Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
651 am EDT Tuesday Oct 21 2014
low pressure over Lake Ontario will slide southeast to the New
Jersey coast this evening. It slowly intensifies Wednesday and
passes southeast of Cape Cod Thursday night. High pressure over the
Ohio Valley late in the week will await the passage of a cold front
Saturday night then build over the Middle Atlantic States Sunday and
Near term /until 6 PM this evening/...
an upper-level trough will continue to amplify into the northeast
through today. An initial strong and compact short wave carving out
the trough continues to work its way across Pennsylvania early this
morning. This feature is well evident on the water vapor imagery as
it lifts east-northeastward, and appears to be stronger than earlier
guidance indicated. As a result, the downstream warm air advection and lower level
convergence/lift has been strong enough to develop and organize some
showers, which are now lifting to our north overall.
A stronger middle/upper level jet diving down the west side of the
trough will push stronger short wave energy along. This will assist
in cutting off the trough from the main flow, therefore this
upper-level trough will be closing off during the course of the day.
This will also take low pressure near the eastern Great Lakes early
this morning, and then move it over our area before it starts to
eventually consolidate offshore. The main forecast challenge is the
timing and coverage of the associated showers. After some initial
showers early this morning mainly across the western zones, there
should be a break before additional showers develop especially
later this afternoon as another strong short wave rounds the base
of the trough. The soundings show some instability developing, but
this is mostly confined to the lowest levels with perhaps an
isolated low-topped storm. Given less certainty, we held off on
adding a thunder mention at this time. We continued slight chance to chance probability of precipitation
for isolated/scattered showers through the day, although a time of
likely probability of precipitation across the western areas closer to the incoming
The airmass is moderating ahead of the main upper-level trough and
the associated warm air advection will yield warmer afternoon temperatures. There
may be a squeeze play though as the warmest air may slide up across
our Delaware-Maryland-Virginia and eastern zones. Therefore, a blend of the NAM/GFS
MOS resulted in the warmest high temperatures in these areas.
Otherwise, the flow should start to shift around from the southwest
to the northwest during the course of the day as low pressure
develops over our area before consolidating some just to our east
Short term /6 PM this evening through 6 am Wednesday/...
a closed low will strengthen across the middle Atlantic and northeast
through tonight. As this occurs, surface low pressure should
consolidate some more off of our coast. The model guidance overall
shows a significant short wave rounding the base of the closed low
overnight. This feature looks to near our southeastern zones toward
daybreak. As this occurs, large scale lift in conjunction with some
potential frontogenetic forcing on the west side of the surface low
should organize and area of rain. This looks to generally be across
the eastern half of the County Warning Area. As a result, likely probability of precipitation are carried
across much of the County Warning Area especially late with the slightly higher
values across the eastern zones.
While isolated thunder cannot be ruled out especially across the
eastern areas as at least some middle level instability could be
present, the chance appears to be rather low and therefore it was not
carried at this time. The potentially developing rain area has the signature
of what could be the start of a pivot region across at least a
portion of our area starting near daybreak. This would be due to the
closed low moving overhead and the strong short wave pivoting up
from the southwest as the surface low organizes some more offshore.
As for low temperatures, we utilized mainly an even NAM/GFS MOS
blend. While the winds should be light for most, a northerly wind
should start to increase some mainly late as the system as a whole
starts to become better organized to our east.
Long term /Wednesday through Monday/...
500 mb: a large and vertically deep closed low along the middle Atlantic
coast to start the period will slide to the east of New England Friday.
A potentially strong short wave is slated to move from south central
Canada Saturday through southeast Canada Sunday then to be followed
by temporary ridging next Monday.
Temperatures: calendar day average temperatures for October were about 2 degrees
above normal through the 20th and difficult to believe this month
can now average normal or below. Daily averages will b a little
above normal through Friday with minimum temperatures significantly above
normal and maximums near or slightly below normal. Above normal temperatures
continue Saturday with a possible brief cooling to near normal Sunday
before warming well above normal early next week.
Forecast basis: unless otherwise noted the long term period of this
forecast is generally based on a 50 50 blend of the 00z/21 NAM/GFS
MOS for the period Wednesday-Thursday...the 00z/21 GFS MOS Thursday
night and Friday...then thereafter (friday night onward) the 0502z/21
wpc gridded guidance. The 00z/21 NCEP guidance maximum temperatures may be
significantly too warm Wednesday, Thursday and next Sunday. The
Wednesday/Thursday 00z/21 NCEP MOS guidance was blended down with the 00z/21
European model (ecmwf) temperatures. Wednesday and Thursday could be even cooler than the
330 am forecast issuance indicated.
