Lexington Weather

Lexington, Massachusetts, USA

75°F

9/22/2021 1:02pm

Lexington Conditions: Overcast
Temperature: 74.8°F / 23.8°C
Dew Point: 68.2°F
Relative Humidity: 80%
Wind: S 10 mph, 10-min avg: 3 mph, gust: 11 mph
Barometer: 30.11 in Falling Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 182305
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
705 PM EDT Sat Sep 18 2021

.SYNOPSIS...
A cold front will trigger a few showers or thunderstorms 
through this evening. High pressure builds in Sunday and will 
bring sunny and dry conditions from Sunday through mid week, 
with a warming trend. An approaching cold front will be 
accompanied by showers and possible thunderstorms sometime 
Thursday into Friday, followed by dry and seasonably mild 
conditions Saturday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
7 PM update...

Widely scattered showers have developed across NW MA this 
evening along a weak cold front which will be dropping south 
across SNE tonight. MLCAPES around 1000 J/kg in the interior
although moisture is somewhat of a limiting factor. Given
instability, approaching shortwave and a weak boundary can't 
rule out a shower or isolated t-storm across portions of 
northern and eastern MA through 03z as the front moves south. 

Otherwise, areas of stratus and patchy fog will likely 
redevelop near the coast tonight, but an improving trend 
expected late tonight as the front moves through with low level
drier air moving in behind it. However, patchy dense fog may 
develop late tonight in the CT valley in northern MA/ 

Previous discussion...

Much of southern New England is now seeing clear skies as 
Tropical Storm Odette accelerates northeastwards well southeast 
of the 70W/40N benchmark. A broken line of showers and
thunderstorms is currently located along an Albany NY- 
Showhegan ME line, out ahead of a cold front sliding south. 
Clearing has allowed 500-1000 J/kg of surface- based CAPE to 
develop across the region but because there is non- existent 
shear or helicity as well as poor mid-level lapse rates, any 
storms that develop are expected to remain sub- severe. In 
addition, subsidence in the wake of the departing tropical 
cyclone has also entrained a layer of mid- level dry air, as 
hinted by the K index values in the mid 20s across the region.
In addition, mesoscale analysis shows that there could be an
area of wind convergence near the I-495 corridor so there could
be additional source of lift to ignite a few showers and 
scattered thunderstorms across Eastern MA and RI. So while we 
have chosen to maintain slight chance to chance PoPs this 
afternoon into early evening, not terribly bullish on convection
chances but enough to mention. Otherwise, forecast remains 
largely on track so only made minor changes to bring it up to 
date with current obs. In addition, beach goers should note the 
high rip current risk today as a result of the swells from 
Odette. 

Cold front then pushes offshore overnight, bringing an end to 
any lingering showers or isolated thunderstorm. Skies clear from
west to east, allowing for a few hours of radiational cooling. 
Blended in the MOS guidance to lower overnight minimum temps, 
although breezy conditions with gusts up to 20 mph at times 
should keep most locations in the upper 50s to lower 60s with 
pockets of mid 60s in urban centers like Boston. In addition, 
there should also be a period where low- level clouds could hang
tight and areas of fog could develop across eastern MA and RI 
before the cold front pushes through.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Sunday looks to be the start of several gorgeous late summer 
days. With high pressure building in, skies should be sunny. 
Highs will be in the 70s with pleasantly dry dewpoints in the 
upper 40s to low 50s. 

With high pressure moving overhead, conditions will be ripe for
excellent radiational cooling. Leaned heavily on MOS guidance to
derive overnight lows in the mid 40s to low 50s, except for mid
50s in the urban centers and immediate coast. Might be a good
idea to close the windows! 

In addition, have opted to hoist Rip Current Statements for the
entire eastern MA coast through tomorrow evening due to
remaining swells and lingering elevated rip current risks from 
Tropical cyclone Odette.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Highlights: 

* Pleasant, dry stretch of weather Mon into Wed with seasonably mild 
  day and cool nights

* Showers & scattered T-storms Thu into Fri

Overview...

