Scientific Forecaster Discussion
fxus61 kphi 270148
Area forecast discussion
National Weather Service Mount Holly New Jersey
948 PM EDT Fri may 26 2017
weak high pressure will briefly affect the area tonight. A weak
low is forecast to move along a developing stationary front to
our south Saturday into Saturday night. Developing low pressure
from the upper Great Lakes into southern Canada Sunday and
Monday will direct a frontal system east across the mid Atlantic
states. Weak cold fronts are forecast to move across the area
Tuesday and Wednesday.
Near term /until 6 am Saturday morning/...
930 PM estf covered for sprinkles through about 11 PM NE and E
central PA. Added more cloud cover. Temps adjusted slightly
through about 3 am.
So, showers/sprinkles possible this evening eastern PA
otherwise bands of cloudiness should thin and diminish during
the night. Winds will continue to diminish.
Short term /6 am Saturday morning through 6 PM Saturday/...
the aforementioned vort Max upstream will move through the region on
Saturday. Models have sped up the timing of this system by quite a
bit, generally on the order of 6 hours. Most operational guidance
and some of the longer-range high-resolution simulations indicate
precipitation moving into the area by tomorrow morning. Increased
pops for the morning hours based on this trend.
Cyclogenesis will occur east of the Appalachians (probably in
virginia) during the day, with a baroclinic zone rapidly developing
to the east of the low. As a result, the warm sector will likely be
south of the area as the zonally-oriented surface boundary will
remain to the south. Not overly excited about thunder chances in
this regime, but there are at least some indications that elevated
instability will exist south of the Mason-Dixon line as large-scale
lift cools the midlevels atop the near-surface stable layer. Kept
thunder chances during the afternoon hours for this area as a
The vort Max will be in the process of being sheared out in the
confluent flow upstream of the upper low in the northwestern Atlantic.
This will result in relatively weak (and weakening) lift as the
system moves through. Thus, precipitation will generally be
light, and given that the warm sector is expected to be south of
the cwa, not expecting more robust thunderstorm development.
However, if the warm sector sneaks into the southern cwa, then a
strong storm or two is possible with the vertical shear being
favorable for storm organization. This is considered to be a
very low probability at this point.
Temperature forecast is tricky with the approaching system and
complications from cloud cover. MOS has come in colder tomorrow (by
about 3-7 degrees), and have generally followed, though I did tweak
temperatures a bit using higher-resolution guidance, which may have
a better handle on how any precipitation affects proximity ambient
temps. Forecast highs range from the low-mid 60s in the Poconos to
the mid 70s in southern Delmarva - with lower than average
confidence given the aforementioned complications.
Long term /Saturday night through Friday/...
an unsettled weather pattern continues for much of the extended
forecast with several periods of rain possible.
On Saturday night, the area of low pressure will be passing to
our south and southeast as it moves eastward along the the
frontal boundary to our south. Any showers associated with this
low are expected to pull away from the area during the evening
hours, with the remainder of the night likely staying dry.
High pressure is forecast to build across the northeast
Saturday night into Sunday, which would keep the frontal
boundary to our south and keep an east southeasterly flow across
the area. This easterly flow would keep cloudy and cool
On Sunday night, the front to our south is forecast to begin
lifting north toward the area, while an occluding frontal
boundary approaches from the west as low pressure moves through
the Great Lakes region and combines with a larger low across
southern Canada. The occluded front may not actually move into
the area until Monday or Monday night, but the chance for rain
will begin to increase from the west overnight Sunday into
A series of cold fronts or surface troughs are forecast to move
across the area basically each day from Tuesday through Friday
as low pressure slowly drifts eastward across southern and
eastern Canada. There will be a chance showers or thunderstorms
each day, which will be focused around the time of any short
wave/vorticity impulse passages.
High pressure tries to build back into the area for Thursday, which
would provide a dry forecast.
Aviation /02z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR sct-bkn aoa 5000 ft. Northwest wind diminishing.
Saturday...VFR ceilings developing during the morning with a band or
two of showers passing east or east southeast around 30 kt
across the tafs sites, generally 15z-21z. Small chance MVFR
conds develop in the showers. At this time no thunder forecast.
Saturday night...MVFR to IFR ceilings possible. Chance of scattered
showers in the evening.
Sunday...MVFR to IFR ceilings in the morning, improving during the day.
Isolated showers during the day.
Sunday night...MVFR to IFR conditions possible with periods of low
clouds and rain.
Monday...MVFR to IFR conditions forecast to continue through the
day, but some improvement by afternoon. Scattered showers and
thunderstorms possible during the day.
Monday night...conditions may lower to MVFR or IFR with low clouds
Tuesday-Wednesday...generally improving to VFR during the day
Tuesday and continuing through Wednesday. Chance of showers or
thunderstorms on Tuesday and Wednesday afternoon/evening.
ni marine headlines through Saturday. West winds with gusts
under 20 kt overnight with winds turning light northeast
This weekend: as it stands now...we should have a low risk for
the formation of dangerous rip currents. Low does not mean
no!Risk. Weak swimmers should always be within sight of a
Expecting a 2-3 ft se swell with ~7 second period both days. A
somewhat stronger east wind is outlooked for Sunday. Even Monday
looks to be a low risk day, however thunderstorms could put a
damper on beach activities on Memorial Day. Water temps are sort
Saturday night-Wednesday...conditions expected to remain below
advisory levels, although winds may gust around 20 knots at times.
astronomical tides continue to run high, and despite an offshore
flow, tidal departures remain about 0.5 to 0.8 feet above astronomical
tide. Therefore, minor coastal flooding was occurring on the
coastal areas of New Jersey and parts of Delaware and areas
along Delaware Bay this evening.
As we go into the weekend, winds are expected to become onshore
again, which could keep tidal departures elevated, possibly
leading to additional coastal flooding with the evening high
tides Saturday and Sunday, especially on Saturday.
Acy 2nd wettest may on record so far with 7.11 inches. The
Phl #11 so far with 6.12 inches.
New Jersey...coastal flood advisory until 2 am EDT Saturday for njz012>014-
Delaware...coastal flood advisory until 2 am EDT Saturday for dez002>004.
near term...drag 948