Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 260411
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1111 PM EST Sun Feb 25 2018

.SYNOPSIS...
A weakening cold front slips southeast through our area overnight.
Strengthening high pressure is then forecast to build into the Mid-
Atlantic states Monday and Tuesday. A cold front will drop down into
the Mid-Atlantic states late Wednesday night while low pressure
develops over Illinois. That low slides to the Mid-Atlantic coast by
Friday morning, then intensifies into a strong storm over the
western Atlantic by Friday night before heading slowly out to sea
next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
1115 pm update: Expanded dense fog advisory to the Maryland
Eastern Shore, as KESN and KW29 are now reporting 1/4-1/2 mile
visibilities.

1030 pm update: Issued dense fog advisory for most of eastern
Pennsylvania (outside of the Philadelphia metropolitan area), as
surface observations are indicating widespread quarter to half
mile visibilities in much of this area. For now, issued through
4 am given the latest hi-res model soundings, but timing of
improvement is very uncertain.

Previous discussion...

An upper air analysis early this evening showed a closed low near
James Bay Canada with a trough axis extending southwestward from it
to the southern Plains. Southwesterly flow is across the East with
an elongated short wave exiting the northern Mid-Atlantic coast. A
strong 250 mb jet is located from northern New England to the
eastern Great Lakes then back into the Mid-Mississippi Valley. At
the surface, a weakening cold front is moving through the Northeast
and Mid-Atlantic region which then stretches southwestward to
coastal Texas.

The 00z raobs from Sterling, VA and Upton, NY still show a rather
strong low-level inversion in place. There is notable drying though
between about 850 mb and 500 mb. There is little in the way of wind
beneath this inversion, therefore the moisture remains trapped. The
guidance does show some dry air advection developing overnight, with
this more pronounced toward morning. Once the weakening cold front
moves through though, the inversion should weaken allowing the
moisture to start dissipating. While we continue to show a gradual
decrease in lower clouds from northwest to southeast overnight, the
aforementioned 250 mb jet will keep mid/high level clouds streaming
overhead. This all complicates the fog forecast. There is fog
already around with it most notable in the more sheltered areas and
in the higher terrain of the southern Poconos. Lots of moisture in
the low levels with a lack of mixing will support fog and this will
probably expand for a time although less confident about widespread
dense fog. The fog should start dissipating toward morning as more
notable low-level drying is forecast to occur. For now, expanded the
mention of fog for awhile tonight.

For the 930 PM update, made some mostly minor adjustments to the
temperature, dew point and wind grids over the next several hours,
starting first with the latest observations. Temperatures should
tend to hold fairly steady in the presence of stratus, with a better
drop occurring toward morning as ceilings rise and thin some.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
While a front will be to our east and south, the gradual eastward
movement of a 250 mb jet will keep mid/high level clouds in place
for awhile. For far southern New Jersey and Delmarva thicker clouds
look to remain for much of the day, especially as a ripple of energy
slides to our south along the front (dry forecast as any rain should
remain to our south). Overall though, sunshine (yes sunshine!) will
gradually be on the increase from northwest to southeast.

There should be enough low-level dry air advection to start the
morning, resulting in any fog quickly dissipating. Maximum
temperatures will be in the 50s with a northwest wind increasing to
around 10 mph.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
We added a coastal flood section this afternoon as this
initially is the primary concern for Thursday-Saturday.

We are also commenting on the late week event in the hydro
section.

Hazards: None explicitly at this time but a few hazards are likely
with the potential for a powerful storm in the Thursday to Saturday
time frame. These potential hazards include a highly probable
coastal flood hazard, severity unknown but potential exists for
moderate coastal flooding along a portion of the Atlantic coast
with multiple high tide cycles of minor tidal inundation flooding
late Thursday into Sunday mornings high tide cycles; Friday-Saturday;
a small stream hydro hazard, mostly in NJ and PA, still a chance
vertical motion from an unusually strong storm can change rain
to a period of wet accumulative snow (WXA hazard) for the
elevations ne PA and nw NJ late Friday; a gale event, the first
2 hour or longer gale since Feb 5, is expected but the exact
time is not yet known. Storm force gusts are possible.

500 mb: east coast ridging follows the departing mid Atlantic
coast short wave Monday night and continues ridging into
Wednesday. Then a weakening closed low in the desert southwest
Tuesday night starts reenergizing in the Ohio Valley Thursday,
becoming a large and strong cyclonic circulation system along
the mid Atlantic coast Friday, and slowly weakening seaward next
weekend.

