Scientific Forecaster Discussion
fxus61 kphi 241039
Area forecast discussion
National Weather Service Mount Holly New Jersey
639 am EDT Wed Apr 24 2019
a cold front will move offshore today. A surface low will progress
from the Central Plains today to the Ohio Valley on Thursday. As it
continues northeastward on Friday, a warm front will move through
the mid-Atlantic, followed by a cold front on Friday night. Another
system may affect the region late this weekend. A stalled front may
linger across the mid-Atlantic for much of next week.
Near term /until 6 PM this evening/...
as of 6 am, the cold front was now over the coastal plains,
very slowly sinking southeast. The progress has been a bit
slower than previously expected, but the front should still be
off shore by mid morning. In its wake, colder and drier air
will filter in to the region. As mentioned by the previous
shift, we will keep a relatively tight pressure gradient across
the region for much of the day as the main surface low digs
towards the Gulf of Maine. Consequently, breezy conditions are
possible through the afternoon.
Cold air advection behind this front is not especially strong. Thus,
expect temperatures, although lower than Tuesday, will still be
slightly above normal. Highs should be in the 60s to lower 70s.
Short term /6 PM this evening through 6 am Thursday/...
surface high builds over region, resulting in light and variable
winds overnight. A mid level short wave trough could be responsible
for increasing mid level clouds late tonight into early Thursday
morning. The dry air and light winds will set the stage for
efficient radiational cooling, so lows should be in the 40s, with
lower 50s possible in Delmarva and the 95 urban corridor.
Long term /Thursday through Tuesday/...
primary forecast challenges in the medium-range period include
precipitation chances Friday, winds on Saturday, and poor model
In general, models are in below-average agreement through the medium
range. This is largely owing to uncertainties in the large-scale
pattern in the Pacific and the inherently higher uncertainty
associated with shortwave troughs progressing in mostly separate
northern and southern streams in North America. There is some
indication this long-duration large-scale pattern will change across
the Continental U.S. Next week, but the evolving pattern is also subject to
large uncertainty owing to the smaller-scale perturbations and their
upscale effects on increased wave amplification. The result is a
much-below-average forecast Thursday Onward, but especially so
Model discrepancies begin to appear Thursday, as a southern-stream
perturbation in the Southern Plains begins to eject east-
northeast to the Tennessee Valley and southeast. There are two
main questions associated with this perturbation's evolution:
the overall strength of the perturbation and its potential
interaction with a northern-stream shortwave trough. The NAM and
GFS have trended toward a stronger perturbation with increased
phasing with the northern stream. This leads to stronger
cyclogenesis on the East Coast Friday and Saturday compared to
the European model (ecmwf) and CMC. The CMC looks too fast and too weak compared
to the other guidance and was generally discarded from the
forecast through the medium range. The 00z European model (ecmwf) also trended
slightly stronger, but is exhibiting far greater run-to-run
continuity versus the GFS (and is also closer to ensemble
solutions). As such, weighted the European model (ecmwf) solution more highly
than other guidance in this forecast, but given the fairly
distinct and consistent trends in the guidance, suspect more
changes to the evolution of the system are coming in subsequent
Regarding the sensible weather details, a predecessor northern-
stream perturbation will move through the mid-Atlantic on Thursday,
and models are showing signs that lift will be strong enough to
generate some showers across the area. Increased pops and sky cover
(the latter substantially) on Thursday, though precipitation amounts
should be light (generally a trace to a couple of tenths). Transient
shortwave ridging should ensue by evening before isentropic ascent
in advance of the main southern-stream perturbation approaches the
region late Thursday night. The trend here has been somewhat more
aggressive with precipitation across the area, but there are mixed
signals among the guidance. The NAM keeps most of the precipitation
west of the area (which tends to be the case when ridge
amplification occurs downstream of phasing systems in the
Midwest/southeast). The GFS is somewhat wetter, but tends to be too
far east with this threat in warm-advection regimes. Nevertheless,
the European model (ecmwf) appears similarly, so raised pops late Thursday night as
Friday and Friday night will likely see some widespread showers and
possibly storms as the attendant cold front moves through the
region. Instability looks fairly limited, as thermodynamic profiles
feature poor lapse rates. Shear is decent but by no means
exceptional, so chances of severe storms look fairly low at this
point. Think at least a slight chance of storms is warranted for
Friday afternoon/evening, however.
Quantitative precipitation forecast is pretty variable among the model suite, but appears tied to
the strength of the developing surface low on the East Coast (and
its proximity to the area). Used a consensus approach for now, which
ends up giving the area a general half-to-one-inch of rainfall.
