Lexington Weather

Lexington, Massachusetts, USA

63°F

7/27/2024 4:29am

Lexington Conditions: Clear
Temperature: 63.3°F / 17.4°C
Dew Point: 55.9°F
Relative Humidity: 77%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 30.03 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 260153
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
953 PM EDT Thu Jul 25 2024

.SYNOPSIS...
A cold front will move offshore thru early this evening, with
scattered showers and thunderstorms moving offshore with it.
Decreasing cloudiness, northwest breezes and falling humidity
levels toward more comfortable conditions are expected tonight
upon frontal passage. Seasonably warm but with comfortable
humidity levels are expected Friday into the weekend. The 
weather pattern then turns unsettled Tuesday and Wednesday with 
increasing chances of showers and perhaps a few thunderstorms.

&&

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

10 PM update...

Cold front has moved in the general vicinity of the Cape where
dewpoints continue to hover around 70F. Dewpoints continue to
fall across interior southern New England, with readings of 52F
in Chicopee and 54 at Boston Logan airport. 

Some early evening fog that developed along the coast remains 
across Nantucket but has begun to dissipate rapidly behind the 
cold front. Anticipate fog will be totally gone by 4Z as the 
front moves seaward. 


7:30pm update...
Hi-frequency obs suggest the cold front is currently draped
from northern Plymouth County southwest to roughly Old Lyme, CT
and continuing a fairly rapid pace to the SE. Dewpoints have
dropped quickly, with several sites behind the front already
showing mid 50s. The drier air will continue to filter across
the region, in spite of the upper lvl moisture plume manifest as
CI band. Winds briefly increasing to around 10 kts with some
gusts to near 20 for a few hours after the frontal passage, but
as the upper lvl CI shifts offshore, this should allow just
enough decoupling for the winds to settle back to around 5 kts
or less.

Previous forecast already fairly well in line with this
thinking. Remnant convection has already collapsed offshore.
Therefore, only minor tweaks to bring current temps/dewpoints
and winds up to date.

Previous discussion follows...

Been slowly peeling away at the extent of low clouds/stratus, 
which now are primarily over the Cape and Islands, and have 
scattered out or dispersed elsewhere in Southern New England. 
METARs and composite radar mosaic shows the leading edge of a 
cold front trailing from Newburyport MA southwest to Storrs CT 
on into the Danbury CT area; marked by a broken line of 
scattered showers with embedded thunderstorms along it. There is
a ribbon of modest instability (around 500 J/kg of mixed-layer 
CAPE) along and just ahead of this frontal boundary into eastern
MA and northern/central RI away from the waters; west of the 
front, winds have shifted to breezy WNW along dewpoints falling 
into the lower 60s in interior portions of Southern New England,
and into the mid 50s into the Adirondacks. Until then, it still
remains humid in with dewpoints upper 60s to low 70s for most 
of Southern New England. 

Expect scattered showers with embedded thunderstorms to move 
into the I-95 corridor during the rush hour to the coastline by 
7 PM. There hasn't been much time for the atmosphere to 
destabilize, so while the along-front forcing is good, the 
paltry instability will keep these storms from reaching severe 
weather limits, but a few could still produce cloud to ground 
lightning and brief downpours. Storms should weaken upon 
approach to the waters where stratus has been most pervasive. 
Cold front will have then pushed offshore by 8 PM, and this will
bring a breezy windshift from SW to WNW upon frontal passage, 
along with the onset of falling dewpoints. 

For the rest of the evening into the overnight, NW breezes are 
expected to continue with speeds around 10-15 mph and gust 
speeds around 20-25 mph thru the late evening hours. These gust 
speeds are similar to observations across eastern and central 
NY, where cool and dry advection should continue to allow for 
shallow mixing even after sundown. Decreasing cloudiness and 
MUCH drier air filters in tonight with dewpoints falling into 
the middle 50s to low 60s. Lows in the mid/upper 50s to around 
60 in the interior, and lower to middle 60s for RI and eastern 
MA where NW breezes should persist the longest.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
325 PM Update:

Friday and Friday Night: 

NW cyclonic flow aloft with surface ridge of high pressure will 
govern conditions for Southern New England Friday into Friday 
evening. Deep dry air should allow for only a few fair weather 
clouds, seasonably warm temperatures and comfortable humidity 
levels. Skies Friday afternoon could take on a bit of a hazy look 
aloft per today's 12z HRRR-Smoke model run, which shows modest 
concentrations of vertically-integrated smoke advecting in stemming 
from the active wildfires over west-central Canada. Forecast surface-
level smoke concentrations are nil, thus we are not expecting the 
hazy skies to lead to any potential health problems as it is 
suspended well aloft. NW breezes should decrease in speed into the 
afternoon, but seabreezes look unlikely (if they develop, would be 
late in the day). Highs Friday in the low to mid 80s inland, with 
mid to upper 70s along the coasts and in the higher terrain. 

