Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
940 PM EDT Thursday Jul 28 2016
low pressure in the virginias will organize and move along a warm
front through the Delmarva early Friday then off the New Jersey
coast by midday Friday. Weak high pressure follows across New York
and New England Saturday. Low pressure develops near the Delmarva
Sunday and moves out to sea Monday. Drier and less humid weather
is expected for much of next week as high pressure builds over
Near term /until 6 am Friday morning/...
930 pm: sva and flash Flood Advisory watches continue. Temporarily quiet. We
expect svr potential to increase in waa toward 11 PM on the
Delmarva. Please do not think watch cancellation means its all
clear all night. Its only all clear through about midnight in the
Flash flood potential continues fairly high and the watch continues. Greatest
concern is se PA, S NJ, NE MD, central and northern de, where it
rained hard this afternoon, and/or where 3 hrly ffg is under 3".
Svr wx threat is mainly Delmarva and S tip of New Jersey through midnight
and then it may expand northward after midnight to near ilg-Acy
axis as low pres in northern Virginia cuts enewd through S New Jersey. Tor parameter
remarkably large toward 09z on the sref in snj.
Lsr's and pns will be updated during the night.
We can benefit from all damage reports, as well as unusual flooding
reports and any rainfall over 2 inches.
Precipitable water increase to 2.3" by 09z philly area. If it thunders at that
time, short term rainfall could easily be excessive for urban
Normally heaviest rainfall occurs near the path of the sfc low,
near the most unstable air which tends to pool near the wind
shift between south-northeast sfc wind convergence zone. Am
expecting modeled qpf to be too far north.
So, another round of heavy rain looks to move in late tonight as
a wave of low pressure tracks northeastward along the stalled
front. Heavy rainfall with this second round may be more widespread.
Also, backed flow in the low levels ahead of the low may provide a
source of low- level helicity. Given very low local heights, a weak
tornado or two may occur toward early morning near the front as
long as instability remains near surface based.
Zone forecast product wording in parts of the area has repetitive phrasing due to
nearly identical grids (except gusty wind replacing damaging wind
Short term /6 am Friday morning through 6 PM Friday/...
precipitable water values of 2.2 to 2.4 inches will overspread
our forecast area from the south for tonight. Conditions are
expected to remain unstable, especially across our southern
counties. Also, surface low pressure approaching from the west
will result in the development of a favorable low level wind
profile for organized convection tonight. The surface wind should
begin to favor the east to south quadrant late tonight, especially
on the coastal plain, while winds around 800 hpa will be from the
The very high precipitable water values should result in heavy
rainfall in any organized convection. As a result, the Flash Flood
Watch remains in effect for southeastern Pennsylvania, central and
southern New Jersey, Delaware and northeastern Maryland.
The cloud cover and high humidity will keep temperatures from
falling below the 70s for tonight except in the Poconos and far
northern New Jersey.
Long term /Friday night through Thursday/...
* scattered showers/T-storms W/locally heavy rainfall this weekend
* drier/less humid weather for much of the next work week
Dry weather expected Friday night with subsidence behind departing
shortwave. May see some patchy fog develop overnight with wet ground
and lingering low level moisture.
Saturday and Sunday...
A couple of additional shortwaves will move across our region this
weekend along with a frontal boundary in the vicinity. Timing is
uncertain this far out, but more scattered showers and thunderstorms
are expected at times this weekend. Pwats will still be between 1.5
and 2 inches, so the threat will exist for locally heavy rainfall.
Clouds and the threat of precipitation should hold high temps mainly
in the 80s both days.
Monday through Thursday...
High pressure will build over the region, bringing US drier and less
humid weather. There still will at least be a low risk for a few
showers early in the work week, but either way dry weather should
dominate through Thursday. High temps will mainly be in the 80s.
Aviation /02z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...VFR showers with probably periods of IFR conds and gusty
winds to 25 kt in sct tstms, especially after 04z/29. Otherwise
sustained winds under 10 kt will tend to become northeast toward
morning. Confidence is low for the timing of heaviest oncoming
convection at any particular taf site, but high that storms will
be in the region through early Friday morning.
Friday...showers and thunderstorms end from west southwest to
east northeast. MVFR or IFR conds 12z-15z in showers/sct tstms
then improving to VFR cigs in the afternoon with a northeast
backing to north wind gust 20 kt.
Sat through sun...shra/tsra will lower cigs/vsbys at times
otherwise VFR expected. Moderate confidence.
Mon...mainly VFR. Moderate confidence.
a Severe Thunderstorm Watch is in effect for the southern waters.
There is a potential for storms through this evening to produce
locally strong winds. An isolated waterspout is also possible
through early Friday morning. The svr risk will probably increase
early Friday morning along the de and S New Jersey coasts as low pressure
crosses the area.
Winds turn northeast to north behind the low Friday morning then
more north to northwest late Friday afternoon.
Expect seas to respond by building to 2 ft in the morning and
possibly 3 to 4 ft during the afternoon in our coastal waters.
Outlook...Friday night through Tuesday...
There is a low risk for marginal small craft seas Friday night but
for now kept them just below 5 feet. Otherwise, winds/seas should
generally remain below Small Craft Advisory thresholds through the
period. Biggest concern for mariners will be the risk for
thunderstorms this weekend. Drier weather should arrive early next
Rip currents...we have forecast low risk Friday but if the wind
turns onshore with gusts 15-20 kt, we may need to increase the
risk to MDT for a portion of the New Jersey coast.
Outlooking: still generally low risk for awhile, possibly low
enhanced. We do note some longer period swells anticipated by the
GFS around the 3rd/4th which may increase the rc risk. Also the
tropics have been quiet this July, but there are a couple of
disturbances in the eastern Atlantic that are being monitored.
ged: a record daily rainfall has occurred so far today.
Phl: heading for top 10 warmest July on record. July avg temp for
phl will be about 80.9f, 8th warmest July in the period of record
Acy: already as of 850 PM this evening...is 7th wettest July on
record with its monthly record of 13.09 set in 1959. The por is back
to 1874. Total for the month as of 850 pm: 8.34" with 1.17 today,
Daily record rainfall
Today Friday the 29th
Acy 2.05 2012 2.21 - 1884
Phl 8.02 2013 3.53 - 1980
Ilg 2.34 1914 1.85 - 1913
Abe 3.00 1916 1.64 - 1979
Ttn 2.35 2012 2.84 - 1961
Ged 1.98 1994 1.07 - 1969
Rdg 3.57 1969 2.51 - 1961
Mpo 2.15 1969 4.59 - 1969
PA...Flash Flood Watch through Friday morning for paz054-055-
New Jersey...Flash Flood Watch through Friday morning for njz001-007>010-
Delaware...Flash Flood Watch through Friday morning for dez001>004.
Maryland...Flash Flood Watch through Friday morning for mdz008-012-015-
near term...drag/Klein 940p