706
FXUS61 KBOX 082142
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
542 PM EDT Mon Jul 8 2024

.SYNOPSIS...

Oppressive heat and humidity are expected to continue into
Tuesday, and could continue into Wednesday as well.There is a
chance for thunderstorms north and west of I-95 Tuesday
afternoon. Isolated strong to severe storms are possible. Still
fairly humid late week, though temperatures are slightly less
hot, along with periods of shower and thunderstorm activity as
the remnants of Beryl moves towards the northeast.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

A very broad and persistent Bermuda high continues to be parked over
the western Atlantic through the foreseeable future preventing us
from seeing any true relief from this heat and humidity more akin to
the southeast U.S. than New England. Moist southwest flow continues
around the high. Hence, warm and very humid conditions continue this
afternoon. Dewpoints are "only" in the mid 60s for northwest MA as
opposed to southern portions of SNE where 70s are more common. This
keeps apparent temps in these areas below Heat Advisory criteria.
Those along the immediate coasts will see some relief thanks to
onshore prevailing SW flow for the south coast and seabreezes along
the east coast. Subsidence should tamp down any showers that may try
to get going late this afternoon. Overnight we won`t see much relief
unfortunately as the high dewpoints limit low temps to the low 70s.
This will also bring the return of fog and low stratus surging
inland much like we saw last night. Thinking continues to be that
this should remain along/just south of the I-95 corridor so mostly
will impact Cape Cod, the islands, and coastal RI/southeast MA.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...

On Tuesday SSW flow continues around the high and looks to increase
a bit, bringing much more moisture overhead with PWATs surging
closer to 2 inches. This moisture out ahead of the influx of
moisture from Beryl will interact with a front overhead and provide
a focus for scattered showers and thunderstorms. Given the anomalous
moisture content (1-2 standard deviations above normal) one of the
main threats from any storms on Tuesday will be heavy downpours that
could lead to localized flash flooding. A Marginal Excessive
Rainfall Outlook is in effect for a good portion of western/central
MA and CT. Similarly, there is a Marginal threat for severe weather
tomorrow afternoon from the Storm Prediction Center focused on a
weak mid level impulse moving overhead during the afternoon which
will help to kick off scattered thunderstorms, some of which could
be severe. Mid level flow will increase to near 50 knots helping to
increase bulk shear from sustained updrafts in combination with 500-
2000 J/kg of CAPE. One limiting factor will be poor mid level lapse
rates which may help to keep storms shallower. The primary threat
from any storms that do form would be flash flooding and strong
winds, then hail to a lesser extent.

As for the heat, the Heat Advisory continues through Tuesday. While
dewpoints will be increasing, actual temperatures will be less
thanks to an increase in cloudcover associated with the weak
shortwave and unsettled weather. This gives us lower confidence that
all locations within the Heat Advisory will hit the criteria, but
odds are favorable at this point. Tuesday night convection comes to
an end as sunset removes diurnal heating from the mix and we have
another warm and muggy night in store. We rinse and repeat with
regard to the return of fog and low stratus, especially for
southeast MA and coastal RI.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Highlights

* Hot and humid on Wed. Heat Advisory may need to be extended in
  future updates. Tropical humidity remains through the week into
  the weekend, but temps return to more seasonable levels.

* Periods of showers and storms throughout the forecast. Heavy
  downpours a risk every day given the tropical environment.
  Stronger storms possible on Wed and perhaps Thu.

Wednesday...

Caught under cyclonic flow during this timeframe, but a ridge axis
will build from the Mid Atlantic during the AM into the eastern
Great Lakes/northern New England by late in the day. The quasi-
stationary front begins lifting northward as a warm front early in
the day. Some uncertainty on where exactly this will be located, but
could be over or north of our region.

Main focus on this period is the dangerous heat and humidity in
combination with the severe weather/heavy rain threat. Both these
risks have some nuances, but given the ongoing heat risk will focus
on this first before diving into the severe weather/flooding risk
details.

Heat/humidity

Anticipating another hot and humid day, but have some uncertainty on
how warm we are able to get due to cloud cover in place. Still
should see 925 hPa temps ranging from 21-25 degrees Celsius under
SWly flow. With the cloud cover in place think the NBM running too
hot, so bumped things down to the 50th percentile of guidance. The
result are widespread highs in the 80s. Tropical humidity persists
with dew points in the 70s. At this point think best shot for the
Heat Adv to be extended would be over portions of the Merrimack and
CT River Valley, which is where heat indices range from 90-95
degrees. Since doesn`t appear to be a slam dunk have held off for
now, but will need to be considered in future updates.

Flooding and severe weather risk

The stalled out frontal boundary nearby/over our region becomes a
warm front and begins to lift northward throughout the day. Guidance
of late has been a bit more progressive with this feature and parks
it over portions of northern New England. Will really need to hone
in on where this boundary sets up as it will be key for the heavy
downpour and severe weather threat. At this point think the risk
likely highest for a good portion of interior MA, especially closer
to the NH/VT/MA border (where have put likely PoPs). Will have an
increasing southerly low level jet and the speed max really drops
off in the vicinity of that area. Low and mid level lapse rates are
fairly unimpressive. Instability will range from a few hundred to
roughly 2000+ J/kg, with the higher values near the NH/VT/MA border
area. Deep layer shear in the 0-6 km layer starting off around 30-40
kts and decreases to 20-35 kts as day progresses. Best overlap with
the shear again across the NH/VT/MA border. There is some question
on where the surface boundary is located, but given the environment
think there could be at least isolated severe storms. At the moment
SPC Day 3 Outlook has us in General Thunder. Leaned more toward the
CSU ML guidance, which does highlight a 15-30% prob of a wind risk.

