Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 110212

Area Forecast Discussion
National Weather Service Mount Holly NJ
912 PM EST Wed Jan 10 2018

Strengthening high pressure centered over the western Atlantic is
ridged back to the mid Atlantic states through Thursday.  A cold
front from the west arrives in our area during Friday. Low pressure
moves northeast along the front through the mid Atlantic states
Friday night into Saturday sending the cold front off the coast by
Saturday night. Arctic high pressure then builds into our area later
Sunday into Monday. Low pressure and a cold front should then move
through our area on Tuesday.


Generally quiet conditions tonight. Light southerly flow will
allow for an increase in clouds through the overnight, and these
clouds should inhibit much fog from forming. Still cannot rule
out some patchy fog in the outlying areas from developing.

Otherwise, temperatures may not fall much tonight given a
considerably more amount of warming is occuring several thousand
feet up. We also have a snowcover to consider as well, so
confidence on the temperatures tonight is not the highest. Low
temperatures look to occur in the evening then slowly rise
overnight with most locations in the 30`s.

Weak isentropic lift needed for precipitation formation looks
to stay north of the region through tonight. If any stray light
showers or sprinkles do reach the southern Poconos and NW NJ
toward sunrise, very light freezing precipitation would be a
potential issue. Ice is not expected at this time.


Thursday will continue to see moisture on the increase with an
approaching cold front and several low pressure systems over the
Ohio Valley. Fairly good agreement is present to keep the
majority of Thursday dry but some areas of light rain may clip
the Lehigh Valley and southern Poconos regions by late in the
afternoon. Any QPF looks very light through the afternoon.

Temperatures look to be the biggest forecast challenge. Low
clouds look to be prominent further inland with a fair amount of
sunshine along the coast. Modeled 925 mb temperatures would
yield highs into the 60`s. Confidence is higher the cloud cover
and any lingering snow keeps the temperatures quite a bit lower
northwest of Philadelphia than what those 925 mb values would
give with most spots in the 40`s and low 50`s. Coastal sections
particularly from Atlantic City to Georgetown may realize the
warmer temperatures close to 60. Forecast is a middle of the
road between the warmer ECMWF MOS and the cooler MAV guidance.


Hazards: No flood watches at this time. Low stream flows,
modest snow water equivalent for contributing runoff and
refreezing by Saturday night should all assist limiting runoff
generally to within bank rises. However, a winter weather
advisory may eventually be needed for portions of the area along
and north of I78, in particular I80 north early this Saturday
morning. A wind chill advisory may be needed in the Poconos Sat
night. A more general snow advisory snow event is possible next
Tuesday or Wednesday.

500 MB: Strong ridging over the ne USA Thu night will shift to
the Maritimes this coming weekend, as a southern stream short
wave in the lower Mississippi Valley midday Friday weakens
northeastward. A second short wave trough will evolve in the
south central USA on Sunday, and then a third short wave trough
becomes evident over the Great Lakes and Ohio Valley on
Tuesday. This last trough moves through the ne USA on Wednesday.

Temperatures: Calendar day averages should be 20-25 degrees
above normal on Friday (near record warmth in some areas), and
then turning much colder Saturday with a calendar day average 5
degrees below normal, then even colder with average temps 10 to
15 degrees below normal Sunday and Monday. A possible snow
event next Tuesday and Wednesday, softens the cold a
bit...somewhere several degrees below normal.

Forecast basis: Unless otherwise noted, a 50 50 blend of of the
12z/10 GFS/NAM was used for Thu night-Fri night, the 12z/10 GFS
MEX MOS for Saturday, and the 15z/10 WPC gridded elements of 12
hr pop, max/min temp, and 6 hourly wind. sky, dewpoint for next
Sunday- Wednesday. I did modify all guidance significantly
colder Saturday through Monday, per the much colder ECMWF
(northerly isobaric gradient is excellent delivery)

The dailies...

Thu night...waa rain with rising temps late at night. How much
rain and how fast it warms remain a bit undecided. Delayed the
start about 6-9 hours from the prior forecast but it should be
raining at a pretty good clip toward dawn Friday. Low
temperature is near sundown Thursday and the very cold Delaware
Bay may further force our NJ-Delaware Valley temps to be lowered
2-4 more degrees than we have forecast at 330 PM today.
Southeast wind becoming south to southwest late. Confidence:
still above average on the scenario despite slowing the rain

Friday...Near record warmth during the day (ABE, MPO, GED, RDG
are particularly vulnerable) ice developing late Friday night
near and north of I80! Looks like the cold front drifts to near
I-95 with a cooling northerly flow developing at night to its
west and an unseasonably warm southerly flow to its east.
Details to be determined. Rain will occur, but as noted by the
previous forecaster, it could be a band or bands of showery
rains early, followed by a more dynamic waa pattern of
increasingly strong fluxes when the southern stream short wave
and low move northeast, with most of this renewed rain at night.
Most may not notice...periods of rain and drizzle would suffice
but a 10 hour break is possible in the aftn-evening. The 06z
NAM and 09z/10 SREF have ice indications toward dawn Saturday
near and north of I-80. Confidence: above average.

