Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 180531

Area Forecast Discussion
National Weather Service Mount Holly NJ
131 AM EDT Sun Mar 18 2018

High pressure currently over the Midwest 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 will move 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.

1 AM Update...Quiet, dry weather persists through the near
term. The tail end of a backdoor cold front will push south into
the area overnight but really won`t have much influence on our
temperatures for Sunday as high pressure moves in bringing
sunshine with highs generally reaching the upper 40s to low 50s,
except a bit cooler over NW NJ and the southern Poconos. NW
winds will generally be around 10 MPH or so. &&

1 AM Update...High pressure maintains control over the area
through this period with mainly clear skies leading to another
cool night as lows will mainly be in the upper 20s to low 30s,
except colder across the far north. For Monday, low pressure
begins to approach as it moves east from the southern plain
states towards Tennessee. This will result in mainly clear
skies to start the day giving way to increasing mid and high
clouds during the afternoon, mainly across the southern half of
the forecast area. However, any precip will hold off through the
day. Highs will be mainly in the mid to upper 40s except upper
30s to low 40s across the southern Poconos and NW NJ. &&


Saturday Afternoon Update (This section will be updated by 4

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.

Through 12z...VFR with light and variable winds.

Sunday...VFR continuing. Northwest winds 5-10 knots, gusts may
reach 15-20 knots in the afternoon.

Sunday night...VFR with with diminishing winds.


Monday...VFR. High confidence.

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.

Sub SCA conditions with fair weather through Sunday and Sunday
night. West to northwest winds will generally be around 10 to
15 knots with seas 2 to 3 feet over the ocean waters and 1 to 2
feet on the Delaware Bay.


Monday...Sub SCA conditions persisting. NW winds shifting to
east at around 10 knots by late day.

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...Fitzsimmons
Short Term...Fitzsimmons
Long Term...MPS
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