Flash Flood Warning 12, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
1004 AM EDT Sun Jun 30 2024

.SYNOPSIS...
A slow-moving cold front could bring a few showers and storms
to the immediate south coast this morning, but these quickly
move offshore. There is a greater risk for strong to severe
thunderstorms this afternoon and evening as this front is moving
by. The main risk is damaging wind gusts and locally heavy
downpours, but large hail and an isolated tornado cannot be
ruled out. Hit or miss showers/storms are possible on Monday,
although drier weather prevails most of the time. High pressure
brings dry weather, seasonably warm temperatures and low levels
of humidity for Tuesday and Wednesday. Turning more humid and
unsettled for the Fourth of July thru the early part of the
weekend with a couple of disturbances offering chances for
clouds, showers and thunderstorms.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10 AM update...

The main concern this afternoon is severe weather. Already a
pretty unstable environment this morning with MLCAPES up to 1500
J/kg across the interior. Lots of breaks in the low clouds away
from the south coast will lead to further heating. This
combined with rich low level moisture with dewpoints in the low-
mid 70s will result in MLCAPES likely increasing to 2000-3000
J/kg which is rather robust instability for SNE. Combined with
deep layer shear 40-50 kt is a very favorable environment for
organized storms to develop including supercells if there are
any discrete storms. CAMs indicate the most likely storm mode
will be line segments or even a QLCS as linear forcing ahead of
the cold front and pre-frontal trough results in storms
organizing into a line. Damaging winds will be the primary
threat, but large hail or an isolated tornado will be possible
with any discrete supercells although this is a lesser threat. A
QLCS tornado is also possible given orientation of increasing
0-3km shear. In addition to severe potential there will be a
heavy rain and localized flash flood threat with any training of
storms.

It appears storms will initially fire along a pre-frontal trough
early to mid afternoon in the interior before approaching the
south coast late afternoon/early evening. Given the marine layer
near the south coast would expect storms to weaken somewhat as
they approach the coast but strong shear may help to sustain
them longer than they otherwise would be in a weaker shear
environemnt. Some of the CAMs also indicating a second line of
storms with the cold front late afternoon and evening.

Previous Discussion...

Highlights

* Main story for today will be a round of strong to severe
  thunderstorms this afternoon into the evening. Main threats
  are damaging wind gusts and heavy downpours, which could lead
  to flooding in some of the more flood prone spots. Cannot rule
  out large hail and an isolated tornado.

* Any AM shower/storms along the immediate south coast push
  further offshore by mid/late AM.

Period anticipated to start with some showers/storms along the
immediate south coast today. Leaned fairly heavily on the latest
NSSL WRF and ARW as they are handling the current shower
activity fairly well. This uptick in showers/storms anticipated
along the south coast as the 925 hPa SWly low level jet begins
to slide offshore. Will see a bit of a shift of the wind
direction resulting in some convergence, but think activity will
mostly be focused along/south of the south coast given this is
where the strongest winds are located. This first rounds
activity is not anticipated to be strong/severe as despite the
increasing instability the deep layer shear is disjointed until
this afternoon. Confidence a bit lower than usual given
guidance is all over the place, but erring on the side of
caution given we are within the warm sector. Any of these
showers/storms could produce heavy downpours with a 2-2.5 inch
PWAT plume overhead and warm cloud layer depths of 4-4.5 km.

Main concern in the forecast comes for this afternoon into the
evening as a strong cold front slides in from the west and a
trough digs into the eastern Great Lakes. This forcing will be
more than enough to trigger activity today. As for moisture we
will see the anomalous PWAT plume start sliding offshore, but
still are quite high with NAEFS indicating us 2.5 to 3 STD above
model climo. Should see PWATs around 1.5-2 inches, and still
have fairly deep 3.5-4.5 km warm cloud layer depths. So heavy
downpours still a risk, but fortunately expecting things to be
fairly progressive. This should minimize the risk for flooding,
but think the typical prone spots are still areas that will need
to be closely monitored.

