Lexington Weather

Lexington, Massachusetts, USA

75°F
9/22/2021 1:20pm 
  • Lexington Conditions: Overcast
  • Temperature: 75.4°F / 24.1°C 
  • Dew Point: 68.7°F 
  • Relative Humidity: 80
  • Wind: Wind from SSW SSW 1 mph, 10-min avg: 3 mph, gust: 11 mph
  • Barometer: 30.10 inFalling 0.02  inHg/hr Falling Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 181749
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
149 PM EDT Sat Sep 18 2021

.SYNOPSIS...
As Tropical Storm Odette passes far to southeast of southern 
New England today, skies will brighten from west to east. But a
cold front will trigger scattered showers or thunderstorms 
toward evening, from northwest to southeast. High pressure 
builds in Sunday and will bring sunny and dry conditions from 
Sunday through mid week, with a warming trend. An approaching 
cold front will be accompanied by showers and possible 
thunderstorms from Wednesday night through Thursday night, 
followed by a drying and cooling trend Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
140 PM UPDATE...

Clearing trend continues across the region as Tropical Storm 
Odette accelerates northeastwards well southeast of the 70W/40N 
benchmark, winds have turned northerly from Worcester point
west. Cold front is now across northern New England and that is
reflected by a broken line of showers and thunderstorms on
radar. Clearing has allowed 500-1000 J/kg of surface-based CAPE
to develop across the region but because there is non-existent
shear or helicity, any storms that develop are expected to
remain sub-severe. In addition, subsidence in the wake of the
departing tropical cyclone has also entrained a layer of mid-
level dry air, as hinted by the K index values in the mid 20s
across the region. So while we have chosen to maintain slight
chance to chance PoPs this afternoon into early evening, not
terribly bullish on convection chances but can't rule it out
either. Otherwise, forecast remains largely on track so only 
made minor changes to bring it up to date with current obs. 
Beach goers should note the high rip current risk today as a 
result of the swells from Odette. And also remember that when
thunder roars, go indoors. Earlier discussion below.

PREVIOUS DISCUSSION...

Low clouds, areas of fog (locally dense in eastern MA & RI) and
mist will slowly lift and burn off this afternoon, as TS Odette
moves farther offshore, which will promote height rises and mid
level flow transitioning to anticyclonic across SNE. Thus, a
drying trend and erosion of the low clouds as the morning 
progresses and especially this afternoon. Fairly warm and moist 
airmass by afternoon, especially across the interior with highs 
in the low 80s and dew pts in the 60s. This will yield MLCAPES 
of around 1000 j/kg. This instability coupled with modest lift 
from approaching s/wv, will generate a line or broken line of 
thunderstorms late this afternoon and evening, roughly 5 pm to 
11 pm from northwest to southeast. Deep layer shear is lacking 
and given instability is only modest, not expecting storms to 
become severe, but something we will have to watch.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
355 AM update...

The line of scattered showers and thunderstorms may make it as
far as the Boston to Providence corridor and then weaken as it
moves off the coast in the early evening. 

Over most of the interior, skies will be mostly clear and 
temperatures will fall into the mid and upper 50s overnight.

It is quite subtle, but models show 925 mb winds veering from 
north to northeast between 6 PM and midnight, which should be
enough to allow overcast low cloudiness and fog to move back
onshore from Boston southward to Cape Cod and Nantucket. Briefly
dense fog may again occur. By midnight, winds turn more to the
north-northwest, allowing the cloudiness and fog to move 
southeastward and eventually off the coast. Due to the cloud
cover in eastern sections, low temperatures will be in the lower
60s.  

Sunday looks to be the start of several fine late summer days.
With high pressure building in, skies should be sunny. Highs
will be in the 70s with pleasantly dry dewpoints in the 49-54 
range.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
355 AM update...

Highlights: 

* Very pleasant, dry stretch of weather Monday, Tuesday and possibly 
  lingering into Wednesday, with mild days and cool nights

* Showers & T-storms Wed night/Thu, possibly lingering into Fri

Synoptic Overview...

