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Area forecast discussion 
National Weather Service Mount Holly New Jersey
926 PM EST Tuesday Nov 25 2014

Synopsis...
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/...
00z surface analysis vs model 6 to 12hr projections favors either the
GFS or the European model (ecmwf). Odd part was the European model (ecmwf) convective quantitative precipitation forecast locations
was too south in Florida. Other models are better with location, none are
good with amounts. There is a plethora of lightning across Florida.
The northwestern extent of the precipitation was pretty good (except for
the can rgem) with a nod to the European model (ecmwf).

Going above the surface, the models are still lagging the
remaining warmth along the eastern Seaboard as 925mb 00z soundings
are 0 to 1c warmer and 850mb soundings are 1 to 2c warmer. At
700mb, there was also a 0 to 1c cold bias upwind with the gso
error at 2c. Of the ECMWF, NAM and GFS, the NAM has the more
prevalent cold bias. Conversely over the Ohio Valley and Midwest
there is a 0 to 1c warm bias that shows up the strongest at 850mb.
At 500mb, the ridging along the East Coast is handled well, the
predicted ridging along the West Coast is about 10m too low, the
Dakota kicker is a clear GFS Victory, while all the models are
pretty good with the la trof, maybe a tad too fast.

The general warming trend that continued with the 18z model run
(phl 1000-500mb thickness valid 18z Wednesday has gone from 546 to 552
dm on the GFS and from 541 to 548 dm on the NAM since yesterday)
suite casts uncertainty to the snow extent on the southeast side
of this event. While there has been warming at or below 925mb, the biggest
push has been at around 700mb creating more of a sleet and less of
a snow profile. This has waffled back and forth and we are waiting
for more consistency before adding sleet. But as a proxy, we
lowered the snow amounts along the i295/95 corridor with the
previous update.

The 00z NAM has reversed this trend and is colder than its 18z
run, although warmer than the full sounding 12z version. Its
initialization and correction of previous cold bias errors looked
good. There remains no denying of the dry air in the soundings and
while dynamic cooling gets most of the good press its
evaporational cooling that is much more clutch in getting it to
snow, highest confidence remains from the fall line northwest and
any locale where elevation can assist.

Tonight...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
future forecasts.

Pockets of power outages a concern...especially just northwest of
Interstate 95.

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
areas.

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
Wednesday morning.

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
vulnerable record).

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
tomorrow morning.

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 /01z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

High impact IFR conditions predicted for Wednesday.

For tonight primarily VFR (high level and then middle level clouds),
possibly becoming MVFR toward morning as precipitation (mainly rain) moves
into the area. Wind direction forecast is more for the overnight.

Wednesday, kabe and krdg IFR conditions with snow as the
predominate ptype by the middle of the morning. Accumulations on
runways likely. Some improvement late. North winds will average
around 10 to 15 knots. Kphl/i95 corridor terminals. MVFR
conditions in rain during the morning lowering to IFR late morning
as snow starts to mix in and probably becomes the predominant
ptype. Some slushy accums on runways possible. We added the
possibility of sleet as a ptype. MVFR improvement late. Northeast
winds will average 10 to 20 knots with peak gusts close to 30
knots. Kmiv and kacy as well as shore point airports, IFR
conditions due to predominately moderate rain. Some snow might mix
in during the afternoon. Northeast winds will average 12 to 18
knots with peak gusts also close to 30 knots, possibly higher at
coastal airports.

Outlook...
Wednesday night...becoming VFR with diminishing winds.

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.

&&

Marine...
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.

Outlook...
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
Sunday night.

&&

Climate...
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
snowfall data.




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.

&&

Phi watches/warnings/advisories...
PA...Winter Storm Warning from 7 am to 10 PM EST Wednesday for
paz054-055-060>062-101-103-105.
Winter Weather Advisory from 9 am to 10 PM EST Wednesday for
paz070-071-102-104-106.
New Jersey...Winter Storm Warning from 7 am to 10 PM EST Wednesday for
njz001-007>010.
Winter Weather Advisory from 9 am to 10 PM EST Wednesday for
njz012-015.
Delaware...none.
Maryland...none.
Marine...Gale Warning from 4 am to 10 PM EST Wednesday for anz431-
450>455.
Small Craft Advisory from 4 am to 10 PM EST Wednesday for
anz430.

&&

$$
Synopsis...Robertson
near term...drag/gigi
short term...drag
long term...Robertson
aviation...gigi/Robertson
marine...drag/Robertson
climate...

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