Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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276
FXUS61 KPHI 241142
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
642 AM EST Tue Jan 24 2017

.SYNOPSIS...
The coastal low will lift northeast, passing east of Cape Cod
this evening. A ridge builds over our region briefly tomorrow. A
cold front passes southeastward off the mid Atlantic coast
Thursday followed by a second cold front moving through our
region late in the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
630 am update: Main band of precipitation has moved north and
west of the CWA this morning, but light rain/drizzle continues
in much of SE PA and adjacent NJ. Already seeing additional more
showery precip developing in Sussex County, NJ, which is likely
a mixture of sleet/snow in the higher elevations. Additional
rain is located offshore the Jersey and Delaware coasts. Though
not expecting steadier precip to reach the coast in the very
near future, high-resolution models do indicate rain showers
will reach the Jersey coast later this morning into the
afternoon. Forecast has all of this well in hand at this point.
Main changes with this update were to tweak temperatures and dew
points to match the latest trends. No changes to any of the
headlines at this point. Previous discussion is below...

Strong coastal low will meander its way northeast today to
offshore Cape Cod by 00Z Wednesday as a high-amplitude ridge
downstream and a cold anticyclone to its north impedes much
progress. Substantial synoptic-scale lift via differential
cyclonic vorticity advection and low-level isentropic ascent
will continue to generate a broad area of precipitation on the
north and northwest side of the surface low. The heaviest
precipitation at this time was generally north of I-80, but
lighter precipitation has developed in SE PA and S NJ in the
past couple of hours. Some occasional instances of light snow
have even occurred in spots as the combination of wet-bulbing
and strong dynamical effects/lift have cooled the column
sufficiently. However, for areas south of the southern Poconos,
any wintry precipitation early this morning will be light,
short-lived, and without impacts, particularly as surface
temperatures warm after sunrise.

High-resolution guidance is rather consistently showing a narrow
band of precipitation developing in the coastal waters moving
westward into portions of New Jersey today in an axis of
enhanced deformation on the upstream side of the surface low.
Although substantial precipitation is not expected, did feel the
need to bump up QPF a tad in portions of northern/central NJ
today given this consistent signal. In addition, with ambient
ascent revolving around the low underneath cold midlevels,
showery precipitation should occur in much of the area today,
particularly in PA/NJ. As such, raised PoPs for the daytime
hours, especially east of a Mount Pocono to Philadelphia to
Atlantic City line. Precipitation should diminish across the
area from southwest to northeast late this afternoon and this
evening as the low pulls away.

Regarding the southern Poconos...statistical guidance has rather
poorly performed in this area in northwest surface flow this
past month, generally being too aggressive in warming daytime
temperatures. As such, I was pessimistic with temperatures here
today, keeping the higher elevations of Carbon, Monroe, and
Sussex (NJ) Counties near or below freezing. With light
precipitation continuing this morning and at least showery
precipitation possible into the afternoon, needed to make a
decision regarding leaving the winter weather advisory as is or
extending it through the day. Several reports of 1-2 inches of
sleet overnight in these areas combined with at least some ice
accumulations -- so travel will be treacherous in this area
today given the pessimistic temperature forecast. Combined with
additional (albeit light) wintry precipitation expected through
the day, felt the need to extend the advisory here through 6
pm. As the column continues to cool this morning, should see
some more snow mix in with the sleet/freezing rain in the higher
elevations this morning, but this will eventually compete with
drying midlevels as the day wears on. Ice crystal growth should
generally cease, potentially leaving light freezing drizzle or
ice pellets the main show before precipitation shuts down late
this afternoon. Accumulations of snow/sleet will likely be an
inch or less, with trace amounts of ice in most locations. Lower
elevations (below 1000 ft MSL) will likely warm to above
freezing.

Regarding winds...as the surface gradient begins to relax this
morning, speeds have noticeably diminished. Although occasional
gusts to 45 mph may occur on the coast this morning, threat is
too marginal to continue the wind advisory, so have canceled it
with this morning`s package. Still will be breezy across the
area today, with gusts 30+ mph likely along the coast and in the
higher elevations of the Poconos.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
As the coastal low begins to move into the open Atlantic east of
Cape Cod, northwest flow becomes established across the region
overnight. The column will dry rapidly as this occurs shutting
any remaining precipitation down quickly. Antecedent overcast
conditions will likely scour out rapidly after dark. Winds
should continue to diminish but will likely be elevated enough
to prevent ideal radiational cooling. Temperatures will cool to
the mid 20s in the southern Poconos to the mid to upper 30s near
the coast.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Wednesday...For a brief period a short wave ridge will build
over our region. Thus, expect dry weather through the day. As
the low level ridge axis shifts east of our region through the
later half of the day, we should see some warm air advection.

