NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Dec 16, 9:27 AM EST, 2 hours 48 minutes earlier.
Area forecast discussion, Dec 16, 9:27 AM EST, 2 hours 48 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
927 AM EST Wed Dec 16 2020
.SYNOPSIS...
A strong coastal low will bring a multitude of high impact
hazards to the region as it races up the east coast today and
will continue affecting our region Thursday. High pressure
returns for the end of the week. Another low pressure system may
affect our region early next week.
&&
.NEAR TERM /THROUGH TONIGHT/...
Morning Update...
No changes to the forecast with the morning update. Mainly dry
weather expected through the morning with precipitation arriving
from southwest to northeast early this afternoon. Will evaluate the
12z hi-res guidance and see how the observations track as the precip
arrives in the next couple hours before making any final tweaks. At
this point, not expecting things to change significantly. 12z RAOBs
to our south (IAD and WAL) suggest the warm nose aloft may be
pushing in a little faster than modeled with temperatures above
freezing around 800mb. However, this is hard to judge until
precipitation begins and the column becomes more saturated, and wet
bulb effects are accounted for both at the surface and aloft. So
will continue to wait and see for now... Previous discussion
follows...
Major coastal storm will impact the region later today through
tonight with a variety of hazards.
Canadian high pressure remains centered over Quebec, and with
northerly flow, a cold, dry airmass is in place over much of New
Jersey and southeast Pennsylvania. Temperatures in the southern
Poconos are in the teens with dew points in the single digits,
whereas across the rest of southeast Pennsylvania and most of
New Jersey, temperatures are in the 20s with dew points in the
teens. Farther south into the eastern shores of Maryland and
Delaware, it is cold, but dew points are slowly creeping up
through the 20s.
Low pressure is developing over the Southeast U.S., and will
slowly track north along the Eastern Seaboard towards the
Delmarva Peninsula by this evening.
Overrunning precip developing ahead of that low will spread
into far southwest zones by late morning, and will continue to
lift to the northeast throughout the day. Most of this precip
should be light to moderate, and the heaviest precip will
develop by this evening. The rain/snow line seems to have lifted
just a bit to the north and west. Not much, maybe 5-10 miles,
and brings mixing throughout the day and into the evening into
much of Philadelphia. Across the southern Poconos, Lehigh
Valley, and western portions of the Delaware Valley including
Berks county, banding of moderate snow bands will develop in the
afternoon, resulting in 3-5 inches of snow during the daylight
hours. For areas farther south, snow amounts will generally be
2-4 inches down to the I-95 corridor from Trenton to Philly to
Wilmington. One note of concern is that given how dry it is,
when precip first starts falling, it will likely not reach the
ground, and will result in dynamic cooling. So many areas under
the Winter Weather Advisory could pick up a quick light snow
accumulation before the rain/snow line develops.
The main event arrives this evening. Low pressure lifts north
through Delaware and towards southern New Jersey early this
evening, and continue to lift to the north and east towards the
Jersey Shore after midnight or so, and then south of Long Island
by daybreak. Meanwhile, a strong H5 low with strong shortwave
energy moves in from the west, allowing the storm to intensify
over the area. A strong 60-70 kt LLJ will lift along the coast,
and with heavy rain along the coast, some of the strongest winds
will mix down to the surface. Will continue the Wind Advisory
for portions of Delaware and eastern New Jersey, as winds will
gust 40-50 mph, and the High Wind Warning remains in effect for
coastal New Jersey, where winds will gust up to 60 mph,
especially right along the shore. Across the interior, winds
will not be quite as strong, generally gusting 30-40 mph. This
could result in near blizzard conditions in portions of the
Poconos and Lehigh Valley, where snow is heaviest, but not
expecting widespread blizzard conditions to warrant a Blizzard
Warning. Snowfall rates will generally be on the order of 1-2"
per hour, but in the heaviest banding, rates in excess of 2" per
hour may occur.
