Lexington Weather

Lexington, Massachusetts, USA

75°F

9/22/2021 12:45pm

Lexington Conditions: Overcast
Temperature: 75.4°F / 24.1°C
Dew Point: 68.7°F
Relative Humidity: 80%
Wind: NW 3 mph, 10-min avg: 3 mph, gust: 12 mph
Barometer: 30.11 in Falling Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 171259
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
859 AM EDT Fri Sep 17 2021

.SYNOPSIS...
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, low humidity and dry weather. A cold frontal 
passage around Wednesday night or Thursday ushers in a cooler 
autumn-like air mass with below normal temperatures for late 
next week into early next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
900 am update...

radar showing a small batch of light showers near the Cape and
south coast of MA, moving NW. Although surface winds are from
the NE, the winds above 1500-2000ft are from the southeast, thus
their direction of motion. Appears the showers are in response 
to a small area of elevated instability in the 750-800mb 
(6000-8000ft) layer, however models suggest this instability 
will fade away by late morning. As such, I've made minor 
adjustments to the hourly PoP field based on latest radar 
trends, but expect the light showers to have faded by 15z before
getting any further inland than roughly a PYM to PVD line. 


718 AM Update: 

Forecast holding up fairly well at this point with no
significant change needed with this update. 

Previous discussion:

4 AM update...

Today...

Most of the 00z guidance 06 hr qpf ending at 06z Fri is too wet
when compared to latest radar imagery across SNE and the adjacent 
waters. Much of the guidance is likely suffering from convective 
feedback.  Thus, will base today's forecast on the drier guidance 
such as the HRRR and GFS. Drier solutions seem reasonable given mid 
level ridging northwest of the offshore cyclone. This is also 
evident on BUFKIT time sections, which reveal lots of mid level dry 
air, likely eroding showers as activity pivots NW and comes onshore, 
especially farther inland where dry air aloft is deeper. Thus, other 
than spotty drizzle this morning, then a spot shower this afternoon, 
mainly dry weather prevails today.

Mostly cloudy conditions, light onshore winds will result in
seasonably cool temps with highs in the 70s. Although, it will feel 
a bit humid with dew pts in the mid to upper 60s. Interesting that 
best chance for breaks of sunshine today may come along the south 
coast, as models indicate drier air wrapping around offshore low and 
clips south coast of MA/RI later this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
4 AM Update...

Tonight...

Offshore low makes its closest approach to SNE, with low level jet 
increasing thermal and moisture advection into southeast MA. Thus, 
low risk for scattered showers across that area, however likely 
remaining dry elsewhere given mid level dry air across the region. 
Remaining mild given moist low levels with dew pts remaining in the 
60s. Gusty NE winds 15-25 mph across Cape Cod and the Islands.

Saturday...

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.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Highlights: 

* 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.

Details:

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.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

12z TAF Update:

Today: High confidence on trends, lower on exact timing. 

Trending toward IFR/LIFR across RI and eastern MA along with
areas of drizzle. MVFR across CT and western-central MA. Cigs
slowly lifting this afternoon from LIFR to IFR across RI and
eastern MA, and remaining MVFR elsewhere. Other than a spot
shower, mainly dry this afternoon. Modest ENE winds, except
lighter inland.

Tonight...high confidence on trends, lower on exact timing. 

Trending back down to IFR/LIFR regionwide. Patchy fog and
drizzle possible along with a few spot showers over southeast MA.
Modest NE wind along the coast, lighter inland.

Saturday...high confidence on trends, lower on exact timing. 

Any IFR/LIFR early in the morning, trends upward to MVFR/VFR in
the afternoon, as winds become northerly. Mainly dry, but a few
showers possible late in the day across CT and western-central
MA. 

KBOS Terminal...high probability for IFR but low risk of LIFR
this morning and again tonight. 

KBDL Terminal...Moderate confidence in MVFR category being the
prevalent conditions.

Outlook /Saturday night through Sunday/

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

Sunday: VFR. Breezy. 

Sunday Night through Tuesday: VFR.

&&

.MARINE...
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 
times. 

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.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT Saturday for ANZ235-237-
     254>256.

&&

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