BOX issues Area Forecast Discussion (AFD) at Aug 4, 6:30 PM EDT094
FXUS61 KBOX 042230
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
630 PM EDT Sun Aug 4 2024
.SYNOPSIS...
Showers and thunderstorms continue this evening before tapering
off tonight. Hot and humid Monday with a low chance for severe
thunderstorms in the late afternoon. Stalled cold front mid
week will bring near daily rain chances. Now tropical storm
Debby looks to impact our region, bringing the potential for
heavy rain and flooding, Friday through Sunday.
&&
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Tonight:
Line of thunderstorms has progressed east into RI and SE MA.
With the loss of diurnal heating and more stable marine air,
storms are slowly weakening. With MLCAPE values dropping to
1000 J/kg or less and the boundary layer starting to decouple
and stabilize, not expecting any more severe winds at the
surface. Lightning activity could last though 8-10pm with the
leftover elevated instability. Models have backed off on showers
and thunderstorms staying alive over the Cape and Islands late
this evening. Overnight lows remain elevated in the upper 60s to
low 70s with dewpoints still in the upper 60s as well.
&&
.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
Monday:
Shortwave moves offshore, but little change is the airmass is
expected as SW flow continues. Skies tomorrow start out sunny, but
mid to high clouds form ahead of another passing shortwave. The main
forcing from the shortwave passes to the north of SNE in VT and NH.
This should keep the best chance for thunderstorms to the north.
However, if any thunderstorms do form in SNE tomorrow, they
will be in an environment sufficient to produce severe weather.
Mean SBCAPE values range from 1500-2500 J/kg tomorrow afternoon
with 30-40 knots of 0-6km bulk shear. Mid-level lapse rates
become more favorable tomorrow approaching 6.5C/km. The primary
severe threat appears to be wind again with DCAPE values around
700-800 J/kg and steep low level lapse rates approaching 10C/km.
There could be some small hail given the marginally steep mid
level lapse rates and strong shear, however, freezing levels are
still very high at 12-13kft. Heavy rain is possible with any
storms that form with PWATS still near 1.5 inches. The main
window for severe thunderstorms tomorrow appears to between 2pm
to 10pm, but uncertainty remains high as most of the Hi- res
guidance is showing the bulk of the thunderstorm activity
staying to the north of SNE.
It will still be hot and humid tomorrow with mostly sunny skies.
High temps reach into the upper 80s to low 90s with dewpoints still
in the upper 60s to low 70s. Heat indicies will approach the mid
90s.
Monday Night
Cold front drops south form northern New England that may bring
more showers and thunderstorms, but confidence is low at this
time. Some of the hi-res guidance does show thunderstorm
activity starting around midnight on Tuesday, but will likely be
dependent on how much instability is leftover. HREF shows
500-1000 J/kg of MUCAPE still available past midnight. With deep
layer shear increasing overnight to 40-50 knots, there is a low
chance these thunderstorms could become severe, however, as the
boundary layer decouples, it will be more difficult for severe
wind gusts to reach the surface. Overnight lows stay warm in
the upper 60s to low 70s again, but dewpoints begin to fall into
the mid 60s.
&&
.LONG TERM /TUESDAY THROUGH SUNDAY/...
* Stalled front Tuesday/Wednesday will yield unsettled weather mid
week, though the potential for severe weather is low
* Best chance for dry weather comes Wednesday night into the first
half of Thursday
* Now Tropical Storm Debby may impact the region late next week or
next weekend; high uncertainty in timing/impacts
We're expecting an unsettled week ahead with persistent mid level
troughing over the northeast and a stalled cold front that will
bring near daily rain chances to the region through Thursday. Beyond
the first few days of the forecast, our attention is focused on now
Tropical Storm Debby, which is slated to make landfall, possibly as
a Hurricane in the big bend of Florida, in the next day.
Through Thursday...
As mentioned by the previous forecaster, cold front gets hung up
near or just south of the south coast Tuesdays and will linger for
much of mid-week. While temperatures will be much cooler than we've
seen the last few weeks, and actually several degrees below normal,
PWATs will remain high, some 1.5-2 standard deviations of normal,
thanks to the tropical moisture plume advective northward from the
deep south. Agree that the risk for the most persistent rain/shower
activity will be south of the MA Turnpike, and primarily across
CT, RI and SE MA, with Wednesday targeted as the wettest day as the
front wobbles a bit further north. Previous forecaster also
mentioned the still deep warm cloud depths, which combined with high
PWATs will lead to downpours, which will pose a threat for urban and
poor drainage flooding. Additionally, the orientation of the frontal
boundary, west to east, could result in some training. Continue to
feel the marginal risk ERO from WPC covers the risk well. Will note
that late Wednesday evening to Thursday morning looks like the best
chance for a prolonged dry period, which would be beneficial in
allowing the region to dry out slightly before the weekend. Did take
this opportunity to lower PoPs in the late Wednesday to Thursday
timeframe to reflect this possibility.
