Lexington Weather

Lexington, Massachusetts, USA

73°F

9/22/2021 1:44pm

Lexington Conditions: Overcast
Temperature: 73.0°F / 22.8°C
Dew Point: 68.2°F
Relative Humidity: 85%
Wind: SSW 2 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

        
000
FXUS61 KBOX 190705
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
305 AM EDT Sun Sep 19 2021

.SYNOPSIS...
A cold front will move across the region during the predawn
hours Sunday and be accompanied by a few showers or 
thunderstorms over eastern MA. High pressure builds later  
Sunday morning and will bring sunny and dry conditions from 
through mid week. 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 THIS MORNING/...
2 AM update...

Short wave trough and cold front moving thru eastern MA early
this morning and accompanied by scattered showers and possibly
an isolated thunderstorms. True post frontal airmass now
advecting into western MA and southwest NH with dew pts in the
50s. This airmass will reach the coastline by sunrise or shortly
thereafter. Mostly cloudy over eastern MA, with partial clearing
westward into RI/CT and western-central MA. This clearing trend
reaches the coastline toward daybreak.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/...
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 NIGHT THROUGH SATURDAY/...

Big Picture...

High amplitude pattern through the week will favor a slow change in 
weather. High pressure surface and aloft over the Eastern Seaboard 
Monday night shifts to the West Atlantic Tuesday. Great Plains 
shortwave digs and deepens over the Midwest Tuesday, forming a 
closed upper low Wednesday which lifts north through the Great Lakes 
Thursday-Friday.

Upper flow shows well-above normal contours, which would support 
summertime temps. But the departing surface high will leave 
southeast surface winds in its wake which should modify temps closer 
to normal levels. Contours fall as the upper low passes to the 
northwest, but remain near season normals. So expect near normal 
temperatures, cooling a little late in the week.

Mass fields are similar through Wednesday, then diverge. The GFS 
quickly becomes 24-hours ahead of the other models with the position 
of the closed low. But all models carry the system through the same 
geography, so the concern on its effects is more with timing. 
Confidence is high through Wednesday, then moderate through Saturday.

Details...

Monday night to Wednesday...

High pressure slowly moves east offshore, but maintains dry weather 
in Srn New England during the period. Mixing reaches between 850-mb 
and 900-mb. Dew points in the upper 40s and low 50s Monday night 
with light wind, so Monday night min temps will be in the upper 40s 
to mid 50s. Dews increase a little Tuesday, so Tuesday night mins 
will be in the 50s. Temps at the top of the mixed layer will be 5-8C 
Tuesday, supporting max temps upper 60s to mid 70s. These temps 
nudge up a little Wednesday, supporting max temps in the low-mid 70s.

Thursday through Saturday...

Low pressure lifts through Ontario Thursday, sweeping the associated 
surface cold front east. GFS moves the cold front through during 
Thursday night, while the ECMWF moves it through late Friday or 
Friday night. Tend to favor the slower ECMWF timing. PW values start 
climbing Wednesday, reaching highest values around 2 inches during 
Thursday and early Friday. This suggests the potential for local
downpours. Convective parameters are so-so Thursday. Will 
forecast low-end chance pops for showers in the CT Valley 
starting Wednesday night. The chance of showers spreads east 
across the region Thursday. Showers continuing through Thursday
night in the west and through Friday in the east. Pops will 
include a chance of thunder. 

Generally dry Saturday, but upper trough sweeps east from the Great 
Lakes and brings a -26C cold pool. Much of this moves northeast into 
Canada. But cooler air does move over New England, and this cold 
advection may destabilize the airmass and bring a few showers, 
especially to Western Mass. 

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

06z Update...

Today...

MVFR/IFR across eastern MA along with scattered showers,
improves to VFR 09z-12z along with dry weather. Elsewhere,
mainly VFR and dry weather. NNW winds becoming NNE around
daybreak and increasing 15-20kt along the eastern MA coast.
Elsewhere 5-15 kt. 

Tonight...VFR. Winds light and variable. 

Monday...VFR with light and variable winds, becoming onshore
along the coast. 

KBOS Terminal...high confidence, dry weather toward 09z and then
MVFR improving to VFR by 16z. Then an extended stretch of dry
weather, VFR, light/variable winds with afternoon seabreezes. 

KBDL Terminal...High confidence.

Outlook /Monday night through Thursday/

Monday Night through 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 Night through Thursday/...

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 this evening for MAZ007-019-022-
     024.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ250-
     254.
     Small Craft Advisory until 2 PM EDT this afternoon for ANZ255-
     256.

&&

$$

SYNOPSIS...WTB/Chai
NEAR TERM...Nocera
SHORT TERM...Chai
LONG TERM...WTB
AVIATION...WTB/Nocera
MARINE...WTB/Chai