Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 261021
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
621 AM EDT Wed Jul 26 2017

.SYNOPSIS...
High pressure across New York and New England today will move off
into the North Atlantic tonight. Low pressure across the lower Ohio
Valley will begin to strengthen Thursday and cause a warm front to
approach the area. This front and the low moving along it will
affect our weather from Thursday night into Saturday. The low will
move away by Saturday night allowing high pressure to build back
over the area for Sunday and into next week. A cold front may cross
the area later Tuesday or Wednesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
6 AM update...It appears that our post on social media from 24
hours ago about finally getting a break from the active wx was a
bit of a jinx (although the weather is not nearly as active as
in recent days).

A Flood Advisory for minor poor drainage flooding was issued
for central DE with a persistent band of showers since yesterday
evening resulting in very localized heavy rainfall. KDOV AWOS
has reported 1.95" of rainfall since 930 PM. This band of heavy
showers has recently drifted a few miles south of the Air Force
base but has also become better organized. Very localized
rainfall totals of 3" is not out of the question before the
showers likely drifts southward and eventually dissipate mid to
late morning.

The same type of mesoscale phenomenon is occurring in the
Lehigh Valley and eastern Berks County but the rainfall rates
are not quite as intense as in Delaware. Mesonet stations
indicate very localized rainfall amounts between 0.75" and 1.50"
have occurred since midnight in the Allentown area. Note that
rainfall estimates from KDIX radar has been grossly underdone as
the radar beam has overshot the core of these very low-topped
showers (e.g., beam height at 0.5 degree tilt can`t see anything
below 6 kft MSL out toward the Lehigh Valley).


Previous Discussion from 4 AM...

High pressure was centered over Northeast states early this
morning. The high will progress eastward today, moving off the
New England coast this afternoon.

Moisture trapped beneath a strong mid-level subsidence inversion has
resulted in low clouds that cover most of the region early this
morning. Clearing was noted to our northeast across New England and
Long Island where low-level dry air has drained southward from the
anticyclone. RH fields from majority of guidance indicates the
leading edge of the clearing may struggle to advance much farther
than northern and coastal sections of NJ today. There may be some
breaks in the stratus mid morning but clouds should increase once
again as stronger daytime mixing leads to development of stratocu.

Despite mid-level subsidence, there is enough lift owing to weak
boundary-layer convergence along a residual trough axis as well as
from a 850-700 mb shortwave disturbance to produce isolated showers
in eastern PA and Delmarva. These showers have been nearly
stationary and efficient rainfall producers (strictly warm cloud
processes). There are actually some impressive localized rainfall
amounts considering how dry the mid levels are, how shallow the lift
is and how unimpressive they look on radar. Mesonet sites in the
Reading-Allentown, PA area and near Dover, DE and Chestertown, MD
range from one- to three-quarters inch. There was even a station
that was reporting just over one inch rainfall overnight just
outside of Allentown. Expect these showers to drift slowly westward
this morning and out eventually outside of our CWA boundaries by
midday. Additional convective showers may develop during peak
heating hours this afternoon from about the Delaware Valley,
westward. Outside of our far western zones, kept PoPs below 20
percent with a mention of isolated sprinkles as a lowering
subsidence inversion will hinder convective updraft growth.

Today is expected to be the second day in a row of temperatures in
the 70s for most of the forecast area. A better opportunity for 80
degrees is northern NJ (I-78, north and east of I-287 where there is
less of a marine influence and where more sunshine is expected) and
in far western zones of the CWA (Reading, PA down into the eastern
shore of MD).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
High pressure centered off the New England will ridge southwestward
and just off the northern Mid-Atlantic coastline tonight. Light
southerly flow around the high will develop over the region.

Expect most of the region to remain dry tonight though an isolated
shower cannot be ruled out before sunset this evening. Additionally,
WAA in the low-levels will yield increasing clouds overnight. Some
of the model guidance (e.g., NAM Nest) indicates strong enough
isentropic lift to yield a few showers early Thursday morning west
of the Fall Line. Confined slight chance PoPs to NE PA for now.

