FXUS61 KPHI 171012

Area Forecast Discussion
National Weather Service Mount Holly NJ
612 AM EDT Wed Jul 17 2019

The remnants of former tropical cyclone Barry will cross the
area later today into Thursday. High pressure will then
generally prevail into the weekend although a weak cold front
associated with a low in Canada may cross the area Sunday. A
more significant cold front will pass through the area early
next week.


No significant changes to the forecast with this update. Will
tweak hourly grids based on latest surface observations.

There are several issues on tap for today. Behind a departing
warm front this morning, and then SW flow ushers an increasingly
hot and humid airmass into the Northeast and Mid-Atlantic. High
temperatures will soar into the low to mid 90s with the highest
temperatures along the I-95 corridor and into southeast New
Jersey and Delmarva. Surface dewpoints climb well into the low
to mid 70s, and even into the upper 70s in parts of the eastern
shore of Maryland due to flow off the Chesapeake Bay. Some
mixing is possible in the afternoon, but dewpoints would only
drop by a few degrees or so, and this really will not have much
of an effect on the heat index.

Max heat index values will approach 100 degrees across the
southern Poconos, Lehigh Valley and far northern New Jersey,
while max heat index values up to 105 are possible across
portions of southeast Pennsylvania and most of New Jersey. The
max heat index across parts of Delmarva could be as high as 110.

Will keep existing Heat Advisories and Excessive Heat Warnings
in place.

The other concern for today is that with the high heat and
humidity, surface-based CAPE values will range from 1000-2000
J/kg, but could be higher in some spots. PWATs will be well in
excess of 2-2.5 inches, and with the approach of the remnants of
Barry later today, this sets the stage for afternoon showers
and thunderstorms. There is not much 0-6 km Bulk Shear,
generally upwards of 20 kt, but with abundant moisture in place,
the primary threat will be heavy rain/flash flooding, but
cannot rule out severe storms with damaging wind gusts.

Though convection begins in the afternoon, the most likely
threat for flash flooding and severe weather will be in the
evening, after 6 pm.


As has been the case in the last several months, the lowest
Flash Flood Guidance values are across much of New Jersey and
southeast Pennsylvania, generally 1.5-2 inches in 1 hour, and as
high as 3 inches in 3 hours, and as high as 4 inches in 6
hours. Based on latest model QPF fields from the HRRR, NAM NEST,
Hi-res ARW and Hi-res NMM, think there will be spots that pick
up 3+ inches of QPF from late afternoon through this evening.
Even the HREF Ensemble max QPF showing pockets of 3-5 inches of

Will go ahead and issue a Flash Flood Watch starting in the
afternoon, but running through 1 am.

There is little consistency among the models as to where the
heaviest precip will fall, but think focus will be along the
I-95 corridor and areas west. For Delmarva and southeast New
Jersey, FFG values are quite a bit higher. Cannot rule out
flooding and flash flooding, but will hold off on the Watch for
those areas.

Warm and muggy tonight with lows in the 70s and dewpoints not
much lower than the ambient air temperature. With the moisture
from the rainfall, can expect fog and low stratus to develop in
the wake of the showers and thunderstorms.


Synoptic Overview: The long term period begins with both the
mid-lvl and lower lvl circulations associated with Barry's
remnants having just crossed the area. An expansive H5 ridge
will then set up over the southern-tier of the US on Friday. The
ridge will remain in place until Sunday at which point a
shortwave trough digging from Canada will act to split it. The
western half of the ridge will retrograde into the southwestern
US while the eastern half is pushed offshore into the Atlantic.
A fairly healthy (by July standards) cold front associated with
the shortwave's surface reflection will cross the area Monday
into Tuesday.

The main story of the extended continues to be the potential
for excessive heat Friday through Sunday. Fortunately it appears
that beginning early next week the overall synoptic pattern
will become characterized by longwave troughing over the eastern
US. This pattern would favor a more mild end of July.


Thursday... Although deep-layer moisture and dynamic lift will
decrease on Thursday as Barry's remnants begin to pull away,
sufficient lingering moisture and instability should be
sufficient for continued showers and thunderstorms. Think the
overall flash flood and severe threats will be lower during the
day on Thursday given the lower moisture/instability parameters
and modest height rises behind the departing shortwave. Thursday
will also be a bit cooler than any of the days surrounding it
due to considerable cloud cover/precipitation, and generally
think highs will struggle to reach 90. That being said heat
indices could still very well top out near 100 in portions of
the urban corridor as dewpoints remain elevated with somewhat
inhibited mixing due to the aforementioned cloud cover.

