Lexington Weather

Lexington, Massachusetts, USA

75°F

9/22/2021 1:04pm

Lexington Conditions: Overcast
Temperature: 74.8°F / 23.8°C
Dew Point: 68.2°F
Relative Humidity: 80%
Wind: S 5 mph, 10-min avg: 3 mph, gust: 11 mph
Barometer: 30.10 in Falling Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 191747
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
147 PM EDT Sun Sep 19 2021

.SYNOPSIS...
Large high pressure over the region today. This will bring dry 
weather and increasing sunshine, as well as low humidity along 
with mild days and cool nights 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 PM THIS EVENING/...
140 PM Update...

Forecast is largely on track so only made minor changes to bring
it up to date with obs. Cold front has passed well offshore and
with 1025mb high building in from southern Quebec, all of 
Southern New England is now basking in wall to wall sunshine
except for a few passing terrain-induced mid-level clouds. Dew 
points will fall into the 40s for most locations as drier air 
aloft mixes down to the surface. Northerly winds with 20 mph 
gusts at times will make it feel like early fall. Overall, 
fantastic day for outdoor activities. Enjoy! 

Previous update...

* Gorgeous stretch of dry weather begins later this morning, 
  featuring mild days and cool nights

High pressure and post frontal airmass beginning to overspread the 
region this morning from NW to SE. Definitely a noticeable change in 
airmass with PWATs falling to about 50% of normal!  At the surface, 
this translates to dew pts falling in the 50s today, even a few 
upper 40s possible in the high terrain of western MA. Quite a 
difference from the 60s yesterday.

Core of cooler post frontal airmass will eventually be across 
eastern MA, where 925 mb temps are about +12C today, then warming to 
+14C to +16C over western MA/CT. This will result in a noticeable 
temp range across the region this afternoon, with highs in the upper 
60s to lower 70s across eastern MA, then warming to the mid-upper 
70s across CT into western-central MA and RI. 

Gusty NNE winds along the eastern MA coast this morning behind the 
front, will slacken and give way to afternoon seabreezes. These NNE 
winds will initially yield scattered to broken strato-cu across 
eastern MA this morning, but eventually very dry air aloft mixes to 
the surface and gives way to abundant sunshine all areas later this 
morning. Away from the coast, light winds will prevail thru the day. 
Just a gem of a day with abundant sunshine, mild temps, low humidity 
and light winds this afternoon, especially away from eastern MA 
coastline.  

For those heading to the beach today, leftover southeast swells from 
Odette will result in one more day of rough surf and increased risk 
of strong rip currents. Thus, rip current statement remains posted. 
Main concern is at east facing ocean beaches of Cape Ann, Cape Cod 
and Nantucket, especially this time of year as beaches no longer 
have lifeguards.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
315 AM update...

Tonight...

The combination of a very dry airmass in place (dew pts in the 40s 
and 50s) and 1025+ mb high pressure overhead, providing light winds 
& clear conditions, will result in a very cool night. Low temps were 
derived from the cooler MOS guidance datasets, which supports lows 
in the 40s across many of the suburbs, 50s in the urban areas. These 
cool temps will serve as a reminder that the first day of Autumn is 
just days away, Wed 9/22 at 3:21 pm EDT. Normal low for this time of 
year is 53 to 58.  

Monday...

Another absolute gem of a day, with abundant sunshine, light winds 
and low humidity with dew pts in the 40s and 50s! Subtropical ridge 
builds to 593 dam heights, centered over southern New England. Still 
a coolish airmass overhead with 1030 mb maritime high extending into 
southern New England. Subsidence inversion may preclude mixing to 
850 mb, therefore with 925 mb temps around +13C to +14C, expecting 
highs in the low to mid 70s, cooler along and near the coast with 
seabreezes.

&&

.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 /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels.

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

1740z Update...

Today...

VFR. NNE wind 10-15 kt with 20-25 kt gust at times before
diminishing by late afternoon into early evening. 

Tonight...VFR. Winds light and variable. 

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

KBOS Terminal...High confidence. VFR. 

KBDL Terminal...High confidence. VFR.

Outlook /Monday night through Thursday/

Monday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Tuesday: VFR. 

Tuesday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Wednesday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SHRA.

Wednesday Night: MVFR/IFR conditions possible. Breezy. Slight
chance SHRA, patchy BR.

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

&&

.MARINE...
Forecaster Confidence Levels.

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

700 AM update...

Today...High forecast confidence.

High pressure builds into New England, with NNW winds eventually 
shifting to NNE late this morning. Gusts build up to 20-25 kt, 
then diminish this afternoon. Seas a combination of NNE wind 
waves and SE swells from Odette. Dry weather. 

Tonight and Monday...High forecast confidence.

Large 1025+ mb high pressure builds over the maritimes, yielding 
fine boating weather with light winds, dry weather and good vsby. 

Outlook /Monday Night through Thursday/...

Monday Night through Tuesday Night: Winds less than 25 kt. 

Wednesday: Winds less than 25 kt. Seas locally approaching
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. Seas up to 5 ft. Slight chance
of rain showers, slight chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Rip Current Risk until 8 PM EDT 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 4 PM EDT this afternoon for ANZ251.
     Small Craft Advisory until 2 PM EDT this afternoon for ANZ255-
     256.

&&

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