As that LLJ lifts north, and the shortwave approaches from the
west, there will be a fair amount of mid-level instability over
the area with NAM MUCAPE values up to 250 J/kg. Think this may
be enough for a few flashes of lightning and rumbles of thunder,
including where it is snowing, and snowing heavily
(thundersnow!). Will go ahead and add isolated thunder to the
forecast.
A surge of warm air in the mid-levels of the atmosphere, with
temperatures warming to as much as +2C from 850-700 mb will also
spread north, generally west of the I-95 corridor, and possibly
as far north as towards Reading and Allentown. This would
result in a period of snow and sleet, generally from around 7pm-
1am, and would cut back on snowfall totals in these areas.
In terms of snowfall, did not make many significant changes to
the previous forecast except to trend snowfall amounts up
slightly. Generally 12-18 inches across the southern Poconos,
far northwest New Jersey, the Lehigh Valley and down to the I-95
corridor, though right along and may 5-10 miles west of the
I-95 corridor, snowfall amounts will be more 6-12 inches due to
mixing. Localized 20+ inches are possible in the Pocono
Mountains. On the east side of the I-95 corridor south of
Trenton, snowfall amounts will generally range from 2-6 inches,
with the highest amounts right along the Delaware River. The
gradient could be tight enough that, for example, Camden, NJ
could get up to 6 inches of snow, while Medford, NJ could only
get an inch or so.
Across southern New Jersey and most of Delmarva, moderate to
heavy rain will develop tonight. In terms of QPF, there should
be 2 areas of max QPF, with 1.5-2 inches liquid QPF across the
southern Poconos, Lehigh Valley, and Berks county, and in excess
of 2 inches along the New Jersey coast. In between those two
zones, a general 1.5 inches of liquid QPF is expected. Flash
Flood Guidance is high enough that there flash flooding is not a
concern, but there may be some ponding of water in poor
drainage areas, and runoff from the heavy rain may add to the
Coastal Flooding concerns. Please see the Coastal Flooding
section of the discussion for more information.
As low pressure begins to move out to sea late tonight, CAA
develops behind the departing low, and precip will change to all
snow throughout before ending.
&&
.SHORT TERM /THURSDAY/...
Snow continues over northern New Jersey and the southern
Poconos early Thursday morning before tapering off before noon.
Little additional snow accumulation possible. For the rest of
the region, conditions will be dry and the sun should begin to
peek out by midday or so.
With fresh snowpack north and west of the Fall Line, will lower
high temps by a few degrees compared to a blend of guidance.
Thursday will be a cold day with below normal temperatures,
generally in the 30s throughout the region.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Heading into the weekend an deep upper level trough will
continue to push the coastal storm towards the northeast. This
brings a broad dome of high pressure over the mid-Atlantic. The
high pressure system will crest over the weekend as a shortwave
ridge tracks through on Saturday. With northerly flow continuing
til Saturday, this points to a chilly couple of days. With a
fresh snow pack over the interior, and high pressure at the
surface, I went quite a bit below our typical blend of guidance.
Typically we'd use an observation based bias corrected blend,
however the bias corrections will struggle for the first couple
of nights with a fresh snow pack cooling the skin temps. This
seemingly holds true for most of our available guidance as a
violin distribution plot indicates most of the deterministic
guidance well above the 50th percentile of the National Blend of
Models. I ended up using a blend of 2:1 NBM deterministic/NBM
10th percentile to significantly drop temps through Saturday.
Heading into Sunday we should start to see some warmer
temperatures as southerly flow returns to the Mid-Atlantic. The
00z suite of guidance continues to indicate the potential for a
clipper system however the northern stream trough doesn't point
towards phasing. This leads towards a mostly dry forecast with
perhaps some light precip over the region Sunday night. Behind
that system, high pressure returns and we'll see another round
of quiet weather.
