Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Home | 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
FXUS61 KPHI 252007

Area Forecast Discussion
National Weather Service Mount Holly NJ
407 PM EDT Sun Jun 25 2017

A large area of high pressure will slowly move through the eastern
U.S. early this week before moving offshore Wednesday and Thursday.
A series of surface lows will move from the northern and central
plains to the Great Lakes and Northeast from mid to late week,
ushering in warmer and more humid air late this week and this coming


Mid and upper level shortwave trough now over the eastern Great
Lakes region will lift northeast through the evening. A few
additional showers may be possible through the early evening hours.
However, after 00Z, the trough should be moving away from the region
and most of the shower activity is diurnally driven, so don`t expect
much to develop past 6 PM.

Temperature wise, as the drier air continues to filter into our
area, expect efficient radiational cooling to result in lows
slightly below normal, ranging from the lower 50s to mid 60s.


Main story through this period is a cold front at the surface which
will approach our region late in the day. There may be enough lift
with this to once again result in isolated convective initiation
across the higher terrain of eastern PA and NW NJ.

In the mid and upper levels, an approaching trough will result in
decreasing 1000 to 500 mb thicknesses which should also translate to
slightly lower temperatures than what we have seen lately. Highs are
expected to range from the upper 60s in the higher terrain of the
Poconos to the lower 80s across central Delmarva.


Main forecast challenges for the long term include a deep trough
passage around midweek and increasing warmth and precipitation
chances late week into this weekend.

A potent shortwave trough is poised to move through the larger-
scale trough in eastern North America early this week, with
passage through the northeastern U.S. in the Tuesday-Wednesday
time frame. Large-scale ascent in advance of the attendant
vorticity maximum will allow for increased cloud cover on
Tuesday and fairly cool temperatures for this time of year.
Forecast highs are around 5-10 degrees below seasonal averages.

Forecast confidence is a little below average on Tuesday,
however, given some lingering disagreement among the operational
models regarding strength/orientation of the trough axis,
extent of large-scale ascent in advance of the trough in the
Mid-Atlantic, and associated frontal timing. The 12Z GFS/CMC are
fairly dry across the area, but the 12Z ECMWF does generate
some precipitation along the front and offshore. Though there is
much improved agreement on the specifics of a developing low
off the coast with the 12Z simulations, the QPF discrepancies do
not bode much confidence in the sensible weather details,
particularly in southern/eastern portions of the area -- which
will be in closer proximity to the developing low and the last
to be passed by the remnant surface boundary moving through the
region. Decided not to stray much from forecast continuity for
this period.

Upstream ridging will move into the East on Wednesday, with a
broad surface high migrating to the coast by 00Z Thursday. This
should allow for a dry day with continued pleasant (slightly
below average) temperatures.

As the surface ridge moves offshore by Thursday, return flow
commences. Temperatures are likely to rapidly warm in this
regime, with substantial warm/moist advection occurring in
advance of another potent vorticity maximum moving into the
Great Lakes and adjacent southern/southeast Canada by this time.
Midlevel ridging in advance of the vort max should permit the
warm sector to surge well north of the area, which will prevent
associated surface fronts from reaching the area likely through
the end of the long term period.

This means the main questions regarding the Friday-Sunday
forecast revolve around heat/humidity and chances for
precipitation, as the rapid warmth of Thursday will likely be
followed by more gradual increases in temps/dew points
thereafter. Additionally, with the Mid-Atlantic becoming more
and more on the fringes of midlevel ridging, large-scale ascent
(pronounced in advance of west-to-east moving vorticity maxima)
will at least glance the area, especially the northern/western
CWA. As such, the models (to varying degrees) suggest increasing
chances for precipitation as a surface boundary sags southward
from Canada through this period.

There is pretty decent agreement on timing/location of the first
vorticity maximum on Thursday (generally in New York/New
England), so kept PoPs pretty low in the CWA during this time
frame (only mentionable in the southern Poconos and vicinity).
However, Friday through Sunday feature increasing chances for
convection as faster southwest flow slowly edges toward the area,
though pinpointing exact locations and timing (the latter of
which will be closely tied to upstream vorticity maxima this
weekend) is a dubious exercise at this time range given the
somewhat unpredictable pattern (and associated poor phasing
agreement among the ECMWF/GFS/CMC). Generally broad-brushed
slight chance to chance PoPs through the CWA this weekend as a
result, with a general blend of the above-mentioned models and
heavy weighting to continuity/WPC guidance.

Regarding heat/humidity, kept the forecast fairly tame during
this period (and close to statistical guidance) given potential
complications from proximity convection. However, if the ridge
remains more prevalent across the area, forecast could be too
cool this period. Regardless, a sultry weekend is anticipated.


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

Mostly VFR conditions are expected through the TAF period. There is
a slight chance showers and thunderstorms after 18Z Monday across
the higher terrain (including KRDG and KABE). If any showers move
over the TAF sites, MVFR or lower conditions are possible, but it is
too uncertain to include in any of the TAFs at this time.

Winds will stay mostly light out of the west or southwest through
the TAF period. The one exception is KACY, where south southeasterly
winds are expected to persist through at least 00Z in the wake of a
sea breeze. There is a chance a sea breeze could redevelop after
18Z Monday and once again shift the winds at KACY.


Monday night and Tuesday: Predominantly VFR, though
isolated/scattered showers/storms are possible. Winds generally
west or southwest 5 to 15 kts with gusts to 20 kts during the
day. Confidence average.

Tuesday night through Wednesday: VFR. Winds generally northwest
5 to 15 kts. Confidence above average.

Wednesday night and Thursday night: VFR. Winds generally
southwest under 10 kts at night and 10 to 20 kts during the day.
Confidence above average.

Thursday night and Friday: Generally VFR, though there is an
increasing chance for showers/storms, especially north/west of
KPHL during this period. Winds generally southwest 5 to 15 kts
with gusts to 20 kts during the day. Confidence below average.


Winds and seas are expected to stay below small craft advisory
criteria tonight and tomorrow. Westerly wind gusts up to 20 kt are
possible on the Atlantic coastal waters overnight, but should stay
below 25 kt.


Monday night through Wednesday night: Sub-advisory conditions
expected. A slight chance of showers/storms on Tuesday and
Tuesday night.

Thursday through Friday: Advisory winds/seas possible as
stronger southwesterly winds become established. A slight chance
of storms on Friday.

The outlook for tomorrow is that the low risk for the formation of
dangerous rip currents will continue.


Very high astronomical tides are expected to continue today and
tomorrow. There is a chance that minor tidal flooding could occur
especially along the northern NJ shore with the high tide cycle this
evening. However, models over the last few days have had a high
bias, and the winds have been light though the day today. Therefore,
do not expect widespread tidal flooding today. Tomorrow water levels
should be slightly lower as well.




Near Term...Johnson
Short Term...Johnson
Long Term...CMS
Tides/Coastal Flooding...Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.