Lexington Weather

Lexington, Massachusetts, USA

75°F

9/22/2021 1:22pm

Lexington Conditions: Overcast
Temperature: 75.4°F / 24.1°C
Dew Point: 68.7°F
Relative Humidity: 80%
Wind: W 2 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 211955
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
355 PM EDT Tue Sep 21 2021

.SYNOPSIS...
High pressure centered south of the Canadian Maritimes moves 
slightly farther offshore Wednesday, resulting in increasing 
humidity and a risk for showers. A slow-moving front 
approaching from the west will be accompanied bring showers into
western sections Thursday and spread across the region Thursday
night and Friday. Improving conditions expected Saturday, one 
or two more periods of showers are possible Sunday. No big warm 
ups or cool downs are expected.

&&

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

Tonight...The surface high pressure and mid level ridge move a bit 
farther offshore, with a deepening mid level trough to our west. 
Across southern New England, S/SE flow will bring increasing low 
level moisture. This allows for an increase in clouds, and the 
chance for patchy light rain and/or drizzle mainly over the 
interior. Higher surface dew points will prompt a more mild night, 
with overnight lows in the upper 50s to mid 60s, with areas of fog 
developing.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...

Wednesday...Surface high and mid level ridge remain centered 
offshore, while deepening mid level trough and associated surface 
low move slowly eastward through the eastern Great Lakes Region/Ohio 
River Valley. S/SE flow persists and pressure gradient tightens 
somewhat. Saturated layer once again remains below 700 mb. Expecting 
partly to mostly cloudy skies, with a continued chance of light rain 
or drizzle mainly over the interior. Above normal temperatures, with 
highs in the low to mid 70s. 

Wednesday night...

Models show mid level trough becoming cut off/blocked and halting 
eastward progression, with surface low retrograding somewhat. Low 
level moisture remains in place, with a continued S/SE flow. Thus 
expecting another night of considerable cloudiness, with patchy 
light rain and drizzle mainly over the interior. Surface dew points 
in the mid and upper 60s, very muggy for mid-September, with 
overnight lows also falling into this range. Areas of fog will 
likely redevelop.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...

Big Picture...

Closed and vertical low pressure over the Great Lakes lifts north 
into Canada. A second upper low then drops southeast from the 
Canadian Prairies across the Great Lakes and Northeast by the 
weekend, with the axis crossing New England Sunday. High pressure 
builds across the Northeast Monday and Tuesday.

Contour heights, reflecting the deep layer, are above normal 
Thursday and Friday, then in the normal range Saturday through 
Tuesday. This may briefly dip below normal Sunday night as the 
trough axis moves through...but then recover to normal Monday as the 
trough moves off through Eastern Canada.

Mass fields are similar through Friday, then show the same trend but 
different positioning over the weekend. Greater differences in the 
fields Monday and Tuesday. Overall confidence is high 
Thursday/Friday trending to low-moderate early next week.

Details...

Thursday-Friday

High pressure centered over the West Atlantic, while low pressure 
moves slowly north through Michigan and Srn Ontario. This pressure 
pattern maintains a southeast flow over Srn New England, bringing 
low level moisture and marine temperatures. Cross sections show 
plenty of convergence below 925-mb and just as much divergence 
between 925-mb and 700-mb. Moisture cross sections show plenty of 
moisture below 800-mb and pronounced dry air above 700-mb. Low-end 
CAPE stretches north over the region, with a limited 200-600 
Joules/Kg available over the region. Given the shallowness of the 
moisture, and the dry air aloft, this seems more like a warm frontal 
pattern with rain/drizzle rather than showers. Mid-level 
moisture is forecast to move into Wrn New England Thursday 
afternoon as does the right entrance region of the supporting 
upper jet. This may be enough to mention chance pops for 
afternoon showers/thunder in the CT Valley/East Slope regions. 

The upper support shifts east Thursday night and Friday, allowing 
the surface cold front to enter Western MA/CT Friday morning. The 
front crosses to the Central Hills by early afternoon and the East 
Coast Friday evening. The cold front should provide good low level 
convergence, with adequate mid level moisture with the front. PW 
values increase Thursday and peak near 1.75 inches along/ahead of 
the front Friday, high enough above normal to support local 
downpours. CAPE builds to 1000-1500 J/Kg Friday, especially in RI 
and Central/Eastern MA. Convective parameters are favorable for 
thunder. Will forecast chance pops for showers/thunder. 

