Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 231106
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
606 AM EST Mon Jan 23 2017

.SYNOPSIS...
Low pressure over the Southeast will drift northeastward to the
Mid-Atlantic by this afternoon. This low will then meander off
the New Jersey coastline Monday night before departing late
Tuesday. Weak high pressure builds across the region on
Wednesday. Low pressure passes north of the area Wednesday
night, and then a deep upper trough builds into the Eastern
United States for the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Strong surface low over the southeastern U.S. will slowly
progress northeastward to offshore the Mid-Atlantic Coast by
this evening. Meanwhile, a surface high will drift southward
across eastern Canada today. In between, the pressure gradient
will continue to increase, leading to strong east to northeast
surface winds through the day. Several sites already reporting
gusts 35 to 45 mph, and these should continue to increase this
morning. Operational models suggest a south-to-north push of
the strongest winds this morning (south of the Mason-Dixon Line)
through early this evening (northern and central New Jersey) as
the surface low moves northeastward off the coast of Delmarva.
Current high wind warnings/wind advisories handle this timing
well and did not change the products with this package.

Meanwhile, light precipitation has already developed overnight
in portions of the CWA, with a fairly persistent band of light
rain in the vicinity of Atlantic City. Expect areas of light
rain and drizzle to continue through the morning hours before
steadier precipitation moves into the area by afternoon. Latest
guidance has the heaviest precipitation moving in from south to
north during the 18Z-06Z time frame. Models are quite variable
with QPF totals, with the 00Z GFS noticeably drier and the 00Z
NAM/CMC on the wetter side. There is also a subtle trend
southward in the highest QPF, especially with the 00Z ECMWF.
Location of maximum QPF will be highly dependent on exact
placement of the coastal front, frontogenetical forcing and the
axis of deformation to the northwest of the surface low, and
exact influence of the low-level jet developing just downstream
of the surface low. It still appears that coastal New Jersey
should see the best chance of highest precipitation. Current
Flood Watch looks OK for now.

This storm will also impact our northern most zones with winter
weather. A Winter Weather Advisory Advisory was issued for
Carbon and Monroe Counties in PA and Sussex County, NJ for this
morning through Tuesday morning. First, temperatures will
continue to fall, most precipitously, across NE PA and NW NJ
this morning as cold air from high pressure over E Canada drains
southward. At MPO, the temperature had dropped 7 degrees since
midnight to 34F at 5 AM. Shallow lift in the low-levels will
provided by upslope flow from a strengthening easterly low-level
jet this morning. The setup should result in periods of light
rain and drizzle in the mountains this morning. For the highest
elevations in Monroe County (above 1500 ft), freezing rain/
drizzle is expected once temperatures drop below freezing after
sunrise this morning. The freezing level will lower to about
1000 ft in elevation by late this morning and afternoon, resulting
in pockets of icing along the ridges in Carbon and Sussex
Counties. The top-down approach using a NAM/GFS blend for
thermal profiles aloft indicates sleet mixing in at times in the
advisory area this afternoon as the pocket of low-level cold air
deepens and steadier precip moves in. A change over to wet snow
is possible at the higher elevations (most likely after sunset
this evening) if there is enough dynamical cooling present to
erode the warm layer aloft. Snow and snow/sleet mix looks to
become heavy for a period tonight. It`s very important to note
that there are still subtle thermal differences between model
guidance, which leads to quite a bit of uncertainty with
regards to how ptypes transition through the event. Therefore,
forecast confidence in snow/ice accumulations is definitely
lower than what we would like to see for this close out and with
a winter headline in effect. Nonetheless, it only takes a light
glaze of ice to produce hazardous travel...which is enough to
warrant the headline. Please see WSW for more details.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The main band of precipitation is expected to be moving across
the area this evening as the deepest lift associated with the
strong system moves through. The highest QPF totals will likely
occur during this time, with storm totals (liquid equivalent)
1-3 inches area wide, much of this occurring in the
aforementioned 18Z-06Z period. As this steadier precipitation
moves through, precipitation in the higher elevations of Carbon,
Monroe, and Sussex County (NJ) will likely transition to sleet
and wet snow. Combined with the strong winds this evening, this
will make for treacherous travel in these areas. As most
precipitation is expected to be sleet, antecedent ice
accumulations and subsequent snow accumulations are currently
expected to remain below warning thresholds. However, much
uncertainty remains as to the exact evolution of precipitation
type with this storm -- i.e., this forecast is of below average
confidence.