&&
.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
This Morning... Generally VFR, with possibly some MVFR cigs
moving into the southern terminals late morning. NE winds
sustained around 10 kts. High confidence
This Afternoon... MVFR cigs will likely arrive at the terminals
in the early afternoon with precipitation and then a
deterioration to IFR following shortly after, generally in the
18-21Z time frame. Expect all snow through the daytime hours at
KRDG/KABE with heavy snow and LIFR conditions expected later in
the afternoon. For the I- 95 sites, expect mostly snow (possibly
moderate-heavy in intensity at times during the mid afternoon)
during the daytime hours although some rain could mix in at ILG
and PHL. Precipitation at KMIV, KACY should remain rain albeit
moderate-heavy at times. ENE winds will be sustained around
15kts with gusts to 25kts. Moderate confidence in category onset
and ptypes, lower confidence on if snow will reach LIFR
intensity at the I-95 terminals.
Tonight... IFR/LIFR initially this evening, with moderate-heavy
snowfall still likely at KABE/KRDG with a small chance of ice
pellets moving in overnight. Ptype will likely transition to
mostly rain at KILG/KPHL but can't rule out a period of ice
pellets early this evening. KPNE/KTTN will likely see more of a
wintry mix with snow, sleet, and rain possible. Late in the
overnight period another round of snow may move back in from
W-E, but confidence on how intense this round would be is low.
NE-ENE winds will be sustained around 20 kts with gusts 30-35kts
(possibly 40+ at KACY). Low confidence on if ice pellets will
impact KILG/KPHL and low confidence on the late overnight snow
potential.
Outlook...
Thursday... Initially, mainly IFR due to lingering snow, ice,
and rain. Precipitation will likely be tapering off as snow for
most locations leading to a continued period of reduced VSBYs
until the afternoon. By late morning/early afternoon we should
see generally improving flight conditions. Winds shifting from
north- northeast to north or north- northwest. Speeds 10 to 20
kt decreasing to 5 to 10 kt. High confidence in overall trends
but low to moderate confidence in the timing of flight category
changes.
Thursday night through Sunday...VFR anticipated with northerly
winds mostly 5 to 15kt. High confidence.
&&
.MARINE...
Winds and seas ramp up throughout the day, and gales will
develop on the waters by early afternoon. Will make some minor
timing adjustments to the marine hazards, and will carry a Gale
Warning for all marine zones for today, and the Storm Warning
for the coastal ocean waters and lower Delaware Bay will be in
effect starting tonight. For southern New Jersey and Delaware
coastal ocean zones, as well as lower Delaware Bay, storm force
winds will mainly occur in the early evening hours, while the
storm force winds may continue into daybreak on the northern New
Jersey coastal ocean zones. For upper Delaware Bay, expecting
gales this afternoon and tonight. There will be VSBY
restrictions in heavy rain tonight, and isolated thunderstorms
are possible as well.
Gale force winds continue through Thursday morning, and
conditions will lower to SCA levels in the afternoon.
Outlook...
Thursday night through Saturday...SCA's will likely be needed
through at least the overnight hours as seas remain elevated.
Winds should subside below SCA criteria early on Thursday night.
Once seas subside early Friday morning, winds and seas should
stay below SCA criteria.
&&
.TIDES/COASTAL FLOODING...
A classic setup is anticipated tonight into Thursday morning
for a high-impact coastal flooding event for the Atlantic coast
and Delaware Bay. As a coastal storm moves northeastward just
off our coast Wednesday night, strong high pressure in eastern
Canada will promote a long-duration tight surface pressure
gradient in the northern Mid-Atlantic and New England. We expect
a prolonged east- northeast flow with sustained winds 30-40+
kt, peaking Wednesday evening and night. Though the timing of
peak surge may not quite match the timing of high tide on the
coast, the magnitude of surge will easily exceed 2 feet, and
flow will not permit escape of water from the coast during low
tide. Additionally, astronomical tides will continue to be high
from the lingering effects of the new moon.
