Area Forecast Discussion
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FXUS61 KPHI 231959
Area Forecast Discussion
National Weather Service Mount Holly NJ
259 PM EST Mon Jan 23 2017
Strong low pressure near the Virginia Capes late today will
drift northeast toward Long Island through Tuesday morning,
before passing east of Cape Cod Tuesday evening. Weak high
pressure crosses our region on Wednesday. A cold front passes
southeastward off the mid Atlantic coast Thursday and with
another cold frontal passage possible again Sunday or next
.NEAR TERM /THROUGH TONIGHT/...
This section will update shortly after 330 PM.
Strong low pressure continues to move northeast along the
central Appalachians this morning and will move offshore of the
Mid-Atlantic later this evening. Meanwhile, high pressure
remains to our north across eastern Canada. A tight pressure
gradient remains across the area today and will continue to lead
to increasing winds. Winds have already gusted 40-50 mph and
will continue to increase through this morning and into this
afternoon, especially from I-95 eastward. We have upgraded the
areas along the I-95 corridor south and eastward to a High Wind
Warning through late this afternoon. Areas north and west of
of I-95 remain in a Wind Advisory.
As several vorticity impulses move across the area in the
diffluent southerly flow aloft, periods of rain and drizzle will
continue into this afternoon. The heaviest precipitation is
expected to move in from south to north during the 18Z-06Z time
frame. Models are quite variable with QPF totals. 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. Light snow has already been reported in some areas
across Sussex, New Jersey. Temperatures are below freezing
across portions of Sussex, New Jersey as well as Carbon and
Monroe, Pennsylvania. So any precipitation that falls in the
higher elevations of these counties will see a wintry mix of
snow, sleet, and freezing rain through today. Therefore the
Winter Weather Advisory continues for these counties.
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 /TUESDAY/...
This section will update shortly after 330 PM.
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
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
.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
500 MB: Weak ridging follows the departure of our coastal storm
Tuesday night and Wednesday, followed by a series of short waves
that carves out a positive tilt trough in the eastern USA.
Temperatures: around 10 degree above normal Wednesday, 10 to 15
above normal Thursday, 5 to 10 above normal Friday, cooling to
near normal Sunday and Monday.
Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/23
GFS/NAM MOS guidance Tuesday night-Wednesday night, thereafter the
12z/23 GFS MEXMOS Thursday and Thursday night, and then the 15z/23
WPC guidance Friday-Monday, at times blended with the 12z/23 GFS
MEXMOS guidance or the long term temps were adjusted because of
the 12z/23 ECMWF 18z 2m temperatures.
Tuesday night and Wednesday: a little leftover mixed precipitation
is possible but it should not last long, if there is any. Conditions
dry out Tuesday night as high pressure builds into the region
and moves offshore during the day Wednesday. Gusty northwest
winds to 20 or 25 mph Tuesday night and Wednesday morning become
west or southwest Wednesday afternoon and diminish a bit.
Wednesday night and Thursday...warm air advection ahead of cold
front. Maybe a few showers. Lots of uncertainty regarding whether
it will rain. Blended the MEX POPS with the previously drier
Later Thursday into the weekend...gradual cool down with
generally dry weather expected. Breezy at times.
Next Monday...by then another cold front has either passed or
is passing and the GFS is the first of our usual reviewed
global models to suggest low pressure forming on the front over
the mid Atlantic states. To say the least, uncertainty and so we
stayed with the dry WPC fcst.
.AVIATION /20Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
This short term aviation section will update shortly after 330
Conditions are generally MVFR this morning across the TAF sites,
but are expected to lower to IFR through this afternoon and
tonight. Low clouds, drizzle, and fog will continue this
morning before steadier rain moves in this afternoon and this
evening. 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.
Strong east to northeast winds are expected through this
evening, with gusts 30-40 kts for ABE, RDG and 40-50 kts
possible for ACY, ILG, MIV, PHL, and PNE through today. Winds
will decrease some this evening and overnight, but remain gusty
A strong low- level jet will generate easterly low-level wind
shear this afternoon and evening across the entire area.
Precipitation will likely diminish from south to north by late
this evening or overnight, but with little recovery of
ceilings/visibilities expected. GUSTS UP TO 50 KT.
Tuesday night and Wednesday...VFR. NW winds 10-15 KT with gusts
to 20 kt with winds backing to west in the afternoon.
Late Wednesday night or Thursday...Isolated showers, possibly
briefly lowering conditions to sub-VFR.
Friday and Saturday...VFR. West winds 10-20 KT with gusts 25 to
This short term marine section will update shortly after 330
The Storm Warning remains in place for the Atlantic waters
adjacent to New Jersey and Delaware. We`ve upgraded the
Delaware Bay to a Storm Warning as winds are already gusting
40-45 knots, and are expected to increase to 50 knots.
The winds will diminish some this evening and overnight, but
will likely remain around gale force through the night for much
of the waters.
Tuesday night...northwest SCA, possible low end leftover gale
early in the evening.
Wednesday...Sub-SCA conditions expected.
Thursday through Friday...Wind gusts 25-30 KT possible.
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.
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
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.
Rechecking monthly normal. The January departure at Philadelphia,
when all is said and done, should end up 5 degrees above normal
or 10th to 17th warmest January in the period of record.
PA...Wind Advisory until 7 PM EST this evening for PAZ060>062-101-
High Wind Warning until 7 PM EST this evening for PAZ070-071-
Winter Weather Advisory until 11 AM EST Tuesday for PAZ054-055.
NJ...Flood Watch through late tonight for NJZ008-010-012>015-020-
Wind Advisory until 7 PM EST this evening for NJZ007>010.
Coastal Flood Warning until 11 AM EST Tuesday for NJZ012>014-
High Wind Warning until 7 PM EST this evening for NJZ015>019-
High Wind Warning until midnight EST tonight for NJZ012>014-
Coastal Flood Warning until 11 PM EST this evening for NJZ016.
Coastal Flood Advisory until 4 PM EST this afternoon for
Winter Weather Advisory until 11 AM EST Tuesday for NJZ001.
DE...Coastal Flood Warning until 11 AM EST Tuesday for DEZ002>004.
High Wind Warning until 7 PM EST this evening for DEZ001-002.
High Wind Warning until midnight EST tonight for DEZ003-004.
Coastal Flood Warning until 11 PM EST this evening for DEZ001.
Coastal Flood Advisory until 4 PM EST this afternoon for
MD...Wind Advisory until 7 PM EST this evening for MDZ008-012-015-
MARINE...Storm Warning until 1 AM EST Tuesday for ANZ452-453.
Storm Warning until 4 AM EST Tuesday for ANZ450-451.
Storm Warning until 10 PM EST this evening for ANZ430-431-454-
Long Term...Drag 258