday 
into much of the upcoming workweek is a large area of high pressure 
that settles over New England. Expecting several days of dry weather 
with strong diurnal temperature ranges: milder than normal high 
temperatures with strong radiational cooling leading to cooler 
nights. With several days of full sun and dry conditions, opted to 
raise highs over NBM by blending in some of the 90th percentile NBM 
highs and bias-corrected temps likely should do fairly well. Lowered 
dewpoints a bit during the peak heating hrs as well. But all in all, 
a pleasant stretch of weather looking ahead into early to mid next 
week.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

09Z update: 

Today...high confidence.

A combination of MVFR/VFR (trending toward MVFR) in scattered 
showers and thunderstorms this morning, then drying trend for 
this afternoon, except south coast especially Cape Cod, where 
showers may persist. Winds shifting to NW to NE with frontal 
passage.

Tonight...high confidence.

MVFR in the evening, then trending down to IFR/LIFR along with
patchy fog. Mainly dry weather. Light ENE winds. 


Friday...high confidence. 

IFR/LIFR to begin the day, but cigs lifting to MVFR/VFR during
the afternoon. Mainly dry weather with modest ENE winds. 

KBOS Terminal...Moderate confidence.

KBDL Terminal...Moderate confidence.

Outlook /Friday night through Sunday/

Friday: Mainly VFR, with local IFR possible. Slight chance
SHRA, patchy FG.

Friday Night: MVFR/IFR conditions possible. Breezy. Slight
chance SHRA.

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

Saturday Night through Sunday: VFR. Breezy. 

Sunday Night through Monday: VFR. 

&&

.MARINE...
Forecaster Confidence Levels.

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

Today...cold front enters the waters this morning, then stalls and 
washes out this afternoon. Showers and patchy fog will lower vsby to 
3-5 miles at times. NW winds become NE this afternoon. 

Tonight...ridge of high pressure offshore builds across the waters. 
Patchy dense fog will yield poor vsby at times. Light ENE winds. 

Friday...ridge of high pressure remains across the waters, yielding 
light ENE winds and mainly dry weather. 

Outlook /Friday Night through Monday/...

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers, patchy fog. 

Friday Night through Saturday: Winds less than 25 kt. Local
rough seas. Slight chance of rain showers. 

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Nocera/Loconto
NEAR TERM...Nocera/KS
SHORT TERM...Nocera
LONG TERM...Loconto
AVIATION...Nocera/Loconto/KS
MARINE...Nocera/Loconto
      

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