Friday/Saturday... Friday will be hot and Saturday will be very
hot as the ridge builds to our SW. Heat indices will generally
be in the 100-105 degree range on Friday, while on Saturday
105-110 readings will be common (with locally higher values even
possible). The only real uncertainty is how efficiently
dewpoints can mix down in the afternoons, but regardless
dangerous heat seems like a certainty. Additionally lows in the
upper 70s to low 80s Saturday and Sunday mornings will not help
with any recovery. Kept PoPs below mentionable both Friday and
Saturday although can't totally rule out some weakly-forced
diurnal storms (e.g. sea breeze or terrain related).

Sunday... A weak cold front may sneak by the area early Sunday
which will result in temperatures slightly cooler than Saturday,
but still very warm i.e. heat indices topping out in the
100-105 range. As the ridge flattens and shortwave energy
approaches from the west the chance of showers and thunderstorms
also increases.

Monday/Tuesday... The main cold front looks to cross the area
in this time frame and will likely be accompanied by some
precipitation given its dynamic support (an amplifying trough
and southward sagging upper jet). Temperatures Monday are a bit
uncertain and will depend largely on frontal/precipitation
timing, however generally think they will top out below any heat
advisory criteria. By Tuesday generally expect high
temperatures in the 80s and dewpoints dropping down into the 60s
with this more mild airmass likely sticking around a bit.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Mainly VFR. SHRA/TSRA possible this afternoon, first at
KRDG/KABE, then south towards KTTN/KPNE/KPHL. MVFR or lower
conditions in SHRA/TSRA, and TSRA will be capable of torrential
rain and strong wind gusts. Winds otherwise SW 5-109 kt, except
at KACY/KMIV/KILG, where afternoon sea breezes could turn winds
to more of a southerly direction.

Tonight...MVFR or lower in SHRA/TSRA through around 06Z, then
MVFR or lower in fog/stratus. SW winds 5-10 kt, becoming


Thursday...Frequent showers and a chance of thunderstorms will
bring a chance of lower CIGS/VSBYS at times. Possibly some
patchy fog Thursday night. Winds generally 5 kts or less.

Friday through Sunday...Mostly VFR with winds generally
southwesterly/westerly 5-10 kts. Some showers and thunderstorms
possible on Sunday.


Sub-SCA conditions on tap for today and tonight with seas
generally around 2-3 feet and SW winds 10-15 kts gusting to 20
kts or less.

This afternoon and tonight, showers and thunderstorms will move
into the waters and could produce locally strong winds, higher
seas, and reduced visibilities.


Sub-SCA conditions are expected through Sunday winds favoring a
southerly/southwesterly direction 10-15 kts and seas generally
3 feet or less. Thunderstorms Thursday may produce locally
higher winds and seas.

Rip Currents...

There is a high risk for the development of dangerous rip
currents at the New Jersey beaches this afternoon and a moderate
risk for the development of dangerous rip currents at Delaware
beaches today.

Southwest winds become south this afternoon at 15-20 kt with
long period swells. Additionally, low tide will occur during mid
to late afternoon. Given these factors plus the proximity to
the full moon, feel the risk will be higher in New Jersey where
shoreline orientation is more favorable.


PA...Flash Flood Watch from 2 PM EDT this afternoon through late
tonight for PAZ060>062-070-071-101>106.
Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for PAZ101-103-105.
Excessive Heat Warning until 10 PM EDT Sunday for PAZ070-071-
NJ...Flash Flood Watch from 2 PM EDT this afternoon through late
tonight for NJZ001-007>010-012-013-015>019.
Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for NJZ009-010-012-013-016-020>022-027.
High Rip Current Risk from 11 AM EDT this morning through this
evening for NJZ014-024>026.
Excessive Heat Warning until 10 PM EDT Sunday for NJZ015-
DE...Flash Flood Watch from 2 PM EDT this afternoon through late
tonight for DEZ001.
Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for DEZ002-003.
Excessive Heat Warning until 10 PM EDT Sunday for DEZ001.
MD...Flash Flood Watch from 2 PM EDT this afternoon through late
tonight for MDZ008.
Heat Advisory from 11 AM this morning to 8 PM EDT this evening
for MDZ008-012-015-019-020.


Near Term...MPS
Short Term...MPS
Long Term...Carr
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