601
FXUS61 KBOX 292348
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
748 PM EDT Sat Jun 29 2024

.SYNOPSIS...
A slow-moving frontal system brings two rounds of scattered
showers and thunderstorms, tonight into early Sunday morning,
and then perhaps again Sunday afternoon and evening. Passing
widely scattered afternoon showers 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. While the timing is still
uncertain, 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 AM SUNDAY MORNING/...
735 PM Update:

Radar imagery at this hour reflects what is amounting to very
light showers largely north and west of I-95, gradually moving
eastward. However METARs and mesonet information indicate little
if any QPF at all with this, with only a handful or so of sites
actually recording measurable precip. Did boost PoP up into the
Likely/low Categorical range at least thru midnight for most
areas, but at least through midnight, this is the textbook
"high-PoP/low-QPF" scenario that you`ll ever find. So while
radar looks imposing, there really isn`t much rain falling from
these echoes. Also backed off on the thunder mention for this
period of time.

However we think that changes for the 2nd half of the evening
into early Sun AM, but mainly for CT-central/southern RI-SE MA.
Watching a cluster of t-storms with cooling cloud tops over
eastern PA NW of the Philly area with additional re- development
over north-central PA, with both areas moving ENE in the WSW
flow aloft. For Southern New England, most models show rising
dewpoints into the lower 70s later in the overnight into the
pre-dawn hrs. With higher dewpoints at night, LCL heights are
lower and with that comes somewhat higher most-unstable CAPE
values (around 500-900 J/kg or so) mainly from HFD-IJD-PVD-PYM
southward. Convective-permitting guidance varies on the handling
of this, with the past several HRRR runs being the most bullish
in the outlined area, with more tepid/muted answers from the
WRF suite and the 18z NAM-3km. So for the 2nd half of the
overnight into early Sunday, ended up reducing PoP elsewhere to
around a chance level (25-30%) but boosted PoP to categorical
for the southern third, added mention of thunder and increased
QPF during the 06-12z Sunday period. Not expecting severe with
this but if current/new development sustains itself in this
setting, we could see overnight lightning and heavy downpours
roughly from Hartford to Providence to Cape Cod.

The rising dewpoints should also lead to more expansive stratus
development for the coastal areas, but pretty strong southerly
winds should keep fog to a minimum.

No changes to lows which look on track attm.

Previous discussion:

Showers assocd with a weak mid level shortwave and anomalous PWAT
axis are moving into northern and western MA this afternoon.
Expect a period of showers across all SNE through the evening
and into the overnight as deep moisture axis with 2-2.25" PWATs
move into the region. Marginal elevated instability develops so
can`t rule out an isolated t-storm, but the best chance may come
toward daybreak along the south coast and Cape/Islands. A few
of the CAMs are showing this potential but there is uncertainty
regarding whether the convection stays offshore or extends
further north along the south coast. Something that needs to
monitored overnight as high PWAT airmass supports heavy
rainfall. Otherwise, it will be a somewhat breezy night as
modest low level jet develops. The SW flow will bring increasing
humidity as dewpoints climb to around 70 overnight. Temps will
be nearly steady in the upper 60s to lower 70s. Stratus and
patchy fog will develop.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Sunday and Sunday night...

* A round of strong to severe t-storms possible Sun afternoon and
  evening

* Damaging wind gusts are the primary threat but large hail, heavy
  rainfall and also a tornado is possible

An area of showers and a few t-storms may be ongoing along the south
coast and Cape/Islands to start the day. There is lower than normal
confidence as convection could end up further south but if it does
extend along the south coast, it will move offshore by late morning.
Otherwise, stratus and patchy fog will burn off away from the south
coast leading to partial sunshine. Given warm sector airmass with
925 mb temps 22-24C, temps should reach well into the 80s to near 90
away in the interior. Stratus will likely linger into the afternoon
along the south coast with temps holding in the upper 70s with SW
flow. Oppressive humidity is likely with dewpoints reaching the low
to mid 70s. Heat indices expected to approach 95 in the CT and
Merrimack valleys.

The main forecast concern will be the potential for strong to severe
t-storms in the afternoon and evening as a strong cold approaches
from the west. The increasing forcing for ascent will likely result
in a greater areal coverage of storms. Mid level lapse rates are
marginal, but the heat and high dewpoints should make up for this
with CAPES likely reaching 1500-2500 J/kg in the interior. This
instability combined with impressive deep layer shear of 40-50 kt is
very favorable for organized storms. Given strong linear forcing
ahead of the front, storms will likely organize into line segments
or even some sort of QLCS as indicated by all the hi-res CAMs.
Damaging wind gusts will be the primary threat, but can`t rule out
large hail or even a tornado if any discrete supercells can develop.
SPC SREF showing moderate probs of STP > 1. Best chance of severe
weather will likely be north and west of I-95 due to marine layer
near the coast and CSU machine learning probs and HREF updraft
helicity are highlighting the interior for severe weather. Also
can`t rule out locally heavy rainfall and flooding from t-storms but
severe weather appears to be the greater threat.

Storms are expected after 2 pm across interior northern and western
MA, gradually reaching the south coast Sun evening. Convection
should be in a weakened state by the time it reaches the coast due
to weaker instability but strong shear may help to sustain
convection a bit longer. Showers and a few storms could linger into
the overnight period over the Cape/Islands, otherwise decent drying
will be moving into the region overnight as cold front gradually
pushes offshore with NW flow developing.