Thursday...the warm air advection will be short lived however,
as the models generally sped up the timing of the cold front,
which is now expected to sweep off shore by mid day Thursday.
Any opportunity for precipitation should be brief, generally
along or just ahead of the front, but even that is somewhat
uncertain.

Friday through Monday...northwest flow pattern develops keeping
the cold air in place. In addition, a second cold front could
arrive in the region late in the weekend. In addition to cold
air advection, these northwest flow patterns also tend to result
in dry air advection, so at this point do not expect much in the
way of precipitation except possibly in the southern Poconos,
where some lake effect snow showers could sneak down into the
region.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

For the 12Z TAFs: MVFR/IFR ceilings/visibilities look to
continue through early afternoon with light rain/drizzle and fog
at most of the TAF sites. May see more showery precip this
afternoon, especially near and east of the Delaware River.
Conditions should improve by late afternoon to MVFR and
potentially VFR, but low confidence on timing. Conditions
expected to improve to VFR tonight, but may deteriorate back to
MVFR at KRDG/KABE after midnight. Northwest winds generally
10-15 kts with occasional gusts to 25 kts, with the strongest
winds likely at KACY, through the day before diminishing to 5-15
kts tonight.

OUTLOOK...
Wednesday...Mostly VFR conditions expected.

Wednesday night and Thursday...Small chance of showers which
could lower conditions to MVFR. Gusty westerly winds behind a
cold front.

Friday and Saturday...Mostly VFR conditions expected.

&&

.MARINE...
Gale conditions are still occurring in the coastal waters near
Long Beach Island, so continued the gale warning from Sandy Hook
to Little Egg Inlet through this morning (with at least small
craft advisory conditions expected thereafter). Small craft
advisory is in effect for the remaining waters through tonight.
Seas remain quite elevated (generally 12-20 feet in the coastal
Atlantic), but they should diminish through the day. Rain is
likely this morning, becoming more showery this afternoon before
diminishing overnight.

OUTLOOK...
Wednesday...Winds should diminish below SCA criteria by mid
day. Once they do so, sub SCA conditions are expected for the
remainder of the day.

Thursday through Friday...West wind gusts 25-30 KT possible and
seas could build above 5 feet.

Saturday...West wind gusts near or above 25 knots possible.

&&

.HYDROLOGY...
Flood Watch was cancelled this morning as precipitation rates
were too light overnight to generate any problems. Although more
rain is expected today, amounts will generally be under a half
inch and should not cause any problems in urban areas and on
area rivers and streams.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Warning remains in effect through this morning
along the NJ and DE coasts and along Delaware Bay. Storm surge
values of 2 to 3 feet are possible at/around high tide this
morning (generally 430 to 6 am). Fortunately, the astronomical
tides are low. We should see tide levels this morning a bit
lower than those experienced last evening.

Strong wave action and the resulting beach erosion will likely
compound the effects of any tidal surge. Waves breaking along the
shore may be around 4 to 7 feet this morning.

Based on the forecast guidance and the expected wind shift to the
north then northwest, we are not anticipating any coastal
flooding on the upper eastern shore of Maryland or on the tidal
Delaware River. The wind shift will result in a decreasing
potential for flooding along the coast and along Delaware Bay
beyond this morning.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 6 PM EST this evening for PAZ054-
     055.
NJ...Coastal Flood Warning until 11 AM EST this morning for
     NJZ012>014-020>027.
     Winter Weather Advisory until 6 PM EST this evening for NJZ001.
DE...Coastal Flood Warning until 11 AM EST this morning for
     DEZ002>004.
MD...None.
MARINE...Small Craft Advisory until 7 AM EST Wednesday for ANZ431-
     452>455.
     Gale Warning until noon EST today for ANZ450-451.
     Small Craft Advisory until 1 AM EST Wednesday for ANZ430.

&&

$$
Synopsis...Johnson
Near Term...CMS
Short Term...CMS
Long Term...Johnson
Aviation...CMS/Johnson
Marine...CMS/Gorse/Johnson
Hydrology...CMS
Tides/Coastal Flooding...Iovino/CMS



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