Waltham, Massachusetts 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

975
FXUS61 KBOX 110600
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
200 AM EDT Mon Aug 11 2025

.SYNOPSIS...
High pressure off the coast will bring increasing heat and humidity
through mid week, peaking on Wednesday, but it will not be as
oppressive as it was earlier in the summer. Scattered showers
and thunderstorms are expected ahead of a cold front later
Wednesday and Thursday, then dry and cooler weather follows
Friday into the weekend, but heat could return Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Updated 2 AM Mon

Key Message:

* Hot and becoming more humid today but probably just shy of
  reaching Heat Advisory criteria this week.

Upper ridge continues to build over region today setting the
stage for hot and somewhat humid weather this week. SW flow gets
underway today with high pressure becoming anchored offshore,
but gradient remains weak enough to allow for sea breezes and
cooling along both coasts this afternoon.

Away from coast, dewpoints climbing into low to mid 60s will
produce heat indices in low to mid 90s this afternoon,
especially in the usual "hot spots" including the Merrimack
Valley in NE MA into Metrowest and the southwest suburbs of
Boston, as well as northern RI and portions of CT River Valley.
Expecting to come close to Heat Advisory criteria (heat index of
95-100F for 2 or more hours) in these areas but we think we`ll
fall short of the 2 consecutive day threshold as detailed below.

For tonight, another mainly clear night ahead with patchy fog
possible near Cape Cod/Islands and perhaps inland as well.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
Updated 2 AM Mon

Key Messages:

* Heat and humidity peak Wed but still short of Heat Advisory
  criteria for 2 consecutive days.
* Afternoon thunderstorms possible Wed.

Upper ridge dominates through midweek maintaining heat and
humidity across region. However, forecast soundings on Tue
indicate fairly deep mixing which should bring down drier air
aloft, likely lowering afternoon dewpoints into upper 50s and
lower 60s, thereby reducing heat index Tue afternoon and keeping
us from reaching 95-100F heat index for 2 consecutive days.
Despite that, it will be hot and somewhat humid, although not to
extent as what we experienced earlier in the summer.

Wednesday looks to be the peak of the heat and humidity as
increasing SW flow around periphery of high pressure offshore
brings higher PWAT values into SNE. Dewpoints should climb into
lower 70s which will result in heat indices reaching into mid to
upper 90s across more of interior. Low level flow looks to be
strong enough to prevent sea breezes along E MA coast.

Another complicating factor deals with uncertainty Wed with
extent of cloud cover from upstream convection across Great
Lakes and potential redevelopment of convection in SNE Wed
afternoon as short wave moves through Northeast. This is just
getting into window for higher-res models but both 12km NAM and
RRFS are fairly aggressive in bringing organized storms into SNE
during afternoon or early evening. Early look at severe
parameters are not overly impressive and ML guidance doesn`t
suggest any organized severe potential, but with a good amount
of instability a few severe storms cannot be ruled out despite
weak 0-6km shear and mid level lapse rates.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Updated 2 AM Mon

Key Messages:

* Low risk of severe storms Thu as cold front moves through.
* Cooler Fri/Sat but heat may return Sun.

Depending on how things evolve Wed with respect to convection,
including cloud debris which could limit heating Thu,
approaching cold front may bring a round of severe storms Thu
but right now odds seem to be fairly low due to timing
differences, even in ML guidance. Favored areas right now
include eastern MA and RI where heating may be maximized but
confidence is low on exactly where that will be.

Front moves offshore early Fri leading to drier and cooler
weather Fri and Sat as upper ridge becomes suppressed, but ridge
tries to reassert itself once again later in weekend, probably
leading to a return of heat and humidity Sun and into next week.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

06z TAF Update: High confidence.

VFR, aside from local MVFR/IFR vsbys in patchy fog across
interior valleys and near Cape Cod and Islands early this
morning and possibly again later tonight into early Tue
morning. Light S/SW winds with sea breezes along both coasts
today and Tue.

KBOS terminal...High confidence in TAF.

KBDL terminal...High confidence in TAF.

Outlook /Wednesday Night through Friday/...

Wednesday Night: VFR. Breezy. Chance SHRA, slight chance TSRA.

Thursday: VFR. Chance SHRA, slight chance TSRA.

Thursday Night: VFR. Slight chance SHRA.

Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Updated 2 AM Mon

High confidence through the period.

Weak gradient will allow for sea breezes near shore today and
Tue before high pressure offshore brings increasing SW flow
thereafter, though winds and seas should remain below SCA
levels. Patchy fog may limit visibility later at night and early
in morning for much of this week.

Outlook /Wednesday Night through Friday/...

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Chance of rain showers, slight chance of
thunderstorms.

Thursday: Winds less than 25 kt. Chance of rain showers.

Thursday Night: Winds less than 25 kt. Slight chance of rain
showers.

Friday: Winds less than 25 kt.

&&

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

&&

$$

SYNOPSIS...JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...JWD
AVIATION...JWD
MARINE...JWD