Lexington Weather

Lexington, Massachusetts, USA

72°F
9/22/2021 2:29pm 
  • Lexington Conditions: Overcast
  • Temperature: 71.8°F / 22.1°CColder 2.9°F than last hour.
  • Dew Point: 69.1°FIncreased 1.1°Fsince last hour.
  • Relative Humidity: 91%Increased 11.0% since last hour.
  • Wind: Wind from SE SE 9 mph, 10-min avg: 3 mph, gust: 13 mph
  • Barometer: 30.08 inFalling 0.02  inHg/hr Falling Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 220212
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1012 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/...
More minor tweaks to rainfall chances based on observed trends.
Latest runs for the HRRR and RAP seem to be verifying reasonably
well, as both captured the light showers across the Cape and
islands this evening. Since these were the only guidance sources
to do so, leaned heavily on their solutions to update the
timing and location of precipitation overnight.

Temperatures were also brought back line with observed trends
and updated expectations from near term guidance.

7 PM Update...

High pressure centered south of Nova Scotia. Local winds show 
a southeast surface flow while winds at 2000 feet or higher, per
the KBOX VWP, are out of due south.

Increasing low level moisture. Radar shows light showers 
popping near the Vineyard and moving toward Cape Cod. Another 
batch of showers near Montauk Pt and along the RI Coast. All 
showers moving north. Of the CAMS, the NAM Nest is the only one 
to fully capture these showers. But it and the other CAMS 
generate a new area of showers moving toward Wrn Long Island 
near midnight that spreads across Srn New England overnight. 

So expect some light rain/showers during the night. POPs at each
core should range from 25-30 pct, within a broader area of 20
pct. 

The forecast expands the coverage of the rain but otherwise
follows the general theme of the previous forecast.

Previous Discussion...

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 /02Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels.

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

Tonight...Scattered to widely scattered light rain/showers moves
north from the South Coast. Greatest coverage should be over CT
and western MA around and after midnight. 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.

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 in trends. Moderate confidence
in timing.

KBDL Terminal...High confidence in trends. Moderate confidence
in timing.

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
maintains a south-southeast surface wind 10 to 15 kts. A few 
gusts to 20 kts possible. Patches of light rain/drizzle with
patchy fog possible. 

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

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