081
FXUS61 KBOX 171126
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
726 AM 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 /UNTIL 6 PM THIS EVENING/...

350 AM Update:

Key Messages:

* Mostly cloudy today, although some breaks in cloud cover in
  eastern MA/RI.

* After morning scattered showers dissipate, mainly dry but can`t
  rule out hit or miss showers north and west of I-95.

* Cooler than normal with cloudiness around, with highs in the mid
  60s to lower 70s.

Details:

Weak sea-level pressure pattern at sfc over Southern New England,
essentially an increasingly moist SE flow as weak sfc ridging
initially in place more or less washes out. Rather broad SW flow
aloft governs our weather as well, between an extension of the
Bermuda ridge to our south and a midlevel trough with axis along the
Mississippi River. Weak spokes of midlevel vort energy embedded in
the SW flow aloft, with one area currently moving through Southern
New England early this morning bringing some pretty hit-or-miss
light showers to areas mainly S of the Mass Pike. Another weak vort
max now over the lower OH Valley will be responsible for another
round of rain showers later tonight.

It`s taken quite a bit longer for cloud cover to set in than most of
the available NWP output indicated, but we should see cloud cover
continue to fill in through daybreak. Although there is some
optimism in some breaks in the cloudiness in eastern New England,
most of the day probably ends up with more cloud cover than sun,
which will really temper how warm we should get today. Though low
70s should be achivable in SE MA and into RI away from the waters,
most areas elsewhere probably struggle to reach 70 degrees. Although
it won`t necessarily warm very much today, it will be turning a
touch more humid later today as dewpoints rise into the lower to mid
60s.

Wouldn`t rule out hit or miss showers around the 700-500 mb SWly
height gradient north and west of I-95 today, but with weak and
generally suppressive midlevel height rises this afternoon, the
coverage of these showers probably ends up being sparse; but the
threat isn`t zero. As will be the case with any shower threats up
until Thurs, the timing of these showers is low confidence and
subject to change. That said, any showers which do develop shouldn`t
lead to any washouts.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...

350 AM Update:

Key Messages:

* Better chances at showers late tonight into early Wed. A few
  showers could produce brief passing downpours.

* Trending drier Wed afternoon with decreasing cloud cover.

* Becomes quite humid tonight and especially Wed. Highs Wed in the
  70s to low 80s.

Tonight and Wednesday:

A somewhat more coherent mid-level disturbance in the broader SWly
flow now over the lower OH Valley deamplifies as it moves ENE
through New England either later tonight and/or into early on Wed.

In this broader SW flow aloft, it remains uncertain exactly when in
this period we`ll start to see rain showers develop; they may begin
as soon as early tonight, but better chances after midnight into
Wed. Increased PoPs into the 40-60% range but could see increasing
these once there`s better consistency on when showers develop. While
the airmass associated with this midlevel disturbance is not
necessarily convectively unstable, thus the thunder risk is quite
low, it is quite moisture-laden. This is evidenced by PWATs of 1.8
to 2 inches and warm-cloud-depths reaching 12.5 to 13kft, which are
factors which favor efficient rainfall processes. Though there
should be dry periods, any passing showers which develop could
surprise with some brief downpours; probably not a washout
everywhere, but some areas could see some decent rain if stuck in
one of these passing showers.

Clouds increase and also turns more humid with lows in the low to
mid 60s tonight. For Wed, cloud cover should initally begin but we
should some breaks in the clouds late in the day over western areas.
Highs Wed in the 70s to a few low 80s, but it will feel quite humid
as dewpoints rise into the upper 60s to low 70s. While not that
warm, Wed is likely the first real dose of true mugginess felt to
this point in early summer.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...

Key Messages:

* Hot and humid Thurs (heat indices 95-100), with strong to severe t-
  storms possible during the afternoon.

* Less humid and seasonably warm and favoring dry weather Fri-Sat.

* Not yet set in stone, but potential multi-day 90+ degree
  temperatures with humidity starting early next week!

Details:

Thursday:

Likely to be the most active day of the workweek, as Southern New
England will be dealing with its first truly hot and humid day with
elevated heat indices, but also the potential for afternoon to early
evening t-storms as a stronger shortwave trough aloft and associated
sfc cold front move through, some which could become strong.

With a good deal of sun and 850 mb temps reaching around 16-18C, we
expect high temps to reach well into the 80s to lower and even mid
90s, leading to daytime heat indices forecast in the mid 90s to near
100. SWly breezes should also mitigate any cooling seabreezes near
the coasts. We probably fall short of reaching heat-headline
criteria, before considering the usual uncertainties which come with
forecasting heat at this time range such as cloud cover; we
encourage practicing appropriate heat-related precautions
nonetheless.

A cold front will be approaching and passing through New England
Thurs afternoon into early evening. Although mid-level lapse rates
are not substantial, model forecast CAPE values are sizable given
the expected degree of heat/humidity in the prefrontal setting, with
CAPEs ranging from 1500-1900 J/kg on the GFS and upwards of 3000
J/kg on the NAM. Deep-layer shear magnitudes are around 30-35 kt,
although the better mid/upper level winds and higher shear
magnitudes lie behind the sfc front. Point-based model forecast
soundings show a well mixed PBL which could lend itself to robust
downdrafts/large DCAPE and outflows. Will continue to monitor the
potential for strong or even severe storms Thurs, and will need to
drill down into the mesoscale model time ranges for more specifics
on the exact timing, but machine-learning progs continue to favor
the potential.

Friday and Saturday:

Something of a cooldown in this period with lower humidity levels
too with high pressure in control. Temperatures mainly in the lower
to mid 80s, so still on the warm side but not nearly as warm as
Thurs.

We`ll have to monitor for any t-storms to our north and west coming
out of Canada but we could finally see a dry Saturday.

Sunday into Early Next Week:

Although the details are not yet set in stone there is general
agreement on a period of hot and humid weather associated with
the development of an anomalously strong 590+ dm upper level
ridge in the northern mid-Atlantic states. 850 mb temps per
ensemble means are at least as high or even a couple degrees C
higher than those expected on Thurs. Too uncertain to pinpoint
thunder potential at this range; but it is possible we could see
a stretch of multi-day 90+ degree temperatures in this period.
Stay tuned!

&&

.AVIATION /11Z TUESDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

12z TAF Update...

Today: Moderate confidence.

Mainly lower end MVFR-IFR conditions dominate today with some
scattered showers with the focus for them this morning. ESE/SE
winds 5-10 knots.

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. 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. S/SW
winds 5-10 kt.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Thursday hrough Saturday/...

Juneteenth: VFR. Breezy. Chance SHRA, isolated TSRA.

Thursday Night: VFR. Breezy. Slight chance SHRA, isolated TSRA.

Friday: VFR. Breezy.

Friday Night through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Conditions remain at sub-SCA levels through Wed evening. SE
winds around 10-15 kt today, which become southerly tonight at
similar speeds and then kick around to SWly on Wed. Seas mainly
4 ft or less on all waters. Periods of showers possible through
Wed but risk for thunder is low. Fog possible tonight and Wed
night.

Outlook /Thursday through Saturday/...

Juneteenth: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers, isolated thunderstorms.

Friday: Winds less than 25 kt. Local rough seas.

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

Saturday: Winds less than 25 kt.

&&

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

&&

$$

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