Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 270336
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1136 PM EDT Mon Jun 26 2017

.SYNOPSIS...
A weak cold frontal passage occurs overnight and early Tuesday. High
pressure will follow into the mid Atlantic states later Tuesday and
Wednesday. This high anchors offshore during the end of this week.
Eventually, a cold front should cross all of the mid Atlantic
states late Sunday or Monday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
A weak front or surface trough continues to slowly shift
into eastern Pennsylvania. It is with this feature along with
some weak short wave energy that is helping to focus some
showers. There had been a few lightning strikes with this,
however the intensity has generally come down. An update was
already sent to increase PoPs for a time into the likely range
across the Lehigh Valley, Poconos and into far northwestern New
Jersey. These PoPs then decrease eastward. There is enough dry
air entrainment to produce locally gusty winds with some of
these showers, and radar imagery shows an outflow boundary
shifting eastward toward the Philadelphia metro area.

At least some of the models are showing additional showers
developing overnight into early Tuesday morning as a weak short
wave rotates through our area. Not all that confident in the
second round of showers and therefore kept the forecast
primarily dry after 06z/2 AM. This may have to be revisited
however.

Overnight temperatures look to be rather cool as the short wave
trough lowers heights/thicknesses across the region. Forecast
lows are around 50 in the southern Poconos, in the middle to
upper 50s in more rural locations elsewhere, and in the lower
60s on the coast and in the urban corridor.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
The vort max in western New York at 12Z Tuesday pivots northeastward
during the day, with a northeast-to-southwest oriented lobe of
higher vorticity approaching the Mid-Atlantic region during the
afternoon. There will be an attendant surface trough moving through
the area during the day. Weak convergence along the associated
boundary during peak heating may initiate a spotty shower or storm
during the day, but guidance looks fairly tame in producing much
precipitation across the area. Nevertheless, the addition of large-
scale ascent in advance of the trough and the rather cold midlevels
suggest that instability may be sufficient to initiate a few storms.

As such, I felt that guidance was a little too tame with
precipitation prospects tomorrow, especially north of the Mason-
Dixon Line (in closer proximity to stronger large-scale ascent).
Kept PoPs on the higher side of consensus (which still keeps it at
slight chance or chance). The 12Z NAM looks too dry, whereas the 12Z
RGEM seems a more reasonable outcome with the stronger large-scale
lift in play. Used a blend of 12Z GFS/RGEM/WRF guidance as a
starting point tomorrow for Wx/PoPs, but confidence is pretty low
given that the strongest dynamics are well to the north of the area.
One limiting factor appears to be the somewhat limited vertical
extent of conditional instability/parcel buoyancy (upper levels cool
rather slowly, and the boundary layer is relatively dry), which will
prevent CAPE from becoming too substantial across the region
(generally well below 500 J/kg across the region).

Forecast temps are a blend of MAV/MET and continuity, with highs
expected to be a little cooler than today owing to increased cloud
cover and the closer proximity of the passing vort max.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Highlights:

1. Heat index values near 100 possible for the urban corridor
Friday and Saturday.

2. Highest chance of thunderstorms right now centered on later
Saturday into Sunday with more uncertainty regarding any strong to
severe storms.

Tuesday night through Thursday night:

High pressure will be moving across eastward across our region
in this timeframe. This will begin a warming trend across the
region as winds become southwesterly on the backside of the
high. Humidity will slowly start to increase as well after one
last refreshing day Wednesday. Wednesday looking at Bufkit
soundings look to be fairly favorable for mixing which may
lower RH values more than what model guidance has at this point.
Soundings also show an increasing potential for wind gusts near
20 mph on Thursday.

Friday through Saturday:

This period largely diverts from blended model and ensemble
guidance outside of the ECMWF suite. It appears the GFS is
overdeveloping convection given the overall set-up below and
normal biases for pattern progression that can be to quick.

