911
FXUS61 KBOX 171839
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
239 PM EDT Tue Jun 17 2025

.SYNOPSIS...
Mostly cloudy through Wednesday as a couple of weak disturbances
bring a threat for hit-or-miss showers, although with several
hours of dry weather in between. Hot and humid weather for
Thursday with heat indices 95 to near 100, with a risk for
strong to severe thunderstorms Thursday afternoon as a cold
front moves through. Drier and seasonably warm Friday and
Saturday. A multi-day stretch of significant heat is possible
starting early next week.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
Key Messages:

* Near to slightly above normal temperatures.

* Hit-or-miss showers for most tonight into mid morning
  Wednesday, with a greater risk across the western half of
  southern New England.

The main feature of our weather tonight into Wednesday will be
the approach of a warm front from the southwest. Until this
front actually moves through, a high pressure over the North
Atlantic will be the main forcing.

Lots of clouds with relatively chilly conditions into this
evening. A mid level shortwave passing overhead will trigger
some light drizzle or light rain showers into this evening. The
greater risk for showers will be with the warm front itself.
That front should move of to our north later Wednesday morning.
Not expecting much in terms of accumulation, with most locations
at one tenth inch or less, where it rains at all.

Once we get into the warm sector, looking at temperatures
starting to rise to near to above normal levels. Thinking by
that point that most areas will be dry heading into Wednesday
night. There is still a low risk for an afternoon or evening
shower or thunderstorm just due to the increased heat and
humidity. These should dissipate after sunset Wednesday.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
Key Messages:

* Potentially dangerous heat Thursday afternoon with areas
  across southern New England experiencing "feels like"
  temperature between the upper 90s into the lower 100s.

* A cold front brings scattered severe thunderstorms to interior
  Massachusetts and Connecticut. There is uncertainty on just
  how far east this threat will extend.

A true taste of summer is on tap for Thursday with temperatures
and humidity on the rise. Plus, a cold front brings potential
for severe thunderstorms across the western-half of southern New
England during the afternoon and evening hours. Let`s breakdown
these hazards.

As advertised, Thursday is peak of the heat and humidity (at
least for this week) and could be dangerous for sensitive
groups. Forecast 850mb temperatures are +19C to +21C. Will be
able to tap into those temperatures with a well-mixed boundary
layer, resulting afternoon temperatures from the lower to middle
90s across interior southern New England. It is a touch cooler
at the south coast where southwest flow coming off the
relatively cooler ocean water, likely highs are in the 80s.
Additionally, it will be humid, dewpoints in the lower 70s is
"air you can wear", apparent temperature range from mid-90s
through the lower 100s.

It`s not out of the question a Heat Advisory may become needed
for parts of southern New England, as there are several hours
during the afternoon with apparent temperatures greater than
100F, which meets the criteria for a single day Heat Advisory.
NWS heat risk shows most of the region with a moderate risk and
major risk for City of Boston and the metro west. Those
sensitive groups or those working outdoors need to practice
appropriate heat-related precautions.

Lastly, there is little relief overnight as many locations fall
into the middle 60s. Urban areas hold the heat with lows in the
upper 60s to perhaps 70F.

The next hazard is the severe threat Thursday afternoon through
the evening hours, roughly 2pm to 10pm timeframe, with an
approaching cold front. There is uncertainty on just how far
east this threat will extend. However, there is certainly enough
energy to get storms going, CAPE values range between 2,000 and
3,000 J/kg depending on which guidance source. There are
favorable mid-level lapse rates of ~7C/km (which continues past
sunset timeframe, this could lead to storms surviving all the
way to the coast), deep-layer shear of 35+ knots, and helicity
of 100 to 200 units.

