Severe Thunderstorm Warning 17, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
158 PM EDT Thu Jun 18 2026

.WHAT HAS CHANGED...
Gale Warnings posted for the eastern coastal waters as well as
the MA bays and sounds.

&&

.KEY MESSAGES...

- Gusty southerly winds with some showers and perhaps a few
  afternoon thunderstorms today. Main threat for severe weather
  still looks to be to our west and south, but is possible
  across our region.

- Dry weather Friday and Saturday with gusty west winds. Highs
  mainly between 75 and 85 with comfortable humidity.

- Potential for a much needed soaking rain to kick off the start
  of the next work week, but that is not set in stone.

&&

.DISCUSSION...
KEY MESSAGE 1...Gusty southerly winds with some showers and
perhaps a few afternoon thunderstorms today. Main threat for
severe weather still looks to be to our west and south, but is
possible across our region.

Showers were overspreading our region early this morning ahead
of an approaching warm front. The timing of this warm front
will be critical to our weather today. Besides providing a focus
for thunderstorms, if our region can get into the warm sector of
a low pressure earlier in the day, the potential for some
strong to severe thunderstorms will go up.

Projected wind shear values are impressive, with values not
usually seen across southern New England. Effective bulk shear
of 40-60 kt and 0-1 km storm relative helicity values of 200-400
m2/s2 raise the threat of strong straightline wind damage and
perhaps a tornado. The primary limiting factor will be how much
instability can be generated.

Showers earlier this morning should also mean lingering clouds
for a time. This is where the timing of getting into the warm
sector of a low pressure over southeast Canada becomes
important. Most-unstable CAPE values between 500-1,000 J/kg are
expected. Those are more typical for our region, and don't
really suggest the risk for widespread severe weather. There
have been numerous cases where strong shear can compensate for
weaker instability.

Putting this all together, a level 2 of 5 risk for severe
thunderstorms remains in place across our region today. The
most likely intersection of instability and strong shear remains
across western MA and western CT during the mid afternoon to
early evening. In sum, low confidence that severe weather
materializes but high potential impact if it does. Timing is
generally noon to 9pm (earlier west, later east).

KEY MESSAGE 2...Dry weather Friday and Saturday with gusty west
winds. Highs mainly between 75 and 85 with comfortable humidity.

Drier and less humid weather anticipated for Friday into
Saturday. However, it should still be gusty into Saturday before
gusts diminish some for Sunday. Sunday could be dry for most as
well, but there is a risk for some showers from mid afternoon
into the evening hours.

KEY MESSAGE 3...Potential for a much needed soaking rain to kick
off the start of the next work week, but that is not set in
stone.

The potential continues for a much needed soaking rain to kick
off the start of the next work week. That will depend upon the
track of low pressure, which is not well defined just yet. Even
the timing remains uncertain.

Drier and very pleasant summerlike weather should return Tuesday
into Wednesday as high pressure builds in from the west.
Temperatures look to be rather seasonable with comfortable
humidity by the middle of next week.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Today...Moderate confidence.

A period of MVFR with even localized IFR conditions possible in
areas near the Cape. The main threat for scattered SHRA and
TSRA will be this afternoon and early evening ahead of the cold
front. A few strong to severe TSRA are possible. Main threat
will be between 19-00Z for showers/storms.

A strong LLJ will result in southerly wind gusts of 25 to 35 kt
by afternoon with a few gusts perhaps near 40 kt.
Localized/brief stronger winds possible if any severe TSRA
develop. LLWS expected towards the Cape and islands.

Tonight...High confidence.

Any lingering SHRA/TSRA should depart the coast by 01Z with
conditions quickly improving to VFR thereafter. Lower cigs/vsbys
may linger until midnight or so for parts of the Cape and
Islands. WSW winds 10-15 kts shifting to the W overnight.

Friday...High Confidence.

VFR. Gusty W winds up to 20-30 kt.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Friday Night through Tuesday/...

Saturday through Saturday Night: VFR. Breezy.

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

Sunday Night: VFR. Chance SHRA.

Monday: Mainly VFR, with areas MVFR possible. Breezy. SHRA
likely, chance TSRA.

Monday Night: Mainly VFR, with local MVFR possible. Breezy.
SHRA likely.

Tuesday: VFR. Breezy. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Friday: High confidence.

Gusty southwest winds expected today. Converted the Gale
Watches to Warnings with the expectation of frequent gusts to
35-40 kt. The RI coastal waters are a more complicated
forecast. The potential for the same strong winds are there,
however the boundary layer mixing might not be as efficient.
Continued Small Craft Advisories there, but will need to monitor
conditions closely. Rough seas and gusty winds likely linger
across the outer southern coastal waters into Saturday, but not
confident enough yet to extend the existing advisories.

Outlook /Friday Night through Tuesday/...

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.

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

Monday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, slight chance of thunderstorms.

Monday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 9 ft. Rain showers likely,
slight chance of thunderstorms.

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning until 7 PM EDT this evening for ANZ230-251.
     Gale Warning until 10 PM EDT this evening for ANZ231>234-250-
     280.
     Small Craft Advisory until 8 PM EDT Friday for ANZ235-237-
     254>256.
     Small Craft Advisory until 7 PM EDT this evening for ANZ236.

&&

DISCUSSION...Belk/Frank
AVIATION...Mensch
MARINE...Belk/Frank