High pressure then ridges in for Friday evening, setting up a 
tranquil evening with clear skies and light winds. Lows should be 
similar to tonight's, in the mid/upper 50s to lower 60s.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights...

* Pleasant weekend conditions.
* Increasing chances for some showers/thunderstorms early next week.
* More widespread precipitation anticipated mid next week.

Details...

Sat and Sun...  
High pres under ridging folding over from the 
central US/Canada. PWATs a sold 1-2 std deviations below normal 
suggest humidity through the column remains relatively low through 
the weekend. Highs in the mid to upper 80s across the interior away 
from the influence of diurnal sea breezes.  

Mon into Tue... 
Agree with previous forecasters assessment of recent 
guidance changes regarding the influence of an offshore cutoff with 
H5 temps dipping as low as -14C.  Noting a still a fair amount of 
spread on the cutoffs location, GEFS mean is suppressed further SE 
than the ECENS and Canadian members suggest as well, which ultimately 
will determine final afternoon CAPE values.  Still, looking at the 
guidance trends in the last 24 hours worth of runs, probs are higher 
for at least some measurable precip during the peak heating early 
next week than they were.  Increasing column moisture also favors 
this as PWATs will begin to reach 1.5-2.0 inches during this period.

Therefore, will continue to highlight at least chance POPs for 
afternoon/evening periods each day.  Mix of showers and occasional 
thunderstorms possible.

Otherwise, with increasing moisture, expect increasing humidities as 
well as peak dewpoints will once again upper 60s to near 70 even 
with afternoon mixing.

Mid to late next week... 
Building ridging across the southwest CONUS 
will favor digging trough across the east, reinforced by shortwave 
energy ejecting eastward from the midwest and Great Lakes region. 
Increasing subtropical flow will once again introduce PWATS near 2.0 
inches.  From all this, anticipating a more widespread 
shower/thunderstorm risk toward the latter portions of the work week.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:

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

00Z TAF Update: 

Tonight: High confidence. 

Last remnants of lowered CIGS across ACK will be giving way to
VFR at all TAF sites by 02Z. Winds WNW with a few gusts between
10-20 kts at times overnight, but expect these too will recede
after 04Z to around 5 kt.

Friday and Friday Night: High confidence.

VFR. NW winds around 6-10 kt to start, with sub-20 kt gusts
during the morning to early-aftn. NW speeds decrease mid to late
in the day; low prob (< 20%) of a seabreeze on the eastern MA
coast late in the day, but should be just enough NW mixing to 
keep the sea breeze offshore. Light NW winds for Friday evening.

KBOS TAF...High confidence. VFR dominates now. Winds WNW through
Fri morning. May be a brief seabreeze impacting the terminal by
early-mid afternoon Fri.

KBDL TAF...High confidence in TAF. W/WNW around 10-12 kt gusts 
20kt. VFR with NW winds around 10 kt, decreasing during the day
Fri.

Outlook /Saturday through Tuesday/...High confidence.

Sunday through Monday Night: VFR. 

Tuesday through Tuesday Night: VFR. Slight chance SHRA,
isolated TSRA.

Wednesday: Breezy. Chance SHRA, isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

325 PM Update:

Rest of the Afternoon into Tonight: High confidence.

SW winds around 10 kt early tonight; a cold front was
approaching the waters with scattered showers and embedded
t-storms along it. These storms could produce occasional
lightning strikes and occasional gusts to 30 kt over nearshore 
waters but should weaken as they move further offshore. 

The cold front will bring a windshift to NW tonight with a 
increase in gust speeds to around the low 20s kt range. Think 
gusts should fall just below small craft advisory criteria but 
could get close to there for a brief period of time thru late 
this evening. NW speeds should settle in around 15-20 kt 
overnight. Seas 4 ft or less all waters. 

Friday and Friday Night: High confidence.

NW gusts 15-20 kt thru late morning, with decreasing speeds into
the afternoon and become 10 kt or less for the evening. Seas 3 
ft or less all waters. Dry weather.

Outlook /Saturday through Tuesday/...High Confidence

Sunday through Monday Night: Winds less than 25 kt. 

Tuesday through Wednesday: Winds less than 25 kt. Slight chance
of rain showers, isolated thunderstorms. 

&&

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

&&

$$

SYNOPSIS...Loconto/Guest
NEAR TERM...Loconto/KS/Guest
SHORT TERM...Loconto
LONG TERM...Guest
AVIATION...Loconto/Guest
MARINE...Loconto/Guest