Precipitation probability guidance is checked against the 21z/20
sref 3hrly probability of precipitation for .01 through 12z Friday and thereafter the
00z/21 gefs 6hr probability of precipitation for 0.05.
The dailies below...
Wed-Thu...periods of showers in the deep cyclonic flow with unstable
middle levels..even small potential of thunder nnj where swi is sub zero
at times. A trowal curls westward around the northwest side of
the deep cyclonic circulation. Rain should be heavy at times
Wednesday along the coast and then possibly Thursday in nnj near
the axis of the 850 mb wind shift (850 mb vorticity axis) from NE
to northwest and coincident with the trowal.
Confidence: above average.
Thursday night-Friday...as the low aloft moves away to our northeast
the potential for rain showers should gradually diminish in our
area. Cloud cover will still be predominant.
Confidence: average since timing of the improvement is uncertain.
This weekend...a short wave with a 12 hour hfc of over 120 meters
passes through New York state and New England Sat and Sat night. 850 temperatures in
its wake cool 6c on the European model (ecmwf) for next Sunday. The short wave
is not expected to have much moisture associated with it. Therefore,
we are not anticipating any rain.
Confidence: average...just a matter of intensity of the short
wave diving southeastward toward the NE USA Sat evening and the resultant
cooler 850mb temperatures on Sunday.
Next Monday...a middle level ridge from the west should build toward
our region when the nicest weather of this 6 day extended period
should occur. Confidence: average for a day 7 forecast.
Aviation /12z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...mostly VFR. Some showers will be around the area as low
pressure develops overhead before starting to consolidate some
just to our east toward evening. While MVFR conditions can occur
for a time with showers, there is some lower confidence on the
timing and areal coverage. There is the potential for a low-topped
thunderstorm to develop this afternoon, however this looks to be
isolated if it were to develop. A south-southwest wind will shift
to the west and northwest during the course of the day, however
speeds are expected to be around 5 knots.
Tonight...VFR ceilings overall. There is the potential for MVFR
visibilities at times with some showers. A more organized area of
rain should develop especially late, where some MVFR ceilings
should start to develop. Lower confidence on the MVFR ceilings. The
winds are anticipated to become north to northeast near 5 knots,
but increase some toward daybreak.
Wednesday through Thursday...conditions varying between VFR ceilings and
MVFR conditions in periods of showery rains. Potential brief IFR
conditions kabe...kttn and kacy. North northwest wind gusts 20 knots
Wednesday and 25 to 30 knots Thursday. Confidence: above average.
Thursday night and Friday...mainly VFR ceilings with a chance of showers.
Northwest wind gusts 20-25 knots. Confidence: above average.
Saturday...VFR scattered-broken at or above 5000 feet. Northwest wind gusts 15 to 20 knots.
Confidence: above average.
today and tonight...low pressure developing overhead will start to
consolidate some offshore tonight. This will result a southwesterly
wind turning to the west and northwest today then north-northeast
during tonight. Given the nearby location of the organizing surface
low, the flow is forecast to be on the lighter side. After some
surge in the seas offshore earlier, the seas are expected to be
below advisory criteria through tonight. The northerly winds may
start to increase though toward daybreak Wednesday.
midweek...a gale watch was issued for most of our waters...for
occasional gusts around 35 knots Wednesday afternoon possibly into midday
Thursday. Only the upper part of Delaware Bay is without a gale
watch and there we issued an Small Craft Advisory.
Future forecasts will determine whether the gale watch is a bit
too robust and we'll need to settle for a strong Small Craft Advisory. For now...be
prepared for the possibility of gales. At this time...am 60 percent
confident that at least the S New Jersey and Delaware coastal waters will verify
a gale by Thursday. Confidence: a bit above average.
Thursday night through Friday night...northwesterly wind gusts of
25 to 30 knots at times. Small Craft Advisory likely. Confidence: above average
Saturday...no marine headlines are anticipated. Northwest wind gusts
15-20 knots. Confidence: above average.
Marine...gale watch from Wednesday afternoon through late Wednesday
night for anz431-450>455.
Small Craft Advisory from noon Wednesday to 6 am EDT Thursday