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Office: PHI

FXUS61 KPHI 220903

Area Forecast Discussion
National Weather Service Mount Holly NJ
503 AM EDT Fri Jun 22 2018

High pressure situated across the region today will gradually move
offshore tonight. Meanwhile, a low pressure system organizing over
the Mississippi Valley leads to a warm frontal passage on Saturday
and a cold frontal passage on Sunday. High pressure over Ontario
builds into the region on Monday and slowly moves off the eastern
seaboard to a position near Bermuda by Thursday.


We will have a brief reprieve from heat today being solidly in the
cool sector. The closed low developing over the Mississippi Valley
and progressing towards the Ohio Valley will be making progress
closer to our region. However, its associated front will stay a
stationary front located south of our region through the day today.

There is a chance (likely across Delmarva, decreasing chances as you
go further north) for showers across the region through the day, but
for the most part the forecasted PoPs are on the lower side of
guidance. The reason for this is that in the mid and upper levels, a
short wave ridge ahead of the main low will be slowly lifting
northeast across our region through the day. As a result, synoptic
scale lift will be limited and to get any precipitation we will be
relying primarily on orographic lift with the onshore flow.

A combination of being in the cool sector as well as onshore flow
will help to keep temperatures 5 to 10 degrees below normal, with
highs generally in the 70s (and upper 60s in the higher terrain of
the southern Poconos).


Chances for rain continue to increase both as the short wave ridge
lifts further away from our region and as the stationary front
begins to transition to a warm front lifting north towards northern
Delmarva. Guidance trended slightly slower with the warm front as it
is expected to remain south of most of the region even overnight.
Instability north of the warm front will be minimal, so
thunderstorms are unlikely.

Thanks to increasing cloud cover, the diurnal swing in temperatures
will be relatively small from today into tonight, with the lows only
5 to 15 degrees below the highs for today.


The main concerns are the severe thunderstorm potential Saturday
afternoon and evening including urban and poor drainage flooding,
additional showers and thunderstorms Sunday afternoon/eve, and
a gradual increase in heat and humidity by the middle of next
week. Except for Sunday, the tendency for an onshore component
of flow will result in high temperatures several degrees cooler
near the immediate coast.

Synoptically, a closed low transitioning into an open wave as it
ejects northeastward into the eastern Great Lakes promotes a deep
layer southwest flow through Sunday, until the longwave trough axis
traverses the area on Monday. This will be followed by a broad mid-
level ridge through Wednesday, which is expected to be interrupted
by a northern stream perturbation on Thursday.

Saturday...Low pressure over the Ohio Valley will lift a warm
front slowly northward through the region. There is uncertainty
regarding how far north the front will make it, and this will
determine the severe weather potential, which will be near and
south of the front. Favorable parameters include ML CAPE up to
1,500 J/kg and Bulk Shear around 45 kt. Damaging thunderstorm
wind gusts are certainly possible, although modest 0-3 km lapse
rates and meager D-CAPE suggest limited coverage. Perhaps of
more concern is the environment closer to the warm front, where
0-1 km shear over 20 kts and low LCLs could favor a brief spin-
up. Hail parameters, including lapse rates in the hail growth
zone, are weak so would expect mainly small hail, except for
updrafts that are able to rotate closer to the warm front, which
would be more favorable for large hail.

South of the warm front, a warm and humid day. Model soundings
support mixing up to 850 hPa. Raised high temperatures into the
mid to upper 80s. Southerly winds will gust 20-25 mph at times.

Saturday night...Thunderstorms will diminish around midnight
with a drying trend during the overnight hours. Patchy fog is
possible in the more sheltered locations northwest of I-95.

Sunday...The cold front will move slowly through the area,
supporting the development of showers and thunderstorms. Both
moisture and instability is limited southeast of the I-95
corridor, so only mentioned a slight chance of thunder.
Northwest of I-95, included a chance of thunder, where there is
sufficient moisture and instability. Compared to Saturday, the
environment is not conducive to severe weather, but gusty winds
and heavy rainfall are possible with any thunderstorms that
develop. With the flow more offshore, high temperatures will be
in the low to mid 80s regionwide.

Sunday night through Thursday....

The aformentioned cold front will finally clear the area Sunday
night into Monday. As such, chance of showers/storms lingers
into Sunday night before a clearing trend for Monday. Highs
Monday will generally be in the upper 70s to low 80s with lower
humidity. The only exception will be eastern NJ, where mid 80s
are possible. Beyond this time, high pressure builds in for
Tuesday and Wednesday bringing mainly clear skies with
seasonable temperatures. The next system doesn't arrive until
next Thursday bringing a chance of showers and storms.


