Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 261937

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


High pressure will build eastward into the region tonight and
throughout the day Tuesday. This high pressure system will become
anchored offshore through the middle and later parts of this week. A
cold frontal boundary is expected to approach from the northwest
this weekend into early next week.



So far, precipitation has failed to develop in the area, and with
stronger large-scale ascent to the west in closer proximity to an
approaching shortwave trough currently in the Great Lakes region,
decided to pull PoPs for the rest of the afternoon and evening. I`m
not sure a spotty shower can be completely ruled out, but satellite
trends indicate a rather benign-looking cumulus field at the moment,
with more pronounced vertical ascent well to the northwest of the
area. A weak perturbation moves through the region around sunset,
but lift in association with it seems rather muted at the moment.

Tonight, the aforementioned shortwave trough marches east, reaching
western New York by 12Z Tuesday. The 12Z GFS/RGEM and (to varying
degrees) some of the higher-resolution guidance indicate a weak
predecessor vort max moving southwest to northeast through the area
during the overnight hours. Spotty showers may develop in proximity
to this perturbation, moving rapidly northeastward just downstream
of it. The 12Z NAM, however, looks quite dry, and the very weak lift
along with some run-to-run continuity issues with the HRRR precluded
me from including PoPs at this point. However, this will be
something to watch during the overnight hours. I did increase cloud
cover in close proximity to the track of this perturbation as a
start, though this may be somewhat overdone.

Overnight temperatures look to be rather cool as the shortwave
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.



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.




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.


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

VFR through the period. SCT-BKN cumulus deck around 6000-8000 feet
this afternoon, with winds west or southwest 5 to 15 kts with
occasional gusts to 20 kts. Tonight, skies may briefly clear for a
time after sunset and then begin to increase late evening onward,
especially northwest of KPHL. Winds mostly light and variable. VFR
CIGs likely tomorrow with isolated to widely scattered storms
possible. Coverage looks too sparse and confidence in occurrence is
not high enough for TAF inclusion at this time. Winds should pick up
tomorrow afternoon, with gusts to 20 kts possible.


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



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.


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.

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




Near Term...CMS
Short Term...CMS
Long Term...Gaines
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