Temperatures: Please see the climate section for updated February
climate stats, which I`ll update further around 430 PM (pcpn
stats). Calendar day averages should be 5 to 10 degrees above
normal Tuesday, Wednesday and Thursday, then big differences
appear between the EC and GFS for Friday-Sunday. Have chosen to
run with the cooler blend of the 12z EC/15z WPC guidance and so
have 5 to 10F above normal Friday and generally less than 5F
warmer than normal for the weekend. This is decidedly cooler
than the the 12z/25 GFS OP cycle.

Forecast basis: Unless otherwise noted above a 50 50 blend of
the 12z/25 GFS/NAM was used for Monday night-Tuesday night, the
12z/25 MEXMOS for Wednesday, and the 15z/25 WPC 12 hr forecast
max/min temps and POP, as well as their 6 hourly sustained wind,
skycover and dewpoint from Thursday through Sunday. Did maintain
our prior higher pops for Wed night/Thu, and Friday night...
continuity.

Models: The 12z/25 op GFS and UKMET look a little warm and
tracking too far north but they are part of the ensemble and
must be considered.

Please see this Sunday afternoons WPC D4-5 QPF and their D6
winter wx graphic to gain some appreciation of the potential
ahead.

The dailies...

Monday night...Clear. Northwest wind gusts 15 to possibly 20
MPH early as instability transfer deepens with cold air advection
aloft, tending to decouple late. Confidence: Well above average.

Tuesday...Sunny and beautiful! Light west wind becomes southwest
late in the day. Confidence: Well above average.

Wednesday...Partly sunny (lots of cirrus) and a few degrees warmer.
Southwest wind may gust 20 mph in the afternoon. Confidence:
Well above Average.

Wednesday night-Thursday night...In association with either a
cold frontal passage or a warm front extension east-
southeastward from the developing Midwest low pressure system,
rain will be developing either late Wednesday night or more
likely by midday Thursday. It may become heavy Thursday
afternoon and night. Winds becoming east and gusty 15-25 MPH in
the afternoon with rapidly falling pressure. Confidence: Above
average on scenario but nothing locked on storm track and
confidence below avg on any snowfall higher elevations of our
northern counties.

Friday into Saturday morning...Overall, probably gusty northerly
winds and periods of precipitation. As of now, rain most of
Friday and thereafter probably periods of showery rain, but
potential exists for some elevation snow. Confidence: average on
details, contending with a latitude tracking problem on what
looks to me to be a powerful (980-987MB) storm s of LI.

Sunday...Partly cloudy. North northeast wind gusty 15-25 MPH.

&&

.AVIATION /04Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...IFR/LIFR ceilings, improving to MVFR then to VFR
overnight. The timing of improvements is low confidence.
MVFR/IFR (locally LIFR) visibilities due to fog, however some
drier air toward daybreak should start to limit or dissipate the
fog. Winds light and variable to calm, becoming locally light
from the northwest late.

Monday...Local MVFR possible early, otherwise VFR. Winds northwest
around 10 knots.

Outlook...
Monday Night...VFR Clear. Northwest wind gusting 15 to possibly
20 kt early, diminishing late. Confidence: well above average.

Tuesday...VFR, virtually clear. Light west wind becomes southwest
late. Confidence: well above average.

Wednesday...VFR mostly mid and high cloud. Southwest winds may
gust 20 kt during the afternoon. Confidence: Above average.

Thursday...Conditions should deteriorate to MVFR or IFR in
rain. East to southeast wind and probably gusts 15-25 kt in the
afternoon. Confidence: Above Average.

Friday...Quite a bit of uncertainty with the latitude of a strong
western Atlantic low the determining factor on conditions. Conditions
can range from VFR cigs to IFR in rain/drizzle. Probably north
winds gusty 20-30 kt. Confidence: below average.

&&

.MARINE...
The conditions are expected to be below Small Craft Advisory
criteria through Monday. Some fog may occur particularly nearshore
and especially across the northern atlantic coastal waters
Light north to northwest winds tonight, then northwest on
Monday generally less than 15 knots.

Outlook...
Monday night...We may need an SCA for nw wind gusts to 25 kt in
the northern NJ waters (ANZ450-51) early?. Confidence: below
average.

Tuesday through Wednesday...Sub-advisory conditions are expected on
the area waters with light west winds Tuesday morning becoming
southwest Tuesday night and Wednesday, gusts under 20 kt.
Confidence: Average.

