NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Dec 1, 9:54 AM EST, 1 hour 6 minutes earlier.
Area forecast discussion, Dec 1, 9:54 AM EST, 1 hour 6 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
954 AM EST Sun Dec 1 2019
.SYNOPSIS...
High pressure extending south from Ontario and Quebec early
this morning will quickly lose its influence over our weather.
Low pressure located in eastern Iowa is forecast to move to the
east, reaching Lake Erie this evening. A secondary low is
expected to develop in the Chesapeake Bay vicinity this
afternoon. The secondary low is anticipated to intensify as it
moves slowly to the northeast, across the coastal waters of New
Jersey tonight, eventually reaching Nova Scotia on Tuesday
afternoon. High pressure should gradually build into our region
from the west and southwest during the balance of the work week.
A cold front approaching from the northwest is expected to
arrive on Friday night.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Morning update...
After days of anticipation, part one of our storm is here. With this
update, I updated everything through 6z tonight, covering the first
part of the storm. Did not make many changes as the going forecast
looked in pretty good shape. Low level dry air has definitely eaten
away at the leading edge of the precip shield, with just spotty
light mixed precip reports so far. However, steadier precip should
arrive in the next couple of hours where it hasn't already. I
increased snow amounts a little bit across the very far north with
this update, out of respect for some of the hi-res guidance (i.e.
the HRRR and RGEM) showing a brief period of heavier snow early this
afternoon as steady precip arrives. Surface observations, as well as
the 12z sounding from OKX which was almost entirely sub-freezing
with a lot of low level dry air, suggest that precipitation will
probably fall as snow for a time up that way. With strengthening
lift early this afternoon, snow could briefly be heavy across far
northern NJ and the Poconos. But these were minor tweaks and things
seem to be evolving generally as expected. Previous discussion
follows.
Today will be a dynamic day as a powerful storm system
currently centered over the Upper Midwest progresses eastward. A
secondary area of low pressure is also expected to develop &
move northeastward today, eventually coming to reside just off
the SE NJ coast at the end of the near-term period. Ahead of the
(parent) low southerly H850 flow is just beginning to advect
higher theta-e air into the region, and radar echoes generated
by this initial isentropic ascent are already moving into our
area as of 3AM. However given dry mid-lvls and a lack of upper-
lvl forcing expect that we will struggle to see measurable
precipitation from these initial echoes. Stronger and more
consistent precipitation is expected to overspread the area from
SW to NE around or just after sunrise as the lower-lvls
moisten, and large-scale ascent increases ahead of an
approaching mid-lvl vort max and UL jet.
The primary forecast challenge of the day remains precipitation
types, and associated impacts. As of 3 AM most locations south
of Philadelphia were above freezing (apart from the Pine
Barrens), while most locations north of Philadelphia were below
freezing. The airmass remains dry with dewpoints generally in
the teens and 20s. Given these observations and forecast thermal
profiles the current thinking regarding Ptypes/impacts is as
follows:
1.) Areas south and east of I-95 should see almost exclusively
rain through the day today. This rain could be heavy at times
(particularly early this afternoon) with amounts approaching
0.75-1 inch near the NJ shore.
2.) Areas along and just NW of I-95 will likely see the initial
precipitation fall as some combination of sleet, freezing rain,
and rain before transitioning over to rain later this morning.
Can't rule out some light ice accumulations in these areas
(particularly the western Philly suburbs) so going Advisory
handles this contingency.
3.) Areas closer to the I-78 corridor will likely see an
extended period of Sleet/Freezing Rain this morning before
transitioning to rain this afternoon. Unfortunately given
uncertainty over how the thermal profiles will evolve,
discerning how much of the precipitation will fall as sleet vs
freezing rain and also when the sleet-freezing rain-rain
transitions will occur is difficult. Consequently capped
forecast ice accumulations over this area at generally less than
a tenth of an inch, however the caveat is that if there is less
sleet (or the transition to plain rain is delayed) higher ice
accumulations are certainly possible. Regardless travel is
expected to be messy in this area this morning due to the
aforementioned wintry mix.
