Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
629
FXUS61 KPHI 161446
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
946 AM EST Fri Feb 16 2018

.SYNOPSIS...
A cold front will move slowly southeastward through our area
today. High pressure builds into the Mid-Atlantic states
tonight, then shifts offshore on Saturday. Low pressure develops
just off the Mid- Atlantic coast Saturday night and speeds away
early Sunday morning. High pressure briefly builds across the
area Sunday, then moves offshore Sunday night. A strong warm
front will slowly lift north of the Mid-Atlantic region early
next week, followed by a cold front late Wednesday and then high
pressure starts to build in on Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A cold front extended across western Pennsylvania and West
Virginia at mid morning. The front will progress to the east and
it should pass through out region late this afternoon.

There was an area of rain showers in Berks County, the Lehigh
Valley, the Poconos and northwestern New Jersey around 9:30 AM
with another area of rain showers in northeastern Maryland,
Delaware and far southern New Jersey. Not much precipitation was
occurring in southeastern Pennsylvania and central New Jersey.
The rain shower activity in our region may lessen a bit more
until about midday, then showers associated with the
approaching cold front should move into our region from the
west. Rain showers are then expected for much of the afternoon.

The wind is forecast to be out of the west and northwest around
5 to 10 MPH until the cold front arrives. The wind is expected
to settle into the northwest behind the cold front and speeds
will likely increase to 10 to 15 MPH.

Temperatures will remain mild until the cold front passes with
readings mainly in the 50s in our northern and central counties
and in the 60s in the south.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
Tonight...any leftover rain ends by 8 PM extreme se DE,
otherwise becoming clear and much colder with lows near normal
for this date! Gusty northwest winds 15-25 mph during the
evening becoming northerly and diminish after midnight.

This part of the forecast was a 50 50 blend of the 00z/16
GFS/NAM MOS blended with the colder EC 2m temps.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Summary...A quick moving but potent storm Saturday evening into
very early Sunday is expected to bring a swath of accumulating
snow; Turning much warmer for awhile early next week with
potentially some record warmth; Cooling for late next week.

Synoptic Overview...As a closed low remains nearly stationary
near Hudson Bay Canada through the weekend, a stronger short
wave will allow for a quick moving but potent Mid-Atlantic storm
late Saturday into very early Sunday. Given the progressive
flow, cold air will not follow this storm, then the pattern
shifts into a much more amplified one with a significant trough
in the West with a strengthening ridge in the East. This will
bring much warmer air to our region during the first half of
next week, however a cold front settles through later Wednesday
with some cooling in its wake.

For Saturday and Sunday...The guidance continues to show a
quick moving but potent system developing just off the Mid-
Atlantic coast later Saturday then quickly tracking
northeastward away from our area early Sunday. While the
guidance is in good agreement that precipitation will occur,
there continues to be more uncertainty with the precipitation
amounts northwestward and also the precipitation type especially
near and east of I-95. We start with clouds increasing during
the day Saturday and if these thicken rather quickly, surface
heating will be slowed. The airmass in place is forecast to be
chilly but also rather dry in the lower levels especially.

The guidance differs some with the large scale ascent to the
northwest of the surface low. This will be key in determining
not only the precipitation amounts but the rates. Some guidance
suggest that a ribbon of enhanced lift occurs associated with a
rapid increase in frontogenetic forcing between about 850 mb and
700 mb. This may become enhanced for a time Saturday night as
our region is within the right entrance region of a potent 250
mb jet. This will be a quick hitting storm, however there is the
potential for a swath of snowfall rates of near 1 inch per
hour. As of now, we are thinking this is near and just north and
west of I-95. However despite the surface high well east-
northeast of our area, some hints of cold air damming lingering
and this could be enhanced some due to wet bulbing effects.
Therefore, a thumping of snow may very well occur for parts of
our region namely near and west of I-95. Farther to the
northwest across our area, there looks to be less QPF but snow
to liquid ratios should be the highest in the vicinity of the
Poconos. Farther south and east across Delmarva and central to
especially southern New Jersey, enough warming should result in
a zone of some mixing (mainly rain/snow but some sleet is
possible briefly) with any snow going over to rain closer to the
coast. In between these zones, a wetter snow if enough
accumulates at a fast rate could result in some power outages.
We continue to not include any ice amounts as freezing rain may
be limited and this is much less certain given marginal surface
temperatures where it could occur.

Given the above, the quick hitting nature of this event and our
latest forecast snow amounts, we opted to go with a Winter
Storm Watch for Saturday night for all of eastern Pennsylvania
(except Delaware and Philadelphia counties) and northern New
Jersey. It is here where we have the highest snow amounts which
could approach warning criteria for some areas but this is
uncertain, as is the western fringe due to potentially less QPF.
Since there is the potential for some higher totals and
enhanced rates and to also help with our messaging efforts, we
felt a watch was a good way to go at this point. To the east of
the watch area, a tier of advisories will be possible. Our
precipitation types were derived from a blend of several models
with some adjustments then made.