Synoptically-driven low-level jet stream will slowly move
northeast away from the region during the evening hours. As this
does so, the strongest lift should depart the area after
midnight, and winds should begin to decrease. Nevertheless, the
proximity of the surface low will permit winds to continue
gusting up to 40 mph across much of the area, especially in New
Jersey and southeastern Pennsylvania. Light precipitation will
likely continue, especially with any wraparound on the northwest
side of the cyclone (indicated by virtually all operational
guidance, though there remain considerable discrepancies
regarding exact placement). As precipitation winds down in the
southern Poconos and adjacent Sussex County, NJ, precipitation
may transition to light freezing rain or drizzle as ice crystals
become more difficult to generate as the midlevels dry out.

Temperatures are expected to stay just above freezing from the
I-95 corridor southeastward to near freezing in the Lehigh
Valley to below freezing in the southern Poconos and adjacent
Sussex County.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
From Tuesday Night on:

Although surface low departs Tuesday morning, upper level low
will lag behind. With several strong shortwaves wrapping around
this low, can expect another shot of precip to impact the region
through Tuesday afternoon. precip should be rain for the bulk
of the area, but a wintry mix is possible for Carbon and Monroe
counties.

Conditions dry out Tuesday night as high pressure builds into
the region and moves offshore during the day Wednesday.

Temps will run a few degrees above normal on Tuesday, generally
in the upper 30s to mid 40s, while a return to above normal
temperatures expected on Wednesday with highs in the 40s and
50s.

Low pressure will organize over the Great Lakes as a deep upper
trough builds into the western U.S. This portends a pattern
change for the end of the week and weekend. Cold front pushes
through the region Wednesday night. For now, expecting dry
conditions. However, GFS is indicating an area of precip
developing offshore with the passage of the front. It is
possible that more organized precip can develop, but models
currently keep it offshore, so will go with a mainly dry
forecast with its passage.

Colder air then builds into the region for the end of the week
and weekend with a return to more normal temperatures. Highs
will fall into the 20s and 30s over the weekend.

&&

.AVIATION /10Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

IFR conditions are expected to occur through tonight across the
TAF sites as low clouds, drizzle, and fog will continue this
morning before steadier rain moves in this afternoon and this
evening. Strong east to northeast winds are expected through
this evening, with gusts 30-40 kts likely (possibly even
stronger at KACY). Precipitation will likely diminish from south
to north by late this evening or overnight, but with little
recovery of ceilings/visibilities expected. There is a slight
chance of some wintry precipitation tonight at KABE/KRDG, but
confidence was too low to include in the TAFs at this time. A
strong low-level jet will generate easterly low-level wind shear
this afternoon and evening across the entire area.

OUTLOOK...

Tuesday...Lingering IFR conditions with rain continuing through
the day. Sleet or snow may briefly mix in at ABE before ending.
Gradually improving to MVFR and then VFR from S to N late in
the day.

Tuesday night and Wednesday...VFR. NW winds 10-15 KT with gusts
to 20 kt.

Thursday...Isolated showers, possibly briefly lowering
conditions to sub-VFR.

Friday and Saturday...VFR. West winds 10-20 KT.

&&

.MARINE...
ANZ454/455 zones were upgraded to a Storm Warning yesterday
evening with all model soundings available in BUFKIT for 44009
show winds only 300 ft above the surface increasing to 50 kt
after sunrise this morning. Although boundary-layer lapse rates
will be weaker in the coastal waters of DE (compared to up
north off central NJ), it won`t take much for storm force winds
right off the deck to mix down to the surface. The heavy rain
expected to move in during the afternoon (possibly embedded
thunder) could aid in downward transport as well.

Easterly winds have steadily increased last night and are
currently gusting in the 30-40 kt range early this morning.
Winds will continue to increase over the next several hours. The
period of strongest winds (when most of the storm- force winds
will occur) will be on Monday from shortly after sunrise through
mid afternoon for the coastal waters of DE, through early
evening for the coastal waters of southern NJ and through late
evening farther north adjacent to central NJ.

OUTLOOK...

Tuesday...Strong SCA conditions expected, with another round of
Gales possible on the ocean waters Tuesday afternoon.

Wednesday...Sub-SCA conditions expected.

Thursday through Friday...Wind gusts 25-30 KT possible.

&&

.HYDROLOGY...
Flood Watch is in effect for this afternoon and tonight for
central and NE NJ, where the flooding risk is highest (heaviest
rainfall and most prone to flooding issues in urban locales).
Impacts will be due to excessive rainfall, as between 1-3 inches
of rain with locally higher amounts, possibly up to 4 inches,
is expected from late this afternoon through tonight.