4.) Areas around and north of I-80 will (per usual) see the
greatest duration of wintry precipitation. It is possible the
precipitation in this area will begin as snow as warm-advection
aloft lags, and there is another possibility that for a time
late this morning as the stronger dynamic ascent arrives the
borderline thermal profiles could cool just enough to support a
return to snow (the HRRR is insistent on this). However, given
the warm temperatures aloft suspect mostly sleet this morning
with perhaps a transition to freezing rain later this afternoon.
Generally have about 1-2 inches of "snow" accumulation in the
forecast although a component of this frozen accumulation could
certainly be high density sleet. Just as with the I-78 corridor
the percentage of sleet vs freezing rain is uncertain and thus
the current forecasted ice accumulations (generally 1-2 tenths)
could be higher if the ptype transitions quicker to freezing
rain.
Naturally there will be a large N-S gradient in high
temperatures today with the Poconos and NW NJ struggling to
reach freezing while temperatures will likely reach the low to
mid 50s in southern NJ/Delmarva.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
The guidance remains fairly consistent with the mid level low.
It is forecast to move from Ohio tonight, to the Delmarva
Peninsula on Monday afternoon, then to the waters off Cape Cod
on Tuesday morning.
The weakening surface low is expected to be located in the Lake
Erie vicinity this evening. The low should continue to fill.
Meanwhile, a developing secondary low is anticipated to be
centered off the southern coast of New Jersey. The secondary low
will likely intensify as it moves slowly to the northeast,
eventually reaching Nova Scotia on Tuesday afternoon.
There will be little change in the forecast for tonight.
Precipitation rates are expected to decrease this evening.
However, a northeast wind will maintain low level sub-freezing
air in parts of far northern New Jersey and the Pocono Region.
Light freezing rain and sleet is anticipated in the elevated
terrain this evening, with mainly light rain in the valleys. The
precipitation type will be light rain in areas to the south of
Interstate 80 all the way down to northeastern Maryland and
Delaware.
As the surface low moves slowly off our coast tonight, it will
continue to draw cold air into our region. As a result, the
transition to freezing and frozen precipitation will work its
way slowly to the south. Precipitation rates in our northern
counties will likely be on the increase as lift is enhanced by
the surface low and an easterly low level jet to its north that
should be feeding into that area, as well as by the mid level
low approaching from the west.
By daybreak on Monday, we are anticipating mainly snow to the
north of Interstate 80, a wintry mix from Intestate 80 down
toward the Interstate 95 Corridor, and all rain from the
Interstate 95 Corridor southeastward. The rain snow line will
likely continue to push slowly southeastward on Monday, reaching
northeastern Maryland, northern Delaware and southern New
Jersey in the afternoon.
The consensus of the model guidance is showing a bit less
moisture with the system than in previous model runs. As a
result, we have lowered snowfall amounts a bit. Regardless, the
Winter Storm Warning and Winter Weather Advisory headlines
remain in place. We continue to expected 5 to 9 inches of snow
in the elevated terrain of the Pocono Region and far northern
New Jersey.
The snow should taper to snow showers on Monday night before
ending from west to east after midnight.
The wind should favor the north tonight, before backing to the
northwest for Monday and Monday night. We kept temperatures a
little below guidance for Monday due to the precipitation and
the northwest wind.
&&
.LONG TERM /TUESDAY THROUGH SATURDAY/...
The period from Tuesday through Saturday appears relatively
uneventful in regard to the weather.
Dry air is expected to follow in the wake of the winter storm
for Tuesday. Temperatures are forecast to remain around 6 to 10
degrees below normal.