On top of the severe risk there is the heavy downpours, which may
lead to localized flooding. Given the tropical environment we are in
this is something we will need to closely monitor not on Wed, but as
the week progresses. The latest NAEFS situational awareness table
guidance showing 2.5-3 STD above model climo with 2 inch PWATS over
our area, whereas deterministic showing anywhere from 1.75-2.25
inches. Warm cloud layer depths will be  4-5 km with NAM/GFS Bufkit
soundings showing a skinny cape profile. Given this environment the
flooding risk concerning depending on where the warm front sets up.
Think the WPC Day 3 ERO of marginal suffices as ensembles at this
point all highlighting best shot for 1+ inches of QPF over northern
New England where the current slight exists. Will need to keep an
eye on the MA/NH/VT border.

Thursday through Sunday...

Remains unsettled through this timeframe with cyclonic flow. The
remnants of Beryl and the moisture associated with it remain nearby
or overhead through at least Sat. Better shot for this moisture to
push offshore by Sun. The big question will be where frontal
boundaries are setup/stalled out. This is where there will be an
increased risk for strong/severe storms and flash flooding.

Still have stuck with the NBM at this point in time as it looks
reasonable given the uncertainties in the forecast. Temperatures
near more seasonable levels, but the humidity remains due to the
prolonged southerly flow. Still moisture laden with PWATS of 1.5 to
3 STD above model climo per the NAEFS and warm cloud layer depths of
roughly 3.5-5 km. Think anywhere where we`ve got a frontal boundary
will be the focus for heavier rainfall and perhaps flash flooding.
This is highlighted per Day 4-5 ERO per WPC and CSU ML Excessive
Rain probs guidance. Given environment could also have the risk for
strong to severe storms, but will again hone on where frontal
boundaries are set up. There are also questions on deep layer shear
as it becomes a bit more marginal based on latest guidance over our
area for much of this period. Something we will need to keep a close
eye on as the week progresses.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...

Forecast Confidence Levels:

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

18z Update:

This afternoon: High confidence.

VFR for all except ACK where there is high confidence that it
remains IFR- LIFR all day. Light south winds, with seabreezes at
BOS and PVD.

Tonight: High confidence in trends, moderate confidence on the
timing and northward coverage of fog/low stratus.

ACK will continue to deal with LIFR in fog and low stratus.
Initial VFR then degrades as stratus and fog at IFR-LIFR levels
will again return inland. Exact timing and how far north the
IFR- LIFR layer makes it is still uncertain; generally should
follow a similar 00-09z timing as the last few nights, with
gradual improvement from NW to SE during the 09-12z Tue
timeframe. VFR outside of these areas, with higher confidence
near/north of the Mass Pike. Light winds.

Tuesday: Moderate confidence.

Any sub-VFR stratus/fog conditions improve to VFR for most,
though still some degraded conditions anticipated for the Cape
and Islands. Potential for SCT TSRA from an HFD-ORH-LWM line
north and west after 18z Tue. S-SW winds 8-12 kt, possible
seabreeze at BOS.

Tuesday night...Moderate confidence.

Expecting IFR/MVFR condition to return to the south/southeast
coastal terminals overnight but confidence on the
north/westward extent of these sub-vfr conditions is low to
moderate.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Wednesday through Saturday/...

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

Wednesday Night: MVFR/IFR conditions possible. Chance SHRA,
isolated TSRA.

Thursday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
isolated TSRA.

Thursday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SHRA, isolated TSRA.

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

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

Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA,
isolated TSRA.

&&

.MARINE...

Forecaster Confidence Levels:

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

Through Tuesday night: High confidence.

Winds and seas to remain below SCA criteria in this forecast
period. Southerly winds will be on the light side tonight,
around 10 kt or so. Southwest winds pick up a little more on
Tuesday, into the 15-20 kt range on all waters. Seas 3 ft or
less on all waters. The main concern for boaters continues to be
reduced visibility from fog, which looks to continue even into
Tuesday night.

Outlook /Wednesday through Saturday/...

Wednesday: Winds less than 25 kt. Slight chance of rain
showers, isolated thunderstorms, patchy fog.

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Chance of rain showers.

Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, isolated thunderstorms.

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

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

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, isolated thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Tuesday for CTZ002>004.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for CTZ002.
MA...Heat Advisory until 8 PM EDT Tuesday for MAZ005-006-011>013-
     017-018.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for
     MAZ005>007-010>016.
RI...Heat Advisory until 8 PM EDT Tuesday for RIZ001>004.
MARINE...None.

&&

$$

SYNOPSIS...BW/BL
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...BL
AVIATION...BW/BL
MARINE...BW/BL