Lexington Weather

Lexington, Massachusetts, USA

8/17/2022 6:36pm 
  • Lexington Conditions: Overcast
  • Temperature: 69.8°F / 21.0°CColder 0.2°F than last hour.
  • Dew Point: 58.3°FDecreased 0.2°Fsince last hour.
  • Relative Humidity: 67
  • Wind: Calm, 10-min avg: Calm, gust: 3 mph
  • Barometer: 29.85 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

FXUS61 KBOX 160132

Area Forecast Discussion
National Weather Service Boston/Norton MA
932 PM EDT Mon Aug 15 2022

High pressure over southeast Canada and developing low pressure 
well south of New England, will continue to maintain mainly dry
and seasonable weather through Tuesday. By Tuesday night into 
Wednesday night, low pressure makes its closest approach, resulting
in cooler weather and a chance of rain, especially for eastern 
Massachusetts. As the low lifts north into Maine, a drying and 
warming trend develops Thursday. Then heat returns Friday and 
Saturday, followed by increasing humidity Sunday and Monday. Dry
weather is expected Friday and Saturday, then a chance of 
scattered showers/thunderstorms Sunday with an approaching warm
front, then a cold front early next week. Although, widespread 
rain is unlikely.



9:30 PM Update...

The forecast remains on track this evening as we begin to
radiate across southern New England. High cloud cover impacting
the eastern half of our forecast area has begun to pivot to our
north and east as surface winds switch to the northeast. As 
mentioned in the previous update, low clouds may develop across
eastern MA overnight due to the wind switch. Should low clouds 
develop, it will put a cap on radiational cooling overnight. 
Western MA and CT are expected to stay partly cloudy to mostly 
clear tonight, thus, will experience some of the chilliest lows 
overnight. Dewpoints are expected to come up a few degrees 
overnight, from the upper 40s and low 50s to the mid 50s to low 
60s, highest across the Cape, which will limit how far temps can
fall before sunrise. At this time, kept low temps consistent 
with the previous update, but lows will be something to keep an 
eye on overnight. 

7 PM Update:

Very pleasant and comfortable evening in progress, with
temperatures mainly in the 70s and dew pts in the 50s, with 40s
across interior MA. This dry airmass, with light winds mostly
clear conditions (especially across CT and western-central MA),
will support radiational cooling tonight. Thus, tweaked
overnight mins toward the cool MOS guidance in this region.
Farther east into RI and eastern MA, thicker mid/high clouds
this evening along with potential low clouds late tonight, as
low level winds become NE, makes overnight mins tricky in RI and
eastern MA. 

Otherwise, forecast straight forward for tonight - dry weather
(rain near Nantucket never reaches the ground due to dry air)
with temps slightly cooler than normal. Earlier discussion


Cumulus clouds continue to develop this afternoon with a low chance 
for an isolated shower or weak thunderstorm. There are two areas we 
are watching this afternoon; the east slope communities of the 
Berkshires and the outer Cape and Nantucket. Few showers have 
developed west of KENX in Albany's area as a result of daytime 
heating and low level lapse rates nearing 9C/km. These showers are 
forming because of orographic lift from the Catskills and Berkshires. 
Secondly, KOKX has a cluster of showers south of Long Island as a 
result of short wave rounding the base of a mid level trough. 

Heading into this afternoon a low chance for showers across the east 
slope communities of the Berkshires, POPs less than 20 percent. As 
the day goes on and we lose daytime heating these showers and clouds 
will fade. As for the cluster of showers over the southern outer 
waters those will continue to move to the northeast and may clip the 
outer Cape and Nantucket by early this evening. PWATs are low, so we 
would not expect any heavy rains to fall out of these showers. 
Values range between 0.60 and 0.90 inches. This is roughly 70 
percent of normal for mid-August. 

Tonight we are dry and should see some clearing, HREF brings in 
nearly clear skies during the overnight hours. Given the departing 
short wave to our east, we've kept sky cover greater for eastern 
Massachusetts. Further west with less cloud cover and light winds we 
could expect good radiational cooling. Overnight lows run a few 
degrees cooler than the average morning low for mid-August, waking 
up Tuesday temperatures will be in the mid to upper 50s, with the 
exception of the urban centers where lows are generally in the low 


The global ensembles continue to support the mid-level trough 
becoming a closed during the second-half Tuesday. Given the cold 
pool aloft we should expect more cumulus clouds to develop Tuesday 
afternoon with a low chance of an isolated shower. This will result 
in slightly cooler temperatures as well. Afternoon highs will 
struggle to reach 80, but will top out in the upper 70 across the 
eastern half of southern New England. Locations in the Connecticut 
River Valley will see a little more sunshine and should be able to 
have a seasonable afternoon with highs 80F to 83F. 

With the vested interest in rainfall many have been keeping a close 
eye on the potential for rain this week. Given the back and forth of 
the deterministic runs of the global models, thought it was fair to 
lean on the global ensembles. As of Monday afternoon there is a weak 
surface low off of the North Carolina coast. Ensembles have the low 
drifting to the northeast on Tuesday and come Tuesday night there is 
deepening of the surface low. The low will have little impact on our 
forecast for Tuesday, outside of added cloud cover during the late 
afternoon and especially during the overnight hours. We have brought 
it slight chance for rain for eastern southern New England during 
the pre-dawn hours of Wednesday as far west as Lowell to Worcester 
to Providence and kept the chance rain for the outer Cape and 

Further discussion on the development and track of this system will 
be discussed in the long term section of the AFD.


