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Area forecast discussion 
National Weather Service Mount Holly New Jersey
459 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/...
the center of high pressure has shifted offshore this afternoon.
Meanwhile, a trough of low pressure was lifting northeastward
across the upper Midwest. Southerly flow in between these two
systems will continue to strengthen over the eastern Seaboard. Warm
and moist air transported by these winds will overrun the retreating
cold air that was previously over our area.

The leading edge of isentropic lift and low-level convergence
matched up well with the band of precipitation that is currently
moving into the Delaware-Maryland-Virginia and eastern PA. The leading precipitation will
continue to move northeastward into the rest of the County Warning Area just in time
for the start of the afternoon commute. Snow had already changed
over to sleet at ridge-ptw-ilg and liquid (rain/freezing rain) over
south-central PA and along the Chesapeake Bay as of 230 PM. The
trend was to speed up the transition to sleet and rain/freezing rain
by a couple of hours from earlier forecasts based on the latest observation
and mesoscale model guidance. Dewpoints are low enough that at
locations where temperatures are just above freezing prior to the onset
will wet-bulb down to at or just below freezing once precipitation starts.
Expanded the Winter Weather Advisory southward into central Delaware-Maryland-Virginia
and far southern New Jersey. A brief period of freezing rain will quickly
develop over these areas since the faster changeover occurred before
the surface temperatures were able to respond to the warming southerly
winds. The timing of the changeover to freezing rain looks to
coincide with the start of the evening commute for the I-95
corridor. Even with minimal icing, travel will be adversely impacted
as untreated roads and sidewalks become slippery, especially
considering the timing.

Unlike the event last Sunday where there was not much wind and warm air advection
in the boundary layer, the strong southerly winds aloft will be able
to have more of an influence in the boundary layer, which will be
much more effective at scouring out the cold air; so the window of
freezing rain will be brief. The one exception may be the I-78
corridor northward where the terrain could trap the cold air in the
typical sheltered valleys. We are expecting the freezing rain to
persist into the overnight in these northern areas and locally
higher amounts of icing between one and two tenths of an inch are
possible.

Much of the coastal plain is forecast to rise into the 40s tonight
with a southeast to southwest wind. Readings are expected to be
mainly in the 30s in areas to the northwest of the Interstate 95
corridor.

&&

Short term /6 am Wednesday morning through 6 PM Wednesday/...
Winter Weather Advisory continues into the first part of Wednesday
morning for the Poconos 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 Poconos to
the upper 40s in southern Delaware.

&&

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...accompained 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 consenus 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 /21z Tuesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

VFR conditions will deteriorate to MVFR as precipitation starts to
overspread the region. Thermal profiles show a warm layer deepening
above the surface which will cause sleet to fall, mixing with snow
early, with and than a quick transition to freezing rain at the
terminals late this afternoon. All terminals are expected to
change over to plain rain by around 03-05z except maybe Abe for a
few hours into the overnight. Conditions will remain MVFR/IFR
through Wednesday.

Winds out of the south this afternoon will continue to gradually
veer towards the southwest tonight. In addition, winds around 2000ft
above the surface are around 50 knots while winds at the surface are
less than 10 knots. Low level wind sheer will be an issue from this
evening through late. Wind sheer will lessen by early Wednesday and
winds will veer to the west and gradually becoming northwest
Wednesday morning.

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 ooz Thursday...at ttn,
pne, phl, and ilg around 03z Thursday...and miv and Acy around 06z Thursday.
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...with last night's solutions looking a little colder and
with less liquid before the changover to snow on Thursday, we became
much less concerned about any flooding on Wednesday into Wednesday
night.

This morning's runs came in wetter 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 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.

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 Weather Advisory until 10 PM EST this evening for
njz021>025.
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 Weather Advisory until 10 PM EST this evening for
dez002.
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 Weather Advisory until 10 PM EST this evening for
mdz015-019-020.
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 from 9 PM this evening to 6 am EST
Wednesday for anz450>455.

&&

$$
Synopsis...franck
near term...Klein
short term...Klein
long term...franck
aviation...franck/meola
marine...franck/Klein
hydrology...kruzdlo

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