Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 170854
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
354 AM EST Sat Feb 17 2018

.SYNOPSIS...
High pressure overhead this morning shifts offshore this afternoon.
Low pressure rapidly intensifies off the mid Atlantic coast this
evening then races out to sea by Sunday morning. High pressure
briefly builds across the area again on Sunday, then offshore Sunday
night. A strong warm front lifts northward across our area later
Monday into early Tuesday, followed by a cold front late Wednesday.
High pressure builds to our north next Thursday with a complex
frontal system developing to our south and west at the end of next
week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
This section updates around 4 am.


The cold front is now well offshore. Skies have mostly cleared
thanks to the dry air advection in northwesterly flow. Tonight`s
lows are forecast to range from around 20 in the Poconos and in
far northern New Jersey to the lower 30s in southern Delaware.

The northwest wind should dry most roads and walkways before
temperatures fall below freezing. However, larger puddles will
remain and there could be some patchy ice in spots.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
This section updates around 4 am.

High pressure is forecast to be centered in eastern Pennsylvania
early on Saturday morning. The high is forecast to move eastward
passing off Cape Cod around midday. Dry weather is anticipated
for the morning with an increase in high clouds ahead of our
next weather system.

Precipitation is expected to spread into our region from the
southwest during the mid to late afternoon hours. Temperatures
should favor the upper 30s and the lower 40s in our region at
the onset. As a result, the precipitation will begin as rain at
most locations.

As we lose daylight on Saturday evening and as precipitation
rates increase, the precipitation should change to all snow
except in our far southern counties. The precipitation rates are
expected to be enhanced by some short wave energy that is being
ejected out of the southwestern states.

We are anticipating a relatively brief but robust precipitation
event for late Saturday into Saturday night. The bulk of the
precipitation is forecast to fall over an 8 hour period. The
surface low is expected to move rapidly from around Cape
Hatteras on Saturday evening to a point about 250 miles
southeast of Cape Cod near daybreak on Sunday.

Snowfall totals are expected to range from 2 to 5 inches in
southeastern Pennsylvania, the Lehigh Valley, the Poconos, and
northern and central New Jersey. Because of the potential for
mesoscale banding, expect some locations within the watch area
will see 6 inches or more. However, it is still far too
uncertain to pin down where the banding will take place. Thus,
have held on to the winter storm watch. An advisory may be
needed for another tier or two of counties to the southeast of
the current watch area as well, but in those areas there is
considerable uncertainty as to how early and how long we will
see a change over to snow.

A fairly tight snowfall gradient is forecast to extend across
the upper Delmarva and southern New Jersey. The event should be
mostly rain in Sussex County, Delaware and in Cape May County,
New Jersey.

The precipitation is anticipated to end quickly late on
Saturday night. A mid level short wave trough that was located
over British Columbia this afternoon will race to the east and
it should pass over our region at that time, kicking the last of
the snow and rain out to sea.

A light and variable wind on Saturday morning should settle
into the southeast on Saturday afternoon. The wind is forecast
to become variable again on Saturday evening before shifting to
the northwest around 5 to 10 MPH late on Saturday night. Low
temperatures on Saturday night are anticipated to be in the 20s
and lower 30s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
**Record warmth possible Tue and expected Wed**

500 MB: a very strong ridge develops along the east coast during mid
week, then slowly weakens Friday and next weekend as a positive
tilt trough develops across the United States.

Temperatures: February through the first 16 days is averaging 3 to 6
degrees above normal and this weeks record warmth will likely make
this February a top 10 warmest in our Mount Holly area history.
I still need to confirm the month around 6 am when we`ll sample
Philadelphia for the month in our climate section. In the mean
time...the warming trend begins in earnest on Monday with
calendar day averages 3 to 7 degrees above normal, then 20 to 25
degrees above normal Tuesday and maybe 25 to 28 degrees above
normal Wednesday, cooling to between 15 and 20 above normal
Thursday and still about 10 degrees above normal next Friday
and Saturday.

Forecast basis:  Except as noted generally a 50 50 blend of the
00z/17 GFS/NAM MOS was applied for Sunday-Monday, the GFS MEXMOS for
Monday night-Tuesday and then the 06z/17 WPC D4-8 12 hr elements of
max/min temp/pop and 6 hrly sky/wind/td were used Tuesday night-
Friday.

The dailies...

Sunday...A nice day. Seasonable. Gusty northwest winds in the
morning to 20 mph quick diminish by midday as the high moves
overhead. Confidence: Well above average.

Sunday night. Frosty with light wind and generally clear sky
atop the melting snow and remaining moisture from the Saturday
night event. Confidence: Well above average.


Monday...A p/sunny start quickly becomes overcast with a period
of rain during the afternoon or evening in eastern PA but lesser
chances elsewhere. South wind. Still sort of cool due to
skycover and little in the way of mixing. Confidence: Well
above average.

Monday night...after rain ends, areas of dense fog possible late
at night. Confidence: average.

Tuesday...Stratus probably limits potential warmth. It becomes
partly sunny during the afternoon. Record max temps possible,
especially Georgetown DE. Confidence: average.

