Areal Flood Advisory 1, NWS Philadelphia/Mount Holly

Area Forecast Discussion
National Weather Service Mount Holly NJ
1121 AM EST Fri Jan 12 2018

.SYNOPSIS...
A strong cold front will move across our area later tonight and
Saturday morning as low pressure moves along it. High pressure
builds in Sunday before shifting mostly to our north Monday. An
area of low pressure is forecast to cross our region Tuesday
into Wednesday, then high pressure should build in from the
southwest during Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
920 AM Update...Showers moving across the area at this time with
southerly winds resulting in areas of fog along the coast and
near the Delaware Bay. For this reason, added in mention of fog
through this morning. Also adjusted temps up based on latest
obs. Otherwise, no changes to the forecast at this time.

Previous Discussion...

Precipitation: Widespread rain has been slower to develop than
previously expected. Still, expect to continue to see an increase in
shower coverage through mid day. At that point, we may have a bit of
a lull in precipitation through the rest of the daylight hours. So
far, upstream obs indicate rather light rain rates with this first
round. However, I am hesitant to change the QPF given the evolving
jet (our region will be under the right entrance region of the jet
through the morning hours). Therefore, kept expected amounts
generally between 05 and 1.00" through the day. There is some
potential for flooding in poor drainage areas . See the hydrology
section for more information on this hazard.

Winds: As for the winds this morning, high resolution guidance has
backed off on the potential for 40 kt gusts. Additionally, upstream
obs have depicted gusts only as high as 25 kt so far even with
enhanced mixing in showers. Although the low level jet will be in
place through the morning with 45 kt winds 2000 ft AGL, it is
unlikely we will have efficient enough mixing to see wind gusts in
the 40 to 45 mph range. Therefore, have cancelled the wind advisory.
However, wind gusts up to 35 mph are still expected and will
continue to monitor this potential through the morning.

Temperatures: With continued southerly flow/warm air advection,
should see temperatures 20 to 25 degrees above normal (!) with highs
generally in the 60s except in the higher terrain of the Poconos and
NW NJ, and along the shore.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
The biggest question tonight into tomorrow is how quickly the cold
front will move through the region, and where it will be when the
surface low lifts northeast along the front. The latest GFS depicts
the low lifting NE of our area while the front is still west of the
I95 corridor. A scenario like this would result in a quicker end of
precipitation (and consequently less chance of wintry precipitation)
as our area would not be in the favored area under the warm conveyor
belt for post-frontal precipitation. However, this scenario appears
to be an outlier and is suspect with how slowly it moves the front
through our area. Therefore, have made only minor changes with the
forecast overnight into tomorrow.

Wintry precip: Wintry precip, primarily in the form of sleet and
freezing rain, looks likely for the Lehigh Valley and areas NW of
there. Ice amounts in these areas look to range from Trace to 0.2".
Unfortunately, the ground is already pretty cold to support freezing
on surfaces. Therefore, hazardous travel conditions are possible
through the morning hours across the Lehigh Valley, Poconos, and NW
NJ.

Expect the precipitation should end well before temperatures
across the I95 corridor drop low enough to support freezing or
frozen precip. Will continue to monitor this, especially with the
evolution of the surface low.

Temperatures: If you sleep in on Saturday, you'll be in for a rude
awakening as guidance suggests that we could see temperatures drop
from the 50s to 30s over only a 3 hour period behind the cold front
on Saturday morning. Highs on Saturday will be 20 to 30 degrees
lower than Friday highs, and will likely be reached early in the day
with temperatures holding steady or falling through the day.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Summary...Rather cold for this entire time frame; an accumulating
snow potential still exits for Tuesday into Wednesday.

Synoptic Overview...An upper-level trough amplifies into the East
through Sunday, then amplifies again during the first half of next
week. Low pressure Tuesday into Wednesday may be potent enough to
produce some accumulating snow for at least portions of our area
especially as it redevelops at a triple point just offshore. There
is less certainty however with the overall evolution of this feature.

For Saturday night...As an upper-level trough continues to amplify
into the East, high pressure presses eastward from the Great Lakes
and Ohio Valley. We will still have a pressure gradient across our
region and this should keep a breeze going. It will be very cold as
temperatures drop into the teens and single numbers. Since there
will be some wind added in, wind chills will drop to 0-5F across
most areas although down to around 10 below zero in the southern
Poconos. Since the coldest wind chills look to be in some of the
higher terrain locations, no wind chill advisory is planned at this
time.

For Sunday and Monday...Low pressure moves across the Canadian
Maritimes to start Sunday with cold air advection across our
region ahead of strong surface high pressure. The East will remain
within an expansive upper-level trough which should start to amplify
from the Midwest to the Great Lakes Monday. As this occurs, a
clipper system moves across the Great Lakes and this should start to
toss some high level clouds our way by late Monday. Some differences
in the guidance starts to take shape Monday night with the evolution
of the incoming upper-level trough and therefore the speed of the
clipper system. The forcing may end up hanging back longer closer to
the stronger height falls and frontal forcing. As a result, Monday
night may be mostly dry and despite some uncertainty did delay the
PoPs a little bit.

