Lexington Weather

Lexington, Massachusetts, USA

70°F

6/21/2025 12:08am

Lexington Conditions:
Temperature: 69.6°F / 20.9°C
Dew Point: 57.4°F
Relative Humidity: 65%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 29.90 in Steady
Rain Today: 0.00 in

Southeast Middlesex County, MA - Extreme Heat Watch
- SEE ALL NEARBY ALERTS -

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 181120
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
720 AM EDT Wed Jun 18 2025

.SYNOPSIS...
Overcast and humid today with a period of morning showers, and
another round of showers mainly south of the Mass Pike early
tonight. Hot and humid weather for Thursday with heat indices 
95 to near 100, with a risk for strong to severe thunderstorms 
Thursday afternoon as a cold front moves through. Drier and 
seasonably warm Friday and Saturday. A multi- day stretch of 
significant heat is possible starting early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
350 AM Update:

Key Messages: 

* Overcast conditions most of today, with foggy conditions early 
  this morning dissipating by mid to late morning. 

* Morning showers give way to brief dry weather this afternoon, then 
  another round of showers and a rumble of thunder late this 
  afternoon mainly south of the Mass Pike. 

* Foggy conditions return again tonight. 

* Humid with highs in the 70s, and muggy nighttime lows in the mid 
  to upper 60s.

Details:

Not much has changed on the large scale compared to 24 hours 
ago; the only real notable change being that its turned 
increasingly more humid with dewpoints in the low to mid 60s. 
We're still stuck in a broad SW flow aloft in a regime of lower-
level moist SE/S flow. The increased moisture has led to a 
blanket of stratus and areas of mist/fog but coverage of truly 
dense fog is too sparse for dense fog headlines. A weak 
disturbance embedded in the SW flow aloft is moving through 
eastern PA, and this will spread an area of what should be 
passing light to moderate showers ENE through the morning, which
will also impact morning commuters. Fortunately stronger 
convection associated with this wave is well south of even the 
Philadelphia, PA area so we're not expecting any thunder with 
this morning activity. We should then see a relative break in 
shower activity by the afternoon. 

There are two main question marks for today. The first is the 
extent to which morning cloud cover lingers and tempers daytime 
heating. RAP-based BUFKIT profiles are pretty saturated this 
morning - makes sense, there's stratus out there - but tends to 
decrease in depth somewhat as we move into the early to 
midafternoon. Probably still ends up cloudier on the whole even 
if low cloud bases lift, due to an abundance of mid/high clouds,
and so high temps were reduced to the mid to upper 70s, but 
these could still be a few degrees too warm. The next is the 
location of another round of scattered showers arriving toward 
late in the day and into the early evening, perhaps with a 
rumble of thunder or two as showalter indices dip negative. This
is associated with another weak disturbance in the SW flow 
aloft; seems to be greater agreement on this feature and its 
associated showers passing over the southern third of SNE (south
of the Mass Pike including CT, RI and southeast MA), which also
will include the offshore waters. After a decrease in PoPs for 
early to midafternoon, they again ramp up into the 30-50% range 
south of the Mass Pike, and around 15-20% north of the Mass 
Pike. 

After this wave passes offshore, we'll likely be dealing with 
another period of fog and stratus with an even more muggy 
airmass. Lows tonight only fall into the mid to upper 60s, which
will also be close to dewpoints.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
350 AM Update:

Key Messages: 

* Hazy, hot and humid! Feeling like 95 to as much as 105 degrees by 
  afternoon. Heat Advisory headlines could be needed.

* Scattered strong to severe thunderstorms as soon as 3 PM, but more 
  likely between 5 to 9 PM as a line of storms moving through 
  interior Southern New England, potentially to the coast. Damaging 
  straight line winds are the main hazard, along with torrential 
  downpours and frequent lightning.  
  
* Much less humid Thursday night and drying out.

Thursday:

Likely will be dealing with a considerable amt of stratus and 
fog, carrying over from the overnight. That being said, Thursday
still looks to be quite active, as this cloud cover should 
erode early, leading to a hazy, hot and humid late morning to 
midafternoon. A strong cold front associated with a pretty 
strong frontal system for late-June will also lead to the 
potential for strong to severe thunderstorms Thurs aftn. Let's 
talk about both below. 

1/ Heat and Humidity

Without question, Thurs stands to be one of the hottest days 
experienced to this point in the summer, but it will also be met
with high humidity levels. 850 mb temps warm on increasing SWly
low level flow to around +17 to +20C. High temps should soar 
into the mid 80s Cape and Islands to the low to mid 90s. Given 
dewpoint temps in the low to mid 70s, it may feel like it is 
100-105 degrees outside in portions of the CT and Merrimack 
Valleys, and could feel like 95-100 degrees across a wider 
portion of Southern New England (excluding the south coast/Cape 
Cod where it may feel closer to 90 degrees). Seabreezes are not 
expected as SW surface winds pick up to around 10-15 mph. We 
considered heat advisories, but consensus among surrounding 
offices was to wait to re-evaluate if dewpoints could mix lower.
We encourage those to take appropriate heat-related precautions
such as taking frequent breaks in shaded and air conditioned 
areas, and checking in on those sensitive to heat such as the 
elderly and pets. 

The high heat and humidity is not expected to break until the 
cold front moves across the area later in the day on Thurs.

2/ Strong to Severe Thunderstorms Late in the Day 

The hot and humid weather will also be accompanied by rather 
steep lapse rates in the 850-500 mb layer at around 6.5 to 7 
C/km, which will create an environment expected to be moderately
to strongly unstable. Afternoon CAPEs using a 100-mb mixed 
parcel range around 1500-2500 J/kg with the highest values in 
interior Southern New England. Quite high. Most models show 
capping which should act to suppress convection until the cold 
frontal forcing arrives; that's the biggest question mark is 
that there's still some disagreement as to when the front 
arrives. We anticipate the main threat for severe weather to 
begin after 3 PM in western New England, and particularly in the
5-9 PM timeframe as a line of storms. 

