NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Aug 9, 5:52 PM EST, 8 minutes earlier.
Area forecast discussion, Aug 9, 5:52 PM EST, 8 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
652 PM EDT TUE AUG 9 2016
.SYNOPSIS...
High pressure will move offshore through tonight. A warm front is
expected to lift north of our area Wednesday, then a slow moving
cold front remains to our northwest Thursday through Saturday. The
main cold front is forecast to approach our region late Sunday, then
slowly move through Monday into Tuesday.
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
The warm front is approaching the region. While the reflection of
the front at the surface is still south of our region, it appears
that the warm front in the mid levels is sliding through Delmarva.
As a result, showers have developed over Delmarva. As a result,
adjusted PoPs to reflect current radar trends (keeping mention of
showers within Delmarva through midnight, before expanding across
the rest of the region overnight.
Still uncertain how widespread fog will be as mid level clouds
(already starting to move in from the south) may limit fog
development below.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
On Wed, the guid indicates that any shwrs/tstms would occur in the
mrng, with any lingering precip in the aftn confined to nrn and wrn
areas. CAPE and LIs are marginal so while there is some potential
for tstms, it does not appear to be all that high. Of course, dew
points will increase behind the front and in an increasingly moist
ams with dewpoints rising into the 70s, it won't take much to get
some pop up tstms. Have adjusted pops in the aftn to mainly be in
nrn areas and removed them over srn sections, based on the mrng
mdl guid. Temps will be making a run at 90 degrees acrs a good
portion of the region. is genly 0.10 or less, with amts psbly
around 1/4 inch up n.
&&
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Summary...An Excessive Heat Watch issued from Thursday afternoon
through Saturday for the urban corridor from Trenton to Philadelphia
to Wilmington. Additional headlines (advisories) should be needed as
we get closer. A several day period of hot and very humid conditions
(Thursday through at least Sunday), along with some convective
chances through early next week.
The synoptic setup is comprised of a ridge centered just to our
south into the weekend. At the surface, high pressure will be
offshore allowing for a southerly flow of increasingly humid air. A
warm front goes to our north early Wednesday, then a cold front
sagging southeastward from southeastern Canada and the Great Lakes
Wednesday looks to remain to our northwest through the weekend. Some
embedded short waves traversing the northern side of the ridge will
help to focus some convection. The track/timing of the short waves
will determine the extent of the convective development. It appears
that the main cold front does not arrive until Monday when the ridge
shifts east and an upper-level trough arrives from the west. We used
a model blend for Wednesday night through Friday, then mostly an
even blend of continuity/12z WPC Guidance thereafter. Some
adjustments were then made following additional collaboration with
our neighboring offices.
For Wednesday night through Saturday...This time frame is
characterized by building heat and humidity. While a ridge remains
just to our south, the stronger flow is forecast to reside farther
north. This lessens the shear with a southern extent, however a
surface front is forecast to be to our northwest Thursday and Friday
before getting a bit closer late Saturday. There looks to be a lee
side trough in place as well. The surface dew points are forecast to
be in the low to mid 70s across much of the area. This combined with
temperatures into the low to mid 90s (cooler in the higher
elevations) lead to forecast heat indices each afternoon to reach or
exceed 100 degrees. The hottest days look to be Friday and Saturday,
with isolated to scattered convection potentially providing some
local relief. Some convection may produce enough cloud debris
especially across the northern areas, which would impact the
temperatures. The WPC heat index guidance shows a medium to high
probability of an exceedance of 100 degrees across a good part of
the area (peaking Saturday). The more concern area is the highly
urbanized corridor from Trenton to Philadelphia to Wilmington. We
are close to criteria on Thursday afternoon but definitely there for
Friday and Saturday. Give that this is a multi-day event and the
potential is definitely there, we opted to go with an Excessive Heat
Watch for the aforementioned corridor Thursday afternoon through
Saturday. For most areas outside of the watch, it looks like an
advisory will eventually be needed.