High-res guidance, NAM3KM and RRFS, illustrate a few discreet
cells developing and then a discontinued line of storms. Primary
concerns are stright-line damaging wind and large hail, a
secondary and lower risk would be a brief tornado and localized
flash flooding. Most of southern New England is highlighted in a
"marginal risk" (level 1 out of 5), but far western
Massachusetts and northwest Connecticut is under a "slight risk"
(level 2 out of 5). Those with outdoor plans Thursday afternoon
and evening should remain weather aware and prepare to seek
shelter indoors in the event of a thunderstorm.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Key Messages:

* Cooler Friday and Saturday with temperatures around-slightly
  above normal.

* Mainly dry Friday. Potentially unsettled Saturday with low
  chances for a few showers or storms.

* Signal for well above normal temperatures next week,
  potentially impactful given higher humidity levels.

Friday-Weekend:

Weak mid level troughing persists over the region Friday and
much of Saturday. Pattern is otherwise quiet with no shortwaves
and limited moisture which will support mainly dry conditions.
Cooler post- frontal airmass expected with 850mb temperatures
5-8 degrees cooler than Thursday. This will yield highs around
normal to just above normal in the upper 70s to mid 80s
Friday/Saturday. By Saturday afternoon, a weak piece of
shortwave energy moves through the flow combined with the
arrival of anomalous moisture. Models show marginal instability
around as well. Models show a weak signal for precipitation with
differences with respect to coverage and timing. This keeps
chances on the lower side until there is more agreement among
model/ensemble guidance on if it will stay drier or wetter.
Can`t rule out a few storms given the marginal instability if it
does lean more active Saturday.

Sunday through Early Next Week:

Ensemble guidance shows good agreement that an anomalously
strong upper level ridge shifts into the northeast by early next
week. Unsurprisingly, this is coupled with a signal for 850mb
temperatures a couple standard deviations above normal. NAEFS,
ECMWF EFI also displays a strong signal for well above normal
heat. Ensemble guidance also shows a stream of above normal
moisture transporting into the region over the top of the upper
ridge. This will help bring in higher moisture air which mean
higher humidity. This will provide an additional factor on top
of the heat that will increase the risk for heat impacts. Rising
heat and humidity begin Sunday extending through at least early
next week. NWS Heat Risk, displays this potential clearly with
values increasing into the Moderate Category Sunday and into the
High Category Monday for most places except the south shore and
Cape/Islands.This is still further out, so stay tuned as
confidence increases in this signal. At this point,
awareness/preparedness are beneficial given the potential for
impactful heat/humidity next week. Find/have a location that
provides cooling/AC and plan more strenuous activities outside
of the hotter parts of the day.

&&

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

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

Through Tonight: Moderate confidence.

Conditions likely trend lower tonight as the cooling boundary
layer. We expect most locations to see IFR conditions with
localized LIFR in spots. Spotty drizzle or light showers
possible. Light S winds.

Wednesday: High confidence in trends but moderate on exact
timing.

IFR stratus from overnight scatters/lifts to BKN VFR bases,
with the bulk of the showers ending early in the day. Risk for
afternoon showers or thunderstorms. S/SW winds 5-10 kt.

Wednesday Night: Moderate confidence.

VFR with areas of MVFR in heavier SHRA/TSRA away from the south
coast, where IFR should persist. MVFR and IFR expected to spread
north overnight in stratus and areas of fog.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Friday through Sunday/...

Friday: VFR. Breezy.

Friday Night: VFR.

Saturday: VFR. Breezy.

Saturday Night: VFR. Breezy. Slight chance SHRA, slight chance
TSRA.

Sunday: Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Relatively light winds and seas through Wednesday night.
Periods of showers possible through Wed, but risk for
thunderstorms is low. Areas of fog with visibility less than 1
mile possible tonight and Wed night.

Outlook /Friday through Sunday/...

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

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

Saturday through Sunday: Winds less than 25 kt.

&&

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

&&

$$

SYNOPSIS...Belk/Dooley/Mensch
NEAR TERM...Belk
SHORT TERM...Dooley
LONG TERM...Mensch
AVIATION...Belk/Dooley/Mensch
MARINE...Belk/Dooley/Mensch