550
FXUS61 KBOX 210644
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
244 AM EDT Sat Jun 21 2025

.SYNOPSIS...
Dry and very pleasant weather tonight and Saturday as a ridge of
high pressure builds into the region. This may be followed by a
potential round of scattered showers and thunderstorms late
Saturday night into Sunday morning, but that activity may remain
to our north. Regardless, the main story will be extreme heat
and humidity beginning Sunday afternoon and continuing through
at least Tuesday. We should see the heat ease up during the
latter half of next week.

&&

.NEAR TERM /THROUGH TODAY/...
Key Messages...

* Winds quickly diminish this evening
* Dry/pleasant tonight with lows mainly in the 55-65 degree range
* Mostly sunny & Warm Sat with highs well into the 80s

Forecast remained on track this evening. Winds were quickly
diminishing this evening. Mainly just some minor tweaks over the
next few hours to better match up with observed trends.

Previous Discussion...

Tonight...

Strong low pressure for late June standards in the Canadian
Maritimes will continue to lift off to the northeast tonight. This
as a weak ridge of high pressure builds in form the west. These two
things coupled with the loss of diurnal heating will allow for the
strong winds to quickly diminish this evening. Airmass in place is
relatively dry too...so with mainly clear skies expect a decent
night of radiational cooling. Low temps will bottom out in the
middle to upper 50s across the outlying locations...while the urban
centers drop into the lower to middle 60s.

Saturday...

Height fields begin to rise on Sat as the shortwave trough shifts
east of the region and mid level ridging begins to build in from the
west. This will result in plenty of sunshine Sat with a warm
afternoon. Highs should top off well into the 80s in most locations.
Dewpoints will begin to climb back into the upper 50s to the lower
60s...so much more of a summer feel than today but nothing out of
the ordinary for late June.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM SUNDAY/...
Key Messages...

* A round of scattered showers & t-storms possible mainly late Sat
  night into first part of Sun...but activity may remain to our north

* Heat & Humidity begins Sun afternoon with highs in the 90s and
  Heat Index Values may reach around 100 degrees

* Extreme Heat watch issued for much of the region beginning Sun
  afternoon for a prolonged stretch of dangerous heat & humidity

Details...

Saturday night and Sunday...

The main concern will be for the potential of an MCS dropping down
on northwest flow aloft with the focus for that late Sat night into
the first part of Sun. This will be a classic setup of northwest
flow in new England as upper level ridging builds into the
Ohio/Tennessee valley. In response...a plains like elevated mixed
layer with 700-500 mb lapse rates on the order of 7-8 C/KM. Surface
instability will not be fully in place yet though...so thinking is
we may have a complex of showers & t-storms drop down and impact
portions of the region with the focus late Sat night into Sun
morning. Greatest risk will be across north/northeast MA on the edge
of the building mid level ridge/cap. However...activity could end
impacting much of our region or stay to our north so this remains
uncertain. If we are impacted by this MCS...thinking is that a lot
of the storms might be elevated considering that the surface
instability has not yet arrived. Nonetheless...given an EML in place
there is at least the low risk for severe weather/wind damage so
something to watch. And the machine learning guidance is indicating
at least low risk. The severe weather risk would increase if we can
get the convective potential persisting into the afternoon.

Otherwise...the main story will be the onset of the heat and
humidity on Sun. There is some uncertainty as to the potential
morning MCS and potential cloud debris...but thinking we should see
at least partial sunshine by Sun afternoon. High temperatures should
reach into the 90s in many locations with dewpoints climbing into
the upper 60s to the lower 70s. Heat Index Values will likely reach
between 95 and 100+ degrees Sunday afternoon.

Given the extended period of extreme heat and humidity...opted to
issue an Extreme Heat Watch in the Sun through Tue time frame. While
Sun will be more representative of Heat Advisory criteria...felt it
was easier to message as one headline.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Key Messages:

* A multi-day stretch of potentially dangerous heat and humidity
  is looking more likely for early to mid next week. Heat
  indices could approach the 105 to 110 degree range Mon and
  Tue.

* Backdoor cool front brings lower temperatures for late next
  week, but also more unsettled with daily thunderstorm chances
  Thu-Fri.

Details:

The main story for this potion of the forecast is a multi-day
stretch of significant and potentially dangerous heat and
humidity. The peak of this dangerous heat and humidity continues
to be primarily Monday and Tuesday, and perhaps lingering into
Wednesday as well. High temperatures themselves on Monday and
Tuesday could approach 100 degrees with lows in the lower to mid
70s. Accounting for humidity, there is potential for maximum
daytime "feels like" temperatures to be in the 105 to 110 degree
range. NWS HeatRisk graphics also conveys Moderate to Major heat
risk levels. It also does appear we`ll see a reprieve from
thunderstorms until later next week. Extreme Heat Watches are
posted for most of southern New England Sunday into Tuesday.

High pressure should move from eastern Canada to the Maritimes
late this week. That could bring a backdoor cool front into our
region. Besides lowering temperatures to more normal levels for
late June, we also wind up with a greater chance for mainly
afternoon and evening showers and thunderstorms.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

06z TAF Update:

Through Tonight: High confidence, though moderate on daytime
seabreeze potential.

VFR. West winds around 5-10 kt; seabreeze could be possible at
BOS as soon as 16z as west winds begin to ease.

Tonight: Moderate confidence.

VFR generally dominates much of the period. Monitoring for a
potential cluster of t-storms that may impact the
northern/northeast part of Southern New England during the
07-12z period. Indicated PROB30s for TSRA at BAF, BDL, ORH, BED
and BOS to hint at this. Exact track of this feature and its
southwest extent, were it to develop, is still uncertain but
could see a need to hit this potential harder and across the
remaining TAFs. Winds become south around 5-10 kt, though could
become more variable and potentially quite a bit stronger in any
TSRA.

Sunday: Moderate confidence.

Any potential early-day cluster of t-storms moves eastward. VFR
generally the rule. An isolated t-storm could develop during
the afternoon, and if it does, it would likely become strong to
severe quickly. Location is highly uncertain and odds favor dry
weather. Winds to become SW around 5-10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Breezy.

Monday through Tuesday Night: VFR.

Wednesday: Breezy. Slight chance SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Tonight through Sunday...High confidence.

Low pressure lifts further northeast and away from the Canadian
Maritimes tonight. This will allow the gradient to quickly
weaken this evening. Gale/Small craft headlines for winds will
come down this evening...but lingering swell will result in some
5 foot seas lingering in parts of our southern outer-waters so
will continue those headlines. Otherwise...upper level ridging
builds into the Ohio Valley this weekend with our region with
westerly flow aloft. Winds/seas should mainly be below small
craft advisory conditions...but may see some nearshore W wind
gusts near 25 knots by Sun afternoon.

Outlook /Sunday Night through Wednesday/...

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

Monday through Wednesday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
     CTZ002>004.
MA...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
     MAZ003>007-010>021-026.
     Heat Advisory from 11 AM Sunday to 8 PM EDT Tuesday for MAZ002-
     008-009.
RI...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
     RIZ001>007.
MARINE...Small Craft Advisory until 5 AM EDT early this morning for
     ANZ254>256.

&&

$$

SYNOPSIS...Loconto
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...Loconto
AVIATION...Loconto
MARINE...Loconto