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Area forecast discussion 
National Weather Service Mount Holly New Jersey
959 PM EST Tuesday Mar 3 2015

Synopsis...
a warm front will lift through the area overnight...followed by a
Arctic cold front during the day Wednesday. The front will push to
our south into Thursday while low pressure develops and moves
along the boundary. High pressure will return to the middle Atlantic
region for Friday and Friday night. A frontal passage is expected
Saturday into Saturday night...followed by high pressure again
Sunday into Sunday night. Another frontal passage is expected
Monday into Monday night.

&&

Near term /until 6 am Wednesday morning/...
930 PM estf: little change made except lowered the expected warmup
overnight on the coastal plain due to the south southwest flow
only pulling modified polar air northeastward from Virginia.

Patchy fog added to the forecast Wednesday morning.

Accumulative sleet, especially in eastern PA of the Lehigh Valley
down into philadelphia's western suburbs and also central and
northern New Jersey where there are many reports of 1/2 to 1 inch
of pure sleet late today with additional 1 inch or more of snow north
of i78. Associated accidents snarled the evening commute in parts
of east PA...and northern New Jersey basically northwest of I-95.

Strong winds aloft will not punch through the strong inversion of
temperatures nearing 10-11c at 2000 feet so overnight warming will be minimal
and basically the southerly flow is drawing on wet bulb modified
polar air.

Much of the rain rate i95 eastward has been heavy enough for considerable
runoff and ice accretion while existing is not major, so far.

Overall, it appears a break is at hand with intermittent precipitation overnight...
rain except light freezing rain near I-78 northward.

We may need to extend some of the southeast PA advisory a little longer than
the current 1 am expiration but that will be considered by our middle
shift.

&&

Short term /6 am Wednesday morning through 6 PM Wednesday/...
Winter Weather Advisory continues into the first part of Wednesday
morning for the pocono's and far northwest NJ, where pockets of
sub-freezing temperatures and freezing rain will persist. Otherwise,
Wednesday will be a relatively quiet period in between two
winter events.

A cold front will approach from the northwest in the morning and
slowly sag southeastward throughout the day. Expect period of rain
throughout the day for much of the area. The coverage of rain will
be highest near this frontal boundary. Quantitative precipitation forecast amounts will be light-
less than a half inch. Leaned toward the cooler met guidance for
temperatures given the persistent clouds and precipitation over the area, as well
as the end to the warm air advection regime with winds veering out of the south and
southwest. Highs Wednesday range from the middle 30s in the pocono's to
the upper 40s in southern Delaware.

18z/3 NAM thermal profile suggest sleet and or snow could begin
mixing with any rain falling near sunset from krdg to kabe
northwestward.

&&

Long term /Wednesday night through Tuesday/...
shortwave energy rounding the base of a middle-level trough situated over
Great Lakes will gradually veer large scale flow to the southwest
over the mid-Atlantic. This will slow the advance of an Arctic
front moving through the area on Wednesday and it will briefly
become stationary to our south on Thursday. Meanwhile...closed
middle-level low ejects northeastward out of Baja California California...accompanied by
several southern stream shortwave disturbances that will lead to low
pressure development along the now stationary front to our south. This
will promote a robust overrunning regime over the mid-atl... fed
by Baja California/greater Mexico moisture and supported by the lf quadrant of a
vigorous 250 hpa jet.

The operational models and their respective ensembles have
signaled this scenario for over a week now...and finally appear
much closer to a consensus on the low-level thermal profile...but
differences remain in their depiction of the quantitative precipitation forecast bullseye and
amounts. At this time...we favor the NAM/European model (ecmwf) quantitative precipitation forecast...and feel the
GFS is overdone. This would place the maximum quantitative precipitation forecast bullseye in the 00z
Wednesday through 00z Thursday time frame along the I-95 corridor. As colder air
works in from the northwest...we expect rain to gradually change
to snow from north to south Wednesday night. Keep in mind there is still
some disagreement on how quickly the low-level thermal profile
cools...as slpr fields indicate the region is in a saddle...which
is supportive of a slow cold frontal passage. At this time...we expect the rain to
changeover to snow in the northwest around 00z...and perhaps as
late as 09z in the lower Delaware-Maryland-Virginia.

As far as ptype and amounts...we're expecting 6 to 8 inches of
snow in the I-95 corridor with 4 to 6 inches north and south
of this area. If a more robust warm nose works in aloft...as
suggested by the NAM...there would be more sleet which would
cut down on the snowfall amounts. On the other hand...although
Omega in the dendritic snow growth zone is not robust...fgen
is indicated in the middle-levels and this could boost snowfall
totals. Finally...it is questionable how much dry air and/or
downsloping due a northerly surface flow will cut down on quantitative precipitation forecast
across the northern part of the County Warning Area. Further to the south over
the Delaware-Maryland-Virginia...snowfall totals could be lower due to a warmer
thermal profile...implying a longer transition from rain to snow.

With this package we have issued a Winter Storm Watch for
the entire County Warning Area from Wednesday night through Thursday evening...with the greatest
uncertainty in meeting snowfall criteria north of the I-78
corridor and over far southeast New Jersey and the southern Delaware-Maryland-Virginia.
We expect the Thursday morning rush hour to be particularly
impacted...especially in the I-95 corridor...where snow could
fall moderate at times.

Snow is expected to end Thursday evening...as the middle-level
trough approaches from the west...and pushes the system
offshore. Much colder air will filter in Thursday night with min
temperatures around 20 degrees below normal. Otherwise...generally
fair weather is expected through the weekend... with a weak cold
frontal passage likely later Sat or early Sunday. The front
has limited moisture at this time...so the forecast is kept dry.
Temperatures may actual approach seasonable levels on Sunday. Early
next week...there is a lot of uncertainty with regard to the
evolution of both northern and southern stream energy...and
the degree of phasing that takes place between the two over
the eastern Continental U.S.. for now we kept this part of the forecast dry
as well.

