Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 241331

Area Forecast Discussion
National Weather Service Mount Holly NJ
931 AM EDT Mon Apr 24 2017

An area of low pressure will deepen along the Southeast coast
through tonight and then move slowly up eastern seaboard Tuesday and
Wednesday. A cold front will approach from the west late Thursday
into Friday. This boundary may return back northward as a warm front
during the weekend.


930 AM update: Brought PoPs northward a bit faster through this

515 am update: Showers are increasing south of a Stevensville,
MD, to Atlantic City line early this morning in what looks to be
a zonally-oriented zone of weak ascent associated with
differential cyclonic vorticity advection poleward of a
weak/small-scale perturbation ejecting from the upper low in the
southeast U.S. Most of the coarser operational guidance appears
to have little idea of this perturbation (e.g., the 06Z NAM),
but the past few runs of the HRRR have begun to pick up on
this. Spread slight chance to chance PoPs a little bit northward
this morning to account for the increasing shower activity.
Precip amounts should generally be quite light during the
morning hours.

Additionally, hi-res models continue to reverse course and are
speeding up the second round of precipitation moving in this
afternoon, now on the doorstep of Philadelphia in the 21Z to 00Z
time frame. Have spread higher PoPs northward during the
afternoon hours as well.

Previous discussion...

Water vapor imagery indicates an upper low making very slow progress
southeastward into northern Georgia early this morning, with an
elongated surface low extending from the SC/GA border SSE into the
Atlantic waters east of Florida. The cutoff upper low is now nearly
neutrally tilted and is expected to become slightly negatively
tilted today as a jet streak rotates around the upper low. Highly
difluent flow will develop just downstream in the Coastal Carolinas
and southern Mid-Atlantic. In conjunction with right-rear quadrant
jet dynamics from a departing New England jet streak, broad/deep
ascent will continue/intensify today along much of the Eastern
Seaboard.  This will foster a gradual intensification of the surface
low as it slowly pivots north-northeast along the Southeast Coast.

Strong low-level onshore flow will develop north of the low, with
isentropic ascent combining with upslope flow to generate widespread
rain and embedded convection along and east of the central/southern
Appalachians today. Our region will be on the far northern fringe of
the stronger ascent today, but the upper low will cease its
southeastward push today as it acquires the negative tilt.
Subsequent to this, the stronger ascent will begin its approach to
the area. This should result in a gradual uptick in precipitation in
the Delmarva Peninsula today with precipitation starting out
light/spotty owing to relatively weak ascent this far north of the
low and the relatively dry low levels that will necessitate
moistening/saturation for steadier precipitation to develop.

Recent simulations of higher-resolution guidance (notably the
HRRR/WRF-ARW) are bringing in precipitation just a little bit
faster (18Z-21Z in the Delmarva Peninsula; 21Z-00Z south of
I-78/I-195) as a perturbation ejects northward from the upper
low this afternoon. However, given my aforementioned concerns
with the low-level air being fairly dry, kept PoPs in the slight
chance to chance range for much of these areas today, with good
chance to likely PoPs confined mostly south of the Mason-Dixon
Line from 3 pm to 6 pm as model agreement improves regarding
precipitation coverage.

Precipitation during this period will be light, but will also act to
put a quick cap on temperatures, which should hold fairly steady in
the central/southern CWA this afternoon as cloud heights lower and
surface diabatic heating is dampened. Highs today are forecast once
again to be highest northwest of the Fall Line (mid 60s), where at
least partial sunshine will remain this morning before overcast
conditions move in, with temperatures in the upper 50s to lower 60s
south/east (lowest near the coast). An increasing east/northeast
wind should be expected (generally 10 to 20 mph by afternoon -
lighter northwest of the Fall Line).


The surface low will make little progress tonight, likely remaining
in the vicinity of the Coastal Carolinas as the attendant upper low
pivots slowly northeastward to near Myrtle Beach by 12Z Tuesday.
Most operational models depict a predecessor vorticity maximum
ejecting northward from the upper low tonight, moving through the
northern Mid-Atlantic region between 03Z and 09Z. Ascent just
downstream (north) of this perturbation will generate steadier/more
widespread precipitation across the area this evening and tonight
with some potential for a brief drier period as the perturbation
departs late tonight. However, accurately timing these perturbations
is a challenge, particularly given the already low-predictability
pattern in place. Thus, although there is some agreement regarding
the timing of the steadier precipitation (generally in the 21Z
Monday to 09Z Tuesday time frame), confidence is simply too low to
delineate a drier period PoPs-wise. Therefore, generally
broadbrushed PoPs during the tonight period, with good chance to
likely PoPs across the area through the night - with the caveat that
it probably will not rain the entire night in any one location.

