031
FXUS61 KBOX 042334
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
734 PM EDT Thu Jul 4 2024

.SYNOPSIS...
A few showers will impact the region into early this evening with
the focus across the interior. A better chance for scattered showers
and isolated thunderstorms will be overnight and at times Friday and
Saturday. That being said...dry weather should still dominate..
Sunday though Monday is warm, humid, and dry. A return to more
unsettled weather by Wednesday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
7PM Update...

Remaining area of showers has now moved off shore with drying
conditions behind. Should be a pleasant evening to be outdoors
with temps in the middle to upper 70s currently, dropping to low
70s by midnight


405 PM Update...

* A few showers into this evening...focused across the interior
  but may briefly impact the Boston to Providence corridor

* Muggy tonight with the main risk for scattered showers & perhaps
  an isolated t-storm or two mainly after midnight

Scattered showers have developed across southwest MA and northern CT
as of mid-late afternoon. This in response to shortwave energy
pushing eastward coupled with some marginal instability. We can not
rule out a rumble or two of thunder across the interior into this
evening...but instability and forcing is rather limited. Further
east onto the coastal plain...just a few brief showers are expected
into the evening. This a result of the shortwave energy weakening as
it runs into subtle ridging.

Appears the better risk for some scattered showers and perhaps a
thunderstorm or two will be after midnight and especially toward
daybreak. This in response to a subtle surface wave that will
interact with very high Pwats on the order of 2-2.25 inches and
several hundred J/KG of MUCape. Therefore...any of the activity will
be capable of producing brief downpours. Because the forcing is
limited but the Pwat plume is very high...the uncertainty revolves
around the areal coverage of the activity and the location of it.
This may need to be more of a nowcast situation...so later shifts
will likely have to adjust the Pops/timing based on radar and
satellite trends.

Moist southerly flow will keep overnight low temps mainly in the
upper 60s to the lower 70s and it will be muggy.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Key Points...

* Very Warm & Humid Friday with highs 85-90 away from south coast
* A few showers/isolated t-storms Fri with brief downpours possible

Friday...

A ridge of high pressure off the coast will result in a very warm
and humid day on Friday. The majority of the day will likely feature
dry weather...but a few showers/isolated t-storms will be possible
at times given the tropical environment in place. We will discuss
that below...but overall expect highs to reach between 85 and 90
Friday away from the south coast/Cape and Islands where highs will
be in the upper 70s to the lower 80s.

Scattered showers and perhaps a few t-storms will probably be
departing the region Fri morning with the subtle shortwave. A few
additional showers/isolated t-storms may popup later Fri morning and
especially during the afternoon given a very high Pwat plume in
place and any diurnal instability. The difficulty is trying to
locate/time this activity given limited forcing in a tropical-like
environment with Pwats of 2-2.25 inches. Models tend to struggle in
these weakly forced environments...but it will not take much for a
few showers/isolated t-storms to develop. Greatest concern would be
during the Fri mid-late afternoon hours into the first part of Fri
evening across the interior MA and CT. This is where surface Capes
may approach 1500 J/KG and CSU machine learning probs along with
HRRR Neural Network/Nadocast indicate some very low severe probs in
our western zones. Again...forcing is limited but given the
environment can not rule out the low risk for an isolated severe t-
storm/localized wet microburst or two. In addition...a very
localized flood threat can not be ruled out either given Pwats 3+
standard deviations above normal. So any t-storm that is able to
develop will be capable of producing torrential rainfall.

Friday night...

Initially will have to watch for a few showers/isolated t-storms
early Friday evening. This activity will probably start to dissipate
though with the loss of diurnal heating coupled with poor mid level
lapse rates. However...we may see scattered showers/isolated t-
storms redevelop overnight given a modest southerly LLJ and very
high Pwats on the order of 2 to 2.5 inches! While much of this time
will feature dry weather...any activity will be capable of producing
brief torrential rainfall given the tropical environment in place.
The very high dewpoints will probably hold overnight low temps in
the upper 60s to the middle 70s...so it will be quite muggy.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Key Points:

* Mid-summer heat and humidity through the period
* Potential for local heavy downpours Saturday and perhaps Wednesday

Latest global ensemble model outputs don`t really show any big
changes from prior forecasts. The large scale weather pattern
indicates a mid/upper level ridge of high pressure will remain
anchored to our east, with the ridge axis from roughly Bermuda north
into the Canadian Maritimes. That puts us on the western periphery
of the ridge, although 500mb heights will remain above normal. That
kind of pattern would keep deep southwest flow across our area,
meaning an extended period of high dewpoint air. It would also be
hard to get any strong upper level dynamics to really impact the
region as any shortwave would be moving into the ridge.

