BOX issues Area Forecast Discussion (AFD) at Jul 24, 1:28 PM EDT855
FXUS61 KBOX 241728
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
128 PM EDT Wed Jul 24 2024
.SYNOPSIS...
Unsettled again today with quasistationary boundary draped
across the waters. Shower and thunderstorms tonight across the
western portions of our CWA ahead of approaching cold front. A
stronger front will move across the region late Thursday and
Thursday evening and may be accompanied by a few showers and
thunderstorms. Dry and seasonable conditions follow Friday with
warming temperatures this weekend, then heat and humidity will
begin to build early next week.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Showers associated with the shortwave passing overhead continue
to blossom over CT moving northeast with more activity further
west over NY. Expecting the bulk of the showers currently over
SNE to exit by 1-2 PM. Areal coverage is uncertain but best
chance of rainfall will likely be near the south coast along the
northern gradient of high PWAT axis. But convection also
possible further in the interior where marginal instability will
be present. Otherwise, SNE will be on the cool side of a
stalled boundary to the south with easterly flow resulting in
lots of clouds and temps in the 70s. It will be rather humid
with dewpoints climbing to upper 60s and lower 70s.
Previous discussion...
Synoptic pattern remains relatively unchanged from Tuesday with
broad trough centered over southern Ontario/Great Lakes and a
stationary boundary in the vicinity of southern New England. There
remains considerable moisture in the lowest levels of the column,
with patchy drizzle present across portions of interior southern New
England this morning.
Shortwave remains on track to eject off the mid-Atlantic coast, with
the northern fringes of the precipitation shield now visible on our
radar skirting the south coast of Long Island; though remaining
south of the Islands for now. Overnight guidance remains a bit
inconsistent with the position of the boundary and the northern
extent of the precipitation shield, which introduces uncertainty in
who will actually see shower activity today. General thinking is
that the best chance for scattered showers this morning will occur
SE of I-95, with scattered showers developing between 17-21Z for
portions of the interior ahead of an approaching cold front.
Did trend the forecast in a drier direction today given the tendency
for guidance to overestimate the northern extent of the boundary
over the last several forecast cycles. In light of PWAT's in excess
of 2" along the south coast and warm cloud depths over 12-14k
ft, the threat for heavy rain has shifted off the coast, with
HREF 6hr PMM showing localized maxima of just 0.25" across the
Islands through 18Z, pretty much eliminating the chance for
urban/poor drainage flooding today. In sum, today wont be a
washout for any particular location.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
* Threat for isolated downpours and thunderstorms overnight tonight
for western MA/northern CT between ~03-07Z ahead of approaching
cold front
* Decreasing potential for isolated severe storms on Thursday;
potential for a "shutout" thanks to stabilizing cloud cover
Tonight...
Trough shifts east tonight along with a cold front that will bring
the chance for scattered downpours and perhaps a rumble of thunder
tonight to the western half of our CWA. Overall, it appears that the
most widespread showers and isolated storms will occur late tonight,
between 03-07Z. With residual healthy PWATs and deep warm cloud
depths, could see localized downpours capable of dropping a quick
half inch of rain across western MA, though the prospect of rain
east of the Connecticut River Valley looks dismal. Given the very
isolated risk for localized downpours, did not consider issuing a
flood watch for any portion of the region.
Patchy fog looks likely tonight, especially along the coast, as
winds shift more southerly overnight.
Thursday...
In contrast to previous updates, the risk for widely scattered
thunderstorms has decreased tomorrow, with most if not all HREF
members trending towards a shutout during the daylight hours of
Thursday. While there appears to be ample forcing with the
approaching front/height falls and modest instability of near
1000J/kg, mid level lapse rates are quite poor, around 5C/km. In
addition to poor lapse rates, expecting it to be quite cloudy for
the better part of the daylight hours until the front leads to rapid
clearing tomorrow evening, which will inhibit destabilization
and storm development. Given conflicting signals, do estimate
that guidance like the HRRR, which essentially shows a shut out
tomorrow, is a bit too conservative in shower and storm
coverage, have again trended the forecast drier for the period.
