Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 241103
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
703 AM EDT Thu Jun 24 2021

.SYNOPSIS...
High pressure will bring lots of sunshine and one more day of
low humidity today. A warm front will lift north late tonight
into Friday morning bringing scattered showers and perhaps a few
embedded thunderstorms. Summer heat and humidity returns this
weekend and especially next week. Typical summertime scattered
showers & thunderstorms will be possible at times, but plenty of
dry weather expected too.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

7 AM Update...

Forecast remains on track so only minor changes have been made. Clear
skies, light winds and a dry air mass have combined for an
unseasonably cool night across Southern New England, with
widespread 40s and localized radiational fog in the typically
fog prone river valleys and ponds. Surface high pressure will
move offshore today and with a low pressure system off the Mid
Atlantic coast, the pressure gradient will result in an
increasing southeast flow by the late morning/early afternoon
hours. Afternoon highs will be a few degrees warmer than
yesterday (mid 70s to near 80) but it will feel very comfortable
for outdoor activities between the low dew points and breezy
conditions along with plentiful sunshine. Get outside if you
can!

PREVIOUS DISCUSSION...

330 AM Update...

* Beautiful weather today with sunshine & low humidity

After a cool start very early this morning, plenty of sunshine
will allow the dry airmass in place to warm-up fairly quickly.
High pressure moving off the coast coupled with a weak wave of
low pressure moving northeast off the mid-Atlantic coast will
result in southeast winds today. As mentioned we do expect
plenty of sunshine today, but late in the day and especially by
early evening some clouds will be advancing onto the
Cape/Islands and south coast.

The onshore flow will keep high temperatures in the lower to
middle 70s along the very immediate coast. Further inland
expect highs to top out mainly in the upper 70s to near 80.
This will also be the last day for quite sometime with low
humidity. So it will feel quite comfortable for outdoor
activities.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
* Scattered showers with an embedded t-storm or two mainly late
  tonight into Friday morning especially across eastern MA/RI

* Low clouds will linger into Friday afternoon but we may
  eventually see some partial clearing with increasing humidity

Tonight and Friday...

Weak surface low near the mid-Atlantic coast will be lifting
northward tonight into Friday morning. While the surface low is
quite weak; it will serve as more of a warm front/low level
moisture gradient. The model guidance indicates an instability
burst late tonight into Friday morning with a rapid drop in
Showalter Indices along with ribbon of deeper moisture. While a
few showers will be possible this evening mainly near the
southeast New England coast, expect the main threat for
scattered showers late tonight into Friday morning. While these
scattered showers will be possible anywhere, areal coverage
will likely be greatest across eastern MA & RI. This is where
there is a bit of a southerly LLJ along with several hundred
J/KG of MUCape. Therefore, we feel there could be a few embedded
t-storms too. PWATS between 1.25 and 1.5 inches may result in
some very brief heavy downpours too. Low temperatures tonight
will be in the 50s, but may not drop below 60 along portions of
the immediate coast and low level moisture increases.

The threat for scattered showers and a few embedded t-storms
will generally come to an end by mid-late Friday morning or
around lunch time. This as the best forcing/deeper moisture
lifts north of the region. Low clouds will linger into Friday
afternoon, but we may eventually see some partial clearing. This
will depend on how quickly the surface warm front can lift
north. We are thinking high should reach well up into the 70s to
near 80. It could get a few degrees warmer than that in the CT
River Valley if partial clearing occurs earlier than expected.
While northeast MA will take the longest to warm-up. That being
said the humidity should return with dewpoints reaching into the
60s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Summer heat and humidity returns with the chance for scattered
  showers and thunderstorms on a daily basis although no washouts
  are expected.

Details...

Model guidance is in consensus that the Northeast US will be under
deep southwest flow with a trough located over the central CONUS and
ridge over the western Atlantic through the period. Multiple short
waves will move through the trough resulting in waves of low
pressure moving along a boundary stretching from the Central Plains
into the Great Lakes Region. With such a synoptic set up, the
overall risk for severe weather is low across Southern New England
given that the better forcing will be to our south and west. PWAT
values rise to between 1.25 to 1.75 inches this weekend, which is
around 75th to 90th percentile according to SPC sounding
climatology. This means that any afternoon and evening showers and
isolated thunderstorms could produce locally heavy rainfall.

A warm front lifts north across our area Thursday night into Friday,
paving the way for the return of summer warmth and humidity this
weekend into early next week. This gels well with the CPC`s 6-10 day
50-60 percent confidence of above normal temperatures across the
Northeast. For reference, average highs for the last week of June
are in the upper 70s to low 80s, so we are looking at highs running
5 to 10 degrees above normal (with up to 15 degrees above normal
during the peak of the heat from Sunday into Tuesday). And even
heading into mid week, temperatures may be a tad `cooler` but no
real relief is on the horizon given that frontal boundaries remain
hung up north of our area. Dew points will remain muggy to
oppressive in the mid 60s to low 70s so enjoy the current reprieve
from summer weather while it lasts. Overall, expect highs to reach
the mid 80s to low 90s away from the immediate south coast with dew
points in the mid 60s to low 70s. So not quite Heat Advisory
criteria but not terribly comfortable either. As for precipitation
chances, there will be the risk for scattered afternoon showers and
thunderstorms every day but no washouts are expected. There is a
somewhat better chance for more widespread thunderstorms on Tuesday
given more robust convective parameters but the chance for severe
weather is low given the discussion above. All in all, a rather
uneventful long-term forecast.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

11Z Update...

Today...High Confidence in VFR conditions. Winds becoming SE at
5 to 15 knots.

Tonight...Moderate Confidence. Conditions gradually deteriorate
to MVFR-IFR conditions from southeast to northwest as the night
wears along. The greatest risk for the IFR conditions will be
across the southeast New England coast and also where they
arrive the earliest during the early evening. Scattered showers
and even a few embedded t-storms will also be possible,
especially late tonight across eastern New England. Light E
winds.

Friday...Moderate Confidence. MVFR-IFR conditions in the morning
with a few lingering showers and perhaps a rumble of thunder.
Gradual improvement is expected during the afternoon with MVFR
to even VFR conditions developing. E winds gradually becoming S
at 5 to 15 knots across most of the region.

KBOS TAF...High Confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: MVFR/IFR conditions possible. Breezy. Slight
chance SHRA.

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Breezy.

Sunday: Mainly VFR, with local MVFR possible. Breezy.

Sunday Night: VFR. Breezy.

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

&&

.MARINE...
Forecaster Confidence Levels.

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

Today through Friday...High Confidence. High pressure moving
east of the waters today will result in winds becoming ESE
around 10 to 15 knots today and tonight. These winds should turn
more S Friday as a warm front crosses our waters. The pressure
gradient should be weak enough so that marine headlines will not
be needed through Friday afternoon.

Outlook /Friday Night through Monday/...

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

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

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

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

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

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

&&

.TIDES/COASTAL FLOODING...
High astronomical tides around 12.0 feet late tonight may
result in some splashover/very minor coastal flooding along the
most vulnerable areas along the eastern MA coast. Do not
anticipate issuing any advisories given the very limited impact.

&&

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

&&

$$
SYNOPSIS...Frank/Chai
NEAR TERM...Frank/Chai
SHORT TERM...Frank
LONG TERM...Chai
AVIATION...Frank/Chai
MARINE...Frank/Chai
TIDES/COASTAL FLOODING...


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