River flooding should be limited unless basin-wide rainfall
averages climb above three inches. The most sensitive responding
rivers still appear to be the Millstone and the Rancocas.

The entire region has some positive things working in its
favor...precipitation has been below normal the last 30 days,
there`s really no consequential snow cover to speak of, there`s
no ice on area waterways, streamflows are either running at or
below normal as is soil moisture, and last but not least, the
ground isn`t frozen. We think all the above will help mitigate
the flood threat.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Warning is in effect for this afternoon/evening
along the NJ coast from Atlantic County northward to Monmouth
County. The latest ETSS has trended higher with tidal forecasts
at both Atlantic City and Sandy Hook. Even though ETSS keeps
Sandy Hook below moderate, we think guidance is trying to play
catch up. The trend has been for the duration of very strong
onshore winds to extend later into the day and even into part of
tonight. The timing of the peak storm surge now looks to coincide
with the late afternoon/evening high tide to yield moderate
tidal flooding in the warned area.

A Coastal Flood Advisory remains in effect for the coastal
areas of Delaware and the remaining portion of the coastal areas
of New Jersey, Delaware Bay and the tidal Delaware River below
the Commodore Barry Bridge area. The advisory covers the high
tide cycles through this evening, but it will likely need to be
extended into the following high tide early Tuesday morning.
There is a concern that moderate coastal flooding develops with
this afternoon/evening`s high tide but did not have as much
support from the guidance as we did at Atlantic City and Sandy
Hook. We will continue to closely monitor the situation.

Increasing wave action and the resulting beach erosion will
likely compound the effects of the surge. Waves breaking along
the shore may build to 5 to 8 feet. The expected surge along
with the wave action and the forecast rainfall will likely
result in additional and more widespread minor flooding around
the high tide from late this afternoon and evening.

The onshore flow is expected to begin weakening from south to
north tonight as low pressure passes off the Middle Atlantic
coast and the tight pressure gradient ahead of the system lifts
into New England. However, lingering minor coastal flooding is
again possible for the high tide early on Tuesday morning being
that the flow is not expected to become fully offshore until
Tuesday afternoon.

Based on the forecast guidance and the expected pattern we are
not anticipating any coastal flooding on the upper eastern shore
of Maryland. Also, the potential for anything more than spotty
minor tidal flooding appears low for the tidal Delaware River
above the Commodore Barry Bridge.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Wind Advisory until 4 PM EST this afternoon for PAZ060>062-070-
     071-101>106.
     Winter Weather Advisory from 11 AM this morning to 11 AM EST
     Tuesday for PAZ054.
     Winter Weather Advisory until 11 AM EST Tuesday for PAZ055.
NJ...Flood Watch from 4 PM EST this afternoon through late tonight
     for NJZ008-010-012>015-020-026.
     Wind Advisory until 7 PM EST this evening for NJZ007>010-015.
     Wind Advisory until 4 PM EST this afternoon for NJZ016>019-021.
     Coastal Flood Advisory until 9 PM EST this evening for NJZ021-
     023-024.
     High Wind Warning until 9 PM EST this evening for NJZ012>014-
     020-026.
     Coastal Flood Warning from 2 PM this afternoon to 11 PM EST
     this evening for NJZ012>014-020-022-025>027.
     Coastal Flood Advisory until 2 PM EST this afternoon for
     NJZ012>014-020-022-025>027.
     High Wind Warning until 4 PM EST this afternoon for NJZ022>025-
     027.
     Coastal Flood Advisory until 10 PM EST this evening for NJZ016.
     Winter Weather Advisory from 11 AM this morning to 11 AM EST
     Tuesday for NJZ001.
DE...Wind Advisory until 4 PM EST this afternoon for DEZ001>004.
     Coastal Flood Advisory until 9 PM EST this evening for
     DEZ002>004.
     Coastal Flood Advisory until 10 PM EST this evening for DEZ001.
MD...Wind Advisory until 4 PM EST this afternoon for MDZ008-012-015-
     019-020.
MARINE...Storm Warning until midnight EST tonight for ANZ452-453.
     Storm Warning until 4 AM EST Tuesday for ANZ450-451.
     Storm Warning until 6 PM EST this evening for ANZ454-455.
     Gale Warning until 7 PM EST this evening for ANZ430-431.

&&

$$
Synopsis...CMS/MPS
Near Term...CMS/Klein
Short Term...CMS/Klein
Long Term...Klein/MPS
Aviation...CMS/Robertson/MPS
Marine...CMS/Klein/Robertson/MPS
Hydrology...Klein
Tides/Coastal Flooding...Klein/Iovino



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