Tropical Storm Debby...
There remains a lot of uncertainty surrounding the late week and
early weekend forecast regarding the track of Tropical Storm Debby
and what impacts might be seen in southern New England. While there
is rather high confidence that Debby gets "hung up" in the
Charleston/Savannah area early this week anticipating
the low, and moisture, will eventually be steered towards our
region. Impacts from Debby may begin as early as Friday
and last through Sunday evening. Ensemble probabilities paint a
picture of a 10 to perhaps 40% chance of 24 hour QPF exceeding 2"
and a few pixels of exceeding 4" of QPF during the same period.
Combining this with deterministic runs, with both the GFS and ECWMF
showing wide swaths of 2-5"+ of rain across the region, its quite
reasonable to deduce, based on the current forecast track of the
storm, that this storm will bring a considerable amount of rain to
SNE. In fact, WPC has placed us in an uncommon day FIVE marginal
risk for excessive rainfall, which may be elevated in later updates.
We will have a better grasp on the expected max precipitation
potential and location of heaviest rain over the next several
days.
&&
.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Rest of Today...High Confidence.
VFR outside of convection with SW winds 5-10 kts and some 20-30
gusts across the Cape/Islands. Round of scattered showers and
thunderstorms this afternoon into the evening. Previous shift
tempo groups looked quite reasonable, so have kept it. Best shot
is from 19-23Z roughly. If a stronger storm moves over a
terminal could have strong to damaging NNW winds and heavy
downpours.
Tonight... Moderate Confidence
Showers and thunderstorms begin to move offshore after 00z with
only lingering showers behind. The Cape and Islands could see
showers and thunderstorms linger through 06z, then possibly
redevelop after 06z to 12z, but confidence is low. VFR across
the interior terminals with IFR/LIFR for the Cape and Islands
Monday...Moderate confidence
VFR with mainly dry conditions except over the Cape and Islands
that could see MVFR/IFR conditions continue through the
afternoon. Not very confident in thunderstorm activity tomorrow
as much of the activity should stay to the north and west. If
storms do form, they would likely form after 18 to 20z and pose
a severe wind threat.
Monday Night...High Confidence
VFR with a low chance for another round of thunderstorms. Winds
turn north at 5-15 knots.
KBOS TAF...Moderate Confidence.
Round of thunderstorms will reach BOS between 19z-21z and could
last as late as 00z tonight. Winds will likely turn NNW as
thunderstorms move through. Some lingering showers are possible
after 00z, otherwise VFR and dry for the remainder of the TAF
period.
KBDL TAF...Moderate Confidence.
Thunderstorms are possible this afternoon through 20-21z.
Otherwise VFR for the TAF period. Low chance for thunderstorms
tomorrow afternoon.
Outlook /Tuesday through Friday/...
Tuesday: VFR. Chance SHRA, isolated TSRA.
Tuesday Night: Mainly MVFR, with areas VFR possible. SHRA
likely, isolated TSRA.
Wednesday: VFR. Chance SHRA.
Wednesday Night: VFR. Slight chance SHRA.
Thursday through Thursday Night: VFR. Chance SHRA.
Friday: MVFR/IFR conditions possible. Breezy. SHRA likely,
isolated TSRA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Tonight through Monday night...High Confidence.
Persistent southwest flow will result in marginal small craft seas
across the southern outer-waters right into Monday night and
will allow headlines for that region to continue. In
addition, small craft headlines this afternoon for Buzzards
Bay,Cape Cod Bay, and Nantucket sound continue though midnight.
Good mixing near the land should result in the potential for a
period of southwest 30 knot wind gusts and choppy seas. Similar
conditions are also possible for Monday afternoon, but likely
will remain below small craft criteria.
Showers and thunderstorms over the northern waters this evening
will move south into the southern water overnight, eventually
moving well out to sea by Monday morning. More showers and
thunderstorms possible near the waters Monday night.
Outlook /Tuesday through Friday/...
Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, isolated thunderstorms.
Tuesday Night: Winds less than 25 kt. Rain showers likely,
isolated thunderstorms.
Wednesday: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, isolated thunderstorms.
Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Slight chance of rain showers.
Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers.
Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Rain showers likely, isolated thunderstorms. Local visibility
1 to 3 nm.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ231>234.
Small Craft Advisory until 8 AM EDT Tuesday for ANZ254>256.
&&
$$
SYNOPSIS...KP/KS
NEAR TERM...KP
SHORT TERM...KP
LONG TERM...KS
AVIATION...KP/KS
MARINE...KP/KS
Source:
BOX issues Area Forecast Discussion (AFD) at Aug 4, 6:30 PM EDT----------------
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