Wednesday...Partly sunny and very warm! Widespread records
likely. Mid 70s. Potential for equaling record warm Feb max
temp. See climate section at 6 am for the records. Chance of
showers late day or evening with the cold frontal passage.
Confidence: Above average.

Thursday and Friday...Considerable cloudiness. chance of showers.
Confidence: Below average on both rain and max temps because of
model differences.

The primary scenario for Thu/Fri as outlined ydy at 330 PM by
our previous long term forecaster: Thursday through Friday the
cold front pushes to our south while high pressure builds to our
north. Thursday should be mostly dry except across the far
south where there could be some lingering lift/moisture behind
the cold front that may combine to bring some light rain to the
area. Friday has the better chance of rain as return flow
develops as the high to our north builds offshore, and a warm
front may lift across the area at the same time a couple of
short waves/vorticity impulses do as well.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

This section updates around 4 AM.

Through Saturday...VFR through early/mid afternoon as clouds lower,
then MVFR/IFR conditions develop after about 20z from southwest to
northeast as snow and rain arrive. Snow is expected at the
terminals, except MIV and ACY where it will mix with rain before
changing to all rain. Winds becoming southeasterly less than 10
knots.

Saturday night...Widespread precipitation including some moderate to
heavy snow at times. IFR or lower conditions, especially with
heavier snowfall rates. Some rain may mix in up to the PHL metro.
Snowfall rates of up to 1-inch per hour possible and this is when
accumulations would be the most efficient. Snow/rain ends quickly
after 06z from southwest to northeast with conditions improving to
VFR by daybreak. High confidence regarding precipitation occurring,
but low confidence on a wintry mix into the I-95 terminals. Light
and variable winds overall, becoming west-northwest overnight less
than 10 knots.

OUTLOOK...
Sunday-Sunday night...VFR. Northwest winds may gust around 15-20
knots Sunday morning-midday. Confidence: Well above average.

Monday...VFR early, lowering to MVFR or IFR during the afternoon or
evening with rain. South to southwest wind. Confidence: above
average.

Monday night...MVFR/IFR conditions continue with rain possible in
the evening, and low clouds/dense fog possible overnight.
Confidence: Above average.

Tuesday...MVFR/IFR conditions possible through midday in low
clouds/fog. Conditions improving to VFR during the afternoon.
Southwest winds may gust around 20-25 knots in the afternoon.
Confidence: average.

Tuesday night...VFR early, then possibly lowering to MVFR/IFR
as low clouds/fog may develop overnight. Confidence: average.

Wednesday...MVFR/IFR conditions possible early with low
clouds/fog. Conditions improving to VFR during the day. chance
of showers late in the day. Southwest winds may gust around
20-25 knots. Confidence:Above average.

&&

.MARINE...
This section updates around 4 am.

The Small Craft Advisory for Delaware Bay has been cancelled as
winds continue to diminish.

High pressure is expected to follow for overnight and Saturday
morning. Low pressure is anticipated to pass off the coast near Cape
Hatteras on Saturday evening.

A gusty northwest to north wind is occuring in the wake of a cold
front. As a result, a Small Craft Advisory remains in effect until
4:00 AM Saturday for the Atlantic coastal waters. Conditions should
remain below the Small Craft Advisory criteria for Saturday and
Saturday night. The area of low pressure on Saturday night is
expected to remain well to the south and east of our coastal
waters.

OUTLOOK...
Sunday..may need a brief SCA for nw winds gusty 20-25 kt early
in the day then sub advisory nw winds in the aftn.
Confidence: average.

Monday...Sub Small Craft Advisory south winds.  Confidence:
Well above average.

Monday night...Sub Small Craft Advisory conditions expected,
although winds may gust around 20 knots.  Confidence:
Above average.


Tuesday-Wednesday...Winds should stay below advisory levels,
but seas may increase to near 5 feet. Confidence: Average.

&&

.CLIMATE...
Record high temperatures could be challenged on Tuesday,
February 20th and probably will be equaled or exceeded
Wednesday February 21st. These records are listed below.


February 20:

Atlantic City...70 in 1930

Philadelphia....70 in 1939

Wilmington......71 in 1930

Allentown.......68 in 1930

Trenton.........70 in 1930

Georgetown......68 in 2002

Mount Pocono....59 in 1930

Reading.........72 in 1930



February 21:

Atlantic City...74 in 1930

Philadelphia....72 in 1930

Wilmington......70 in 1953

Allentown.......67 in 1953

Trenton.........70 in 1930

Georgetown......71 in 2014

Mount Pocono....60 in 1930

Reading.........71 in 1930


More records will post by 6 am.


Atlantic City rainfall already ranks 11th wettest in February
history with 6.50" in 2010 the wettest. By sunrise Sunday,
Atlantic City should rank about 8th wettest in its historical
database.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST
     Sunday for PAZ054-055-060>062-070-071-101>106.
NJ...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST
     Sunday for NJZ001-007>010-012-013-015>019.
DE...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST
     Sunday for DEZ001.
MD...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST
     Sunday for MDZ008.
MARINE...None.

&&

$$
Synopsis...Drag 353
Near Term...Iovino
Short Term...Iovino
Long Term...Drag 353
Aviation...Drag/Gorse 353
Marine...Drag/Gorse 353
Climate...353



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