As for the convection, the pattern favors more northwest than south
as the southern areas may deal with more of a cap. The stronger flow
is forecast to be to our north and therefore shear will be less and
pulse type convection may end up being the main mode. The guidance
is showing several short waves embedded in the flow on the northern
periphery of the ridge, some of which could be convectively induced
and therefore timing and initiation may be challenging. The airmass,
especially if there is little to no cap, should have good buoyancy
given the high humidity and therefore it may not take much to induce
some convective development (terrain influences may focus some
development first). Also, the presence of a lee side trough should
help focus at least some convection during peak heating farther
south. The question is how widespread it will become, and as of now
it is looking isolated to scattered. Given the expected heat and
high moisture content in the air, localized downburst convective
winds are the main threat (convection will probably produce a lot of
lightning). PW values increasing to 2-2.5 inches will result in
locally very heavy rain (farther south the storm motion should be
slower).
For Sunday through Tuesday...Excessive heat may continue Sunday
especially from near and south/east of Philadelphia. However, the
pattern is forecast to change as the ridge shifts east and weakens
some as an upper-level trough arrives from the west. This transition
timing is less certain, however overall it appears the front slowly
moves through Monday into Tuesday. As a result, some convection
should occur especially each afternoon and evening with some cooling
relief especially Monday and Tuesday.
&&
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
VFR conditions should continue through at least 06Z. After 06Z, an
approaching warm front will result in increasing mid level clouds
and isolated showers across the region. Thus, expect conditions to
lower to MVFR for most if not all of the TAF sites by 12Z (mostly
for ceilings, but visibility restrictions could also be present).
Some guidance shows ceilings as low as IFR, but not confident to
put it in the TAFs for now.
For tomorrow, Rain is possible through the morning. The rain could
linger up n, along with the lower ceilings through the day. Right
now, we are ending the rain and returning conditions to VFR across
most of the region by 18Z. Isolated thunderstorms are possible in
the afternoon, but confidence is too low to mention in the
individual TAF sites for now.
Wind will genly be from the se to sw direction for the remainder
of today at 10 kts or less. Speeds will decrease a bit tonight and
become more swly on Wed around 10 kts.
OUTLOOK...
Wednesday night...VFR overall. Some mainly evening showers and
thunderstorms are possible especially north/west of KPHL with local
and brief restrictions.
Thursday through Sunday...A chance of local MVFR visibilities early
each morning due to local haze/fog, otherwise mostly VFR except
during times of some mainly afternoon and evening showers and
thunderstorms.
&&
.MARINE...
Conds remain quiet on the area waters, with around 2 ft for the
remainder of today and increasing 2 around 4 ft by the end of Wed.
Wind will become more swly and increase as well. Gusts cud
increase to around 20 kts by the end of the short term pd, however
we will remain below sca criteria.
OUTLOOK...
For Wednesday night through Sunday, high pressure offshore will
result in a southerly wind. This flow is forecast to increase as we
go through Thursday, Friday and Saturday. The airmass is forecast to
be rather warm/hot and therefore we capped the gusts below 25 knots
given lower confidence in how much mixing can be obtained. The
prolonged southerly flow will build the seas some, however these
were kept below 5 feet for now. Some showers and thunderstorms may
occur at times Thursday through Saturday however these may be mainly
inland.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Watch from Thursday afternoon through Saturday
afternoon for PAZ070-071-102-104-106.
NJ...Excessive Heat Watch from Thursday afternoon through Saturday
afternoon for NJZ015-017>019.
DE...Excessive Heat Watch from Thursday afternoon through Saturday
afternoon for DEZ001.
MD...None.
MARINE...None.
&&
Synopsis...Gorse
Near Term...Johnson/Nierenberg
Short Term...Nierenberg
Long Term...Gorse
Aviation...Gorse/Johnson/Nierenberg
Marine...Gorse/Nierenberg