If any isolated storm were to develop on the prefrontal trough in 
the mid to late afternoon, it would likely become strong to severe 
quickly. But the main threat is from a line of strong to severe 
storms moving in from eastern NY after 5 PM with the cold 
front, as indicated by most convection allowing models. Of some 
concern is that flow fields have trended a little stronger (850 
winds now near 50 kt in some models), and bring deep shear 
values to around 40 kt. While the greater threat for severe 
weather is mainly west of I-95/I-495, some threat is possible 
toward sundown in the Boston to Providence corridor, especially 
if convection can get going sooner. 

Damaging wind gusts are the primary hazard, but with a linear 
storm mode and the increased low level windfields, a brief 
tornado embedded in the line could exist too. Street flooding 
could be possible in isolated instances, but the storms should 
be moving along at a good clip. Hail's secondary risk too, but 
the linear storm mode and the hot airmass probably limits this 
potential. Included gusty winds, small hail and heavy downpour 
hazards in enhanced wording, and messaged as scattered 
thunderstorms to bring greater awareness. 

Those with plans outside during the afternoon, especially from 
the Berkshires/Litchfield Hills eastward to Metrowest, will want
to keep close tabs on the forecast for the afternoon.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Key Messages: 

* Seasonably warm but less humid Fri into Sat.

* Growing confidence in a multi-day stretch of heat and
  humidity, starting as soon as Sunday but peaking Mon thru Wed.
  Air temps could reach the mid to upper 90s with humidity
  making it feel even hotter. Heat headlines could be needed.

* Mainly dry, but will need to monitor for possible T-storms. Timing 
  is still uncertain.

Details: 

Quasi-zonal midlevel pattern begins on Fri per ensemble means, but 
is expected to amplify rather significantly for late in the weekend. 
This occurs as a strong upper trough moves into the Pacific 
Northwest states, causes a building heat ridge over the center of 
the country to amplify downstream to anomalously strong levels, with 
the center of this heat ridge moving eastward into the mid-Atlantic 
states. 

As for temperatures, Friday and Saturday are generally seasonably 
warm and less humid. As the upper ridge amplifies and settles over 
the mid-Atlantic region, there is growing confidence in a multi-day 
stretch of high heat and humidity. We may begin to see temperatures 
return into the 90s on Sunday, but the heat peaks in the Mon thru 
Wed timeframe. Latest ensembles continue to point to heat and 
humidity levels at least as high as Thurs, if not higher, and also 
over a stretch of a few days. Heat headlines could be needed at some 
point if confidence continues to increase.

We will have to monitor for embedded shortwave trough "ridge 
rollers" along an active storm track over the northern tier of 
states, rotating around the heat ridge. Some models indicate Sat 
night into Sun as one possible period to watch, but confidence is 
lower on those details. Thunderstorms could be possible if we lie in 
the track of these disturbances but it is too difficult to pinpoint 
at this point in time.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

12Z TAF Update:

Today: Moderate confidence. 

IFR/LIFR will linger through about 15z this morning with slow 
improvement toward MVFR/VFR levels expected after 15z. A line of
showers is gradually moving east this morning, but should
dissipate by the noon hour, otherwise mainly dry this 
afternoon. IFR/LIFR will likely linger across the Cape and 
Islands today with S/SW winds 5-10 kts.

Tonight: Moderate confidence. 

IFR/LIFR stratus and fog sets in by 04z with periods of light 
rain and drizzle. Light SW winds. 

Thursday: Moderate confidence. 

Early IFR-LIFR fog/stratus should disperse by early to mid 
morning. VFR by late morning to early afternoon with hazy 
conditions from very humid air then gives way to a round of SCT 
strong to severe T-storms mainly after 18z. Anticipating a line
of strong to severe t-storms entering western MA/CT gradually 
weakening as they move east. Storms may struggle to reach the 
east coast. Frequent lightning, strong to locally damaging gusts
and IFR/LIFR visby in rain are the main risks from storms. SW 
winds increase to 10-15 kt.

KBOS Terminal...Moderate confidence in TAF. 

IFR/LIFR this morning. An area of showers moving east will help
conditions improve to MVFR by mid morning. MVFR/VFR this
afternoon. IFR/LIFR again tonight with periods of light rain and
drizzle.

KBDL Terminal...Moderate confidence in TAF. 

IFR/LIFR with showers this morning. MVFR/VFR by late morning and
remaining there through the afternoon. IFR/LIFR again tonight
with periods of light rain and drizzle.

Outlook /Friday through Sunday/... 

Friday: VFR. Breezy. 

Friday Night through Saturday: VFR. 

Saturday Night: VFR. Slight chance SHRA.

Sunday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Winds and seas through Wednesday night should remain below small
craft advisory thresholds. S winds around 10-15 kt with seas
around 4 ft or less. Could need small craft advisories on Thurs
at least from building seas to near 5 ft, although SW winds will
become around 20-25 kt.

Foggy conditions this morning should begin to disperse by this
afternoon, but fog is likely to return again tonight. Potential
for strong to severe thunderstorms in the nearshore waters late
Thursday.

Outlook /Friday through Sunday/...

Friday: Winds less than 25 kt. Local rough seas. 

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

Saturday through Sunday: Winds less than 25 kt.

&&

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

&&

$$

SYNOPSIS...Loconto
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...Loconto
AVIATION...KP
MARINE...Loconto