Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KPHI 270642
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
242 AM EDT Tue Jun 27 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 /THROUGH TODAY/...
An upper-level trough is forecast to sharpen eastward today as it
moves into the Northeast and Mid Atlantic. This will drive weak low
pressure well to our north, however a few weak cold fronts or
surface troughs will continue to push eastward. The first one early
this morning has teamed up with just enough instability and lift to
focus an area of showers and some thunder primarily up the I-95
corridor. It appears that the Chesapeake Bay played a role in this,
helping to boost the MLCAPE just enough to get lightning going. This
activity is expected to move northeast of our area through about
12z/8 AM. We continue with an area of likely PoPs in this
corridor. We then turn our attention to the west.

A more pronounced area of short wave energy is forecast to arrive
from west to east late this morning and through the afternoon. This
looks to be accompanied by either a weak cold front or surface
trough. While the trough aloft is forecast to be sharpening some as
it shifts eastward along with gradual cooling aloft, the bulk of the
lift is more focused to our north. The majority of the model
guidance is therefore showing much of the convection just to our
north. There should be however enough moisture and at least some
instability through the day to pop a few showers or thunderstorms.
Since the overall coverage is less certain still, we kept PoPs in
the slight chance to low chance range.

High temperatures are a blend of continuity, MOS and some high-res
guidance. Given lower heights and thickness values, the afternoon
high temperatures are forecast to be a bit cooler than
yesterday.

&&

.SHORT TERM /TONIGHT/...
The axis of an upper-level trough is forecast to move across our
area during the overnight. Any shower or thunder is expected to end
early as the main short wave energy is offshore, then some increase
in the subsidence should begin to overspread the area as surface
high pressure starts to arrive toward daybreak.

The sky is anticipated to become mostly clear as a result with any
clouds due to daytime heating flattening and then dissipating in the
evening. Low temperatures are mostly a blend of continuity and
MOS.

&&

.LONG TERM /WEDNESDAY 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.

Wednesday 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 /07Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through today...VFR overall. An area of showers and embedded
thunderstorms will move across the KILG, KPHL/KPNE to KTTN areas
through about 09z. The visibility can be significantly reduced for a
short period of time in any heavy shower/storm. Isolated to perhaps
scattered showers/storms are possible again this afternoon, but
confidence regarding timing is low and therefore a mention is not
included in the TAFs. Mostly light and variable winds, becoming
westerly and increasing to around 10 knots late this morning and
afternoon. Some local gusts up to 20 knots this afternoon are
possible.

Tonight...Any shower/storm ends early, otherwise VFR with any
lingering clouds tending to dissipate. West to northwest winds
mainly 5 knots or less.

OUTLOOK...
Wednesday 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...
The conditions are expected to remain below Small Craft Advisory
through tonight. A southwesterly wind will become westerly late in
the day and tonight as a weak cold front moves offshore. This
offshore flow will increase at times tonight with some gusts to near
20 knots. A few isolated thunderstorms will be possible mainly this
afternoon.

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 for today should result once again in a
low risk for the development of dangerous rip currents.

&&

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

&&

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.