Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 201323

Area Forecast Discussion
National Weather Service Mount Holly NJ
923 AM EDT Sun Aug 20 2017

High pressure will build in from the Ohio Valley today and then move
offshore on Monday. A cold front will approach from the Midwest on
Tuesday while high pressure remains anchored off the eastern
seaboard. The front is expected to move through the area on
Wednesday. Canadian high pressure builds into the region late in the
week and into next weekend.


Only minor updates needed to reflect current temperature and dew
point trends, but otherwise the forecast remains on track.

The axis of the upper trough will move to the east of the area this
morning with flow aloft becomes more zonal behind the passing
trough. Meanwhile, surface high pressure will start to build in from
the west.

Some drier air will be across the region today and with little to no
moisture around, we should see plenty of sunshine and no rain. The
dry air will be slightly cooler than what was in place the past few
days but maximum temperatures will only be about 2-3 degrees cooler
than they were on Saturday. Expect highs to rise into the mid to
upper 80s across the forecast area with some upper 70s to lower 80s
across the southern Poconos and northwest New Jersey. With drier air
in place, it will feel much more comfortable outside.


The center of the high pressure will cross overhead tonight and then
push offshore early Monday. Flow across the region will be light and
skies should be relatively clear overnight. Cloud cover will start
to increase toward daybreak, mainly across Delmarva, as return flow
around the high starts to bring some moisture back into the area.

Temperatures will respond to the relatively clear skies and light
winds overnight and we should radiate pretty well. Lows will drop
down into the low to mid 60s across much of the region with some
upper 50s across the higher elevations. Temperatures in
Philadelphia, nearby suburbs, and along the coast will be slightly
warmer and in the mid to upper 60s.

Even with moisture starting to increase, do not see much in the way
of fog development overnight although a few patchy spots may
develop, mainly toward daybreak.


High pressure moves offshore on Monday. Southerly return flow around
the high develops across the Mid-Atlantic region. Meanwhile, a weak
shortwave disturbance embedded in an belt of stronger westerlies
residing to the north of the upper ridge is expected to approach
from the Ohio Valley during the afternoon and then pass through the
area Monday night. A few showers and storms could develop on Monday,
especially west of the Delaware Valley (Delmarva-Reading-Lehigh
Valley) late in the afternoon and evening when forecast soundings
show instability increasing in concert with the arrival of deeper
moisture from the southwest. Kept coverage isolated for the
afternoon hours with the greater lift from the shortwave trough
still to our west. Slightly higher coverage of showers and storms
may arrive in our far western zones during the evening.

Solar Eclipse Viewing Monday Afternoon... It is important to note
the wide range of model solutions for cloud cover across the most of
the area, basically ranging from clear skies (with a minimal cumulus
deck and thin cirrus clouds) to cloudy skies. In general, RH fields
in the 850 mb layer (closely represents the ceiling height of the
cumulus field at the top of the mixed layer) for Monday afternoon
have trended higher with the most recent 00Z models compared to 24
hours before. This trend seems loosely tied to models showing an
earlier exit of the high and a more pronounced warm-air/moisture
advection pattern on the western side of the high. Unfortunately,
this means we increased cloud cover with the latest forecast update-
to partly sunny roughly south/west of Philadelphia and to mostly
sunny farther north/east. While confidence in the official
(deterministic) sky cover percentages (seen in the NDFD graphics on
our website) is are not not particularly high, (in a probabilistic
framework) the likelihood of a having a decent view of the eclipse
increases the farther north and east you go.

Low pressure is expected to track from the Great Lakes into
southeastern Canada on Tuesday. A cold front extending southward
from the low will move through the Midwest states during this time.
The focus for showers and storms on Tuesday will be along a pre-
frontal trough to the lee of the Appalachians. Accordingly, the
highest PoPs reside across our far western zones in eastern PA and
far northwestern NJ. Tuesday is still expected to be the hottest day
of the period with highs generally in the lower 90s.

Showers and storms across northwestern zones may continue into the
overnight and progress farther south/east across the forecast area
later Tuesday night as the cold front approaches. The highest PoPs
Tuesday night once again reside across the northwestern zones. While
showers and storms are forecast with the front moving through on
Wednesday, a morning-early afternoon timing of the fropa may prevent
us from realizing our full convective potential (in terms of
thunderstorm coverage and severity) than if it coincided more
favorably with peak heating later in the day.

Showers and storms will end from northwest to southeast late in the
day Wednesday into Wednesday night as the front moves through the

Canadian high pressure gradually builds into the area late in the
week. Dry conditions and below normal temperatures can be expected
Thursday through Friday with high pressure firmly in control of our
weather pattern. Highs will be in 70s and lower 80s each day with
lows in the 50s and 60s.


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

VFR conditions are expected at the terminals through the TAF

Relatively clear skies will be across the region
today, with just some cirrus around as high pressure moves into
the area. Mainly clear skies will continue through tonight. Fog
may develop in a few locations overnight tonight but confidence
is low and it has been left out of the TAFS at this time.

Light winds this morning will become west to northwest by mid
morning at around 10 knots or less through this afternoon. A few
gusts around 15 knots are possible this afternoon. Winds will
subside again and become slightly more southwest this evening
before becoming light and variable overnight.


Monday...Predominately VFR. Isolated showers and storms could
develop late in the day and at night, mainly south and west of
Phila terminals.

Tuesday...Mainly VFR. Isolated to perhaps scattered showers and
storms in the afternoon, especially across western terminals
(RDG and ABE). Showers and storms could move into I-95 terminals
Tuesday night with a cold front approaching from the NW.

Wednesday...Showers likely with a chance of storms. Localized
sub-VFR restrictions a possibility with this activity. Improving
to VFR with showers ending from NW to SE late in the afternoon
and evening. Expect a wind shift from SW to NW to occur in wake
of fropa.

Thursday...VFR and light winds.


Tranquil conditions will continue across the area waters through
tonight. Seas will be around 1 to 2 feet. West to northwest winds
around 10 to 15 knots will start to turn more to the south/southwest
this afternoon and evening. A few gusts around 20 knots are possible
through tonight.


Monday...Winds and seas below SCA thresholds.

Tuesday...SCA possible late afternoon and night with S winds
increasing to near 25 kt ahead of cold front. Building seas to
4-6 ft are forecast for the Atlantic coastal waters.

Wednesday...Winds should decrease to below 25 kt but SCA
conditions may continue into the morning as it may take longer
for seas in the coastal waters to drop below 5 ft.

Thursday...No marine hazards expected.

Winds will be northwest through this morning before turning to the
southwest this afternoon. The overall risk for today looks to be low
at both the Delaware beaches and the New Jersey shore but with
a more southwesterly flow developing a few of the more prone
locations, like LBI, may see a slightly increased risk through
this evening.


We will be watching the tidal departures closely for the Monday
evening high tide, as winds will once again shift to more southerly,
and astronomical tides should be relatively high thanks to the new
moon on Monday. At this point minor flooding is possible, though
some guidance continues to show water levels falling short of minor
flooding thresholds.




Near Term...Meola/Di Spigna
Short Term...Meola
Long Term...Klein
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