Somerville, Massachusetts 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

794
FXUS61 KBOX 200541
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
141 AM EDT Thu Aug 20 2026

.WHAT HAS CHANGED...
Updated Aviation section.

&&

.KEY MESSAGES...

- Dry conditions through tonight. Unsettled weather returns
  Thursday with periods of showers.

- Another round of unsettled weather possible for this weekend.

&&

.DISCUSSION...
KEY MESSAGE 1...Dry conditions through tonight. Unsettled
weather returns Thursday with periods of showers.

An upper level low slowly tracks across eastern Canada well to
our north today and into Thursday. As it tracks eastward, it
will induce troughing across the region accompanied by increased
SW flow this afternoon. Despite the pattern transition, expect
mainly dry conditions for the rest of the afternoon and tonight.
A weak boundary moves through overnight, but would at worst
case result in an isolated shower tonight.

The higher risk for seeing showers will likely arrive Thursday a
few embedded shortwaves move through the southwesterly flow. A
plume of above above normal advects into the region with ascent
further supported by a 500mb jet. This will support scattered
showers developing from west to east in the afternoon/evening.
An additional wave moves through overnight producing another
push of showers. There is still spread across guidance with
regards to the track of these shortwaves. This will influence
rain probabilities and where the higher risk is.

Some guidance indicates a more southerly track of the waves
with the higher risk for widespread showers further south or
clipping the south coast and Cape/Islands. Across both GFS and
ECMWF ensembles, there are probabilities 50 to 80% for at least
0.01" draped across southern New England with the higher
probabilities for the south coast/Cape and Islands. The second
wave that moves through overnight is further south in track
across most guidance, so the better shot will be south coast,
Cape and Islands getting brushed with a round of showers. With
very marginal instability, mainly in the west and S/SE southern
New England there is a low probability of a few isolated
thunderstorms or embedded convection.

In terms of amounts, global ensembles range amounts anywhere
from a 0.01 to 0.50" with with the greater potential for the
higher end of that range confined south and southeast. Models
like the GFS trend a little drier and track the better forcing
south which has led to lower QPF overall (< 0.25"). On the other
hand, HREF PMM totals indicate potential for higher amounts
0.25-0.75" in spots across the south coast/southeast MA likely
indicating at some convection leading to isolated downpours. So
all in all, no major concern for impacts, but could see periods
of showers later tomorrow into the overnight period.

KEY MESSAGE 2...Another round of unsettled weather possible for
this weekend.

Looking like an extended period with a risk for rainfall from
Friday into Sunday due to a stalled front somewhere along the
coast. The greatest risk for rainfall is towards southeastern
New England. The details are still not certain, since much will
depend on exactly where this front stalls. The most likely
location is across the coastal plain of RI and southeast MA,
which would steer a low pressure right through our region. The
most likely time for that would be sometime Saturday into
Sunday. As humid as we are, it is possible we could see 1-2
inches of total rainfall in spots near the front. Farther
northwest of this front, total amounts are most likely less than
one half inch. Localized thunderstorms are also possible,
especially during the afternoon and evening hours this weekend.

Once this front finally moves offshore some time Monday, the
high pressure that follows should provide dry weather with more
comfortable humidity for the first half of next week.

&&

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

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

Through 12z...High confidence.

Mainly VFR with light WSW winds. Patchy MVFR remains possible
for the Cape, and the Islands will likely remain LIFR/IFR for
much of this period, improving as we get closer to sunrise.

Today...High confidence.

VFR. Scattered showers arrive from west to east after 18-20Z
with a few thunderstorms possible. Confidence not high enough to
include TSRA in TAF, but SHRA included as PROB30 for now.

Tonight...Moderate confidence.

VFR with areas of MVFR to IFR towards the Islands. Scattered
showers continue moving east through the overnight period. These
would most likely linger towards the Cape and Islands, along
with SE MA. Winds may briefly pick up slightly following the
frontal passage, with gusts possibly reaching 15 kt at the most.

Friday...Moderate confidence.

Mostly VFR with MVFR/low-end VFR towards the Cape and Islands.
Guidance has shown a possible return of showers as low pressure
passes to our south and east. These would likely impact more
eastern terminals, including into RI. Onshore NE winds 5-10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR. Chance SHRA.

Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance
SHRA, slight chance TSRA.

Sunday: VFR. Chance SHRA, chance TSRA.

Sunday Night: VFR. Chance SHRA.

Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Thursday Night...High confidence.

Southwest winds 8-12 kts with  gusts up to 20 kts this afternoon.
Light WSW flow overnight with light southerly flow on Thursday. Seas
1-3 ft. Patch fog possible overnight tonight.

Outlook /Friday through Monday/...

Friday through Friday Night: Winds less than 25 kt. Chance of
rain showers.

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Rain showers likely, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Rain showers likely, slight chance of
thunderstorms.

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

Monday: Winds less than 25 kt.

&&

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

&&

$$

DISCUSSION...Belk/Mensch
AVIATION...Hrencecin
MARINE...Belk/Mensch