Strong mid level ridge along the east coast into New Eng early next 
week moves slowly east Wed/Thu as a high amplitude trough moves into 
Gt Lakes with cut off low developing. GFS remains most progressive 
of the guidance envelope sweeping the trough and attending cold 
front through on Thu while ECMWF/GGEM have a much slower evolution 
with fropa not until Fri/Fri night. Given the GFS progressive bias 
in amplified patterns we will trend the forecast toward the slower 
ECMWF solution and its ensembles which favor mainly dry weather into 
Wed, then increasing risk of wet weather with showers and possible t-
storms Thu into Fri. Additionally, with ECMWF indicating upper low 
opening up and lifting well to the NW into eastern Canada, airmass 
behind the fropa will be modified so not expecting a significant 
cooling trend into next weekend.

Precipitation...

Strong high pres will maintain dry weather Mon and Tue and possibly 
into Wed as well. By Wed, the surface high will be well east of New 
Eng with increasingly moist southerly flow possibly bringing a few 
light showers in the west. However, much of the area should remain 
dry. Forecast confidence decreases beyond Wed due to timing 
differences on upstream trough and approaching cold front. ECMWF 
ensembles also vary a bit on timing so we will have chance showers 
Thu into Fri although there is uncertainty regarding which time 
period will have the greatest risk of showers. Pre-frontal PWAT 
plume approaching 2 inches suggests some heavy rainfall is possible 
with the front. Drying trend expected by Sat. 

Temperatures...

Despite strong mid level ridge, high pres from Maritimes with 
ridging extending into New Eng will result in seasonably mild days 
Mon into Wed with highs low/mid 70s. Cool nights Mon night and Tue 
night. Still mild and somewhat humid Thu ahead of the front and 
depending on timing of fropa, the mild weather could linger into 
Fri, especially eastern New Eng where highs could reach into the 
70s. Not much push of cool air in the post frontal airmass on Sat 
with 850 mb ensemble mean temps around 10C so highs should reach the 
70s again.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

00z Update...

Overall moderate confidence in the forecast through tonight. 
High confidence Sunday.

Through midnight...Areas of IFR stratus and patchy fog will 
redevelop across Cape Cod and portions of eastern MA. An 
isolated shower or t-storm possible in northern and eastern MA. 

After midnight...Gradual improvement along the coast as stratus
and fog erodes. Otherwise VFR, except localized fog developing
in the CT valley.  

Sunday...Starting off MVFR for Cape terminals but improving to
VFR across all terminals by 15z. North winds 5-10 kt with gusts
up to 20 kt across Cape terminals. However, gusts gradually
diminish after 18z. 

Sunday night...VFR. Winds light and variable. 

KBOS Terminal...Moderate confidence tonight, then high 
confidence. MVFR cigs expected to redevelop this evening with 
chance of IFR, then improving after 06z. 

KBDL Terminal...High confidence.

Outlook /Saturday night through Sunday/

Monday through Tuesday Night: VFR. 

Wednesday: Mainly VFR, with local MVFR possible. 

Wednesday Night: Mainly MVFR, with local IFR possible. Slight
chance SHRA.

Thursday: Mainly MVFR, with local IFR possible. Breezy. Chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Generally high confidence in the forecast. Even as Tropical 
cyclone Odette continues to pull further away into the Atlantic 
Ocean, lingering swells and 4-7 ft seas warrants the
continuation of Small Craft Advisories for most outer waters
through a good part of Sunday.

Chance of a shower or thunderstorm this evening. Areas of dense
fog over the waters east of Massachusetts overnight tonight, 
reducing visibility to less than 1 nm at times.

High pressure moves overhead late Sunday, resulting in light and
variable winds and seas falling below 5 ft overnight Sunday. 

Outlook /Monday through Thursday/...

Monday through Monday Night: Winds less than 25 kt. Seas
locally approaching 5 ft. 

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

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

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

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Rip Current Risk through Sunday evening for MAZ007-019-
     022-024.
RI...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 8 PM EDT Sunday for ANZ250-254.
     Small Craft Advisory until 2 PM EDT Sunday for ANZ255-256.

&&

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