The system quickly moves away early Sunday and all the
precipitation should be done by 5 AM. As the flow turns
northwest and increases some with drier air arriving, clouds are
expected to clear out for much of the area. The flow is
progressive however and therefore Sunday afternoon will be on
the milder side with plenty of snow melting occurring.

For Monday and Tuesday...A pattern change results in much more
amplified flow, with a large trough settling in across the West
while a ridge strengthens in the East. This allows for
significant warming in the East, with potentially some record
warmth on Tuesday.

High pressure offshore Monday will allow for a return flow
along with warm air advection. Low pressure in the Central
Plains allows a warm front to extent eastward Monday. This warm
front is forecast to be more pronounced with time aided by
downstream warm air advection. As this occurs, the bulk of the
model guidance shows an area of ascent with a warm air advection
burst which results in a swath of showers/rain Monday afternoon
before lifting northward. The warm front should work northward
Tuesday with possible showers with it. The extent of the showers
on Tuesday will depend on how quickly the warm front advances
northward and the associated lift shifts to our north. It is
possible that we start Tuesday with some areas of low
clouds/fog, however the overall pattern is rather warm and
therefore the warm front should progress steadily northward.
Based on the northward advancement of the warm front with at
least partial clearing, Tuesday afternoon could get quite toasty
for much of the area. It is during this time where some
locations could be near record warmth. Regarding the chance for
showers, we continued with a time of chance PoPs and Tuesday
could end up being dry as we get more into a true warm sector.

For Wednesday and Thursday...As an upper-level trough slides
eastward from the Northern Plains Wednesday, an elongated
surface cold front will be shifting eastward with some ripples
of low pressure riding along it. This boundary will be running
into a lingering ridge near and east of the southeast U.S.
coast, therefore the eastward progression of the front may be
slowed some. The northern side of the ridge however is forecast
to flatten some and this would allow the cold front to arrive in
our later Wednesday. The front may get hung up to our south
Wednesday night and Thursday as strong high pressure builds to
our north and a ridge remains across the Gulf Coast states. We
therefore continue with a mention of showers Wednesday (no
thunder at this time), and some slight chance to low chance PoPs
on Thursday as some guidance shows a wave of low pressure
riding the front. Wednesday is expected to be another rather
mild day ahead of the front and it could be warmer than
currently forecast, then cooling occurs at night and Thursday.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

This morning...Generally VFR with scattered light showers.
Light west to northwest wind.

This afternoon...Generally VFR CIGS in last bands of showers
asstd with possible MVFR VSBY, ending late in the day from nw to
se. Northwest wind increases considerably toward 22z when gusts
should become widespread 15-25 kt.

Tonight...VFR. Gusty northwest winds 15-25kt early become
northerly toward 09z/17 and max gusts diminish to 15 kt or less.


OUTLOOK...
Saturday and Sunday...Clouds increase and lower during
Saturday, then MVFR/IFR Saturday night through very early Sunday
morning as snow moves through. There could be some mixed
precipitation near and and east of the I-95 terminals, with any
snow or rain/snow mix going to rain at MIV and ACY. Some
accumulating snow is forecast for most terminals with the higher
amounts just west of the I-95 terminals. Conditions are
expected to quickly return to VFR Sunday as clouds clear. High
confidence on precipitation, lower confidence on where mixed
precipitation occurs.

Monday...MVFR/IFR possible as showers move through. Southwest
winds may gust to around 20 knots during the afternoon.

Tuesday...Possibly some low clouds and/or fog to start,
otherwise mainly VFR. Southwest winds may gust up to 25 knots
during the day.

&&

.MARINE...
SCA Atlc waters is marginal and mainly for seas today. De Bay
was delayed till later in the day.

Fog may be a marine hazard this morning.

Northwest wind gusts 25-30 kt should develop late this
afternoon or evening and continue much of tonight. Thereafter,
winds trend north and gusts diminish to 20 kt toward sunrise
Saturday.

OUTLOOK...
Saturday and Sunday...The conditions are anticipated to be
below Small Craft Advisory criteria despite low pressure
tracking to our south and east.

Monday and Tuesday...The conditions are anticipated to be below
Small Craft Advisory criteria overall, however southerly winds
start to increase late Monday through Tuesday. Seas may start to
build close to 5 feet on the outer waters late Tuesday.

&&

.CLIMATE...
Some record high temperatures could be challenged on Tuesday,
February 20th. These records are listed below.

Atlantic City...71 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

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Watch from Saturday evening through late Saturday
     night for PAZ054-055-060>062-101>106.
NJ...Winter Storm Watch from Saturday evening through late Saturday
     night for NJZ001-007>010-012-015.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ452>455.
     Small Craft Advisory until 6 AM EST Saturday for ANZ450-451.
     Small Craft Advisory from 4 PM this afternoon to 6 AM EST
     Saturday for ANZ430-431.

&&

$$

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.