Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 242143
Area Forecast Discussion
National Weather Service Mount Holly NJ
443 PM EST Fri Feb 24 2017
A strong cold front will move through the Mid-Atlantic on Saturday.
High pressure will build into the region on Sunday before moving
into the Atlantic on Monday. A surface low will develop in the
central U.S. on Tuesday and move northeast to the Great Lakes by
Wednesday and southeast Canada by Thursday. An associated warm front
is forecast to move through the Northeast on Wednesday before
another strong cold front sweeps across the East Coast on
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Big question tonight will be when and where low clouds and fog
develop. Winds are expected to shift out of the southeast overnight.
This slight shift with more of a onshore component, could result in
low clouds advecting from the ocean on shore. Having said that,
guidance was fairly poor with the extent of low clouds this morning,
so not sure if it will be quite as widespread as what some guidance
With the onshore flow and potential cloud cover, temperatures should
stay quite mild, with low generally in the 50s across the region.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The first question will be how quickly early low clouds will
dissipate. Even if clouds persist into late morning though, models
have recently had a cold bias in warm air advection events, so have
gone slightly above guidance for highs during the day. If clouds
hang on longer or if the cold front moves in earlier, current
forecast would be too high.
The big story in the short term will be the cold front. Latest
guidance suggests it should be moving across our area from 18Z (for
far western Berks Co and the southern Poconos) to just approaching
the shore by evening. Forecast still looks on track for a line of
showers with embedded thunderstorms along and just behind the front.
The biggest question mark with thunderstorms will be surface based
or mixed layer instability. CAPE values will be marginal at best so
if we do end up being cooler than forecast, instability (especially
surface based instability) could be negligible. Given the model cold
bias though, I think we will have enough instability to support
Primary hazard continues to be gusty winds given the high shear
environment (0-6km bulk shear values could be above 50kt). Will
continue mention in the HWO and add mention to the forecast for this
Although there could be brief periods of heavy rain with the line,
the showers and storms should be moving through fast enough to
limit any flooding risk.
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
The long term period continues to look active with a period of
unsettled weather likely during the early to middle portions of next
week. A pattern change is likely toward week`s end.
By 00Z Sunday, a negatively-tilted shortwave trough in the Northeast
and adjacent portions of Canada will continue to pivot northeastward
to the maritime provinces of Canada by 00Z Monday. A narrow cold-
frontal rain band will likely be moving offshore during the evening
hours Saturday as deep ascent downstream of the associated vorticity
maximum progresses to the northeast. Strong cold air advection will
commence upstream of the front, and winds will sharply turn west or
northwest Saturday evening, becoming gusty as the fast flow aloft
mixes to the surface underneath the frontal inversion.
Sprawling (but progressive) high pressure moves through the eastern
U.S. on Sunday. In advance, gusty winds will continue during the day
in the Mid-Atlantic as boundary-layer mixing diurnally maximizes
underneath mostly clear skies. However, winds will quickly diminish
late Sunday afternoon and evening as the surface high pressure
center approaches the coast. Temperatures will be much colder than
the anomalous warmth experienced in recent days. Consensus of the
guidance has edged downward a bit for highs on Sunday, and given the
strength of the cold air advection in play -- leaned a bit in this
direction, despite the relatively clear skies expected. May see
some stratocumulus development in the Poconos, as is typically
observed in this setup.
The upper-level pattern next week appears to start out as fast west-
southwesterly flow with several embedded vorticity maxima ejecting
from a longwave trough in the West. Operational models remain in
quite a bit of disagreement timing these perturbations as they move
through the central and eastern U.S. The aggressive appearance of
the first perturbation seen in yesterday`s GFS is nowhere to be
found with today`s 12Z simulation. In fact, the GFS overall looks
like an outlier with the overall pattern next week. Given this as
well as this model`s poor run-to-run continuity compared to the
ECMWF/CMC/UKMET of late, pretty much disregarded it with today`s
As such, it looks like there are three main systems that may affect
our region next week. The first occurs some time on Tuesday, with
the most likely timing in the morning hours, as a vort max quickly
moves into the Northeast during the day. Strong isentropic ascent
along a quasi-zonal baroclinic zone in advance of this system
produces a swath of precipitation moving through the region.