However, potential exists for higher totals (per the GFS and nam),
so cannot rule out some heavy rain potential. Notably, pws will be
high (generally 1.25+ inches), with potential for some convective
training should the more amplified GFS/NAM solutions be realized.
As the cold front sweeps offshore Friday night, attention turns to
wind potential late Friday night and Saturday. BUFKIT soundings do
not show particularly impressive mixed-layer depths in the Post-
frontal regime; however, winds just off the surface will be cranking
(particularly in the more amplified GFS/NAM scenarios). As such,
potential to mix down 30+ kt winds is present. Bumped up winds
substantially during this period (though not to the above-mentioned
thresholds yet). As is typical in these regimes, the wind gusts
realized may be stronger than a 3+ day forecast would suggest.
Any remaining model agreement derails by Sunday, as another
shortwave trough approaches the area. The GFS is considerably more
amplified than the European model (ecmwf)/CMC (though there appears to be some
trending toward increased amplification). The GFS is wetter for our
area as a result (with the strongest lift farther north in the
ECMWF/cmc). Slight-chance to low-chance pops appear warranted for
Sunday and Sunday night at this point until model agreement improves.
Thereafter, a period of high pressure is expected in the northeast,
but its duration and its ability to force an associated cold front
southward are in question. The GFS and the European model (ecmwf) indicate the front
may get held up in close proximity to our region as subtropical
ridging begins to take shape in the southeast and adjacent western
Atlantic. This suggests several rounds of precipitation may be
possible next week. Kept mentionable pops the rest of the period,
though capped probabilities at low-chance thresholds given large
uncertainty in frontal position and timing of midlevel perturbations
poleward of the developing ridge.
Temperatures through the long term were changed little from the
Aviation /12z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...VFR conditions are expected with only few to scattered high
clouds. Winds will generally be out of the northwest through the
day. Gusts near 20 kt are possible, primarily between 15 and 21z.
Tonight...VFR conditions are expected. Winds will start
northwesterly near 10 kt, but quickly become light and variable.
Thursday...mainly VFR, though there is potential for MVFR/showers.
Winds becoming southeast or south around/below 10 kts. Low
Thursday night...sub-VFR probable with increasing chances for
showers late, especially northwest of phl. Light south or southeast
winds. Some potential for low-level wind shear. Moderate confidence.
Friday...periods of sub-VFR possible with a good chance of showers
and possibly thunderstorms. South to southwest winds 5 to 15 kts,
possibly with a few gusts to 20 kts or so. Moderate confidence.
Friday night...showers/storms ending, with VFR conditions ensuing.
Winds becoming west to northwest and increasing to 10 to 20 kts,
with higher gusts possible. Moderate confidence.
Saturday and Saturday night...VFR with strong northwest winds
Saturday (possibly gusting to 30+ kts) subsiding to light/variable
by late Saturday night. Moderate confidence.
Sunday...mainly VFR, but increasing chances for showers. Southwest
winds 5 to 15 kts. Low confidence.
winds and seas are expected to remain below Small Craft Advisory
criteria today and tonight. Northwesterly gusts around 20 kt are
possible through the day, and seas may get close to 5 feet on the
Atlantic coastal waters, but we expect conditions to stay just below
Thursday and Thursday night...sub-advisory winds/seas expected. A
slight chance of showers.
Friday and Friday night...south to southwest winds approaching
advisory criteria, with seas rising to near/above 5 feet. A good
chance of showers/storms. Winds switching to northwest late Friday
night, at which point advisory conditions are probable.
Saturday...advisory conditions probable with northwest winds slowly
subsiding during the day. Seas near/above 5 feet but decreasing
Saturday night...sub-advisory winds/seas expected.
Sunday...increasing south to southwest winds, with gusts approaching
advisory criteria. Seas may rise to around/above 5 feet.
behind the cold front early this morning, we are expecting drier air
to filter in. Consequently, on the coastal plains, minimum relative humidity values
could be as low as 30 percent today. Additionally, wind gusts around
20 mph are possible across the region today. One uncertainty though
is the state of fuels. Greenup continues across the region, and many
areas have seen rain (albeit light rain) in the last few days.
Depending on the state of fuels, the New Jersey coastal plains could
get close to elevated fire weather conditions today. We may
coordinate with our New Jersey state fire partners later this morning to
determine if an Special Weather Statement for elevated fire weather conditions would be