What we will primarily experience as sensible weather for the area
will be the humidity. NBM data looked quite good, so that would
suggest dewpoints will rarely be below 65F through the period.
Saturday and then again Tuesday and Wednesday should be the most
humid of the days, with dewpoints in the 70-75F those days. Luckily
we aren`t looking at extremely high temperatures (mid to upper 80s
those 3 days), so heat index values will "only" be in the lower 90s.
Those most humid days are also when we will have the higher chances
of showers and thunderstorms. Given precipitable water values
expected to be 2-2.25" on Saturday and again mid-week, that sets us
up for the potential for locally heavy rainfall.  Ensemble model
probabilities show about a 20-30% chance of seeing 0.5" in a 6 hour
period for Saturday and again Wednesday. Although those values don`t
seem high, they are a respectable signal given the lower resolution
of global models.  With Saturday, surface CAPE values should reach
500-900 J/kg especially across western half of SNE so some
convection should develop. Wind fields are not strong and mid level
lapse rates are only 5-5.5C/km so not expecting any severe weather,
but an isolated strong storm is not out of the question. As
mentioned previously, it`s the high PW values that suggest local
heavy downpours as the primary issue.

Looks to be mostly dry Sunday and Monday, so those days will have
the best potential to top out in the 90-92F range in many inland
areas. Though it will be humid, dewpoints will be not be high enough
to result in oppressive conditions. Another surge of higher moisture
comes back in for Tuesday and Wednesday, thus additional showers and
a few storms are expected. Much more uncertain for Thursday, so
stuck with NBM which offers a climatologically normal 30-40% chance
of showers and a few t-storms. However this time of year there are
rarely strong synoptic signals to really latch onto, so sticking
with a "climo" forecast is the best bet.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:

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


Tonight...High Confidence.

Mainly VFR this evening, but pockets of MVFR to even IFR at ORH
are popping up as dewpoints continue to climb this evening.
Expecting more widespread MVFR-IFR ceilings to begin to develop
after midnight as the boundary layer cools and low level
moisture continues to increase. More scattered showers and
perhaps a rumble of thunder will be possible after midnight.
Any of this activity will be capable of producing brief
downpours. S-SW winds 5-15 knots becoming light tonight.

Friday and Friday night...Moderate Confidence.

MVFR-IFR conditions early Fri morning should improve to MVFR/VFR
levels by mid afternoon. However...do expect MVFR-IFR
conditions to redevelop Fri night with even some localized LIFR
Cigs/Vsbys. A few showers and perhaps an isolated t-storm or two
will be possible into mid-morning Fri & some re-development is
possible Fri PM. Any of this activity will be capable of
producing brief downpours. Timing/location of this activity is
quite uncertain...but regardless thinking dry weather dominates
the vast majority of the time. S winds mainly 5-10 knots.

KBOS TAF...High confidence in TAF into this evening followed by
moderate confidence thereafter.

VFR with periods of MVFR possible this evening.
Becoming MVFR/IFR later tonight with a low chance for a passing
shower. MVFR/VFR tomorrow with scattered showers or
thunderstorms possible in the afternoon

KBDL TAF...High confidence in TAF into this evening followed by
moderate confidence thereafter.

VFR this evening. Becoming MVFR/IFR later tonight with a low
chance for a passing shower. MVFR/VFR tomorrow with scattered
showers or thunderstorms possible in the afternoon

Outlook /Saturday through
Tuesday/...

Saturday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
slight chance TSRA.

Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA, isolated TSRA.

Sunday through Monday Night: VFR.

Tuesday: VFR. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight through Friday night...High Confidence.

High pressure east of the waters will generate persistent S-SW
winds of 10 to 15 knots with perhaps some 20+ knot wind gusts at
times. Conditions though should generally remain below small
craft thresholds...but given long southwest fetch we may see
some marginal 5 foot seas develop across our southern outer-
waters Fri night. We may also have to watch for an isolated
t-storm or two...but widespread convection is not expected.

Outlook /Saturday through Tuesday/...

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Chance
of rain showers, isolated thunderstorms.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.

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

Monday through Tuesday: Winds less than 25 kt.

&&

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

&&

$$

SYNOPSIS...Frank/Nash
NEAR TERM...Frank/KP
SHORT TERM...Frank
LONG TERM...Nash
AVIATION...Frank/KP/Nash
MARINE...Frank/Nash