BUFKIT soundings and convection-allowing models are providing some
indication of elevated instability and more convective precipitation
near/off the coast tonight. Though much of the ascent in our region
tonight looks large-scale (generally stratiform in nature), the thin-
CAPE profile from parcels rooted near 900-850 mb would permit the
development of scattered convection, which the latest SPC-WRF, WRF-
ARW, and HRRR simulations all appear to be depicting this evening.
Have included a slight chance of thunder roughly from Stevensville,
MD, to Atlantic City for the tonight period.

QPF should generally be light -- a tenth to a third of an inch in
general, but amounts may be locally higher with any convection that
develops in the southern portions of the area.

Temperatures are forecast to be rather steady overnight with the
expected overcast conditions and precipitation. Steady
east/northeast winds should continue to increase as the proximity to
the slowly intensifying surface low gradually increases. Winds of 10
to 20 mph are likely, with some gusts to 30 mph or so possible near
the coast.


A vertically-stacked low over the eastern Carolinas Tuesday morning
is expected to weaken as it track slowly up the Mid-Atlantic coast
Tuesday and Wednesday. As discussed yesterday, a westward shift in
the low track has been noted recently in the models. There still
remains modest spread in the low track among the difference ops
models and ensemble guidance. Rain looks to become widespread across
the area Tuesday-Tuesday night. QPF amounts vary considerably from
model to model, which could be tied to the seemingly minor
differences in storm track, storm structure (especially the low-
level jet and TROWAL on the eastern and northern side of the
cyclone) and mesoscale boundaries (e.g., coastal front). There is a
potential for a swath of enhanced rain along and just on the cool
side of the coastal front so monitoring the position of this
boundary will be important. Some of the wetter solutions (e.g., NAM,
CMC) bring the coastal front inland (close to I-95) and show more
pronounced lift within the TROWAL and at the nose of the low-
level jet, resulting in a local max rainfall band of 2-3".
Meanwhile, there is another mode in the precip distribution
among the guidance (e.g., GFS, NAM nest, WRF ARW and ECMWF) that
indicates the heavier rain confined to coastal areas as the
coastal front remains just offshore. The latter camp of guidance
generally advertises QPF of 1.0-1.5" within this heavier band,
with less amounts farther inland. Forecast reflects a compromise
of both scenarios with the heaviest amounts (QPF 1-2") falling
over the coastal plain. A slight chance of thunderstorms was
introduced to the forecast in Delmarva and S NJ where elevated
instability may exist assuming the coastal front advances this
far northwest. Temperatures on Tuesday will also be dependent of
the position of the coastal front with highs in the 50s (60s)
northwest (southeast) of the boundary.

Models disagree about what happens with the coastal low by the time
it reaches our latitude late Wednesday-Thursday with some solutions
curving the storm out to sea, some keep in an a northward track into
New England and while others dissipate it just off the northern Mid-
Atlantic coast. Either way we should dry out sometime during the day
Wednesday and remain precip free Wednesday night and Thursday.
Forecast confidence for high temperatures on Wednesday are below
normal as it will depend on how quickly skies clear out behind the
low. The current thinking is there will be a notable temperature
gradient across the area Wednesday afternoon with SW zones
(Delmarva/SE PA) having the best opportunity to see sunshine
arriving in time for afternoon heating than locations farther N/E.
The SREF/CMC matched this idea fairly well with highs in the 70s S/W
of Philly and 60s to the N/E. Warmer air aloft arrives on Thursday
owing to strengthening subsidence underneath a shortwave ridge.
However, the forecast has trended cooler for Thursday with E-SE
winds in the boundary layer advecting cooler marine air inland
(which is backed compared to yesterday`s runs).

