708
FXUS61 KBOX 190013
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
813 PM EDT Wed Jun 18 2025

.SYNOPSIS...
Overcast skies and muggy conditions persist through the rest of
the day with lingering showers pushing across central MA today.
A Heat Advisory is in effect for Thursday with heat indices
ranging from 97 to near 105. There is also 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 THURSDAY/...
Key Messages:

* Overcast conditions remain through the evening with a quick
  round of scattered showers across western MA.

* Muggy again tonight with lows in the upper 60s to low 70s and
  temperatures near 70.

Details:

Another round of scattered showers is moving into western
MA/northwest CT moving NE this afternoon associated with another
weak disturbance in the SW flow aloft. Should only result in a
quick shower... not expecting any lightning/high winds/etc.
Muggy conditions continue through the afternoon and evening,
with little relief expected over the next day or so.

High humidity continues tonight as dewpoints remain in the high
60s, possibly even into the low 70s across the interior,
meaning dewpoint spreads will only be around 1-2 degrees at
most. Broad SW flow continues aloft while moist southerly flow
continues in the lower levels. This is the primary driver behind
the overcast stratus, and will also lead to a wider area of
patchy fog tonight... especially along southern RI, the South
Shore, Cape and Islands.

&&

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

* Heat Advisory in effect for tomorrow with temperatures feeling
  like 95-105.

* Scattered strong to severe thunderstorms as soon as 3 PM, but
  more likely between 5 to 9 PM. Damaging straight line winds,
  torrential downpours, and frequent lightning are the main
  concerns.

* Much less humid Thursday night and drying out.

Thursday:

Stratus and fog likely lingers through mid-morning, but should
scatter out/dissipate quickly once the winds start picking up.
Thursday still looks to be quite active, as the eroding of the
cloud cover will lead to a hazy, hot and humid late morning to
midafternoon. A strong cold front associated with a rather
strong frontal system for late-June will also lead to
potentially strong to severe thunderstorms Thursday afternoon.

1... Heat and Humidity...

Without question, Thursday stands to be one of the hottest days
in southern New England up to this point in the summer, and it
will only feel worse with high humidity levels. 850mb
temperatures warm to around +17C-20C on increasing southwesterly
flow. High temperatures should soar into the mid 80s across the
Cape and Islands and into the low to mid 90s across the rest of
the CWA. With the prolonged moist, southerly flow... dewpoints
will hover in the low 70s, making it feel like 97-105 degrees
outside in the CT and Merrimack Valleys as well as the I-95
corridor. It will likely feel like 95-100 degrees across a wider
portion of Southern New England (excluding the south coast/Cape
Cod where it may feel closer to 90 degrees). Seabreezes are not
expected as southwesterly surface winds pick up to around 10-15
mph. A Heat Advisory is in effect, and we encourage those to
take appropriate heat-related precautions such as taking
frequent breaks in shaded and air conditioned areas, remaining
hydrated, and checking in on those sensitive to heat such as the
elderly and pets.

The high heat and humidity is not expected to break until the
cold front moves across the area later Thursday evening.

2... Strong to Severe Thunderstorms Late in the Day...

Guidance is indicating 850-500mb lapse rates will be quite
steep, around 8-10 C/km, creating an environment that`s
moderately to strongly unstable. Afternoon MLCAPE values range
around 1500-2500 J/kg, with the highest values in interior
Southern New England. Most guidance indicates a moderate amount
of capping, which should act to suppress convection until the
forcing arrives as a cold front arrives... which leads to one of
the biggest questions in the forecast: there is still some
disagreement as to when the front arrives. We anticipate the
main threat for severe weather to begin after 3 PM in western
New England, and particularly in the 5-9 PM timeframe as a line
of storms.

If any isolated storm were to develop on the prefrontal trough
ahead of the main line of storms in the mid to late afternoon,
it would likely become strong to severe quickly. However, most
guidance (including high-resolution and convection allowing
models) are indicating the main threat forming a line of strong
to severe storms moving in from eastern NY after 5 PM with the
cold front. Of some concern is that flow fields have trended a
little stronger (850 winds now near 50 kt in some models), and
bring deep shear values to around 40 kt. We have high confidence
that the greatest threat for severe weather is west of
I-95/I-495, but some threat is possible towards sunset in the
I-95 corridor, especially if convection can get going earlier in
the afternoon.

Damaging wind gusts continue to be the primary hazard, but with
a linear storm mode and the increased low level windfields, a
brief tornado embedded in the line could exist too. Street
flooding could be possible in isolated instances, but the storms
should be moving along at a good clip. Hail is a secondary risk
as well, but the linear storm mode and the hot airmass might
limit hail growth potential. Included gusty winds, small hail
and heavy downpour hazards in enhanced wording, and messaged as
scattered thunderstorms to bring greater awareness.

Those with plans outside during the afternoon, especially from
the Berkshires/Litchfield Hills eastward to Metrowest, will want
to keep close tabs on the forecast for the afternoon.

&&

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

* Cooler and windy Friday with gusts 25-40 mph.

* Mainly dry Friday. More unsettled for Saturday with shower
  chances.

* Signal for well above normal temperatures early next week with
  potential to reach dangerous/impactful levels.

