Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 181949

Area Forecast Discussion
National Weather Service Mount Holly NJ
349 PM EDT Fri May 18 2018

A nearly stationary front just to the south of the region and
several weak areas of low pressure will bring generally unsettled
conditions to much of the area through the weekend. A cold front
will cross the area later Sunday followed by high pressure moving in
for early Monday. Another low with its associated fronts will affect
the area Tuesday into Wednesday before high pressure returns

The main front remains well south of our region. With the surface
high continuing to build south and a secondary backdoor cold front
moving through the region, the main stationary front should remain
well south of our region (in a stark contrast from yesterday where
we thought there was a potential for the front to lift back into
northern Delmarva).

Three factors are lowering our risk for heavy rain tonight. First,
with the stationary front to our south, this should help to keep the
focus of shower development south of our region. Second, a low level
jet should develop west of our region, keeping moisture transport
and the heavy rain threat north of the front west of our region
(though Berks county could be on the fringe of heavy rain). Finally,
drier air has worked its way in behind the secondary back door cold
front. Consequently, some of the heavy rain signals (high
Precipitable water, high mean RH, etc.) have decreased. There
remains some risk for flooding generally south of the PA
Turnpike/I195 corridor (see hydro section below for details on this.

None the less, showers should slowly work their way north through
the overnight hours, so rain across the region looks likely for

A warm front should finally lift through our region slowly during
the day tomorrow. Although we will technically be in the warm
sector, temperature-wise it may not feel like it as the front is
expected to move through very late in the day, and lingering cloud
cover will temper the warm up.

Chances for rain continue through the day, and if we see any breaks
in the clouds, we could have enough instability for


Saturday night through Monday...

To start the period Saturday evening, upper level trough will be
approaching from the west with an associated surface low moving
through Ontario into Quebec helping to lift a warm front across the
area. As a result, temps will hold fairly steady in the 60s or even
rise Saturday night with dewpoints also creeping up. There will be
some scattered showers around in response to weak impulses moving
through the upper level flow and these could still be capable of
producing brief periods of moderate to heavy rain due to the
abundant moisture in the atmosphere. There also looks to be enough
instability for the chance of some thunder, especially north and
west of the I-95 corridor.

For Sunday, a cold front will gradually work southward across the
area with time...reaching the I-95 corridor in the afternoon and
clearing the coast in the evening. There will continue to be
abundant deep moisture in the atmosphere until this front passes
with PWATS over 1.7 inches. Also, surface Tds will be in the 65 to
70 range in the warm sector so very muggy! The front along with its
attendant upper trough and diurnal instability will be the
focal point for more showers and storms to develop...especially
late morning through about mid to late afternoon. Biggest
question will be how much instability there will be and this
will depend on if we get any sunny breaks ahead of the front.
Currently, still doesn`t look like a lot of sun so this should
limit instability to the 500-1000 j/kg range of CAPE. Still,
combined with the high PWATs any storms will be capable of
producing very heavy rain resulting in continuing flooding
concerns exacerbated by the already saturated conditions. Also,
some indication from NAM soundings that Corfidi Vectors will
result in slow moving and/or backbuilding storms. Urban and
small stream type flooding would be the biggest concern but
can`t rule out flash flooding as well. Highs Sunday will
generally be in the upper 70s to low 80s but again, combined
with muggy dew points until the front passes.

Sunday night...finally some relief as the cold front clears the
area bringing an end to any showers and least
temporarily. In fact, expect a period of mainly clear to partly
cloudy skies overnight as high pressure begins to nose in. With
the saturated conditions and diminishing winds this will likely
result in areas of fog, especially in valleys. Lows generally
range from the mid 50s to mid 60s...coolest north, warmest

For Monday, reprieve continues as most of the day looks to be
dry dominated by transient high pressure. However there will be
some mid and high clouds around ahead of the next approaching
wave and very late in the day some showers could approach
eastern PA zones from the west so we mention slight chance POPs
here. Highs will generally be in the 70s to near 80 but with
much lower and more comfortable dew points in the drier airmass.

Monday Night Through Friday...

As the next upper level wave and associated surface low approaches,
this will lift the warm front back to the north across the
region into Tuesday bringing the return of some showers and
possible thunderstorms. This will be followed by the system`s
cold front Tuesday night into Wednesday bringing continuing
chances for showers and storms though it won`t be a steady all
day rain. The front should finally clear the area by Wednesday
night and at this early vantage point, high pressure with fair
weather looks to return by Thursday. It should be stressed
though that the models have been struggling in the extended (day
6-7) so this could still change as the moist, rainy tropical
airmass plaguing us will never be pushed too far southward away
from the area...even right through next week. &&

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

Rest of today and tonight...Models continue the trend of being too
pessimistic with ceilings. Therefore, TAF is above the GFS Lamp and
other guidance, with most sites VFR through the late afternoon, but
should gradually lower through the evening. Expect most locations to
be IFR by 06Z. Northeasterly winds gusting to 25kt will slowly
diminish through the evening, but remain northeasterly. Moderate
confidence on the flight categories, but low confidence on the
timing of any changes in flight categories.