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

Today...Through the day, we are expecting easterly onshore
winds. As a result, low clouds over the ocean are expected to
advect in through the day. Some of the guidance, including the
GFSLAMP, the NAM, and high resolution models such as the HRRR,
depict the low clouds already over much of the Coastal Plains. I
have chosen to stay closer to the operational GFS, partially
because the previously mentioned guidance isn't verifying well
currently, and also because the GFS solution is closer to what I
would expect in this scenario (the front will remain well south
of the region through the day and even overnight tonight). As a
result, the current forecast is for IFR or MVFR ceilings at
KACY as early as 09Z, at KMIV closer to 18Z (after sunrise, I
expect the progress of low clouds to slow). For the Delaware
Valley TAF sites (KILG, KPHL, KPNE, and KTTN), persistent MVFR
ceilings could develop from south to north after 18Z. Having
said that, a small area of MVFR ceilings developed between 08
and 09Z over KTTN and KPNE. With the higher cloud cover, it is
hard to know exactly the extent of this area, but it doesn't
appear to be a part of the main marine layer. Therefore, I don't
have much confidence it will linger after 12Z. For KABE and
KRDG, current forecast is for VFR conditions through out the
day. Showers are possible primarily south of PHL and ACY,
however, do not expect much in the way of visibility
restrictions if any showers move over a TAF site. Low confidence
especially in the timing of lower ceilings developing.

Tonight...MVFR and eventually IFR ceilings should develop across the
region (including KABE and KRDG) overnight. Showers become
increasingly likely as well. However, as with today, I am not
expecting too much visibility restrictions as any rain should be
light and the continued easterly winds at or near 10kt should limit
fog/mist development. Moderate confidence on most aspects of the
forecast, but low confidence on the timing of any flight category

Saturday...Conditions gradually improving to MVFR, with VFR possible
by afternoon, especially I-95 corridor terminals. But SHRA and TSRA
will lead to brief ceiling/visibility restrictions. Easterly winds
gradually backing to the southwest. Southerly winds 10-15 kt gusting
around 20 kt southwest of ABE and RDG, with higher gusts possible in
TSRA. Medium confidence.

Saturday night...TSRA possible until 06Z. Thereafter, MVFR in low
clouds and patchy fog, mainly ABE, RDG, MIV, and ACY. Southwest
winds less than 10 kt. Low confidence.

Sunday...MVFR in low clouds and patchy fog improving to VFR early.
Another round of SHRA at all terminals, with TSRA possible at TTN-
ABE-RDG, may lead to brief ceiling/visibility restrictions. West-
southwest winds around 10 kt. Medium confidence.

Sunday night...Spotty MVFR possible due to low clouds and SHRA,
especially at ABE and RDG. Otherwise VFR. Winds veering to the
northwest around 10 kt. Low confidence.

Monday - Tuesday...MVFR in patchy fog possible Monday night into
early Tuesday morning. Otherwise VFR. Northwest winds around 10 kt
on Monday becoming light and variable Monday night, veering to the
northeast on Tuesday. Medium confidence.


Easterly winds are expected to increase from south to north from
this morning through tonight. Seas will subsequently increase as
well. Therefore, expect to reach small craft advisory conditions on
most of the coastal waters by mid day. For the coastal waters
adjacent to the far northern NJ shore, SCA are not expected to be
met until very late tonight.

On the Delaware Bay, winds and waves are expected to stay below SCA
criteria. However, gusts above 20 kt will be possible at the mouth
of the Bay this afternoon into this evening.


SCA conditions continue until noon Saturday over the Atlantic
Waters of DE and NJ before winds/seas gradually diminish. There
will also be the potential for reduced visibilities in showers.
For Saturday through Saturday night, warm front lifts north of
the waters with east winds shifting to south and eventually SW.
Winds should stay below SCA levels but seas may reach around 5
ft by Sunday morning. Late day Saturday into Saturday evening,
it's possible some strong thunderstorms could affect portions of
the waters bringing local wind gusts to 34+ knots.

Breezy SW winds continue Sunday with seas hovering near or just
below SCA levels. Additional late day storms are possible. A
cold front crosses the waters Sunday night into early Monday
with sub SCA conditions expected behind this feature for late
Monday through Tuesday.

Rip Currents...

A moderate risk is forecast today due to the strengthening
onshore wind.


With a persistent onshore flow through early Saturday,
increasingly positive departures from astronomical tides are
expected. The high tide of most concern is Friday night, when
minor flooding is possible.


MARINE...Small Craft Advisory from noon today to noon EDT Saturday for
     Small Craft Advisory from 8 AM this morning to noon EDT
     Saturday for ANZ453>455.
     Small Craft Advisory from 2 AM to noon EDT Saturday for ANZ450.


Near Term...Johnson
Short Term...Johnson
Long Term...LF
Tides/Coastal Flooding...