Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 220558
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
158 AM EDT Sat Jul 22 2017

.SYNOPSIS...
A very warm and humid air mass will remain over the area
through this weekend and possibly into Monday. Meanwhile, low
pressure along a roughly west to east frontal boundary will
result in periods of showers and thunderstorms. By Tuesday, high
pressure will build over the area bringing somewhat milder and
drier conditions through through the middle of next week. A
seasonally strong cold front may cross the area later in the
week.

&&

.NEAR TERM /THROUGH TODAY/...
Storms are developing rapidly in the Delmarva Peninsula.
Lightning flash rates with the stronger cells are ramping up,
and rain rates are locally 1-3 inches per hour. High-octane air
is in place, with MUCAPEs > 2000 J/kg and PWs 1.75-2.25+ inches
in this region. Large-scale ascent just downstream of a weak
perturbation moving through the southern Mid-Atlantic is likely
aiding in storm development, with smaller-scale effects
associated with subtle surface boundaries and a nocturnally-
favored warm-air advection regime. Bumped up PoPs even more in
this area, and think there is some potential for locally heavy
rainfall accumulations (though totals will be mitigated by
reasonable storm motions and the scattered nature of the
convection). Storms are expected to move into far southern New
Jersey by late tonight.

Meanwhile, the forecast today has been updated to reflect latest
thinking on timing of convection moving through during the late
afternoon. Hi-res guidance is beginning to converge on the most
favorable timing generally after 3 pm from west to east across
the area. The 00Z NAM Nest is hinting at the potential for
multiple rounds of convection, with perhaps another convective
complex this evening. This increases hydro concerns,
particularly given the troubling high QPF output from the NAM
Nest (and NAM), RAP, UKMET, and CMC. Don`t think this is flood
watch material (yet), but with more convection expected later
this weekend ... think the concerns will increase with time.
Would not be surprised to see a few flooding issues this
afternoon/tonight as well, though this may be more of the
urban/poor-drainage variety, for the most part.

Chance of severe storms today is certainly present. Forecast
soundings show 2000+ MLCAPE and deep-layer shear of 30+ kts this
afternoon, with effective shear approaching/exceeding 40 kts by
early evening. With moist midlevels and a well-mixed boundary
layer this afternoon, wet microburst potential is present, with
cold-pool organization likely given degree of instability/shear.
Shear looks to increase this evening, so the potential will be
present for embedded mesovortices within any convective
lines/clusters that develop. Think the risk of severe is high
enough for inclusion in the grids at this point.

Will update discussion with latest thinking on the heat today in
about an hour.

&&

.SHORT TERM /TONIGHT/...
Low-confidence forecast overnight with potential for a round or
two of storms in a moderately unstable environment. With
adequate shear for severe storms, mention of this potential
remains through the early morning hours. However, with the first
round of convection likely to move through during the early
evening, there is some question as to what occurs thereafter. If
another convective complex enters the region, threat of heavy
rainfall (with some suggestion of this in most model output
currently) will be elevated, especially north of the Mason-Dixon
Line (as a weak surface boundary slowly lifts northward).
However, there is a decent chance there will be too much
stabilization with the first round of storms to support
additional convection. Decreased PoPs after midnight but did not
eliminate them anywhere given the uncertainty.

With a slow-moving boundary, midlevel flow oriented parallel to
it, very high PWs (2+ inches, generally), and potential for
training/upwind propagation effects, locally excessive rainfall
is possible. Too much uncertainty exists for a flood watch at
this time, especially given the relatively dry antecedent
conditions. Nevertheless, some local instances of poor-
drainage/urban flooding are probable and isolated flash flooding
is possible.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Updated discussion will be available by 4 am.

The extended period starting Saturday night features a
transition in the flow aloft from roughly zonal flow across the
northern US to a somewhat more amplified pattern with troughing
over the east US by the latter part of next week. In terms of
weather this translates to very warm/hot and humid conditions
this weekend transitioning to a milder and somewhat drier period
for at least part of next week.

A quasi-stationary east-west frontal boundary is expected to
persist across the area through the weekend, although it may
lift north a bit on Sunday. Low pressure along the front will
favor low-level convergence and areas of showers and
thunderstorms. PWAT values near 2 inches will support some
heavy downpours and possible flooding. Also SPC has a slight
risk of severe for Saturday and Sunday. Timing is tricky but
there is some indication of more likely precip for Saturday
night and possibly again Sunday night.

By Monday morning a fairly pronounced shortwave trof is forecast
to be over the Great Lakes and move east to New England by
Tuesday morning. The GFS is a bit slower than the ECMWF, but
either way the trof should force a cold front across the mid-
Atlantic and offshore by Tuesday morning. With the frontal
passage on Monday there will be a continued good chance for
aftn/eve t-storms. Max temps on Monday ahead of the front look
to be still a few degrees above normal.

Tuesday and Wednesday should be relatively pleasant with shrtwv
ridging aloft and surface high pressure moving across the area.
The high is associated with a somewhat cooler and drier air
mass. Looks like some low-level warm advection develops
Wednesday night and the forecast carries a chance of showers at
that time. Chance of precip increases on Thursday as another
shrtwv trof and associated cold front approach from the NW.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
For the 06Z TAFs...VFR through the morning, except for
potential/temporary visibility restrictions at KMIV/KACY
through 12Z. Clouds increase through the day, but CIGs should
remain VFR outside convection. Storms expected to move east
through the area late this afternoon into early evening, with
timing generally between 21Z and 03Z. More storms are possible
thereafter, but confidence is low. Additionally, sub-VFR
CIGs/VSBYs may develop after storms move through, though
confidence is too low at this point for TAF mention. Expect
frequent updates to the TAF today as details become clearer.

There is potential for strong to severe storms today with gusty
to damaging downburst winds and torrential rainfall.

OUTLOOK...

Saturday Night through Monday...Generally VFR conditions
expected, but temporarily lower cigs/vsbys in scattered showers
and thunderstorms are likely at times.

Tuesday and Wednesday...VFR conditions.

&&

.MARINE...
The wind is expected to favor the southwest around 10 knots
tonight and it should back to the south for Saturday. Wave
heights on our ocean waters will remain around 2 feet and waves
on Delaware Bay should be 1 to 2 feet.

Scattered showers and thunderstorms may affect the coastal waters of
Delaware and far southern New Jersey, as well as Delaware Bay,
tonight.

OUTLOOK...

Generally sub-SCA conditions are expected Saturday night through
Wednesday. However winds and seas may be locally higher in
scattered thunderstorms over the waters from Saturday night
through Monday.

RIP CURRENTS...

The energy associated with the underlying 10 to 14 second
southeasterly swell continues to weaken and the 6 second south
southeast swell has become prominent. As a result, we will continue
to carry a low risk for the development of dangerous rip currents
for the balance of today and the same should be true for
Saturday.

&&

.EQUIPMENT...
Dewpoint values at KDOV continue to run a few degrees over
actual.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...AMC
Near Term...CMS
Short Term...CMS
Long Term...AMC
Aviation...AMC/CMS
Marine...AMC/Gaines/Iovino
Equipment...Staff


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