Showers look to be widely scattered to isolated with perhaps a
few embedded thunderstorms if clearing occurs earlier than
forecast. Overall, think the severe threat is rather low, but
can't rule out isolated storms capable of gusty winds;
corroborated by the CSU machine learning probs showing a 5% risk
of damaging winds. SPC continues to maintain a "general
thunder" risk for our region, attributing the lack of severe
potential to the aforementioned cloud cover. Will note that the
GFS remains the "wet" outlier for Thursday, maintaining it's
estimate of more widespread scattered showers and thunderstorms.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Key Points...
* Dry and seasonable conditions Fri with a warming trend this
weekend
* Heat and humidity builds early next week
Thursday night and Friday...
Cold front near the coastal plain Thu evening will move off the
coast by midnight. Low risk for a few showers and perhaps a t-storm
near the south coast in the evening within the moisture axis ahead
of the front, then a drying trend will take place overnight with
clearing skies and decreasing humidity as the front moves offshore.
Post-frontal airmass on Friday with seasonable temps and sct-bkn
diurnal cu developing. Overall a mostly sunny day with seasonable
temps 80-85 and near normal humidity for late July with dewpoints
upper 50s and lower 60s.
Saturday through Tuesday...
An extended period of mainly dry weather is anticipated this weekend
into early next week as high pres takes control. However, will have
to watch a weakness in the upper flow off the New Eng coast
downstream of the building ridge Sun and Mon. Latest ensemble
guidance is overwhelmingly dry during this time so will maintain a
dry forecast. Forecast confidence decreases Tue due to timing
uncertainty with next approaching shortwave and assocd moisture
increase. Majority of ensembles keep it dry Tue for now.
Warming temps Sat and Sunday well into the 80s but with manageable
humidity, then there is a signal for increasing heat and humidity
early next week, especially away from the coast as temps aloft warm.
However, the upper level low just offshore Sun-Mon may lead to
onshore flow and somewhat cooler temps along the coast.
&&
.AVIATION /18Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18Z TAF Update:
This afternoon... moderate confidence
Mainly MVFR-IFR cigs will persist today, possibly improving to
a brief period of VFR in the CT valley. But IFR expected to
dominate in eastern MA later in the afternoon. Scattered
showers.
Tonight... Moderate confidence.
MVFR/IFR. NE/N winds. Scattered showers come to an end after
sunset but cant rule out some lingering drizzle. Fog development
very likely, especially along the south coast and across the
islands.
Thursday... Moderate Confidence. Early morning IFR/LIFR will
lift to VFR by mid morning. Cold frontal passage later Thursday
will lead to a wind shift with scattered TSRA. Confidence is low
in TSRA reaching the coastline.
Thursday night...High confidence.
VFR. Light W/NW winds.
KBOS TAF... Moderate confidence in TAF.
IFR redevelops this evening but potential cigs drop below 1000
ft before 23z. Shower activity should be done by 20-21Z.
KBDL TAF... Moderate confidence in TAF. Will see conditions
deteriorate to IFR again overnight with fog development likely.
Outlook /Thursday Night through Sunday/...
VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Through tonight... High Confidence.
Stationary boundary draped across the southern waters will provide a
source for shower and storms this afternoon along with periods
of poor visibility. Winds from the ESE to SE with speeds 10 to
15 knots and seas between 2 and 4 feet. Wind shift occurs late
Wednesday evening into Thursday morning as a weak front moves
across the waters, wind shift from SE to SW, wind speeds less
than 10 knots with seas 3 to 4 feet.
Thursday.... High confidence.
Marine fog/stratus will linger for a good portion of Thursday
morning with more southerly winds. Isolated shower or
thunderstorm possible along a cold front, with rapid clearing
expected behind the front as winds shift more to the west. Seas
remain below SCA criteria between 2-4ft.
Thursday night...High confidence.
Cold front crosses the waters overnight. NW winds 10-15 kts.
Seas 2-4 ft.
Outlook /Friday through Sunday/...
Friday through Sunday: Winds less than 25 kt.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
$$
SYNOPSIS...KJC/KS
NEAR TERM...BW/KS
SHORT TERM...KS
LONG TERM...KJC
AVIATION...BW/Guest
MARINE...BW/Guest
Source:
BOX issues Area Forecast Discussion (AFD) at Jul 24, 1:28 PM EDT----------------
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