Friday-Weekend:

Mid level troughing persists over the region Friday with NW
flow aloft. Subsident flow should bring in drier air in place
for Friday which will keep conditions dry for Friday. A 40-55 kt
LLJ positions across southern New England. Model soundings
shows sufficient mixing a little below 850mb which should help
bring down gusty W-NW winds. The ECMWF ensemble EFI shows a
signal for anomalous wind gusts with the stronger signal more
confined to the higher terrain of New England. This means we are
likely to have a gusty day, with lower potential for higher
impact winds for southern New England. Ensemble guidance show
gusts in the 25-40 mph range with the higher end of that range
over the higher elevations (east slope of Berkshires and
Worcester Hills). Temperatures trend lower than Thursday in the
cooler post-frontal airmass with highs in the upper 70s to mid
80s.

Saturday, a mid-level ridge approaches from the west. A weak
piece of shortwave energy rounds the ridge dropping across the
region later Saturday-Saturday night. Combined with higher
moisture, this should bring a more unsettled day. Models show a
weak signal for precipitation with differences with respect to
coverage and timing. Most guidance keeps the higher chances for
scattered showers/few storms for later Saturday. Can`t rule out
a spot shower during the day or even a storm given the marginal
instability (MLCAPE < 500 J/kg).

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 moisture
transporting into the region over the top of the upper ridge.
This will help bring in higher moisture air which mean higher
humidity. Another element will be warm temperatures overnight
providing little relief from the hot daytime temperatures.
Together, these elements will raise the risk for heat-related
impacts to those in southern New England.

Rising heat and humidity begins Sunday extending through early
next week. NWS Heat Risk is quite reflective of the rising
temperatures and humidity with values increasing into the
Moderate Category Sunday and into the High Category
Monday/Tuesday for most places. Even the Cape/Islands enter the
Moderate Category by Tuesday. What this means is that early next
week will have a high risk for heat related impacts, especially
to heat sensitive people, urban populations, and those who lack
cooling sources. 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 /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight: High confidence.

Areas of SCT decks early this evening becoming MVFR-IFR after
00z. IFR/LIFR stratus and fog sets in between 02z-06z with
periods of light rain and drizzle especially over the Cape and
Islands. Light SW winds.

Thursday: Moderate confidence.

IFR/LIFR Fog and stratus should be quicker to burn off
tomorrow, generally dissipating between 13-16z. The Cape and
Islands will be slower to improve where IFR conditions will
persist. VFR by the afternoon for most place. Bit trickier for
the Cape/Islands on what will prevail. It`s possible it may
bounce between VFR or IFR/LIFR if it goes between SCT-BKN during
the day.

SW winds 10-15 kts with gusts 20-30 knots. Thunderstorms form
in Western MA around 21-23z and move east through about 02-04z.
Greatest area of uncertainty is between ORH and BOS where
Thunderstorms may not quite make it to the eastern sea board.

Thursday Night: Moderate Confidence.

Winds turn west with the passage of a cold front, allowing CIGS
to remain VFR with the exception of the Cape and Islands. Cold
front may not clear there until Friday morning finally allowing
CIGS to improve to VFR.

KBOS Terminal...Moderate confidence in TAF.

MVFR re-develops 00-02z. IFR stratus and fog should move in
around midnight, with LIFR CIGS between 08z-12z. CIGS should
improve to VFR quickly after 15z tomorrow with gusty SW winds
around 20-25 knots. Thunderstorms may reach the terminal
around/after 00z Friday, but more likely to occur out west. Due
to lower confidence included as a PROB30 -TSRA.

KBDL Terminal...Moderate confidence in TAF.

MVFR for the evening. IFR/LIFR status and fog move in after
midnight tonight. Should see clearing to VFR by 15z tomorrow,
then chance for thunderstorms late tomorrow afternoon/early
evening

Outlook /Friday through Monday/...

Saturday: VFR.

Saturday Night through Sunday: VFR. Breezy. Chance SHRA, slight
chance TSRA.

Sunday Night: VFR.

Monday through Monday Night:

Tuesday: Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Winds and seas through Wednesday night should remain below small
craft advisory thresholds. S winds around 10-15 kt with seas
around 4 ft or less. Small craft advisory in effect for Thurs
and Fri both from building seas near 5 ft and SW winds around
25-30 kt.

Foggy conditions again tonight. Potential for strong to severe
thunderstorms in the nearshore waters late Thursday evening into
the overnight period.

Outlook /Friday through Monday/...

Saturday: Winds less than 25 kt.

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

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Slight chance of rain showers.

Sunday Night through Monday: Winds less than 25 kt. Seas
locally approaching 5 ft.

Monday Night through Tuesday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory from 11 AM to 8 PM EDT Thursday for CTZ002>004.
MA...Heat Advisory from 11 AM to 8 PM EDT Thursday for MAZ003>007-
     010>019-026.
RI...Heat Advisory from 11 AM to 8 PM EDT Thursday for RIZ001-002.
MARINE...Small Craft Advisory from 4 PM Thursday to 8 AM EDT Friday for
     ANZ250-254>256.

&&

$$

SYNOPSIS...McMinn/Mensch
NEAR TERM...McMinn
SHORT TERM...McMinn
LONG TERM...Mensch
AVIATION...Mensch/KP
MARINE...McMinn/KP