Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 172239

Area Forecast Discussion
National Weather Service Mount Holly NJ
639 PM EDT Sat Mar 17 2018

Low pressure over the Tennessee Valley moves off the Mid-
Atlantic coast tonight. High pressure currently over the
Northern Plains states will gradually build to the south and
east on Sunday, and will move off the Mid-Atlantic coast
sometime Monday or Monday evening. Canadian high pressure then
re-establishes itself north of the Great Lakes. Meanwhile, low
pressure currently over the Southern Plains will build east and
emerge over the Mid-Atlantic Monday night. It then intensifies
as it moves out to sea on Tuesday. Another low may form over the
Mid-Atlantic Wednesday or Wednesday night. High pressure
returns from the north to close out the work week. Low pressure
then approaches next weekend.


After some light precipitation this afternoon/early evening,
quiet conditions are expected overnight. Light rain associated
with an approaching short wave/vorticity impulse and low
pressure sliding eastward along a frontal boundary to our south
will continue to move east late this afternoon into this
evening. Once this short wave passes to our east, any
precipitation will come to an end around sunset. A backdoor cold
front is then expected to approach from the north this evening,
and may stall across the area overnight, although it will
remain dry. Only an increase in mid-high level clouds is
expected overnight across the northern half of the area.
Overnight lows are expected to remain few degrees below normal.


Sunday is expected to be another quiet day. The backdoor cold
front that pushes into the area overnight remains stalled out
across the area, before an area of low pressure approaches later
in the day, which will begin to push the front fully across the
area. Sunday is expected to be dry, with very little in the way
of cloud cover expected. Daytime highs are expected to remain a
couple of degrees below normal.


Overview: Complex storm system still on track to impact the
region Tuesday through Wednesday, but details are still
uncertain. 12Z GFS and 12Z ECMWF keep the storm and most of the
precip to the south, as well as the 12Z CMC-GDPS. The 12Z NAM,
however, is somewhat of an outlier, bringing the heavier and
more widespread precip farther north.

High pressure builds over the over the region Sunday night
builds to the south and east and moves off the Mid-Atlantic
coast on Monday. Conditions will be dry through Monday.

Meanwhile, low pressure over the Midwest on Monday will track to
the east through the Tennessee Valley Monday night and will
emerge between the North Carolina coast and the Delmarva
Peninsula by Tuesday morning. At the same time, high pressure
centered over central Canada will press to the south and east
and will be centered just north of the Great Lakes Tuesday
morning, and this high slowly builds east during the day.

In terms of sensible weather and QPF there are still too many
variables to determine what will happen with this storm. There
are several things going against a significant snowfall:

1) A lack of Arctic air in place ahead of the storm.
2) Higher late March sun angle (Tuesday is the First Day of
Astronomical spring).
3) The position of the high to the north is too far west to
lock in cold enough air at the surface.

That does not mean that we are out of the woods, however. The
12Z NAM, though an outlier, is a bit farther north with the
position of the low and has the high a bit farther to the south
and east. This brings more in the way of cold air into play, and
brings the heavier precip up into central and NJ and southeast
PA as opposed to southern NJ and Delmarva. This track would
bring more snow to the region during that time.

Opted to use a blend of Superblend and WPC guidance given the
uncertainties that lie with this storm, and did not make many
appreciable changes to the previous forecast.

This results in snow north of I-78, and otherwise snow, rain, or
a wintry mix elsewhere. Generally rain during the day and snow
at night.

Please continue to use our snowfall probability maps to get an
idea for the worst case scenario regarding this storm, as
although main forecast keeps snow amounts low, there is the
chance for significant accumulations. It is a low chance, but
still a chance.

Primary storm moves out to sea Tuesday night, and then another
storm forms off the Mid-Atlantic coast on Wednesday. This one
seems too far south to impact the region, but wrap-around
precip may spread west.

High pressure closes out the work week and another low
approaches for the weekend. Temperatures will be well below
normal through the period.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions continue through tonight. West to
northwest winds gust around 20-25 knots late this afternoon into
this evening. Gusts drop off after sunset, but remain around
5-8 knots through the night.

Sunday...VFR conditions continue through the day Sunday.
Northwest winds 5-10 knots, gusts may reach 15-20 knots in the


Sunday night through Monday...VFR.

Monday night and Tuesday...Onset of precip will be Monday
evening over the Delmarva as rain. By late Monday night and
Tuesday, widespread MVFR/IFR conditions. Snow for KRDG/KABE, and
wintry mix for KTTN/KPNE/KPHL/KILG. Wintry mix and then plain
rain for KACY/KMIV. Low confidence on precip type, though. NE
winds 15-20 kt with gusts up to 30 kt.

Tuesday night through Wednesday...Additional precip possible
with continued MVFR/IFR conditions. Low confidence.

Wednesday night and Thursday...VFR.


The Small Craft Advisory was dropped at 6 p.m. The trend towards
lowering winds and seas continues. Sub-SCA conditions expected
through Sunday.


Sunday night and Monday...Sub-SCA conditions.

Monday night and Tuesday...SCA conditions with gales likely
Tuesday. VSBY restrictions in rain, but wintry mix possible.

Tuesday night and Wednesday...Gales continue Tuesday night. SCA
conditions Wednesday. Additional VSBY restrictions in rain/snow.

Thursday...SCA conditions continue.


Relative humidity values on Sunday will drop into the 20s
again, but winds are expected to be less than Saturday. Even
though fuels have been drying, they are expected to remain
above critical levels as well. No enhanced statements are
expected at this time.




Near Term...Robertson
Short Term...Robertson
Long Term...MPS
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