For Tuesday and Wednesday...This time frame revolves around an
amplifying upper-level trough into the East. There remains some
timing issues as well as the southern extent of the trough
amplification. Some guidance such as the 00z ECMWF has a stronger
short wave within the base of the trough that results in a farther
south extension of the trough axis. This tends to slow the entire
system as it shifts east, which ends up with a weaker clipper
generally just off our coast. Given the stronger short wave that
rolls east, allows the trough to take on a negative tilt late
Wednesday. This ECMWF solution then results in a new surface low off
the Mid-Atlantic coast which then strengthens northeastward and
tosses precipitation back across the area. Some other guidance is
less robust with the short wave in the base of the trough and
therefore the trough axis remains more neutral with the clipper
moving right along. Some of the GFS ensembles however do show the
trough taking on more of a negative tilt. Given less certainty
regarding the evolution of the upper-levels, the details are less
clear. There is a chance of some accumulating snow during at least a
part of this time frame, whether it is Tuesday or delayed until
Wednesday. As a result, we made no significant changes to the PoPs
during this time frame (chance range).

For Thursday...Based on the idea of the upper-level trough remaining
more neutral allowing surface low pressure to be closer to the
Canadian Maritimes, we bring in high pressure as it builds from the
southwest. If the trough is more amplified including a negative
tilt, surface low pressure is stronger and slower therefore snow
would linger into at least the start of this time frame. Since some
guidance may be over amplified with this system, we stayed close to
continuity and WPC. This translates into a dry forecast but
remaining cold. It looks like a northwesterly wind will add a
wind chill factor.

&&

.AVIATION /16Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Widespread MVFR with localized IFR possible at times. Low
stratus is expected through the period. Visibility restrictions will
be possible in periods of rain. There is a small chance of
thunderstorms through 18Z, but the chance is too low to include in
the TAFs at this time. Low level wind shear will continue to be
an issue through about 18Z. South southwesterly winds of 50kt
about 2000 ft AGL have been sampled by radar.

Tonight...Widespread IFR conditions are possible, for both ceilings
and visibility (mainly due to periods of precipitation). An abrupt
shift from southwesterly to northwesterly winds is expected around
06Z or later as a cold front moves through the region. Wind gusts to
25 kt are possible behind the front.

Saturday...MVFR or lower is possible to start the day, with fZRA and
sleet possible at KABE and KRDG. Conditions improving to VFR by mid
day if not sooner. Northwest winds gusting as high as 25 kt. Average
confidence.

Outlook...
Saturday night...VFR. Northwest winds 10-15 knots. High confidence.

Sunday and Monday...VFR overall with some increase in clouds mainly
later Monday. Northwest winds 10-15 knots Sunday diminishing at
night, then becoming light northeasterly Monday. Moderate confidence.

Tuesday...A period of snow and IFR conditions possible. Lower
confidence.

&&

.MARINE...
920 AM Update...Added in fog to the forecast through today and
also issued Dense Fog Advisory for ocean zones and the lower
Delaware Bay based on latest observations and web cams.

Previous Discussion...Small craft advisory conditions are
expected to develop soon. Gusts near 35 kt possible but warmer
air moving over cold water should limit this possibility. Still,
its possible special marine warnings may be needed later today
if thunderstorms develop or there is a well defined short line
of heavy showers.

Periods of rain and patchy fog may result in visibility restrictions
at times today and tonight.

In addition, significant ice melting and breakup on bays and along
the coast will continue through this time. Temperatures are expected
to stay above freezing through the overnight period.

On the upper Delaware Bay, winds may drop below SCA criteria by this
afternoon. Otherwise, SCA conditions are expected to continue into
tonight. An abrupt shift to northwesterly winds is possible either
very late tonight or Saturday morning.

Saturday...SCA conditions continue with northwesterly winds through
the day.

Outlook...
Saturday night...Small Craft Advisory conditions should continue.
Light to perhaps moderate freezing spray.

Sunday...Winds should fall below advisory criteria, with seas
subsiding below 5 feet on the ocean by late day. Mainly light
freezing spray.

Monday and Tuesday...The conditions are anticipated to be
below Small Craft Advisory criteria.

&&

.HYDROLOGY...
Rainfall...with the 1 to 2 inches of rainfall expected, many of
our 48 hydrological forecast points will see rises in the 2 to
6 foot range. These rises will remain well within bankful.
Outside of our 48 forecast points, poor drainage flooding is
possible through tonight.

River ice...The nearly 2 weeks straight of sub-freezing temperatures
has created a build up of ice on local rivers and streams. We wanted
to address the ice and the prospects of ice jams.

