Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KPHI 151340

Area Forecast Discussion
National Weather Service Mount Holly NJ
940 AM EDT Tue Aug 15 2017

A weak front will affect the weather across our region today and
tonight. High pressure will build in for Wednesday and remain into
early Thursday. Another low and its associated fronts will affect our
weather Thursday and into Saturday. More high pressure arrives for
Sunday and persists into early next week.


Perturbations within fairly weak west-southwest flow aloft will
move through the southern portions of the region today.
Widespread light rain/showers have developed from the Blue Ridge
northeastward into southeast Pennsylvania and central/southern
New Jersey this morning. Radar trends are indicating convective
cells increasing in coverage/intensity in the past couple of
hours, with a few lightning strikes off the Delmarva coast
(earlier). Instability will be increasing today as warm air
advection continues in advance of the perturbations, even with
low overcast conditions continuing. A surface front is just
south/east of the CWA and will serve as a focus for enhanced
(albeit not strong) convergence through the day, and large-scale
ascent will be plentiful given differential cyclonic vorticity
advection downstream of the perturbations, combined with the
aforementioned warm air advection and upper-level divergence
associated with a departing jet streak in the Northeast.
Additionally, low-level moisture will continue to advect into
the region via onshore flow north of the surface boundary (and
aided by TC Gert in the western Atlantic). The KWAL 12Z sounding
observed a PW near 2.2 inches, and these higher PWs are expected
to surge northward to the I-76 corridor late this morning.

This sets the stage for a swath of decent QPF this afternoon,
with the 06Z NAM indicating this would occur near and south of
the I-76 corridor (where surface moisture convergence is
projected to maximize during the early afternoon). With large-
scale dynamics quite favorable for deep/sustained lift and
marginal but sufficient instability advecting northward
(enabling convective enhancement), there are ingredients in play
for moderate to potentially even locally heavy rainfall totals
in southern portions of the area. Once the convective evolution
of the precipitation today becomes clearer, may need to add some
heavy rain wording to the forecast for relevant portions of the

For now, broadbrushed QPF in the quarter to half inch range
generally south of I-195/I-276, but think local totals up to and
possibly exceeding one inch are possible, especially south of
the Mason-Dixon Line (as suggested by HRRR and 00Z WRF
simulations). Will be interesting to see how this plays out


Few changes made to this period, with precipitation chances
rapidly diminishing after 6 pm. With skies expected to clear
quickly from west to east and residual moisture near the
surface, ingredients are certainly in place for at least patchy
and perhaps more widespread fog formation.

Made some modifications to sky cover to adjust for projected
timing of clearing and made some tweaks to fog coverage/timing
as well based on changes to the sky cover grids. Otherwise,
going forecast is in good shape.


High pressure, both at the sfc and aloft, will be building across
the Middle Atlantic region Wednesday and Wednesday night. This
should bring fair weather for the period, with a warm day expected
with highs in the mid to upper 80s over the area. Lows Wednesday
night will be a summer-like across the southern areas with readings
only dropping into the upper 60s to low 70s. Across the north, lows
on Wednesday night will range from the upper 50s to mid 60s.

A slow moving low pressure system will begin to affect the water
with increasing clouds and higher chc for showers/tstms beginning
Thu and persisting into the early part of the weekend. It will not
be a total washout thru this period, but mostly just showery
conditions. The best chcs for precip will be Thu night and Friday,
mostly for the N/W areas. Temperatures for this period will be
normal or a little above normal (Friday). A cold front will bring
slightly cooler and drier air back to the region Saturday.

Some of the models are showing that a weak wave of low pressure will
move along the front Sunday and this may bring some clouds and
slightly better chc for a few showers across Delmarva early Sunday.
The norther parts of the fcst areas will remain dry. High pressure
will gradually build in later Sunday and it should remain into early
next week. Temperatures for Sunday and Monday will be mostly near


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

For the 12Z TAFs...Low-confidence forecast, with timing of CIG
improvement very uncertain. Patches of VFR have occurred this
morning, but with showers entering the region, timing sub-VFR
conditions is virtually impossible. Expect at least brief sub-
VFR VSBYs through much of the day, with slow CIG improvement,
particularly when rain showers affect the terminals. In the
meantime, MVFR to IFR CIGs are likely this morning, especially
at MIV/ACY. A slight chance of storms persists this afternoon,
but confidence is still too low for inclusion in TAFs. After
brief clearing this evening, areas of fog are likely to develop

Wednesday through Wednesday night...Mainly VFR. MVFR possible
in late night and early morning low clouds and fog.

Thursday through Friday...Mainly VFR, but MVFR possible in
showers and thunderstorms. Late night and early morning low
clouds and fog possible.

Saturday...Some morning MVFR possible, then VFR behind an
exiting cold front.


Few changes to the marine forecast this morning. Main change was
to increase northwest winds a little late tonight and to include
some patchy fog along the coast of New Jersey and in Delaware
Bay via advection processes late tonight. Otherwise, previous
discussion below remains relevant.

Seas will be slowly building during the next 12 hours as Hurricane
Gert makes its closest, yet distant, approach to our area. It will
remain well offshore, but longer-period swells should continue
to increase during the period. We continue the Small Craft
Advisory for hazardous seas on our ocean waters from late this
afternoon until noon Wednesday.

Scattered showers through today, with a small chance of lightning
this afternoon. Winds/seas may be erratic near any thunderstorms
that occur. Additionally, there may be some visibility restrictions
in showers, though widespread/dense fog is not anticipated. Winds
should generally be from the southeast around or under 10 kts
today turning light northwest later this coming Tuesday night.

Wednesday thru Saturday...Low end SCA seas possible Wednesday
morning due to offshore Hurricane Gert. Otherwise sub-SCA
conditions expected through this week and into the early part of
the weekend. Sct tstm may bring locally higher winds and seas.

Gert is forecast to pass well to our southeast later today.
Swells from the Hurricane will likely bring at least a moderate
risk for the development of dangerous rip currents today into
Wednesday. Swim only in the presence of lifeguards where safety
prevails. Otherwise, you`re on your own with any rescue
potentially critically delayed. There have been numerous
drowning fatalities this year along the NJ coast.


MARINE...Small Craft Advisory for hazardous seas from 4 PM this
     afternoon to noon EDT Wednesday for ANZ451>455.
     Small Craft Advisory for hazardous seas from 10 PM this
     evening to noon EDT Wednesday for ANZ450.


Near Term...CMS/Drag
Short Term...CMS/Drag
Long Term...O`Hara
Marine...CMS/Drag/O`Hara is the U.S. government's official web portal to all federal, state and local government web resources and services.