We should be in a cyclonic flow pattern for Wednesday into
Thursday. As a result, there may be some stratocumulus overhead
at times, with a passing sprinkle or flurry possible. Daytime
highs should favor the 40s, with readings not getting above the
30s in the elevated terrain.
A cold front is forecast to approach from the northwest on
Friday and it should arrive on Friday night. Some light rain
showers or snow showers are possible. An area of high pressure
is expected to follow on Saturday.
&&
.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...Lowering Cigs are expected through the morning hours,
with MVFR conditions expected to spread from S-N in the 13-16Z
time frame, with IFR conditions likely filling in by 18Z. Expect
a period of sleet/freezing rain at ABE and RDG through the
morning (and perhaps in the early afternoon). Although some
brief sleet/freezing rain is possible at the other terminals
this morning, the bulk of the precipitation should be rain at
these sites. Winds will be easterly around 10-15 kts (highest
near the coast). High confidence on winds and flight categories,
moderate confidence on ptypes and flight category timings.
Tonight...IFR conditions with periods of rain. The rain is
expected to transition to a wintry mix at KABE and KRDG after
midnight. North wind around 10 knots. Medium confidence.
OUTLOOK...
Monday...IFR with mainly snow at KABE. IFR with rain eventually
changing to snow at our remaining TAF sites. An accumulating
snow is possible at KABE. Northwest wind increasing to 10 to 15
knots. Medium confidence.
Monday night...Snow ending with conditions gradually improving
from IFR to VFR. Northwest wind 10 to 15 knots with gusts around
20 knots. Medium confidence.
Tuesday...VFR. Northwest wind 10 to 15 knots with gusts around
20 knots. Medium confidence.
Tuesday night...VFR. West wind 5 to 10 knots. Medium
confidence.
Wednesday...VFR. West wind around 10 knots. Medium confidence.
Wednesday night...VFR. West wind 5 to 10 knots. Medium
confidence.
Thursday...VFR. West wind around 10 knots. Medium confidence.
&&
.MARINE...
SCA conditions developing across the area by this afternoon
with easterly wind gusting 25-35 kts this afternoon and seas
building to 4-5 feet. A brief period of gale force gusts is
possible to the north of the intensifying coastal low pressure
this afternoon into early this evening, so went ahead and
converted the Gale Watch to a Gale warning north of Barnegat
Light. Note although the Gale warning starts at 15Z, gusts will
likely not actually approach gale force until 20-21Z.
Tonight...A Gale Warning is in effect for our ocean water off
Monmouth County and Ocean County for this evening. Northeast wind
gusts around 35 knots are possible there. Otherwise, a Small Craft
Advisory is in effect for our ocean waters from the Atlantic City
area southward.
OUTLOOK...
Monday and Monday night...A Small Craft Advisory is in effect
for a northwest wind with gusts of 20 to 30 knots, and waves on
our ocean waters of 4 to 6 feet.
Tuesday...Northwest wind with gusts of 20 to 30 knots. Waves on
our ocean waters are expected to be 4 to 6 feet.
Tuesday night and Wednesday...No marine headlines are
anticipated.
Wednesday night and Thursday...A west northwest wind may gust
around 25 knots.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Warning until 1 AM EST Tuesday for PAZ054-055.
Winter Weather Advisory until 1 AM EST Tuesday for PAZ061-062-
105-106.
Winter Weather Advisory until 2 PM EST this afternoon for
PAZ060-070-071-101>104.
NJ...Winter Storm Warning until 1 AM EST Tuesday for NJZ001-007-008.
Winter Weather Advisory until 1 AM EST Tuesday for NJZ009-010-
012-015.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST Tuesday for ANZ452>455.
Gale Warning until 10 PM EST this evening for ANZ450-451.
Small Craft Advisory until 7 PM EST this evening for ANZ430-
431.
&&
Synopsis...Iovino
Near Term...Carr/O'Brien
Short Term...Iovino
Long Term...Iovino
Aviation...Carr/Iovino
Marine...Carr/Iovino