Low pressure is expected to pass well to our NW through the Great
Lakes and into E Canada late this week. This track will allow the
pre-frontal warm sector to further become established Thursday night
and Friday. Highs in the 80s are anticipated for Friday, except
cooler along the coast due to a sea breeze. Friday also contains a
chance for showers and storms with a cold front approaching from the

The aforementioned front should weaken and stall over the region
Friday nigh and into the weekend as it encounters downstream ridge
blocking over the western Atlantic. The position of this boundary
will greatly influence temperatures and chances for rain this
weekend. Models advertise a strong temperature gradient near this
boundary with highs in the 80s (maybe even close to 90) in the warm
sector south of the front and 60s (maybe even 50s) on the northern
side. The forecast reflects the idea that the warm front should be
able to lift north of the area on Saturday. By Sunday, the boundary
may start to sag southward (as a backdoor cold front) into our
northern zones in response to high pressure building southward from
Canada. The best chance for showers and storms will reside close the
front. With uncertainty in the boundary position, PoPs were kept low
for the upcoming weekend (20-30 percent).


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

09Z update:  VFR conditions are expected at the terminals
through at least early afternoon, but CIGs will gradually
approach MVFR by late afternoon or early evening from south to
north. Rain is expected to move in to KILG/KMIV/KACY around or
after 18Z and from KPHL northward between 21Z and 03Z.
Conditions will continue to deteriorate through the evening,
likely becoming IFR near or after 06Z Tuesday. Primarily E or NE
winds will increase through the day, becoming 10 to 15 kts
after 15Z, with potential for occasional gusts to 20 kts or so,
especially at KMIV/KACY.


Tuesday and Tuesday night...High confidence for IFR with periods of
rain. E-NE winds 10-15 kt with gusts up to 25 kt, especially near
the coast.

Wednesday...A gradual improvement from IFR to MVFR from SW to NE is
anticipated during the morning. MVFR CIGs may linger thru much of
the afternoon, particularly from PHL, N and E. Light winds

Wednesday night and Thursday morning...Low clouds may remain trapped
beneath a low-level inversion. This could result in continued MVFR
CIGs and even expansion of MVFR back farther to the S/W. Lower
confidence in IFR but it`s still a possibility, especially late
Wednesday night-early Thursday.

Thursday afternoon through Friday...Predominately VFR. SEly winds
on Thursday become Sly Friday.


East to northeast winds will increase to small craft advisory levels
early this morning in the Delaware coastal waters, with these
stronger winds pushing northward into Delaware Bay and the New
Jersey coastal waters during the afternoon. Winds are expected to be
strongest late this afternoon and early this evening, with gusts
approaching but likely staying predominantly below gale-force during
this time. There may be a subtle decrease in winds late tonight, but
conditions should remain above advisory thresholds. Seas will rise
above 5 feet today and will continue to build through the night.

Rain is expected to progress south to north through the coastal
waters late this afternoon through tonight. Some restrictions in
visibility are expected, especially tonight. Isolated thunderstorms
may occur in the marine zones tonight.


Tuesday and Tuesday night...SCA was extended to cover this period
with high confidence for winds and seas to meet advisory criteria.
There is a potential for easterly winds to gust to near gale force
briefly during the afternoon. Confidence of gales is not
particularly high with the strongest pressure falls confined south
of our waters and with stable mixing profiles. Will continue to
mention the possibility of gales in the HWO. Seas look to build to 7-
9 ft in our coastal zones.

Wednesday through Friday...Winds will become light. However, seas
will likely still remain elevated at or above 5 ft through Thursday
night or even Friday morning.


Positive tidal anomalies are expected with onshore flow persisting
through Tuesday night. Ensembles included in the Stevens Flood
Advisory System indicate a high probability for minor coastal
flooding at our tidal points along the oceanfront and DE Bay
with the Tuesday afternoon-evening high tide. Coastal Flood
Advisories may eventually be needed as tidal departures of
around a foot above astronomical tide would be sufficient to
produce minor coastal flooding with the new moon on Wednesday.

A few of the outlier ensemble members in the Stevens tidal
guidance predict moderate coastal flooding, particularly at
Reedy Point and Lewes. However, there is a much higher
likelihood that the magnitude of flooding would be limited to
minor since onshore flow will not be particularly strong
(predominately be below gale force) and the low pressure center
will be relatively weak (MSLP above 1000 mb) at the time it
moves in close proximity to our region.


MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ452>455.
     Small Craft Advisory until 6 AM EDT Wednesday for ANZ430-431-


Near Term...CMS/33
Short Term...CMS
Long Term...Klein
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