138
FXUS61 KBOX 010715
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
315 AM EDT Tue Jul 1 2025

.SYNOPSIS...

We`ll have two rounds of potential for showers and thunderstorms
today, first early this morning, then a much better chance this
afternoon into the evening. Today will be very warm and humid with
heat indices in the 90s. Warm but a bit less humid Wednesday and
Thursday then drying out with more seasonable temperatures heading
toward the 4th of July. An upper level disturbance may bring
scattered showers and thunderstorms Thursday, otherwise dry through
the end of the week. Heat and humidity return late weekend into
early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...

Key Messages...

* Fog and low clouds expand over southeast New England this morning
  with humidity increasing.

*  Early morning showers and embedded storms possible through around
   9 AM. Although severe weather isn`t expected, storms could be
   capable of downpours and lightning which could impact the morning
   commute.

* Another round of showers and thunderstorms develops by late
  afternoon/early evening, first in the interior (after 3 PM),
  moving southeast by sundown and offshore after midnight. Main risk
  is from torrential downpours capable of urban and street flooding,
  with possible flash flooding if training happens. More limited
  localized wind damage threat in wet downbursts.

* Very warm and humid.

The most active period of the forecast looks to be today into this
evening as a broad, vertically stacked trough digs into the
northeast. This will bring two rounds of showers with embedded
thunderstorms, first early this morning then again this afternoon
and evening.

This morning a warm front is draped from northwest to southeast
across the region, slowly moving north through the next several
hours. SW flow behind this front brings a plume of deep moisture
overhead (PWATs over 2 inches by 5am while dewpoints are already in
the upper 60s and low 70s). The result is a moist and conditionally-
unstable airmass ripe for showers and some garden variety
thunderstorms; best chance for thunder is in CT and RI between now
and 8 am where elevated instability is greatest (CAPE values 500-750
J/kg). However, confidence in coverage is low, especially given that
hi-res guidance has not verified well so far, indicating more
showers over SNE than we have at present. Given weak forcing
mechanisms and the elevated nature of the instability, not expecting
severe weather with any storms that do form this morning, but there
exists the risk for some localized ponding of water under any heavy
downpours. These showers should exit the east coast by around 10 am.

The only other thing to note this morning is the fog and low stratus
that has expanded generally along and south of the I-95 corridor
given those very elevated dewpoints. These high dewpoints will
contribute to the very high humidity, making highs in the upper
80s/low 90s feel more like the mid to upper 90s this afternoon.
Fortunately "feels like" temperatures like this are a short lived,
only lasting one day so Heat Advisory headlines are not in place.

The second and more potent round of thunderstorms comes with better
forcing and greater instability this afternoon and evening. Still
not overly confident on exactly where the storms will form this
afternoon but generally they should pop first over the
interior/northwest SNE then spread south and east through the late
afternoon and evening. Ahead of the cold front CAPE values in the
warm sector jump to 1000-2500 J/kg owing to the high temps and
dewpoints, though forecast soundings show it is of the tall, skinny
variety more associated with heavy downpours and localized flash
flooding than damaging winds or hail. Thus, this is the greatest
risk later today, though can`t rule out a secondary threat from
damaging winds in wet microbursts. Marginal bulk shear and poor mid
level lapse rates will keep the severe risk in check. Synoptic
forcing from placement beneath the rear entrance region of a 100kt
300 mb jet will provide plenty of lift and while the cold front
arrives after midnight, a cool front/surface boundary may provide
low level forcing, potentially lining up paralel to steering flow
which could increase the risk for training storms. The region is
under a Marginal Risk (level 1 of 4) of Excessive rainfall (i.e.
flash flooding potential) and a Marginal Risk (level 1 of 5) of
severe storms. Uncertainties remain with regard the slower timing of
the cold front, as well as how much convection we actually see this
morning and how quickly the cloudcover clears out to allow for
efficient diurnal heating. Certainly not a high confidence forecast
today with respect to timing and placement details. The showers
continue into the early morning hours Wednesday with the front
slowly getting closer but instability and the severe risk drops off
considerably after sunset.

&&

.SHORT TERM /WEDNESDAY/...

Key Messages...

* Warm Wednesday with the best chance for some showers or
  thunderstorms along and adjacent to the southeast coast.