The combination of these conditions will likely lead to at
least widespread minor flooding on the Atlantic Coast/Delware
Bay with the Wednesday evening high tide and then a round of
moderate flooding on the Atlantic coast and Lower Delaware Bay
for Thursday morning's high tide. The spread among guidance
(both between the three major surge forecasting systems, and the
spread within the Stevens Institute guidance) remains rather
high, especially for this evening's high tide. In fact some of
the guidance is hinting that this high tide could be very
similar in magnitude to the Thursday morning high tide (which
will be the higher of the two tides from an astronomical
perspective). All in all a challenging forecast given that the
peak onshore winds will occur in between the two high tide
cycles, which may make them both relatively significant, but
also somewhat limit the highest end potential. Given that the
guidance envelopes still span minor to major flood for most of
the Atlantic feel that keeping most sites in moderate for now is
a reasonable compromise (although did bump up the Wednesday
evening high tides a bit to account for the timing uncertainty).
Consequently also extended the CFW for Middlesex county to
encompass the Wednesday evening high tide cycle.
Things become a bit more uncertain as we head up the Delaware
Bay, and am not certain that the gauges at Bowers Beach and
Bivalve will hit moderate flood stage Thursday morning given
that they will have more time in the NW flow regime on the back
side of the low, but current forecast remains close enough to
moderate flood that will retain the warning.
The Delaware River coastal flooding potential remains nebulous
as these sites as this regime is less optimal for significant
tidal flooding there, however, can't rule out advisories for
Thursday AM's tide as we get closer to the event.
Tidal flooding is not anticipated for the eastern shore of
Chesapeake Bay at this time.
Bottom line: All interests in areas susceptible to tidal
flooding should monitor the latest forecasts and be prepared for
a significant coastal flooding event on Thursday morning.
&&
.CLIMATE...
Here are the daily snowfall records for Wednesday, Dec 16 and
Thursday, Dec 17:
Dec 16
Allentown...4.5" in 1973
Atlantic City Airport...0.8" in 2010
Philadelphia...7.1" in 1896
Wilmington...3.5" in 1896
Dec 17
Allentown...2.1" in 2013
Atlantic City Airport...0.2" in 2013
Philadelphia...7.3" in 1932
Wilmington...2.2" in 1973
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Warning until 10 AM EST Thursday for PAZ054-055-
060>062-070-071-101>106.
NJ...Winter Storm Warning until 10 AM EST Thursday for NJZ001-
007>010-012-013-015.
Coastal Flood Warning from 7 AM to 3 PM EST Thursday for
NJZ021.
Wind Advisory from 6 PM this evening to 1 AM EST Thursday for
NJZ021-022-027.
High Wind Warning from 6 PM this evening to 6 AM EST Thursday
for NJZ014-026.
Coastal Flood Warning from 7 PM this evening to 3 PM EST
Thursday for NJZ012>014-020-022>027.
High Wind Warning from 6 PM this evening to 1 AM EST Thursday
for NJZ023>025.
Winter Weather Advisory until 10 AM EST Thursday for
NJZ016>019.
Wind Advisory from 6 PM this evening to 7 AM EST Thursday for
NJZ020.
DE...Coastal Flood Warning from 7 AM to 3 PM EST Thursday for
DEZ002.
Coastal Flood Warning from 7 PM this evening to 3 PM EST
Thursday for DEZ003-004.
Wind Advisory from 4 PM this afternoon to 1 AM EST Thursday
for DEZ002>004.
Winter Weather Advisory until 10 AM EST Thursday for DEZ001.
MD...None.
MARINE...Storm Warning from 6 PM this evening to 1 AM EST Thursday for
ANZ431-452>455.
Gale Warning from 1 PM this afternoon to 6 PM EST this evening
for ANZ431-450>455.
Storm Warning from 6 PM this evening to 6 AM EST Thursday for
ANZ450-451.
Gale Warning from 1 PM this afternoon to 6 AM EST Thursday for
ANZ430.
&&
Synopsis...Deal
Near Term...MPS/O'Brien
Short Term...MPS
Long Term...Deal
Aviation...Carr
Marine...Deal/MPS
Tides/Coastal Flooding...WFO PHI
Climate...WFO PHI