Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1211 PM EDT Thursday Sep 18 2014
a weak frontal boundary will move through our area this afternoon
and evening. High pressure will then build in tonight into Friday,
before shifting offshore Friday night. A warm front will lift to
our northwest Saturday, then a cold front is expected to arrive
late Sunday into Monday. This front will settle to our south and
east during Monday, then an expansive area of high pressure is
scheduled to build in for Tuesday and Wednesday.
Near term /until 6 PM this evening/...
with the midday update, we added some isolated showers to the
forecast for the next couple of hours across portions of far
southeastern Pennsylvania, far northern Delaware, and Cecil County
Maryland. Latest radar imagery shows a small area of light showers
and sprinkles holding together and slowly moving east into these
areas. In addition, hourly temperatures and dewpoints were also tweaked
through the afternoon using the lav/lamp guidance to account for
latest observations and trends.
Otherwise, the middle-Atlantic is located toward the base of an
upper trough as seen on water vapor satellite imagery. This large-
scale trough is associated with an upper low located just to the
east of Hudson Bay. A cold front stretching from the Gulf of St
Lawrence southeastward into upstate New York was rather diffuse on the
13z surface analysis. The cold front will move southward into the middle-
Atlantic region this afternoon.
12z iad, apg and okx soundings all observed a shallow saturated
layer between 800-700 mb with moisture trapped below the middle-level
subsidence inversion. Modest lift associated with a shortwave trough
has lead to a band of broken-overcast stratocu over eastern PA, northwest New Jersey
and the eastern Maryland so far this morning. Tried to time eastward
progression of the trough and associated stratocu in hourly sky
grids with the morning update. Latest rap model, which had
initialized well with current cloud cover, hangs on to the morning
cloud the longest into the afternoon over southeastern PA and the
Forecast maximum temperatures today in the middle 70s are only a couple of degrees
Short term /6 PM this evening through 6 am Friday/...
the middle-level trough axis moves off the coast this evening. A surface
high behind the cold front begins to build in across the area.
Temperatures will settle in the lower 40s across the southern
Poconos to the middle 50s across the coastal plain. This is a few
degrees below normal.
Could be a bit more cloudiness across the southern zones, but no
precipitation is expected. Some patchy fog is possible toward
Northwest winds on the lighter side will shift out of the north or
northeast by the Friday sunrise.
Long term /Friday through Wednesday/...
the overall synoptic setup is comprised of the flow becoming more
zonal Friday across the northern half of the nation. Some ridge
development though out west looks to allow a short wave to dive into
the Midwest Saturday, which results in a trough amplifying across
the Great Lakes and northeast Sunday through Monday. This trough may
then start to lift out late Tuesday. It appears the main frontal
passage occurs Sunday night into Monday, then an expansive surface
high is scheduled to build across the region Tuesday and Wednesday.
We used a general model blend for Friday through Sunday, then
blended in the 00z wpc guidance thereafter. Some adjustments were
then made after additional collaboration with our surrounding
For Friday...strong surface high pressure should be shifting off the
New England coast with our low-level flow turning east and
southeast. There is some moisture forecast to be lingering well
offshore especially to our southeast, and the onshore flow into our
area may result in some cloudiness especially at night. The
potential complication is that the low-level flow may actually veer
to more southeast or south through Friday night, focusing an
increase in moisture northwest of the fall line. It is here where
perhaps some stratus and/or fog develops as the low-level moisture
pools beneath an inversion. As of now, not quite sure how much
stratus develops due to some question on how quick the lower levels
moisten up. For now, we increased the clouds especially at night a
bit more. The combination of a new airmass and the onshore flow
should result in a cool day across the cwa, and potentially breezy
along the coast.
For Saturday and Sunday...high pressure becomes elongated into the
middle Atlantic as it weakens and shifts farther offshore. This occurs
as some warm air advection gets underway ahead of the next system and heights rise
some. This should result in a northbound warm front that mostly
slides just to our northwest, and warmer air arrives across our County Warning Area
especially Sunday with a more southwesterly low-level flow. A short
wave diving across the Great Lakes will drive a surface low
eastward. The amplification of the upper-level trough should take
the main surface low well to our north though during Sunday. The
associated cold front however will arrive into our area late Sunday
with some showers. A band of showers may end up weakening with a
southeastern extent due to much of the large scale lift peeling
away to the north. The instability looks low therefore we
continued with no thunder mention at this time. We generally favored a
blend of continuity and wpc guidance, which paints slight chance to
low chance probability of precipitation across the County Warning Area mainly Sunday night. Meanwhile, a lead
short wave near the Carolinas may organize a surface low along an
old baroclinic zone. This feature should track northeast and have
no real affect on our sensible weather.
For Monday and Tuesday...the main upper-level trough may amplify
some more across the northeast and middle Atlantic regions Monday.
Based on some ridging entering the plains, an expansive surface high
is forecast to build across the Midwest and Great Lakes later Monday
into Tuesday, with this feature arriving into our area during
Tuesday. Some guidance shows some instability type showers occurring
just to our north Tuesday as another short wave rotates around the
base of the upper-level trough. Regarding pops, kept some slight chance
to low chance probability of precipitation especially Monday morning with the departing cold
front. We carried a dry forecast for Tuesday as the upper-level
trough may start to lift out some with any focus generally to our
For Wednesday...as a ridge slides across the Great Lakes, a
potentially strong surface high builds across the northeast and middle
Atlantic regions. If the center of this high ends up across New
England, then we will have an onshore flow once again. As of now,
leaned much closer to the 00z wpc guidance for all weather elements.
Aviation /16z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas
VFR today and tonight. Stratocu between 5-9 kft progresses eastward
across the terminals today. Northwest winds under 10 knots today will veer to
the north this evening and NE by early Friday morning.
Friday...VFR through the day, however some low clouds or even some
fog could develop overnight within an onshore flow. Our confidence
is on the lower side.
Saturday and Sunday...some low clouds and/or some fog to start each
day, otherwise mostly VFR. A cold front arrives Sunday night with
possible showers and local sub-VFR conditions.
Monday...some showers possible especially in the morning, otherwise
mostly VFR with a wind shift to the northwest.
buoy 44065 came back on line about 10 PM last night. Hopefully buoy
44025 will not be far behind.
Today...long period swells generated by Edouard continue to reach
the East Coast today. Latest observation show 13-14 second wave periods this
morning. Seas expected to remain just below Small Craft Advisory levels wit wave
heights are 4 feet while northerly winds will be around 10 knots
(gusting to 20 knots late in the day).
Tonight...swells are still forecast to be with US through tonight.
Seas will be less than 4 feet. Light northerly winds will increase
to 20 knots overnight especially across the northern waters.
Friday...a northeasterly wind surge with an initial tightening of
the pressure gradient and cold air advection should then ease some as the winds
turn east-southeast by late in the day and at night. The surge could
be enough for some 25-knot gusts and seas building to 5 feet on the
coastal waters for a time. As of now the winds and seas are
marginal and therefore will hold off on an advisory.
Saturday and Sunday...the winds should turn from the south during
this time frame as high pressure departs to the east. The next cold
front arrives Sunday night, however an increase in the winds should
occur ahead of it but they should be below advisory criteria. The
seas however could be right near 5 feet Saturday from the earlier
Monday...the conditions should be below advisory criteria overall as
winds shift from the north as high pressure starts to build in late.
in house guidance suggests a moderate risk today for New Jersey/Delaware because of
long period swells generated by Edouard.