590+ dam ridge along the eastern seaboard provides dry, pleasant 
weather Mon/Tue and possibly lingering into Wed. Then a high 
amplitude trough moves across the Great Lakes into the Northeast. 
However the guidance is split on the timing and evolution of this 
trough. The ECMWF and its ensembles are slower with the trough 
passage, maintaining dry weather across the region through Wed. 
Meanwhile, the GFS and its ensembles are faster/more progressive, 
with risk of showers as early as Wed. Although, this is a typical 
fast bias of the GFS and GEFS in the medium range. Also models 
differ on how cold post frontal airmass will be Fri. GFS/GEFS blast 
trough thru New England with subzero 850 mb temps across northern 
NY/New England Fri. Meanwhile, the ECMWF and its ensembles have 
trough sheared and lifting off to the northeast, with much weaker 
cold air advection across SNE.  

Temperatures...

Maritime airmass yields seasonably cool temps Monday with highs 70-
75 and lows in the 50s. Then airmass gradual warms Tue into Wed with 
highs 75-80. Real pleasant stretch of weather featuring mild days 
and cool nights, along with light winds and abundant sunshine each 
day. Likely still mild Thu ahead of approaching cold front with 
highs in the 70s and dew pts into the 60s. As mentioned above, 
uncertainty on magnitude of post frontal cold air advection Fri. 
Thus will follow a model blend here (which will offset the fast/ 
progressive bias of the GFS/GEFS) which offers highs 65-70 Fri, 
lower 70s southeast MA, which is only slightly cooler than normal. 
Although, a cooler scenario is possible if the GFS/GEFS solutions 
verify. 

Precipitation...

590+ dam ridge along the eastern seaboard maintains dry weather Mon 
and Tue. As mentioned above, uncertainty/model differences on timing 
of upstream trough. Given time range here and to offset the 
fast/progressive bias of the GFS/GEFS in the medium range, we will 
follow a model blend which offers chance pops from Wed night thru 
Thu night, followed by a drying trend Fri in the post frontal 
airmass.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

1740z Update...

Overall moderate confidence in the forecast today and tonight. 
High confidence Sunday.

Through mid-afternoon... Fog lifting and IFR becoming MVFR 
ceilings in eastern sections. Skies breaking out to VFR in 
western and central sections.

Late afternoon and evening...
VFR western and central sections, but a line of scattered
showers and thunderstorms is expected to sweep across portions
of MA, northern CT, and northern RI, with local MVFR vsbys.
IFR/LIFR overcast cloudiness will move back into the east coast
between 22Z-00Z, impacting KBOS, KHYA, KFMH, and KACK until
roughly 04Z-06Z. 

Sunday...VFR.

KBOS Terminal...Moderate confidence. MVFR conditions expected 
to develop after 16Z with VFR briefly possible after 20Z. 
IFR/LIFR redeveloping 22Z-00Z and lasting until 04Z-06Z Sunday.

KBDL Terminal...Moderate confidence.

Outlook /Saturday night through Sunday/

Sunday Night through Tuesday Night: VFR. 

Wednesday: 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.

Generally high confidence in the forecast. Tropical Storm 
Odette passes far offshore to the southeast today. It is 
expected to generate enough of a swell, combined with some winds
touching 25 knots (especially near the Cape and Islands), and 4
to 7 ft seas...so Small Craft Advisories remain in effect. They
have been added for the waters east of Ipswich Bay and for 
Stellwagen Bank through Sunday. For the waters south of RI, the 
Advisory is expected to expire Saturday night. 

Scattered showers, mainly southeast of Nantucket today. Chance
of a shower or thunderstorm this evening. Areas of dense fog
over the waters east of Massachusetts through tonight, reducing
visibility to less than 1 nm at times.

Outlook /Sunday Night through Wednesday/...

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

Monday through 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.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Rip Current Risk until 6 PM EDT this evening for MAZ022-
     024.
RI...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 6 PM EDT Sunday for ANZ250-254.
     Small Craft Advisory until 8 AM EDT Sunday for ANZ255-256.

&&

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

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