Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
359 PM EST Tuesday Nov 25 2014
low pressure near Florida this evening will strengthen as it passes
near the middle Atlantic coast Wednesday, before racing to the northeast
Wednesday night. High pressure is forecast to build across the
East Coast Friday into early Saturday, before squeezing to the
south Saturday night. A cold front is expected to sag down across
the area Sunday night, before slowly moving through the area
Monday into Monday night.
Near term /until 6 am Wednesday morning/...
tonight...temporary clearing late this afternoon in east PA soon seals
up with thickening and lowering clouds as dynamics with the Florida
low and Gulf Coast short wave flood northward. Rain develops rapidly
from south southwest to north northeast after midnight as low
pressure heads northeastward for the NC coast. Northern fringe reaching
krdg/kabe at 11z may begin as rain/snow mix per marginal warm
boundary layer temperatures (a wall of deep relative humidity moving northward in our
County Warning Area after midnight).
50 50 blended 12z/25 NCEP MOS guidance.
Short term /6 am Wednesday morning through Wednesday night/...
update to this section coming by 330 PM.
**High visibility high impact snow event for elevations of east PA
and northwest New Jersey 9a-9p and likely a developing impact event to near sea
level just northwest of I-95 Wednesday afternoon/evening**
This forecast was a 50 50 blend of the 12z/25 warmer GFS and colder
European model (ecmwf) partial thicknesses and then adjusting the wet bulbing
12z/25 blended MOS temperatures colder, especially with the column all
snow down to 5000 feet and ice nuclei seeded for crystals by cirrus.
Low pressure system has been trending faster onset/departure
since first discussed in details this past weekend.
Snow grids are posted.
We saw the warmer 18z NAM (700mb above zero down here near phl).
The snow amounts near I-95 from Cecil County Maryland through
Wilmington Delaware and then northeast through Philadelphia up
to New Brunswick are highly uncertain. The advisory area snow
amounts are shaky and we may not much more until noon tomorrow?
The advisory may need to be expanded southwest into NE Maryland in
Pockets of power outages a concern...especially just northwest of
Suggest travel into eastern Pennsylvania and northwest New Jersey
be completed no later than 7 am Wednesday, or wait until after 7 PM
Wednesday night (ie...after roads are cleaned up/cleared of slushy
snow). Its that 12 hour window during the day Wednesday that is
going to become a significant travel problem, especially the hilly
Its unusual to forecast heavy wet snow in a 1000-500mb 543-546 thickness
without a big (1030mb) surface high to the north but this is a situation
that I think heavy snow will occur, especially elevations for reasons
below. Thermal profiles and precipitation microphysics with cirrus seeding marginal
-2 to -8c below 15,000 feet!! this is another test of that precipitation
microphysics which has worked so well in the past.
An intensifying 500mb 12hr 180 M hfc moving northeastward combines with
isallobaric gradient to pull 0-6km 1c or less wbz air progressively
southeastward to near i95 by 18z and to the Atlantic coast by 00z/27
(wednesday evening-too late for any snow snj and Delaware coasts).
Elevations change to snow first from northwest to southeast
Piva and comma head suggest some banding potential vicinity krdg to
kmpo during the day. Also support from csi product of the NAM but
again its the NAM. The maximum uvm right now is tending to show up below
the ideal dendritic growth zone in the multi model profiles but
that can change.
A record daily snowfall for Allentown PA is likely to occur (a
Snow water ratios per internal collaboration 6 to 1 I-95 maybe
8 to 1 krdg to kabe and 10 or 12 to 1 for the Poconos.
Small chance of minor tidal flooding along the Delaware coast for
tomorrow mornings high tide.
Small chance we'll need a Wind Advisory for the Delaware coast
Snow/precipitation ends from SW to NE Wednesday evening with some
remaining banding snow over NE PA and northwest New Jersey.
Flurries may occur late at night?
Long term /Thursday through Tuesday/...
on Thanksgiving, there will remain a chance for isolated showers for
portions of the area, although we expect most areas to remain dry.
There could remain some snow showers in the northerly flow across
our northern areas, and as a short wave rounds the base of the
trough aloft, it may help create an area of showers across the
southern portions of the area as well. Most areas should stay
precipitation free, but if it does snow across the northern elevations, a
light accumulation could occur with some showers. By Thursday
night, any precipitation is expected to end as we begin to lose
good moisture aloft.