&&

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

* Hit-or-miss widely scattered showers Mon, but most stay dry with
  lowering humidity levels.

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

* Turning more humid and more active for the Fourth of July, and
  potentially into Fri/Sat too, with clouds, showers and storms
  possible. Exact timing still uncertain.

Details:

Monday:

Surface ridge associated with a seasonably-strong 1024 mb high
pressure over the Gt Lakes region eventually builds in. Will still
see generally improving conditions, although shortwave
disturbance aloft and associated cooler pool of air aloft should
produce enough if still meager instability to pop diurnal
cumulus clouds and hit-or- miss showers in widely scattered
coverage. Can`t really rule out a shower anywhere in Southern
New England, although central MA eastward has somewhat better
chances with somewhat higher surface dewpoints (lower 60s).
However not expecting washouts with these showers. Conditions
improve significantly toward drier weather by the evening both
with waning diurnal instability and passage of the shortwave
disturbance aloft, with falling dewpoints into the 50s thru
evening on northerly winds. Highs in the mid/upper 70s and lows
in the mid/upper 50s.

Tuesday and Wednesday:

500 mb heights briefly rise in this period, along with high pressure
settles in over the Northeast in this period. This high shifts
offshore later into Wed night. Leads to a couple of tranquil and
pleasant early-July days with comfortable humidity levels and highs
in the low to mid 80s.

As high pressure moves offshore late Wed evening, SW flow and
weakening 500 mb heights (upper ridge shifting east) should bring
increased cloud cover and rising dewpoints/humidity levels. Flow
aloft also strengthens a bit; ahead of a frontal system for the
Fourth of July, that preceding stronger flow aloft could bring a
canopy of cloud cover in potentially quicker than current forecast
calls for on Wed evening. Thus lows also warmer in the mid to upper
60s.

Fourth of July Holiday:

While primary weak-amplitude disturbance aloft in fast westerly flow
passes to the north into Quebec/northern New England, the Fourth of
July is looking active as a sagging cold front moves southeast
through New England. This front should act on an increasingly humid
air mass to favor showers and t-storms. Greatest uncertainties at
this time lie in the timing and how warm/degree of instability, and
kept PoPs pretty broadly in the Chance range for now and then re-
assess once there is better clarity on each of those. But as it
looks now it does look like Thurs and potentially into Thurs night
features clouds and showers/storms. With dewpoints in the upper 60s
to mid 70s, it should also be quite humid. Kept highs in the mid 80s
for now as well, although clouds and shower/storm coverage will
dictate adjustments to highs as well. Lows in the mid to upper 60s.

Friday and Saturday:

Forecast for Fri is uncertain as there looks to be another
disturbance aloft which tries to send a warm front back into
Southern New England either Fri or Fri evening/Sat. GFS is the
quickest in progressing this feature into our area into Fri, while
the ECMWF/Canadian GEM and its ensembles are slower, more into Sat.
Kept PoP pretty broadly into the Chance range here given the
uncertainties which are close to NBM values; these too will also
need to be adjusted pending trends in subsequent guidance.

&&

.AVIATION /23Z SATURDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

00z TAF Update:

Tonight: Moderate confidence.

Conditions gradually deteriorate from VFR to VFR/MVFR levels
through midnight for most, with more degraded/MVFR-IFR more
likely for the southern TAFs. Current area of -SHRA are light
and not restricting visby and expect that to continue. The
exception is after 06z with the risk for +SHRA/TSRA from HFD-
IJD-PVD-PYM southward into the Cape and Islands. Risk for
lightning and localized downpours for these areas. Can`t rule
out a shower north of these areas but not expecting much north
of this line. SSW winds around 10-15 kt with gusts 20-25 kt.

Sunday...Moderate confidence.

Conditions improve to VFR interior by 16z. However, IFR may
linger through the afternoon along the south coast. SW gusts to
20-25 kt. Showers and possibly a few t-storms should be exiting
the Cape/Islands in the morning. Then another round of
showers/t-storms expected to develop after 18z in the interior,
reaching the south coast in the evening. A few strong to severe
storms possible with damaging wind gusts the primary threat.

Sunday night...Moderate confidence.

IFR with showers/t-storms in the evening along the south coast,
then improving to VFR overnight. Wind shift to NW 00-06z.

KBOS TAF...Moderate confidence in TAF. Generally OVC VFR with
P6SM -SHRA, with deterioration to at least MVFR ceilings
overnight. Improving to VFR around 16z Sun with another round
of showers/t-storms after 20z. SSW winds around 10-15 kt with
gusts 20-24 kt.

KBDL TAF...Moderate confidence in TAF. MVFR cigs lowering to IFR
this evening as light showers move across the region. Low risk
for embedded thunder after 06z. Improving to VFR by 15z Sun
with another round of showers/t-storms after 20z.

Outlook /Monday through Thursday/...

Monday: VFR. Breezy. Slight chance SHRA.

Monday Night through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

Wednesday Night: VFR. Breezy. Chance SHRA.

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

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Sunday night...High confidence.

Gusty S winds tonight becoming SW on Sunday with gusts to 25 kt at
times. SCA continued for all waters. Winds shift to NW overnight Sun
night behind the cold front. Vsbys reduced in developing showers and
fog tonight, with fog lingering on Sunday especially south coastal
waters. A few t-storms possible over southern waters late tonight
and Sun morning, then more showers and t-storms moving across the
waters late Sun and Sun night.

Outlook /Monday through Thursday/...

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

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, isolated
thunderstorms.

&&

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

&&

$$

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