Lexington Weather

Lexington, Massachusetts, USA

72°F

9/22/2021 2:29pm

Lexington Conditions: Overcast
Temperature: 71.8°F / 22.1°C
Dew Point: 69.1°F
Relative Humidity: 91%
Wind: SE 9 mph, 10-min avg: 3 mph, gust: 13 mph
Barometer: 30.08 in Falling Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 200555
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
155 AM EDT Mon Sep 20 2021

.SYNOPSIS...
Large high pressure remains over New England Monday and Tuesday, 
bringing dry weather with mild days and cool nights. The high begins 
to drift offshore Wednesday, with increasing humidity and scattered 
showers possible. A slow moving frontal system approaching from the 
west will be accompanied by widespread showers at times Thursday 
into Friday. Improving conditions expected Saturday but a few 
showers could linger in eastern Massachusetts and Rhode Island.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...

2 AM update...

Temps falling thru the 50s with MVY the coolest location at 46!
Previous forecast remains on track, thus no changes with this
update. Earlier discussion below.

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

Chilly night ahead, especially outside of the urban areas where
temps are already down in the 50s. 1024 mb high over VT is 
providing a dry airmass, light winds and clear conditions.
Hence, ideal radiational cooling conditions. Previous forecast 
is for many suburbs to fall into the 40s overnight, with 50s in
the urban areas. These temps remain on track, thus no change to
previous forecast. 

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...

Monday...

Anomalous 592 Dm high builds into New England and becomes centered 
over our area. This is 99.5th percentile according to the NAEFS 
Situational Awareness Table. Nonetheless, daytime highs should be 
near or just above climatological normal due to southeast winds as 
1028mb surface high slides just offshore. With 925mb temperatures 
+12 to +14C, expect low 70s near the coast and mid to upper 70s in 
the interior locations. Due to subsidence inversion, expect 
plentiful sunshine with a few mid to high level clouds filtering in 
from the southwest during the afternoon hours. That is associated 
with weak disturbances associated with the remnants of Tropical 
Cyclone Nicholas getting caught in the weakness between the 592 Dm 
ridge centered over the Northeast US and an approaching amplifying 
H5 trough from the Great Plains. 

An interesting statistic is that Boston has never failed to record a 
sub-70 degree high past September 20. The latest first sub-70 degree 
high heading into astronomical fall is September 20 1947. With 
southeast flow and SSTs in the mid 60s, it will be a close call 
whether Boston will officially hit 70 degrees, given that the 
climate site is just off the waters at Logan Airport. Have gone with 
a maximum forecast of 70 degrees, but it is a toss-up between 69 and 
71F. We shall see if the record stands by the end of tomorrow! 

Monday night...

Surface high slides further offshore, allowing winds to turn 
southerly. Dew points rise from the 40s into the 50s and along with 
increasing cloud cover, overnight lows will generally bottom out in 
the 50s, with mid to upper 50s near the coast and in the urban 
population centers. If cloud cover is less extensive, then upper 40s 
in the typical cool spots like the MA coastal plains are possible.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

Highlights...

* Mainly dry and seasonably mild Tue/Wed
* Scattered showers at times Thu into Fri and continued mild
* Improving conditions Sat, but showers may linger in the east

Tuesday into Wednesday...

Strong mid level ridge over New Eng shifts to east on Wed while 
surface high remains sets up east of Nova Scotia with ridging 
extending west into SNE. Dry and seasonably mild weather Tue with 
sunshine mixing with some lower strato-cu as persistent SE flow 
brings a shallow layer of low level moisture. Then Wed, some of the 
guidance is indicating a few showers developing in the interior as 
low level moisture deepens a bit. Given that large scale forcing is 
limited with ridging nearby and lots of dry air in the mid levels, 
much of the day should be dry with just a low risk for a few showers 
in the interior. Temps Wed will be a bit milder with highs mid/upper 
70s

Thursday into Friday...

Models are similar in the overall pattern with high amplitude 
trough/mid level low across the Gt Lakes opening up and lifting to 
the NE into eastern Canada as strong ridge persists to the east. As 
a result of the amplified pattern, there is consensus of a slow 
moving front approaching from the west with deep southerly flow 
across New Eng. Shower chances will increase during this period. 
Expect scattered showers Thu, especially in the west but given that 
best forcing remains to the west with neutral to slightly rising 
heights not expecting widespread showers and it may remain mostly 
dry in eastern New Eng Thu. The most likely period for numerous 
showers and a few t-storms will be Fri as low level jet and 
accompanying PWAT plume moves into region ahead of the frontal 
system. While some heavier rainfall is possible, the axis of 
heaviest rainfall is expected to be north and west of SNE. Temps 
will be in the 70s Thu/Fri with humid conditions as dewpoints will 
be well into the 60s.

Saturday and Sunday... 

Low confidence forecast next weekend as there are differences on the 
amplitude of the upstream trough across the Gt Lakes which impacts 
the timing of the front moving offshore. GFS and ECMWF suggest 
improving conditions Sat, but many EC ensemble members have showers 
lingering on Sat with front stalling near eastern New Eng. Will 
maintain chc pops in the east on Sat but lots of uncertainty. For 
Sunday, we went with dry forecast in NW flow, but can't rule out 
more showers assocd with a shortwave passage. Seasonable temps 
expected next weekend as there is not push of colder air. In fact, 
ensemble mean 850 mb temps 10-12C.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels.

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

06z Update...high confidence thru Tue.

Today...VFR, dry weather and light winds, becoming onshore
during the afternoon. Patchy fog in the interior valleys and low
lying terminals near shore. 

Tonight...VFR, dry weather and light winds. Patchy fog in the 
interior valleys and low lying terminals near shore.

Tuesday...VFR, dry weather and SE winds. VFR cloud bases
possible across CT into western-central MA, possibly RI.

KBOS Terminal...High confidence. VFR. 

KBDL Terminal...High confidence. VFR.

Outlook /Tuesday night through Thursday/

Tuesday Night: Mainly VFR, with local IFR possible. 

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

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

Thursday through Thursday Night: Mainly MVFR, with areas VFR
possible. Breezy. Chance SHRA.

Friday: Mainly MVFR, with local VFR possible. Breezy. Chance
SHRA, isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

340 PM update...

Rest of today...High forecast confidence.

High pressure builds into New England, with NNE winds 10-15 kt
and isolated 20 kt gusts diminishing by this evening. 5-7 ft
seas and SE swells from Odette will diminish after midnight.
Remaining Small Craft Advisories are scheduled to come down 
after midnight.

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. 

Monday night...Winds shift to the south at 5-10 kt with
localized reduced vsby due to patchy fog possible. 

Outlook /Tuesday Night through Friday/...

Tuesday: Winds less than 25 kt. 

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

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft. 

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

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

Friday: Winds less than 25 kt. Rough seas up to 7 ft. Chance of
rain showers, isolated thunderstorms.

&&

.CLIMATE...
The latest first sub-70 degree high heading into astronomical 
fall for BOS is September 20 1947. It is possible we may break 
the record by tomorrow. 

The latest first sub-70 degree high heading into astronomical
fall for PVD is September 26 2015.

Both BDL and ORH have already recorded sub-70 degree highs
earlier this month.

&&

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

&&

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