Cambridge, Massachusetts 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

109
FXUS61 KBOX 130656
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
256 AM EDT Wed Aug 13 2025

.SYNOPSIS...
Patchy fog this morning gives way to another hot and humid day
on Wednesday. An approaching cold front will bring a chance for
showers and thunderstorms later Wednesday afternoon and evening.
Additional showers and thunderstorms are possible into
Thursday along with a low risk for excessive rainfall and urban
flash flooding. Seasonable temperatures and lower humidity
levels are expected for Friday and Saturday, although
temperatures briefly warm up again for Sunday. After midweek,
our next chance for rain is with a cold front which looks to
move in on Monday.

&&

.NEAR TERM /UNTIL 1 PM THIS AFTERNOON/...
Near Term

Wednesday

Key Messages:

* Increasing Heat and Humidity Today

* Thunderstorm chances increase early to mid afternoon, mainly west
  of I-95

Heat and humidity peaks this afternoon as southwest flow ahead of an
approaching cold front will support dewpoints increasing into the
upper 60s to low 70s across southern New England today. The air mass
in place remains unusually warm for mid-August with 925 hPa
temps in the low 20s Celsius. This will support another day with
afternoon high temperatures in the upper 80s to low 90s. Since
we anticipate higher dewpoints today, we can expect higher head
indices as well in the mid to upper 90s.

As the aforementioned cold front approaches from the west, southern
New England will see increasing chances for showers and
thunderstorms during the early to mid afternoon hours. Latest HREF
model run (00Z) and its associated members support thunderstorm
activity as far east as Boston. Confidence in showers and storms is
highest west of Worcester, where forcing for ascent should arrive in
time to take advantage of diurnal instability. The last several runs
of the HREF model dating back to 00Z Tuesday, have trended toward a
faster arrival of the cold front which has resulted in thunderstorm
chances gradually increasing further east. With southwest flow
supporting a surge in PWATs to 2+ inches, any storms that do develop
will have the potential to support heavy downpours. Severe
thunderstorm potential is very low since both mid-level lapse rates
and deep layer wind shear will be very weak. Nonetheless, we
can`t rule out an isolated strong storm as precip loading, steep
low-level lapse rates, and high surface based instability could
result in isolated instances of strong downburst winds.

&&

.SHORT TERM /1 PM THIS AFTERNOON THROUGH 6 PM THURSDAY/...
Key Messages

* Thunderstorm activity winds down overnight, though some showers
  linger.

* Heat and humidity persist into Thursday.

* A cold front crosses Thursday afternoon, bringing another round of
  showers and storms.

Tonight

Any thunderstorm activity this evening will diminish after
sunset as daytime instability fades. A few lingering showers may
persist overnight as an upper-level trough axis and the associated
surface cold front inch closer from the west. Expect a warm and
muggy night, with lows in the upper 60s to low 70s. Patchy fog is
possible along the south coast.

Thursday

A surface cold front will gradually sweep from northwest to
southeast Thursday afternoon. The day will be unsettled, with mostly
cloudy skies and periods of scattered showers and thunderstorms.
Isolated to scattered showers are possible during the morning, with
activity possibly becoming more active by the early to mid-afternoon
hours.

Instability and deep moisture will support thunderstorms capable of
heavy downpours, raising the risk for localized urban and poor-
drainage flooding. HREF probabilities for 6-hour rainfall exceeding
10-year ARIs are notably high, 1540%, across eastern and
southeastern MA into RI. While confidence in excessive rainfall is
moderate at this range (~36 hours out), it could increase later
today; if so, a Flood Watch may be considered for urban flash flood
potential.

Despite the flood risk, severe thunderstorm potential remains low
due to weak mid-level lapse rates and insufficient wind shear to
support organized convection.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...

Key Messages:

* Cooler with lower humidity levels Fri/Sat with drier weather.

* Warming trend with rising humidity levels Sun precedes a cold
  front on Mon.

* Trending cooler/drier during the middle of next week

Friday and Saturday

High pressure builds in behind Thursday`s
cold front and brings much more comfortable weather to southern
New England on Friday and Saturday. A fairly strong onshore
flow on Friday will support much cooler temperatures along the
east coast in the mid to upper 70s. The interior will be
substantially warmer in the low to mid 80s. After several muggy
evenings this week, Friday night will feature very comfortable
sleeping temperatures with lows in the upper 50s to low 60s.
This will be followed by a seasonable day on Saturday with high
temperatures ranging from the low to upper 80s. Expect abundant
sunshine on both Friday and Saturday.

Sunday and Monday

Steady southwest flow redevelops Saturday night into Sunday and
will support another surge in both temperatures and dewpoints
across southern New England. High temperatures in the mid to
upper 80s and and dewpoints in the mid to upper 60s will likely
support heat indices in the low 90s across much of the region
Sunday afternoon. This bout of heat and humidity will be short-
lived as another frontal boundary is expected to push through
the region during the Sunday night/Monday time frame. This would
be accompanied by another opportunity for showers and
thunderstorms across the region, but details are vague at this
time range.

Next Week

As the aforementioned cold front crosses our area, southern New
England is likely to experience more fall like weather as
probabilities for high temperatures exceeding 80 degrees Mon-
Wednesday are less than 50 percent, per latest long-term
ensemble forecast guidance. Stay tuned for further details.
&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

06Z TAF Update:

Through 12Z...High Confidence

Mainly VFR with light winds. Some low-stratus fog may support
periods of IFR/LIFR at Cape/Islands terminals through sunrise.

Today...High Confidence

VFR. Any IFR/LIFR near the south coast should lift by 13-14Z.
Chance for thunderstorms after 18Z, mainly over the western
terminals, though there is also at chance at both BED and BOS.

Tonight...Moderate Confidence

Any thunderstorm activity will wind down after sunset. A few
showers may linger. Ceilings fall to IFR/MVFR mainly south of
I-90.

Tomorrow...Moderate Confidence

Mainly VFR. Some MVFR over the south coast, Cape, and Islands.
Periods of scattered showers with embedded thunderstorms during
the afternoon.

KBOS terminal...High confidence in TAF. Lower confidence in TSRA
chances, but would likely be in the 20-00Z time frame at
BED/BOS.

KBDL terminal...High confidence in TAF. Moderate confidence in
TSRA at BDL after 18Z.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Breezy. Chance SHRA.

Friday: VFR. Breezy.

Friday Night through Saturday Night: VFR.

Sunday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

High confidence through the period.

Winds increase out of the southwest to 10 to 15 knots this
afternoon with a slight chance for an afternoon/evening
thunderstorm. Storms should diminish shortly after moving out
over the waters if they make it that far.

Seas remain in the 2-3 foot range. A cold front will cross the
coastal waters late tomorrow afternoon resulting in a wind shift
to the northwest. Overall quiet conditions over the marine
zones for the foreseeable future.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Chance of rain showers,
slight chance of thunderstorms.

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

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for Ctz003.
MA...None.
RI...None.
MARINE...None.

&&

$$

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