Unfortunately, neither the formation or dissipation of an ice jam
can be predicted with certainty, although we know what can influence
them. The best approach is awareness and to take notice of day to
day changes on a river or stream of concern.

Ice jams can develop near river bends, near the confluence of
tributaries, where slope changes, and around bridges. Solid ice
cover is not an ice jam. There needs to be a restriction with flow.
Even with solid ice cover, water can flow freely under the ice.

A combination of well above normal temperatures for several days,
heavy rainfall, and additional runoff from snowmelt is a worst case
scenario outside of the actual hydrology. Results and impacts will
be very different if rivers and streams were running high or were
above normal or as they are now, running low or below normal.
The more capacity a channel has, the more that channel can
withstand rises (absorb the water within bankful).

Today into Saturday, 1 to 2 inches of rainfall is predicted
across the region. Due to warmer temperatures and rainfall, some
ice movement can be expected since melting is occuring.

There hasn't been much water equivalent in the lingering snow.
Across our inland basins, there is really only 1 to 5 inches on
the ground where there is snow. Snow water equivalent or SWE, is
no more than an 1/2-inch. In addition, our river levels are
running below normal for this time year. In many cases, on the
Delaware for instance, levels are 1/4 bankfull or less. So there
is a lot of capacity within the banks to withstand the volume
of increased runoff rises.

As mentioned, awareness is the best approach. The high impact
ice jams we tend to hear about are those that break and cause
flash flooding downstream of a broken jam. Again, thanks to the
high capacity of channel we have, due to low flows, and the
width of the channel itself (e.g. Delaware, Passaic, Raritan,
Schuylkill), the threat of flash flooding continues to be very
low.

What has been more common in our Hydrologic Service Area (HSA),
is water building up behind a restriction or ice jam. Again,
while this cannot be precisely forecast, the process is a slow
one. It could take a day or days for water to build up behind a
jam, rather than hours. Preparation time is usually available.

Outside of our mainstem rivers, where the threat of flash flooding
is minimal, the flash flood threat (or flood threat) can increase
for smaller creeks or streams due to less available volume in the
channel. But on the flip-side, there is usually less water in these
tributaries to cause widespread impacts.

Temperatures get cold once again during Saturday and beyond and
this will temporarily lessen the chance for any ice movement.

&&

.CLIMATE...
For today, January 12th:

Location   Record High Temp    Record Warm Minimum Temp
--------   ----------------    ------------------------

ACY          67 in 2017               49 in 2017

PHL          72 in 1890               49 in 2017

ILG          68 in 2017               46 in 2017

ABE          60 in 1932*              43 in 2017

TTN          68 in 1890               47 in 2017

GED          70 in 1975               N/A

RDG          63 in 2017*              47 in 2017

MPO          56 in 1975*              N/A

*Preliminary Record Event Reports (RERs) were issued this morning
for ABE, RDG, and MPO, for record highs that were either tied or
broken. Temperatures may fluctuate further, and final reports will
be issued later today.

             Record Daily Rainfall
Location   (values under 1.8 inches)
--------   -------------------------

RDG              1.70 in 1915

ACY              1.19 in 1915

ILG              1.26 in 1900

GED              1.61 in 1991

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 4 AM to 10 AM EST Saturday for
     PAZ060>062.
     Winter Weather Advisory from 1 AM to 10 AM EST Saturday for
     PAZ054-055.
NJ...Winter Weather Advisory from 4 AM to 10 AM EST Saturday for
     NJZ001-007.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ431-
     450>455.
     Small Craft Advisory until 6 PM EST Saturday for ANZ431-
     450>455.
     Small Craft Advisory until 1 PM EST this afternoon for ANZ430.

&&

Synopsis...Gorse
Near Term...Fitzsimmons/Johnson
Short Term...Johnson
Long Term...Gorse
Aviation...Gorse/Johnson
Marine...Fitzsimmons/Gorse/Johnson
Hydrology...
Climate...LF et al.

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Special Weather Statement
National Weather Service Mount Holly NJ
1137 AM EST Fri Jan 12 2018

NJZ013-014-020-022>027-121900-
Western Monmouth-Eastern Monmouth-Ocean-Atlantic-Cape May-
Atlantic Coastal Cape May-Coastal Atlantic-Coastal Ocean-
Southeastern Burlington-
Including the cities of Freehold, Sandy Hook, Jackson, Hammonton,
Cape May Court House, Ocean City, Atlantic City,
Long Beach Island, and Wharton State Forest
1137 AM EST Fri Jan 12 2018

...Locally Dense Fog through this afternoon...

Areas of fog with visibility of around one half to one mile
continue across eastern and southern New Jersey late this
morning. Localized areas of dense fog with visibility of one
quarter of a mile or less are being reported, mainly closer to the
coast. Be alert for sudden changes in visibility if you are
traveling through this afternoon as the visibility may vary
greatly between short distances. Be sure to allow extra space
between you and the vehicle in front of you and remember to only
use your low beam headlights.

Meola