On this forecast: I think my freezing rain amounts are too high
and there should be more snow or sleet in this forecast. I think
tomorrow our top down will yield more snow-sleet than ice,
presuming pcpn lingers long enough.

Thunder is possible Friday or Friday night but as our previous
fcstr noted, not enough confidence to introduce to the grids?

Saturday...Sharply colder? Rain probably ending early in the
day (except ice or snow ne and ecentral PA and far nw NJ ).
Warmest temperatures of the day prior to 7 AM. Much much colder
Saturday night with partial clearing. Wind becoming northwest by
days end.

Wind chill may drop to 15 below in the Poconos Saturday night.
Not enough time to collaborate even colder temps than currently
fcst. Confidence:

Sunday...Fair and cold! Northwest wind gusty 20 mph.
Confidence: above average.

Monday...Sunny, then increasing high clouds late in the day or
at night. Light wind. Confidence: a little above average

Tuesday and Wednesday...Interesting...a seemingly weak separation
of short waves may end up being a a decent Alberta clipper
redeveloping along the Delmarva and yield a light to moderate
snowfall event in parts of our area. Probably worth monitoring
if you have travel plans next Tuesday or Wednesday. I did raise
guidance POPS 10-20%. Impressive short wave. Confidence:
above average.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight: VFR through tonight with winds under 10 knots. High

Thursday: Ceilings start to lower around sunrise with MVFR/IFR
ceilings for all terminals except KACY and possibly KMIV. Some
uncertainty is still present on how quick the ceilings drop and
how low. Right now most locations look to hover between MVFR and
IFR for most of the day. Winds will stay under 10 knots and any
areas of light rain look to hold off till later TAF periods.
Moderate confidence.


Thursday night...MVFR or IFR conditions in developing rain,
especially after midnight. Some fog development could occur
late. Southerly wind with possible LLWS late. Confidence:
above average

Friday and early Saturday...MVFR/IFR/LIFR conditions due to low
clouds, fog and periods of rain. The rain ends toward midday
Saturday (possibly as sleet or freezing rain at KRDG/KABE) with
conditions improving as a cold front moves off the coast. South
wind Friday shifts northwest Saturday. There may be some LLWS
on Friday?  Confidence: Above average.

Sunday...VFR for the most part. Northwest wind. Confidence:
above average.

Monday...VFR.  Light wind. Confidence: Above average.


Quiet tonight and Thursday on both Delaware bay and the coastal
waters. Waveheights will be around a foot or two into Thursday
but then rise a little late in the day as a cold front and low
pressure system begin to approach the region. Winds under 10
knots and an SCA is not anticipated through Thursday. High


Thursday night...Winds and seas increase later at night, and a Small
Craft Advisory has been issued. We may need a GLW for lower De Bay?
Fog may develop at night as warmer air arrives. Confidence:

Friday and Saturday...Small Craft Advisory Friday may need
extension into Friday night and Saturday. This period will start
with gusty southwesterly winds, with an abrupt shift to northwesterly
winds later Saturday. Since the airmass will be much warmer,
the stronger winds are not anticipated to mix down and therefore
gusts should remain below gale force. A long period se swell of
5 to 9 feet and period 8 to 9 seconds should develop. Fog may
be an issue for navigation through Friday night. Confidence:
above average.

Sunday...Small Craft Advisory conditions may linger for the morning.
Freezing spray. Confidence: average.

Monday...No marine advisories anticipated.


The nearly 2 weeks straight of sub-freezing temperatures has created
a build up 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).

For the Thursday night through Saturday timeframe, one to two inches
of rainfall is predicted across the region. Daytime temperatures are
expected to peak in the 50s or 60s for a couple of days. Those two
factors alone will begin to move some ice or at least begin the
melting process.

But, while it`s been cold, there hasn`t been much water
equivalent 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 banks to withstand
the volume of increased runoff rises.

As mentioned, for the next several days, 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 is
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 later in the weekend through next
week and will temporarily lessen the chance for any ice movement.


Just in case: for Friday January 12

Record high temperatures both Max and Min (note they occurred
last year!)

ACY 67-2017     49-2017

PHL 72-1890     49-2017

ILG 68-2017     46-2017

ABE 60-1932     43-2017

TTN 68-1890     47-2017

GED 70-1975     POR too short

RDG 63-2017     47-2017

MPO 56-1975     POR too short

record daily rainfall: Only noting values under 1.5 inches

ACY 1.19 - 1915

ILG 1.25 - 1900


Pottstown did not rise into the 50s today. We have a ticket on
the sensor and have stopped temperature reporting for this site.


MARINE...Small Craft Advisory from midnight Thursday night to 5 PM EST
     Friday for ANZ431-450>455.
     Small Craft Advisory from midnight Thursday night to 1 PM EST
     Friday for ANZ430.


Near Term...Gaines/MPS
Short Term...Gaines
Long Term...Drag
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