Mixing temperatures Thursday around 10-11C support max temps mid to 
upper 70s. Similar temps aloft Friday but more rain suggest max 
temps more broadly in the 70s.

Saturday-Sunday...

Cold front moves offshore Friday night but lingers over the nearby 
offshore waters through the weekend. Passage of the second trough on 
Sunday may generate a wave along the stalled front. Also, the upper 
trough and its cold pool aloft may generate enough instability for 
showers on Sunday. Will forecast chance pops for showers over the 
southeast waters over northern/eastern areas Saturday and northern 
areas Sunday. Will also forecast chance pops for showers over Nrn MA 
Saturday night and Sunday. 

Monday-Tuesday...

Upper low moves off to the northeast, with showers diminishing 
Sunday night. All models generate a wave passing south of New 
England Monday. ECMWF then conjures up another system coming through 
the Great Lakes Tuesday, while the GFS shows high pressure. With
low confidence in this part of the forecast, will keep the 
forecast dry at this stage with mostly clear skies but with 
limited confidence as noted above. 

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels.

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

1930Z update...

High confidence through 00Z, then moderate confidence. 

Through 00Z...Mainly VFR SCT-BKN cloud cover, with localized MVFR 
CIGs possible especially at ORH. E/SE winds mainly 10 kt or less.

Tonight...Conditions lowering to MVFR/IFR across most terminals 
overnight into Wednesday morning, from low clouds and areas of fog. 
Localized VSBYs as low as 1/2 mile. Patchy -RA, mainly after 04Z. 

Wednesday...IFR/MVFR CIGs improving to VFR, except for far interior 
terminals, where CIGs only improve to MVFR. Patchy fog lingers until 
12-14Z. Patchy -RA mainly over the interior. SE wind 5-10 kts with 
gusts to 20 kts possible south coast/Cape/Islands.

Wednesday Night...Conditions once again lower to MVFR/IFR across 
most terminals, with low clouds and areas of fog. Slight chance -RA. 

KBOS Terminal...High confidence. VFR through this evening, then CIGs 
lowering to IFR overnight with -RA possible. Reduced MVFR VSBY in 
fog is possible. CIGs improving to VFR by midday Wed. 

KBDL Terminal...High confidence. VFR through this evening, then 
deteriorating to IFR CIGs with fog overnight and chance for -RA. Fog 
dissipates by 14Z, with CIGs improve to MVFR during Wed morning.

Outlook /Thursday through Sunday/...

Thursday: Mainly MVFR, with areas VFR possible. Breezy. Slight
chance SHRA, isolated TSRA.

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

Friday: Mainly MVFR, with areas VFR possible. SHRA likely,
chance TSRA.

Friday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA.

Saturday: Mainly VFR, with local MVFR possible. 

Saturday Night through Sunday: Mainly VFR, with areas MVFR
possible. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Tonight through Wednesday Night...High Confidence

Tonight and Wednesday...High pressure centered offshore keeps rather 
tranquil conditions over the waters for this timeframe. SE winds 10 
to 15 kts prevail for much of this period, with a few gusts to 20 
kts possible. Mainly dry conditions forecast over the waters with 

Wednesday Night....Seas building to 4 to 5 feet on the southern 
outer coastal waters, thus SCA headlines may be needed there. 
Otherwise SE winds 10 to 15 kts continue. Patchy fog possible with 
visibility as low as 1-2 miles. 

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers, isolated thunderstorms. 

Thursday Night through Friday: Winds less than 25 kt. Seas up
to 5 ft. Chance of rain showers, slight chance of
thunderstorms. 

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers. 

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers. 

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers. 

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers. 

&&

.CLIMATE...
The latest first sub-70 degree high temperature heading into 
astronomical Autumn for BOS was September 20, 1947. Still
waiting on the first sub-70 max temperature after Summer.

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...WTB/NMB
NEAR TERM...NMB
SHORT TERM...NMB
LONG TERM...WTB
AVIATION...WTB/NMB
MARINE...WTB/NMB
CLIMATE...