Forecast low temperatures are in the low to mid 60s area-wide. The
setup for radiational fog looks to be most favorable in the coastal
plain, where the arrival of thicker cloud cover may hold off until
daybreak Thursday morning.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
The rather benign weather from Wednesday will spill over into
Thursday as high pressure moves offshore. The return flow around the
high will cause moisture to increase over the region. Accordingly,
we will have a partly sunny becoming mostly cloudy fcst. Pops will
increase through the day from W to E with slgt chc pops for the N/W
looking OK for the morning and the further S/E with chc pops for the
afternoon. There will be enough instability for tstms and the SPC has
indicated a slgt chc for severe storms for later Thu and Thu night.
Pops Thu night will rise to likely/categorical as the warm front
nears the area.

Friday through Saturday morning will feature another round of
showers and thunderstorms with more heavy rains expected. Most of
the operational models and the ensembles show 2 to 4 inches of rain
over parts of the CWA. There is still plenty of variability on
timing and location will have to be sorted out over the next few
cycles, but its expected that some Flood or Flash Flood Watch
products will be needed. The ground is still wet from the rains
earlier this week.

Improving conditions will take hold across most areas Saturday
afternoon as high pressure from the N/W begins to build over the
area. A couple lingering showers over Delmarva and SE NJ are
possible though. Temperatures Saturday will be seasonably cool with
highs in the low 80s S/E and mid to upper 70s N/W.

Fair weather is expected much of the Sunday through Tuesday period
as high pressure affects the area. Temperatures will return to near
normal readings by then as sunshine should be more abundant over the
region.  The next system, a cold front, may bring some showers to
the area Tue afternoon, but confid in this is rather low attm.

&&

.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

There is extensive low clouds present across the region but CIGs
have been highly variable overnight, ranging from 1500-3000 ft AGL
(MVFR) and at times scattering out (VFR). Lower CIGs just west of
the Delaware Valley (including ABE) are hovering between 600-1000 ft
AGL. Localized visibility restrictions in isolated showers have been
confined to ABE and RDG. IFR restrictions at these two terminals
should be temporary through about 11 or 12Z.

CIGs may briefly improve to VFR at some locations between 12-15Z
before stratocu yields MVFR CIGs in the the 1500-3000 ft range late
this morning and early afternoon. Despite broken cloud cover for
most of the day, expect TAF sites to improve to VFR with CIGs above
3000 ft between about 17Z and 20Z.

VFR this evening. There is considerable uncertainty with the CIG and
VSBY forecast for overnight tonight. Some of the guidance shows low
clouds and MVFR or even IFR CIGs developing from west to east
between 06-12Z Thursday. However, there could also be visibility
restrictions in patchy fog during this time, especially at the fog-
prone terminals.

A light E to NE wind around 5 kt this morning will veer out of the
SE 5-10 kt this afternoon and then S tonight.

OUTLOOK...
Thu...VFR. Increasing clouds.
Thu Night thru Sat Morning...Frequent MVFR/IFR with
    showers/tstms.
Sat Afternoon thru Sunday...Mostly VFR.

&&

.MARINE...
Winds and seas will remain below SCA thresholds today and tonight.
Seas are around 3 to 4 feet on the ocean with onshore winds around
10 to 15 knots.

OUTLOOK...
Thu thru Friday...Mostly sub-SCA conditions. Tstms with locally
gusty winds and higher seas Thu night thru Fri night.

Sat thru Sun...SCA seas on the ocean expected. Seas diminishing
Sun night.

RIP CURRENTS...

Based on in-house regression guidance, the forecast calls for a
low risk for the formation of dangerous rip currents today.
However, with a residual onshore flow around 10 mph and waves
in the surf zone around 3 ft, the risk may approach moderate for
the NJ beaches later this morning.

&&

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

&&

$$
Synopsis...O`Hara
Near Term...Klein
Short Term...Klein
Long Term...O`Hara
Aviation...Klein/O`Hara
Marine...Klein/O`Hara


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