Tomorrow...Will likely start the morning with IFR ceilings, but
should see improvement to MVFR conditions and possibly VFR conditions
in the afternoon. Winds are expected to shift to southerly through
the day, and speeds should be 5 to 15 mph. Moderate confidence.


Saturday night thru Sunday...Mostly sub-VFR conditions expected
with showers and psbl thunder at times in addition to low cigs.

Sunday night...Areas of fog possible with associated

Monday...mainly VFR.

Monday night through Wednesday...chances for showers/storms with
associated restrictions returns as next frontal system moves

Localized gale force gusts have developed along the Ocean County
shore, so have issued a gale warning for those locations through
early this evening. Otherwise, small craft advisory conditions are
expected to continue on the coastal waters through at least the day
tomorrow. On the Delaware Bay, winds could diminish below SCA
criteria by late this evening, then stay below SCA conditions
through the remainder of tomorrow.


Sat night thru Sun...Lingering SCA across the ocean
possible. Showers.

Sunday night thru Wednesday...Mostly sub-SCA. Sct Showers.

Surface high pressure pushed south today across northern New Jersey
and northeast Pennsylvania. This kept the rain across the southern
portion of the HSA all day. While this southern rain will slowly
track to the north tonight back across much of the area, rainfall
totals are no longer forecast to be as high as they once were across
northern NJ and east-central PA. For these reasons, the Flood Watch
has been cancelled north of the PA Turnpike and I-195 corridor in NJ.

Further to the south, over three inches of rain have fallen in spots
today alone across the Delmarva. Lesser amount in southern NJ. This
is on top of the rainfall that occurred the last few days.

Periods of rain remain in the forecast tonight through Saturday.
Going forward, another inch can be expected just about anywhere.
With the heaviest rain falling across southern Delaware and the
adjacent Maryland Eastern Shore the last few days, the threat of
mainstem river flooding across New Jersey and southern Pennsylvania
has diminished. Low lying and poor drainage can still be expected to
develop through Saturday. Flash flooding also remains possible as
thunderstorms are expected on Saturday. Continue to keep an eye on
later forecasts. All of our hydrologic forecasts can be accessed via
our Advanced Hydrologic Prediction Service website. While the threat
of mainstem river flooding has indeed diminished, it has not been
completely eliminated.

An east to northeast wind along the coasts of New Jersey and
Delaware will continue into tonight. The onshore flow is expected to
result in a 1.0 to 1.5 foot surge along the coast. As a result, we
are anticipating minor flooding around the time of tonight`s high
tide. The Coastal Flood Advisory remains in effect for the coastal
counties of New Jersey and Delaware and for the counties along
Delaware Bay. The Coastal Flood Advisory has been expanded to
include the tidal Delaware River, based on the latest guidance.

We should lose the onshore flow during the course of the day on
Saturday. Therefore, the potential for coastal flooding will
diminish over the weekend and no additional advisories are

No tidal flooding is forecast for the upper eastern shore of
Chesapeake Bay.

PA...Flood Watch through Saturday afternoon for PAZ060-070-071-
     Coastal Flood Advisory from 3 AM to 7 AM EDT Saturday for
NJ...Flood Watch through Saturday afternoon for NJZ016>027.
     Coastal Flood Advisory from 9 PM this evening to 3 AM EDT
     Saturday for NJZ012>014-020>027.
     Coastal Flood Advisory from midnight tonight to 4 AM EDT
     Saturday for NJZ016.
     Coastal Flood Advisory from 3 AM to 7 AM EDT Saturday for
DE...Flood Watch through Saturday afternoon for DEZ001>004.
     Coastal Flood Advisory from 9 PM this evening to 3 AM EDT
     Saturday for DEZ002>004.
     Coastal Flood Advisory from midnight tonight to 4 AM EDT
     Saturday for DEZ001.
MD...Flood Watch through Saturday afternoon for MDZ008-012-015-019-
MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ450-
     Gale Warning until 10 PM EDT this evening for ANZ451.
     Small Craft Advisory until midnight EDT tonight for ANZ430-431.


Near Term...Johnson
Short Term...Johnson
Long Term...Fitzsimmons
Tides/Coastal Flooding...Iovino is the U.S. government's official web portal to all federal, state and local government web resources and services.