Arlington, Massachusetts 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

988
FXUS61 KBOX 200759
AFDBOX

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

.WHAT HAS CHANGED...
Increased forecast QPF slightly for later today.

&&

.KEY MESSAGES...

- Unsettled weather returns today with a frontal passage and
  periods of showers

- Another round of unsettled weather possible for this weekend.

&&

.DISCUSSION...
KEY MESSAGE 1...Unsettled weather returns today with a frontal
passage and periods of showers.

Shortwave disturbances track towards southern New England within SW
flow aloft today. The jet at around 500 mb picks up more towards the
afternoon and, given its placement, provides better ascent. A plume
of above-normal moisture will also advect into the region, with some
guidance having PWAT values up to around 1.7" by the afternoon and
evening hours. With all of these factors together, scattered showers
are expected to move through the region later today into tonight.

Despite this being the day of the event, there are still some
discrepancies between the guidance regarding rainfall totals. The
general consensus is that a front passes through the region late
today into tonight, and with it, scattered showers. Aside from the
GFS, more models have come to agree the higher rainfall totals in
our area will be towards the south coast onto Cape Cod and the
Islands. Totals there range from 0.25" to around an inch, with some
outliers from the CAMs that exceed that. The HREF PMM even has 24
hour totals along the south coast up to 2". This is all most likely
due to these models (and the HREF`s members) picking up on the
possible development of showers with more convective
characteristics. HREF probabilities, though, do keep the higher
chances for 1" an hour rates out of southern New England. GFS-based
guidance remains much drier than the other model suites, keeping any
appreciable totals off to our southwest. ECMWF and Canadian
ensembles, though, both agree on around 20 to 50 percent
probabilities for totals to reach or exceed 0.5" towards the south
coast into RI, SE MA, and the Cape, with probes up to 60 percent
towards the Islands in the ECMWF guidance. NAEFS guidance also
tracks the anomalous moisture plume into these same areas, so
rainfall totals reaching at least 0.5" are not out of the question.
Elsewhere in southern New England, totals will be much lower given
the drier air filtering in there.

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

With the drier air coming in behind the front, the interior likely
dries out a bit from any showers going into Friday. Weak low
pressure off the coast of Long Island could track more NE and lead
to scattered showers for SE MA, RI, and the south coast onto Cape
Cod with cyclonic flow keeping conditions moist and
unsettled.

Guidance continues to show the front stalling out somewhere off the
east coast; a much larger shortwave moves in from the Great Lakes
Saturday into Sunday, greatly amplifying the SW flow aloft as well.
Discrepancies regarding the front`s placement do remain, but the
most likely track at this time has low pressure tracking right over
southern New England. This would lead to more widespread rainfall
across the region between Saturday and Sunday. NAEFS and other
ensemble guidance once again has us under more anomalous moisture,
so totals that reach around an inch or two remain within the realm
of possibility. Depending on the timing of these waves and low
pressure, thunderstorms could be possible if they can take advantage
of daytime heat.

The front looks to move offshore sometime Monday, allowing for high
pressure to develop in its wake and bring drier weather overall.

&&

.AVIATION /07Z 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 Night through Monday/...

Friday Night: VFR. Chance SHRA, patchy BR.

Saturday: Mainly MVFR, with areas IFR possible. SHRA, isolated
TSRA.

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

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

Sunday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, slight chance TSRA.

Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Friday...High confidence.

Light southerly flow today with E flow possible in the afternoon
over the eastern waters. Winds shift more N, then E following
the frontal passage Thursday night, generally to around 15 kt.
Gusts could approach 20 kt over the southern waters heading into
Friday morning. E to NE winds continue into Friday night. Seas
1-3 ft. Showers possible this evening through Friday.

Outlook /Friday Night through Monday/...

Friday Night: Winds less than 25 kt. Chance of rain showers,
patchy fog.

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Rain showers, slight chance of thunderstorms. Local visibility
1 to 3 nm.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Rain
showers likely, slight chance of thunderstorms.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers, slight chance of thunderstorms.

Sunday Night: Winds less than 25 kt. 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...Hrencecin
AVIATION...Hrencecin
MARINE...Hrencecin