The progression of the cold front has slowed a bit for Wednesday,
likely draped along the I-95 corridor in the morning then getting
hung up near the coastal plain through as late as Wednesday evening;
this keeps the south/southeast coast in the warm sector with PWATs
continued near 2 inches; likely that we`ll continue to see showers
and some embedded thunder Wednesday morning/early afternoon over the
southeast coast. The front will usher in drier air on Wednesday as
well with dewpoints falling from northwest to southeast through the
day. This will make it feel less humid than Tuesday, especially
north.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Key Messages:

* Very warm Thu with scattered showers and t-storms possible

* Drying out with more seasonable temperatures and less humidity for
  the 4th of July

* Heat and humidity return Sun-Mon

Details...

Upper trough amplifies over the NE Thu-Fri as upper low digs
southward through Quebec and into northern New Eng Fri. Increasing
forcing for ascent ahead of the trough will lead to chance of
showers/t-storms Thu afternoon, especially interior. Given
increasing mid level wind field and favorable mid level lapse rates
approaching 6.5 C/km, we think there will be a conditional severe wx
threat if there is enough moisture/instability. The magnitude of
instability is uncertain and SREF probs are rather meager with
CAPES. Instability will be the key for Thu, but CSU and NCAR ML
guidance is indicating some risk for severe. Very warm temps are
expected Thu but dewpoints may mix out and fall through the 50s
which would be a inhibiting factor. Stay tuned. Then cold front is
expected to move offshore by Fri with NW flow bringing less humid
airmass and more seasonable temps for the 4th of July.

Looking ahead to next weekend into early next week, upper trough
exits followed by rising heights and ridging building back into the
region. This will result in increasing heat and humidity, especially
by Sun and Mon.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

06Z TAF Update:

Through 12Z: High confidence in TAF/trends, moderate on timing.

IFR-LIFR stratus and fog continues to advances northward from
Cape Cod and immediate south coast overspreading RI and SE MA to
south of BOS by 07z, with MVFR further inland. SHRA/possible
embedded TSRA moves in from eastern/southern NY after 07z, TS
possible anywhere but somewhat better chance for the BDL/HFD-
PVD/SE MA area. These storms would not become strong were any to
develop but could still pose risk for lightning and brief IFR
visby downpours.

Today: High confidence in TAF/trends but moderate on timing.

Early-day SHRA/possible TS thru 15z then trends to VFR. Second
round of TS more likely after 20z, but could start as soon as
18z in interior Southern New England. Storms could produce
torrential downpours and brief gusty winds. SW winds around 5-10
kt with occasional gusts.

Tonight: High confidence.

SHRA/TS moves southeast towards the southern waters thru 06-09z,
possibly longer for Cape and island terminals, with gradual
improvement to VFR thereafter. Possible BR/FG south coast until
SHRA/TS clears the southern waters.

KBOS Terminal...Moderate confidence in TAF. Stratus will
approach the airport from the south after 07z. Confidence in
IFR in general is low to moderate (possible it stays just
south) but higher confidence that IFR cigs materialize than
vsbys, so continuing a BKN010 in the TAF to indicate near IFR
conditions. Early morning SHRA possible from 09-14z, outside
chance at TS but better chance south and west. VFR after approx
15-22z with SW breezes but risk for SHRA/TS returns by 22z.

KBDL Terminal...Moderate confidence in TAF. VFR thru 08z, SHRA
with embedded TS possible 08-12z. VFR thereafter approx 13-19z
with S winds but risk for SHRA/TS returns by 19z.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR.

Thursday: VFR. Slight chance SHRA, isolated TSRA.

Thursday Night through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Today and tonight: Moderate confidence.

SCA remains valid for today as SW winds increase to around 25
kt, with seas building to 4-6 ft. Early-morning showers and
thunderstorms possible, then dry weather for most of the
afternoon. The risk increases for showers and thunderstorms
again for late in the day into the mid Tuesday evening hours,
some could be capable of frequent lightning, heavy downpours and
locally rough seas.

Wednesday: Moderate confidence.

Lingering showers and thundertorms possible much of the day,
especially on the southern waters. SW winds 10-15 kts with seas
3-5 ft on the southern waters where the SCA continues; 2-3 ft
elsewhere.

Outlook /Wednesday Night through Saturday/...

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

Thursday through Friday Night: Winds less than 25 kt. Seas
locally approaching 5 ft.

Saturday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 11 AM this morning to 8 PM EDT
     Wednesday for ANZ250-254.
     Small Craft Advisory from 11 AM this morning to 6 AM EDT
     Wednesday for ANZ255-256.

&&

$$

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