Friday into early Saturday, high pressure begins to build across the
East Coast, which is expected to keep the weather mostly
precipitation free. The models do push a weak short wave/vorticity
impulse across the area Friday night, with the GFS showing more
moisture to work with than the European model (ecmwf). For now we will keep the
weather dry until we get better collaboration between the two.
The high pressure pushes to our south later Saturday into early
Sunday, before a frontal boundary approaches the area Sunday night
into Monday. Models are hinting at a slight chance of showers
Saturday night into Sunday as a couple of weak short wave/vorticity
impulses slide across the area. However, the greater threat is
expected to be Sunday night into Monday night as the frontal
boundary sags across the area. Models indicate that some moisture
is lost as the front sags across the area, but there will remain a
chance in the forecast for now.
As high pressure builds back across the northeast on Tuesday, dry
weather should return to the area through Tuesday.
After below normal temperatures for the first half of the
extended through Saturday, a warming trend is expected for the
latter half of of the weekend into early next week with
temperatures returning above normal.
Aviation /21z Tuesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR with ceilings lowering to 4000 feet from south to north after
06z then rain begins as per the tafs. Light northwest winds turn
north northeast after 06z and gust 15-20 knots by daybreak.
Wednesday and Wednesday evening... widespread MVFR and IFR conditions in rain
changing to snow except mainly snow at krdg/kabe. Precipitation will
be heavy at times and IFR conditions in all snow should occur for
a time Wednesday in the I-95 taf corridor from kttn through kpne
Pavement accumulations of several inches are expected at krdg and
kabe where a short period of one quarter mile heavy snow is
forecast. Even sloppy wet snow accumulation is possible at kphl.
This is a wet snow where it occurs except more crystalline vicinity
the Poconos where it will be slightly colder!
Confidence is high about the event, but imperfect regarding precipitation
phase change from rain to snow in the Philadelphia area.
Thursday-Thursday night...mostly VFR. Isolated rain or snow showers
possible during the day which may temporarily lower conditions.
Friday-Sunday...mostly VFR. Wind gusts 15-20 knots each day; northwest
Friday, SW Saturday & Sunday.
early tomorrow morning, gale conditions are forecast for the
lower Delaware Bay and the Atlantic coastal waters...especially
southern New Jersey and the Delaware coastal waters.
Wednesday...a full blown Gale Warning is in effect for the coastal waters
and lower Delaware Bay per an intensifying storm moving northeastward from
the NC coast. Small Craft Advisory conditions expected on the upper Delaware Bay
with a possible need for an upgrade to a short fuse Gale Warning.
The 06z, 12 and 18z NAM are advertising a period of storm force
winds vicinity 44009 around midday Wednesday (near 50 kt). We will
not know for sure since no 44009 wind data.
Wednesday night...gale conditions will continue early, then seas and wind
will decrease and and Small Craft Advisory will be likely needed as the storm moves
northeast of Cape Cod.
Thursday-Thursday night...winds are expected to be below Small Craft
Advisory levels, but seas may remain elevated into Thursday night.
Friday-Friday night...sub-Small Craft Advisory conditions expected
as high pressure builds over the area.
Saturday-Sunday...near Small Craft Advisory conditions possible as
high pressure pushes to our south and cold front approaches late on
regarding the midweek coastal storm bringing snow to a large
portion of our region, we have included daily snowfall records
below for our four primary climate sites that have a long-standing
unbroken period of record (por) with respect to historical
Daily snowfall records:
November 26th: November 27th: period of record:
Acy: trace 1977,1957,1955 1.2 inches 1978 1874
Phl: 6.0 inches 1898 6.9 inches 1949,1938 1872
Ilg: 0.1 inches 1950 4.5 inches 1978 1894
Abe: 1.7 inches 1925 7.0 inches 1938 1922
Other cooperative site shorter historical record can be found at
the National Weather Service Mount Holly website under now data.
PA...Winter Storm Warning from 7 am to 10 PM EST Wednesday for
Winter Weather Advisory from 9 am to 10 PM EST Wednesday for
New Jersey...Winter Storm Warning from 7 am to 10 PM EST Wednesday for
Winter Weather Advisory from 9 am to 10 PM EST Wednesday for
Marine...Gale Warning from 4 am to 10 PM EST Wednesday for anz431-
Small Craft Advisory from 4 am to 10 PM EST Wednesday for
near term...drag 4p
short term...drag 4p