NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Jun 29, 4:43 AM EST, 4 hours 17 minutes earlier.
Area forecast discussion, Jun 29, 4:43 AM EST, 4 hours 17 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
543 AM EDT Tue Jun 29 2021
.SYNOPSIS...
Bermuda high pressure will slowly move to the south and east through
the end of the week. A cold front will approach the region from the
west on Thursday then stalls nearby through the weekend.
&&
.NEAR TERM /THROUGH TONIGHT/...
The excessive heat continues today, with the upper ridge remaining
right near the central Mid-Atlantic coast. With time, there will be
a very gradual weakening trend as it shifts slowly southward.
However, its effects will be felt in full today, with strong
subsidence (particularly in the central/southern CWA) and increasing
thicknesses allowing for temperatures to warm a couple more degrees
versus yesterday while dew points remain near 70 degrees. Model
soundings suggest mixing will likely result in dew points falling a
couple degrees in the afternoon, and for this reason, decided to
lower forecast heat indices a degree or two. However, this is really
splitting hairs. It will be hot and humid today...definitely a day
to seek air conditioning. Forecast highs today are mostly in the
90s, with lows tonight in the lower to mid 70s.
Few changes were made to the heat products today. Added Salem County
in New Jersey to the heat advisory, as they were too close for
comfort with the 105+ heat index criterion to keep out.
Additionally, added the coastal portions of Monmouth County to the
heat advisory, as guidance seems to be uniformly suggesting heat
indices reaching the century mark this afternoon. Heat indices
yesterday in these two zones were quite close to criteria, an
indication of a subtle low bias with statistical guidance. This bias
was also observed on the Eastern Shore of Maryland. Per coordination
with LWX, did hold off on expanding the advisory for these areas, as
guidance remained a few degrees below thresholds today. However,
think locations near the coast will be flirting with 105-degree heat
indices this afternoon.
Other concern today is chances for convection this afternoon.
Convection-allowing models are fairly skittish overall, but
consensus suggests coverage may be a little better than what was
observed yesterday. For now, have slight-chance to chance PoPs
northwest of the Fall Line. Model soundings suggest pulse-like
storms with slow storm motions producing very localized downpours.
Any convection should diminish quickly by sunset.
&&
.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
The forecast becomes more complicated on Wednesday, as a digging
trough in eastern Canada begins to suppress the ridge plaguing our
region the past several days. An attendant cold front in advance of
the vort max will likely initiate scattered to numerous storms from
the Ohio Valley to New England by early afternoon. The front will
make very slow progress southward, but the convection may begin to
outpace it via the development/consolidation of cold pools. This
process may take most of the afternoon before reaching even our
northern CWA. Thus, another hot/humid day is expected for the area
in advance of the convection.
I am a little concerned that conditions may be a little worse on
Wednesday than guidance is advertising. Though mixing will likely
lower dew points again during the afternoon in central/southern
portions of the CWA, I suspect models are underplaying the moisture
pooling in the warm sector in the immediate downstream of the
front/convection. For this reason, I went slightly higher than
consensus with forecast temperatures/dew points on Wednesday. Though
in a strict sense, most of the area within the excessive heat
warning for today is forecast to be just shy of criteria on
Wednesday, I was compelled to extend the warning through Wednesday
for these areas given that (1) this will be day 4 of heat products
for the urban corridor and (2) the effects of long-duration heat
events tend to become exacerbated with time, even if slightly lower
heat indices are observed at the tail end of the event.
Only other change to the heat products for Wednesday was to extend
the advisory for Sussex and Warren Counties in New Jersey, as
guidance is advertising widespread 100+ heat indices in advance of
the approaching convection. An advisory was considered for our
Maryland counties, but per coordination with LWX, held off for now
to await later guidance.
As the convection organizes while approaching the northern CWA
during the afternoon, the environment downstream will become quite
unstable (with NAM Nest model soundings suggesting 2500+ J/kg MLCAPE
during the afternoon). Though shear will not be particularly high,
thermodynamic profiles are quite favorable for strong cold pools,
with a well-mixed boundary layer beneath fairly dry midlevels. SPC
has upgraded the northern CWA to a slight risk of severe storms,
which seems reasonable given the ambient environment as well as CAM
guidance indicating organization to the storms upstream during the
afternoon. Strong/damaging wind gusts would be the primary concern,
though with PWs 1.5-2.0+ inches, heavy rainfall will be possible
with the most intense convection, with localized flooding not out of
the question in areas of cell training.
Convection will likely weaken by late evening into the overnight
hours, but may maintain organization later than usual owing to the
moderate-to-strong instability expected during the day (as well as
potentially elevated surface winds inhibiting nocturnal
stabilization). Kept mentionable PoPs through the overnight hours to
address this concern.