As previous forecast shift mentioned and as is one of the flies
in the ointment our mid level lapse rates are quite marginal.
This should be made up for by the increasing heat with 925 hPa
temps of 20-22 degrees Celsius and low to mid 70 degree
dewpoints. This will result surface based CAPEs of 1500-2500
J/kg across the interior. Deep layer shear (0-6 km layer) will
increase 40 to 50 kts and low level lapse rates will be around
6-8 degrees Celsius. Given these parameters in combination with
the forcing will be very favorable for more organized storms to
develop. At this point based on CAMs appears that there could be
a brief window wMain theme of the forecast is still intact. Made some
adjustments to account for latest observed trends.
here a few discrete supercells develop before things quickly
congeal into a line, given the strong linear forcing.
Anticipating that risk for large hail comes in that brief window
where there are supercellular storms. Overall though am
anticipating more of a QLCs event as advertised by all the CAMs.
This will really elevate the damaging wind gust threat. Though
should note that we will have roughly 100-150 of 0-3 km SRH and
low LCL heights. SPC SREF still showing moderate probs (30-50
percent) of STP >1. Think this risk will likely come with both
the discrete and linear activity. Best shot for severe weather
still along and NW of I-95 as the marine layer should limit near
the coast. Can see this quite clearly with latest HREF 2-5 km
updraft helicity moderate probabilities (30-50 percent) away
from the coast where the probs quite rapidly drop. Latest ML
guidance all really hitting the wind risk per the Nadocast, NCAR
HRRR Neural Network Convective Hazards and CSU. Though all do
also indicate that their is also a lower risk for hail and an
isolated tornado.

Timing wise previous shifts forecast looks quite reasonable.
Still thinking will see an update 18-20Z and activity gradually
moving southeastward toward the coast by the evening, 00-02Z.
Uncertain how strong the activity will be along coastal areas
given the stable marine layer, but the deep layer shear could
sustain activity a bit. Just not sure if once it reaches the
coastal areas how much of the wind will be able to get to the
ground. High temps generally in the 80s for most today.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Highlights

* Any lingering showers/storms along and near the south coast
  pushing offshore before midnight. Dry heading into Mon AM.

* Widespread scattered hit or miss showers/storms on Mon.

The trough over the eastern Great Lakes digs into the Mid
Atlantic/Upstate NY by early Mon. The trough cuts off and lifts
into southern New England by early afternoon before pushing
offshore during the evening. The cold front slides offshore
tonight and a surface trough digs extends into southern New
England on Mon.

Should see any lingering shower/storms across the region pushing
offshore tonight as the cold front sweeps through. As previously
mentioned there could potentially be a few strong storms given
the deep layer shear. Still have questions especially SE of I-95
given the marine layer in place. Regardless, will see activity
ending as it moves offshore. Drier weather anticipated after
midnight in wake of the cold front. Lows will mostly be in the
60s.

Main concern on Monday is that will have a roughly -15 to -20
degree cold pool moving in overhead with the cutoff low. This in
combination with daytime heating will result in fairly
widespread CU and hit or miss showers/storms developing. Given
the environment would not be surprised if we are able to get
some small hail, though do wonder if moisture will be a bit too
limited. Given the focus on the immediate near term did not dive
too deeply into this, but will need to be looked at in future
updates. Though at the moment current ML probs don't really
hammer this risk. Expecting a much more comfortable airmass to
push in as dew points fall into the 50s/low 60s. High temps a
bit cooler than seasonable with readings in the 70s.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights:

* Dry and warm with tolerable humidity levels for Tue and Wed.

* Turning more humid and active for Independence Day, and
  potentially into Fri/Sat too, with clouds, showers and storms
  possible. Exact timing remains uncertain.

30/00Z guidance suite was in rather good agreement with the overall
synoptic pattern. Broad mid level ridging to start out this portion
of the forecast is expected to transition to broad troughing towards
next weekend. At the surface, a large high pressure should be over
southern New England Tuesday into Wednesday. This high pressure
should be far enough offshore to permit a cold front to approach
from the west on Independence Day, but it is a little complicated.
The offshore high pressure may not permit this front to make it all
the way into, or through, our region before being driven back north.

Dry weather with near to slightly above normal temperatures Tuesday
and Wednesday. Increasing risk for some showers and isolated
thunderstorms Wednesday night into Thursday. Also expecting
increased humidity with above normal temperatures Thursday into
Saturday. The timing and coverage of these showers and thunderstorms
remains the most uncertain aspect of this forecast. With many
outdoor plans for the holiday, it would be best to monitor the
forecast over the coming days.