Temperatures may be cold enough for snow or a wintry mixture in the
southern Poconos (at least in the morning), but confidence in the
details remains low. As this first system moves through, transient
cold air advection should follow before a stronger low moves into
the Great Lakes on Wednesday. Winds should quickly switch to a more
easterly component, which may favor a period of cold air damming
Tuesday night. Isentropic ascent should commence again during this
time, with another round of precipitation possible by Wednesday. The
details of this second round of precipitation are even murkier, as
much will depend on the eventual position of the warm front via cold
air damming processes not well simulated this far in advance, the
track of the surface low in the Great Lakes, and the track of a
southern stream vorticity maximum to the south/west of the surface
At the moment, it appears three main regions of precipitation will
develop. The first will be tied to the surface low and the
associated cold conveyor belt, which looks to be in the vicinity of
the Great Lakes and New England. The second will be associated with
the isentropic ascent along and poleward of the baroclinic zone
positioned roughly in the Ohio Valley and southern Mid Atlantic. The
third will be along the developing cold front that will sweep across
the East Wednesday night and Thursday. The exact locations of these
precipitation maxima are anyone`s guess, but there are some
indications that a relatively decent precip event could occur in an
east-west band from the Ohio/Tennessee Valleys to the vicinity of
Delmarva Wednesday and Wednesday evening. Bumped up PoPs in the
southern CWA during this time frame. The cold frontal passage
Wednesday night or Thursday could also be tied with some heavier
precipitation -- possibly with localized enhancement via convection.
Also bumped PoPs across the CWA late Wednesday night and Thursday
morning, though confidence in the timing is well below average.
The temperature forecast is of ... below average confidence, to say
the least. Temperatures look generally warmer than average Monday
through Wednesday, but there is some question regarding the observed
warmth Tuesday and Wednesday owing to the potential for
precipitation and the establishment of cold air damming in portions
of the area. For now, kept temps on the warmer side of guidance,
but there is a possibility these numbers are overly optimistic on
By week`s end, a longwave trough becomes established across the
East. This means a return to much colder (i.e., more typical)
conditions across the region. Establishing northwest flow could
bring a few clipper-like systems to the area late next week through
early the following week.
.AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR conditions are expected to continue through at least 03Z. After
03Z, as winds shift to more southeasterly (resulting in more of an
onshore component), could see low stratus move over the TAF sites.
As it does so, could also see some fog development underneath the
stratus. Timing is uncertain, so have mentioned the period of
greatest risk (08 to 14Z) in the the TAFs. After 14Z, any fog should
dissipate and ceilings should begin to slowly lift.
The next concern will be after 18Z as a cold front approaches from
the west. An abrupt shift to westerly winds, widespread showers (and
associated restrictions in visibility), and scattered thunderstorms
are possible with and just behind the front. KRDG and KABE may see
this between 18 and 21Z, while the rest of the TAF sites will be
impacted 21Z or later.
Saturday night and Sunday...Conditions quickly improving to VFR
behind the cold front, with showers/storms moving off the coast.
West or northwest winds 10-20 knots with gusts to 30 knots likely.
Above average confidence.
Sunday night and Monday...VFR with west winds around 10 kts becoming
southwest by Monday afternoon. Average confidence.
Monday night through Wednesday...Periods of showers with associated
sub-VFR conditions possible, especially during the day Tuesday and
again Wednesday afternoon. Primarily light southerly or
southwesterly winds, but a more easterly component may develop on
Tuesday north of a developing warm front, and gustier southwest
winds may occur Tuesday night or Wednesday as the warm front moves
through the region. Very low confidence on timing and
Seas will slowly build overnight and should be above 5 feet on the
ocean waters near or shortly after sunrise. Winds will also increase
after sunrise with gusts above 20kt possible. The strongest winds
(and an abrupt shift to westerly winds) are expected to come with
and behind a cold front which will move over the Delaware Bay late
tomorrow afternoon. The front is not expected to arrive on the ocean
waters before tomorrow evening.
Saturday night...Cold front will move through the coastal waters
with showers/storms likely. Strong/gusty winds near storms. Small
craft advisory (SCA) conditions likely as winds become west or
northwest behind the front. Gusts may approach gale force. Average
Sunday...Continued SCA conditions likely with winds slowly
diminishing during the afternoon. Above average confidence.
Sunday night and Monday...Sub-SCA conditions likely. Above average
Monday night through Wednesday...Generally sub-SCA expected, though
winds and seas may approach criteria after potential warm frontal
passage Tuesday night or Wednesday. Periods of rain/showers likely,
especially during the day Tuesday and again Wednesday afternoon.
Very low confidence.
Record event reports have been issued for 6 of our 8 climate
sites, including an all time monthly for the shorter POR at
Allentown. Please see each for detail.
ABE 77, RDG 77 GED 76 TTN 75 MPO 67 PHL equal at 74
We may add more information by 6PM.
MARINE...Small Craft Advisory from 7 AM Saturday to 6 PM EST Sunday for
Small Craft Advisory from 6 PM Saturday to 6 PM EST Sunday for