345 PM update...


* Best chance for widespread rain appears Wed/Wed ngt in eastern MA

* Heat returns Fri & Sat w/highs 85-90, then warm & humid Sun & Mon

* Drying trend Thu, dry Thu, Fri & Sat. Chance of scattered 
  convection Sun with warm front, then cold front early next week


Some of the guidance has trended farther west regarding the threat 
for widespread rainfall Wed/Wed night, associated with developing 
vertically stacked low offshore. Specifically the 12z EC, ARW and a 
few 12z GEFS members have trended farther west with heavy rain into 
eastern MA. The new 12z ECENS has only 7 of its 50 members with 
heavy rain into eastern MA. That's only 14% of its membership. Seems 
like the 12z op EC has some convective feedback beginning 12z Tue 
over the 40N/70W benchmark, which then leads to heavy convective 
rains into eastern MA Wed/Wed night. Forecast confidence this period 
(Wed/Wed night) is below average given this precip is convectively 
driven vs baroclinic driven. During this severe drought, we have 
seen multiple times promising rains disappear in model guidance as 
the potential event nears. This could be the case here again, given 
the lack of baroclinity with this August system. Therefore we will 
go with chance pops Wed/Wed night, highest over eastern MA to 
reflect potential rainfall, but also reflect the uncertainty, 
especially with event 36+ hrs out in model time.  By Thu, even the 
more robust 12z EC agrees with remaining guidance for mid level low 
to lift northward, with rising heights across SNE. Thus, any risk 
for morning rain Thu should be replaced by a drying trend for the 
afternoon. Then confluent flow Fri followed by short wave ridging 
Sat, as subtropical ridge builds northward, will support dry weather 
Fri/Sat. Ensembles agree on an airmass change around Sunday, with 
PWATs rising rapidly above normal. Thus chance of warm frontal 
convection Sunday, followed by risk of convection early next week 
with approaching northern stream trough exiting the Great Lakes. 
Although given convectively driven, widespread rain is not expected. 


Offshore low Wed will result in cooler NE flow, with ensembles 
indicating 850 mb temps noticeably cooler than normal. This combined 
with clouds and possibly showers, highs Thu may only be in the 70s, 
especially eastern MA given onshore flow. However, by Thu, with mid 
level low exiting into ME, WNW flow develops across SNE. Therefore 
should see temps rebound to normal Thu afternoon, upper 70s to lower 
80s, even a few mid 80s possible. Then Fri and Sat, WNW flow aloft 
advects warmer than normal temps per ensembles, supporting highs 85-
90 both days. Although humidity will remain in check, with dew pts 
only in the 50s, especially away from the coast and potential 
seabreezes. Humidity definitely increases Sun and Mon, as ensembles 
indicate PWATs rising rapidly above normal. Hence, warm and humid 
Sun and Mon, with highs in the 80s and dew pts in the 60s. 
Definitely a summer feel.


Forecaster Confidence Levels:

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

23z TAF Update: no change to previous TAFs, VFR cloud bases, dry
runways and light east winds becoming NE overnight. Earlier
discussion below.


Tonight...High confidence.

VFR. East wind becomes north/northeast 5 to 10 knots, the exception 
are terminals on Cape Cod and Islands where wind is 10 to 15 knots. 

Tuesday...High confidence. 

Mostly VFR with MVFR conditions possible for eastern MA terminals as 
early as 18z. North wind becomes northeast 10 to 15 knots, gust 20 
to 25 knots for terminals on the Cape and Islands.

Tuesday Night...Moderate confidence. 

VFR for western terminals. MVFR/IFR conditions with -RA possible for 
eastern MA terminals, including the Cape and Islands. Northeast wind 
10 to 15 knot with gust 20 to 25 knots. Terminals away from the 
coast have northeast wind 5 to 10 knots. 

KBOS...High confidence. 

KBDL...High confidence.

Outlook /Wednesday through Saturday/...

Thursday through Saturday Night: VFR. 

Sunday: Isolated SHRA, isolated TSRA.


Forecaster Confidence Levels:

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

4 PM Update...

Tonight...High confidence.

Dry conditions. Seas generally 1 to 3 feet. East wind becomes 
north/northeast 10 to 15 knots and sub advisory gusts for southern 
outer waters. 

Tuesday...High confidence. 

Mostly dry conditions but increasing rain chances during the second 
half of Tuesday afternoon for the waters southeast of Nantucket. 
Small Craft conditions for southern outer waters as waves build to 3 
to 5 feet and gusts reach 25 knots. 

Tuesday Night...Moderate confidence. 

Chance of light rain for outer waters as low pressure system 
develops. Small Craft conditions extend to all outer waters as wave 
heights build 4 to 6 feet and wind gusts reach 25 to 30 knots. 
Outlook /Wednesday through Saturday/... 

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

Thursday Night through Saturday Night: Winds less than 25 kt. 

Sunday: Winds less than 25 kt. Isolated rain showers, isolated


MARINE...Small Craft Advisory from 11 PM Tuesday to 8 AM EDT Thursday 
     for ANZ250.
     Small Craft Advisory from 8 PM Tuesday to 8 AM EDT Thursday 
     for ANZ254.
     Small Craft Advisory from 6 AM Tuesday to 8 AM EDT Thursday 
     for ANZ255-256.


NEAR TERM...Gaucher/KS
SHORT TERM...Gaucher
LONG TERM...Nocera

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