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
Another night of model runs and another night of things remaining
unclear through the weekend.
The good...the worst of the heat should be ending Thursday as cloud
cover increases across the region. Daytime max temps may reach into
the lower 90s, especially in the more urbanized areas, and with dew
points still in the lower 70s it will certainly be muggy out but we
should remain below advisory criteria. As we head into Friday and
the weekend, temperatures will fall to below normal levels remaining
mostly in the 70s to near 80, though the southern Poconos and parts
of northwest New Jersey may stay in the upper 60s.
The bad..The cold front starts to sag into our area on Thursday.
There is uncertainty as to when the front will make its way into the
region and how far east it progresses before stalling. With several
shortwaves moving across the region along with sufficient daytime
heating, the main concern will thunderstorm development across our
area. SBCAPE values may near 2000+ J/kg and with strengthening low
and mid level winds, we may have the potential for some storms to
become severe. The other issue remains the potential for heavy
downpours as the PWATs remain substantially above normal and range
int eh 2-2.5 inch range. With the front remaining nearby through the
weekend with multiple shortwaves moving through the region, we will
remain rather wet and stormy, though things look to improve some as
we near the end of the weekend.
The ugly...the weekend isn't looking good and the models are not
doing a good job of handling the mid/upper levels well at all. There
has been considerable flip-flopping (mainly with the ECMWF) with the
potential upper level low from run to run and over the past several
nights it has been a good sign and lowers overall confidence in the
forecast. The timing and strength of this low will have marked
impacts for our weather over the holiday weekend.
As we head into the new week, things start to clear out as most long
range models show the upper low starting move off to the northeast
around Monday. Should that be the case, temperatures should start to
rebound some and warm back up near normal levels.
&&
.AVIATION /10Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Today...An area of stratus has developed off of the Delaware
Bay and is moving to the northeast this morning. IFR cigs will
remain possible at KMIV and KACY through 12z. Otherwise, mainly
VFR conditions expected. Isolated afternoon/evening showers or
thunderstorms possible, mainly near RDG and ABE. Southwest winds
around 5 to 10 knots, with some gusts near 20 knots through the
afternoon. High confidence.
Tonight...Mainly VFR conditions expected. Light southwest winds
around 5 knots or less. Fog possible late tonight. High
confidence.
Outlook...
Wednesday... Mainly VFR. Fog/stratus possible Wednesday night.
Afternoon showers and thunderstorms possible with sub-VFR conditions
and heavy rain. South to southwest winds around 5 to 10 knots with
gusts around 15 to 20 knots possible. Moderate confidence.
Thursday through Friday...Periods of MVFR or lower in showers and
thunderstorms and heavy rain. Southwest winds around 5 to 10 knots
on Thursday, becoming north to northeast around 5 to 10 knots on
Friday. Low confidence.
Saturday...Mainly VFR. Showers and thunderstorms possible with sub-
VFR conditions. North winds around 5 to 10 knots. Low
confidence.
&&
.MARINE...
Mostly sub-advisory conditions are expected through Wednesday night.
South to southwest winds of 10 to 20 kt will occur daily, with the
highest speeds/gusts during the afternoon hours. Seas will generally
range from 2 to 4 feet, though they may approach 5 feet away from
the coast on Wednesday. Dry weather should persist through most of
the day Wednesday, but storms are possible late in the day through
the overnight hours, especially off the New Jersey coast.
Outlook...
Sub-Advisory conditions are expected to continue across the area
waters. Seas start to build and near 5 feet late Thursday through
Saturday.
Rip Currents...
Expect similar conditions today as what we have seen the past
several days, with southerly winds of 10 to 20 mph and mid-period
swell. Expect a moderate risk for the formation of dangerous rip
currents along the New Jersey coast (though the risk is expected to
remain low in Monmouth County), while the risk is forecast to be low
for coastal southern Delaware. Similar conditions are forecast on
Wednesday.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Warning until 8 PM EDT Wednesday for PAZ070-071-
102-104-106.
Heat Advisory until 8 PM EDT Wednesday for PAZ060>062-101-103-
105.
NJ...Excessive Heat Warning until 8 PM EDT Wednesday for NJZ008>010-
012-013-015-017>019.
Heat Advisory until 8 PM EDT Wednesday for NJZ001-007-014-016-
020-027.
DE...Excessive Heat Warning until 8 PM EDT Wednesday for DEZ001.
MD...None.
MARINE...None.
&&
Synopsis...Meola
Near Term...CMS
Short Term...CMS
Long Term...Meola
Aviation...Meola
Marine...CMS/Meola