Thursday...An easterly SCA likely with a possible gale for the
NNJ waters. Confidence: Average.

Friday...Quite a bit of uncertainty with the latitude of a strong
western Atlantic low the determining factor on conditions. Certainly
SCA with potential for a northerly Gale , especially NNJ waters.
Confidence: below average.

Special note: This big western Atlantic storm is likely to drive an
increasing northeast swell of 10 to 15 ft toward the NJ and DE coasts
this weekend, exception 5 to 10 FT off Monmouth County sheltered a
bit by Long Island. This could affect marine operations next
weekend, especially at the entrance to Delaware Bay.

&&

.HYDROLOGY...
A large and strong storm will affect our area Thursday into
Saturday. The bulk of the rain is anticipated Thursday into
Friday with widespread 1-2" amounts expected in eastern PA and
NJ, particularly near and north of I-78. If the track of the low
permits, there could be significant backlash precipitation late
Friday but at this time, we are not forecasting that option.
Therefore, we may need to deal with some small stream flooding
in northern NJ and and east-central/northeast PA later Friday
if we receive basin average rainfall of nearly 2 inches. No
action is needed at this time and we`ll see if the models remain
similar through the week. Its easily possible that we`ll be
able to escape any significant small stream flooding, especially
if the storm tracks further north than now forecast. So, while
the MMEFS outlook is significant, we are opting for lower than
these outcomes, especially based on recent performance history,
in part due to a slight high bias to the qpf.

Suggest all interests monitor future forecasts issued Monday-
Thursday for information trends and any office briefing packages.

&&

.TIDES/COASTAL FLOODING...
Low pressure is forecast to approach from the west and it should
pass off the Middle Atlantic coast around Thursday night. The system
is expected to strengthen as it moves slowly out to sea on Friday
and over the weekend. It is anticipated to have some impact on the
coasts of New Jersey and Delaware at that time.

Tides will already be running high due to the full moon that will
occur on Thursday. Heavy surf and the potential for a developing
strong northeast wind may result in some coastal flooding along with
beach erosion.

Based on the latest forecast guidance, it appears as though there
may be some minor coastal flooding on Thursday evening, mainly along
the northern and central parts of the New Jersey coast. The
potential for widespread minor flooding exists from late Friday into
early Sunday along the entire New Jersey and Delaware coast and into
Delaware Bay and the tidal Delaware River. Depending on the system`s
development, some moderate flooding could occur. However, the event
is several days away and the forecast details are subject to change.

If you live along the coast or along Delaware Bay, please pay close
attention to the forecast over the coming days.

&&

.CLIMATE...
**Top 3 warmest February on record and top 10 wettest February
on record.**

February projected climate ranking as of 7 am today-Sunday Feb
25, based on our mid shift fcst through the 28th and mins this
Sunday morning; and for rainfall, amounts 7 AM today. RDG and TTN
not included due to too much missing data.

ABE #3 warmest 38.0 or 7.3F warmer than the 30.7 norm.
39.2-2017
38.6-1998
38.0-2018
36.8-1954

ACY #1 warmest 42.8 or 7.5F warmer than the 35.3 norm.
43.0 -2017
42.8 -2018
40.6 -1954

ILG #3 warmest 41.2 or 6.2F warmer than the 35.0 norm.
43.1-2017
42.3-1903
41.2-2018
41.2-1976

PHL #3 warmest 42.3 or 6.6F warmer than the 35.7 norm.
44.2-2017
42.3-2018
42.2-1925
41.8-1998
41.4-1890

Water equivalent February pcpn as of 4PM today. These rankings
below are the lowest they will be for the month and possibly the
final rankings for February.

PHL ranked #5 with 6.02". All time 6.87-1896

ILG ranked tied #4 with 6.00". All time 7.02-1979

ABE ranked #4 with 5.50". All time 7.62-2008

ACY ranked #2 with 6.43". Wettest is 6.50-2010.

MPO 4.71
RDG 5.53
TTN 5.52
GED 3.97

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Dense Fog Advisory until 4 AM EST Monday for PAZ054-055-
     060>062-101>103-105.
NJ...None.
DE...None.
MD...Dense Fog Advisory until 4 AM EST Monday for MDZ008-012-015-
     019-020.
MARINE...None.

&&

$$
Synopsis...Drag
Near Term...CMS/Gorse
Short Term...Franck/Gorse
Long Term...Drag
Aviation...Drag/Gorse
Marine...Drag/Franck/Gorse
Hydrology...
Tides/Coastal Flooding...
Climate...



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