&&

Aviation /03z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Overnight...icing layer is thinning below 5000 feet as warmer air
moves northeast and works downward. Deicing needs should continue
for several more hours krdg/kabe/kttn and possibly even kphl area.

MVFR variable IFR conds in freezing rain changing to rain from south to north
with light southeast to south surface winds but low level wind shear with strong
winds near the inversion around 2000 feet. Please see tafs for details.



Wednesday...MVFR or IFR conds in intermittent rain/drizzle/fog.
Potential for a period of LIFR fog in the frontal boundary during
the morning as warm air temporarily lays close to the near
freezing snow and ice surfaces. Sleet/snow should begin mixing
with any on going rain vicinity kabe and krdg around 23z.



Outlook...
Wednesday night and Thursday...rain will gradually changeover to sleet...
then snow across the terminals from north to south. The changeover is
expected to occur at Abe and ridge between around 23z/4-01z/5...at
ttn, pne, phl, and ilg around 03z/5...and miv and Acy around
06z/5. There is still some uncertainty as to what time the change
takes place. Expect MVFR conditions with IFR possible in snow and
low ceilings...particularly at ttn, pne, phl, and ilg.

Thursday night and Friday...snow will gradually end from west to
east across the terminals Thursday evening. Expect MVFR/IFR conditions
to improve to VFR by Thursday night and remain VFR through Friday.

Saturday and Sunday...MVFR possible in low clouds and rain showers/snsh
with the passage of a cold front late Sat into early Sunday...
otherwise VFR conditions expected.

&&

Marine...
southerly winds will increase this evening. Although storm-force
winds will be located only 1,000 feet above ground level and gale-force winds 200 feet
agl, a strong inversion will not be favorable for these winds to mix
down to the surface. However, the pressure gradient tightens enough
for sustained winds to approach Small Craft Advisory criteria tonight. Seas are also
forecast to build to 4-5 feet overnight. Therefore, a Small Craft Advisory was issued
for the coastal waters.

The winds turn more westerly on Wednesday and decrease just ahead of
a cold front. Do not expect Small Craft Advisory gusts. However, elevated seas near 5
feet may linger into the morning. Confidence wasn't high enough to
extend the advisory at this point.

Outlook...
Wednesday night through Friday...winds and seas will likely meet Small Craft Advisory thresholds
Wednesday night through at least Friday morning.

Friday afternoon through Sunday...winds and seas are expected to remain
below Small Craft Advisory thresholds.

&&

Hydrology...
please monitor latest quantitative precipitation forecast forecasts from wpc and marfc.

Tuesday forecast cycles came in wetter for Wednesday afternoon
and Wednesday night, especially south of philly. The GFS, for
instance, has 0.75 to about 1.00 of liquid across the Delaware-Maryland-Virginia and
southern NJ, south of the i295 corridor. We're once again taking
notice.

A changeover to sleet or snow isn't expected to occur until early
Thursday morning.

While there isn't much of a snowpack across the southern half of our
cwa, the ground is frozen, and any runoff will be very efficient.

If the 0.75 to 1.00 materializes, we feel nuisance poor drainage and
low lying flooding will be the most likely outcome. Modeled solutions
suggest we would need about 2.50 inches of liquid, either in the form
of all rain or rain and melting snow to have our southern creeks and
streams leave their banks.

With more snow or sleet expected across the northern half of the
cwa, we're not concerned about flooding there.

With the rainfall, the flow on area creeks and streams will increase.
This could start to move ice, and in a worst case scenario, create
restrictions or jams.

Continue to monitor the forecast. Keep in mind that snowmelt and the
warmer temperatures expected tomorrow, by themselves, would not
cause flooding. The amount of rain that we receive will be the
determining factor.

&&

Phi watches/warnings/advisories...
PA...Winter Weather Advisory until 6 am EST Wednesday for
paz060>062-105.
Winter Storm Watch from Wednesday evening through Thursday
evening for paz054-055-060>062-101>106.
Winter Storm Watch from late Wednesday night through Thursday
evening for paz070-071.
Winter Weather Advisory until 1 am EST Wednesday for paz070-
071-101>104-106.
Winter Weather Advisory until 9 am EST Wednesday for paz054-
055.
New Jersey...Winter Weather Advisory until 6 am EST Wednesday for
njz007>010.
Winter Storm Watch from Wednesday evening through Thursday
evening for njz001-007>010-012-015.
Winter Storm Watch from late Wednesday night through Thursday
evening for njz021>025.
Winter Storm Watch from late Wednesday night through Thursday
evening for njz013-014-016>020-026-027.
Winter Weather Advisory until 1 am EST Wednesday for
njz012>020-027.
Winter Weather Advisory until 9 am EST Wednesday for njz001.
Delaware...Winter Storm Watch from late Wednesday night through Thursday
evening for dez002>004.
Winter Storm Watch from late Wednesday night through Thursday
evening for dez001.
Winter Weather Advisory until 1 am EST Wednesday for dez001.
Maryland...Winter Storm Watch from late Wednesday night through Thursday
evening for mdz015-019-020.
Winter Storm Watch from late Wednesday night through Thursday
evening for mdz008-012.
Winter Weather Advisory until 1 am EST Wednesday for mdz008-
012.
Marine...Small Craft Advisory until 6 am EST Wednesday for anz450>455.

&&

$$
Synopsis...franck
near term...drag/Klein 10p
short term...drag/Klein
long term...franck
aviation...drag/franck/Klein 10p
marine...drag/franck/Klein
hydrology...

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