This uncertainty lingers late this week. As such, stayed close the
NationalBlend solution for now, as it is ensemble-based. This meant
a broad chance for showers and thunderstorms Friday into Saturday,
but am not expecting it to be raining this entire time.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

Today...Moderate confidence.

Conditions improve to VFR for most by roughly 13-16Z. The only
exception is for PVD-OWD where MVFR to LIFR stratus lingers.
May have a few showers/storms exiting the Cape and Islands
during this timeframe.

Another round of showers/storms expected to materialize roughly
17-19Z across the interior. These will push southeastward toward
the south coast by the evening. Strong to severe storms
possible, which if impacting a terminal would bring the risk of
damaging wind gusts and conditions briefly deteriorating to MVFR
to LIFR. Have handled this risk with a PROB30 for all terminals,
except across the south coast where am uncertain on how quickly
the line weakens given the stable boundary layer as the marine
layer will be in place. For now utilized a VCTS for this. Will
have SW winds at 101-5 kts with gusts of 20-30 kts. Winds
shifting to the W/WNW toward the evening across the interior.

Tonight...Moderate confidence.

MVFR to IFR with showers/t-storms in the evening along the
south coast, then improving to VFR overnight. Wind shift to NW
00-06Z. Speeds of 5-10 kts with gusts diminishing toward 03-06Z,
but will then increase to 15-20 kts toward daybreak.

Monday...High confidence.

VFR to start, but as CU develops will have borderline MVFR/VFR
cloud bases. Should see hit or miss showers/storms developing,
which could potentially produce some small hail. Winds out of
the NNW to N at 5-10 kts with gusts of 15-20 kts.

KBOS TAF...Moderate confidence in TAF.

Will improve to VFR roughly 15-17Z. Scattered showers/storms
developing 19-23Z and could bring strong to perhaps damaging
winds and brief heavy downpours. Improving to VFR as
showers/storms move toward the south coast by the evening. S to
SW winds at 10-15 kts with 20-25 kt gusts. Winds shifting to the
W/WNW during the evening once the showers/storms slide through.

KBDL TAF...Moderate confidence in TAF.

Will have increasing chances of showers/storms this
afternoon/evening, some of which could bring strong to perhaps
damaging wind gusts. Have handled with a PROB30 from 17-23Z.
These storms moves S/SE of the terminal during the evening and
will fairly quickly improve to VFR. Winds out of the S at 5-15
kts. Will shift to the W/WNW this evening in wake of
shower/storm activity. Could have some 15-20 kt gusts this
afternoon (not associated with the thunderstorms).

Outlook /Monday Night through Thursday/...

Monday Night through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: VFR. Breezy. Slight chance SHRA.

Independence Day: Mainly VFR, with local MVFR possible. Breezy.
Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Tonight...High confidence.

Gusty S to SW on today with gusts to 25 kt at times. SCA
continued for all waters. Winds shift to NW tonight as the cold
front pushes through. Fog lingers today, especially across the
south coastal waters due to increasing dew points. A few
t-storms possible over the waters this morning, then more
showers and t-storms moving across the waters late Sun and Sun
night. This later activity has the potential to still be strong
to severe producing strong wind gusts.

Monday...High confidence.

Winds out of the N to NW. Will see speeds of 10-20 kts earlier
in the day diminishing to 5-10 kts late as high pressure builds
in. Seas diminishing to 2-4 ft late.

Outlook /Monday Night through Thursday/...

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

Tuesday through Wednesday: Winds less than 25 kt.

Wednesday Night: Winds less than 25 kt. Slight chance of rain
showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ230-
     236-251.
     Small Craft Advisory until 8 PM EDT this evening for ANZ231-
     250.
     Small Craft Advisory until 11 PM EDT this evening for
     ANZ232>235-237.
     Small Craft Advisory until 6 AM EDT Monday for ANZ254>256.

&&

SYNOPSIS...Belk/BL
NEAR TERM...KJC/BL
SHORT TERM...BL
LONG TERM...Belk
AVIATION...Belk/BL
MARINE...Belk/BL