Mid-level ridging will continue along the east coast in this
timeframe. As a result, an approaching cold front will be kept to
the northwest. With development of a CAP marked by several hundred
J/KG of CIN and 700 mb temps modeled near 9C it will be hard to
develop any thunderstorms across the region with the front still
well to our northwest. The higher terrain northwest of the I-95
fall line may be able to overcome this enough for an isolated
thunderstorm or two to fire with peak heating.

The main story will be the increasing heat and humidity. Looking at
the modeled temperatures at 850 and 925, low to mid 90`s are
entirely possible across the urban corridor both days with
slightly cooler temperatures elsewhere. A continued increase in
humidity will keep lows from falling much below the low and mid
70`s at night in spots. Heat index values near 100 are
possible as well. Be prepared to drink plenty of water, wear
light weight and loose clothing along with having a plan to
spend time in air-conditioning this weekend to beat the heat.
Due to some uncertainties with thunderstorm coverage, frontal
timing and the region more accustomed to heat will leave this
out of the HWO for now. Wind gusts from the southwest near 20
mph are currently expected for both afternoons as well.

Saturday night through Monday:

The cold front mentioned in the Friday-Saturday timeframe will
continue to push southeast into the region eroding any CAP along
with acting as a trigger for higher coverage of showers and
thunderstorms Saturday night and Sunday. Heavy downpours will be
possible with thunderstorms due to the very moist airmass
expected to be in place. While instability looks sufficient for
thunderstorms, the amount of shear currently modeled is limited
for any widespread strong to severe storms at this time. The
front is likely to dissipate by Monday leading to a typical
summertime set-up with isolated pop-up thunderstorms.

&&

.AVIATION /04Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Overnight...VFR overall. Some showers will shift north and
northeast of KRDG and KABE through 06z. A brief drop in
visibility and ceilings may occur with these showers. Some
additional showers may develop after 06z from KILG northeastward
to KTTN, however confidence as of now is on the lower side.
Light and variable winds overall.

Tuesday...VFR overall. A chance for some showers and thunderstorms
exists, especially in the afternoon, but confidence with
respect to timing and location is too low to include in the
current TAFs. Winds will increase out of the west at 10 to 12
knots with gusts up to 20 knots. Winds will start to subside a
bit late in the day.

OUTLOOK...
Tuesday night through Thursday: VFR. West wind 10 knots or less
through Wednesday shifting to southeast from 10-15 knots on
Thursday with gusts around 20 knots in the afternoon.

Thursday night through Saturday: Mainly VFR. Southerly winds 10-15
knots on average. Any isolated thunderstorm may briefly lower
celings and visibilities later Friday and Saturday during the
late afternoon and evening. Gusts around 20 knots in the
afternoons.

&&

.MARINE...
Sub-advisory conditions are expected through the period. Winds will
primarily be in the southwest quadrant, with speeds generally 5 to
15 kts with occasional gusts to 20 kts or so. There is a slight
chance of storms tomorrow, especially in the Delaware coastal
waters, as a low develops offshore and a weak surface trough moves
through the region during the day. Coverage of any precipitation
should be sparse at best.

OUTLOOK...

Seas are likely to increase to around or just over five feet with
southerly flow sometime on Thursday. These may continue into
Saturday. Otherwise, seas under five feet. Some wind gusts
around 25 knots are currently likely from the southerly flow
late Thursday through Saturday.

.RIP CURRENTS...
The forecast conditions through this evening should result in a
low risk for the development of dangerous rip currents.

&&

.EQUIPMENT...
There is a nationwide network outage affecting the National
Weather Service point and click web pages. IT support is
working to correct the issue.

&&

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

&&

$$
Synopsis...Drag/Gaines
Near Term...CMS/Gorse/Meola
Short Term...CMS
Long Term...Gaines
Aviation...CMS/Gaines/Gorse/Meola
Marine...CMS/Gaines
Equipment...



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