Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 232231
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
531 PM EST Thu Feb 23 2017

.SYNOPSIS...
A cold front will move into the Northeast tonight before stalling,
then moving northward as a warm front Friday night. A strong cold
front will move through the Mid Atlantic on Saturday. High pressure
will pass through the region Sunday and Monday. A front will set up
near the area for much of next week, with several lows expected to
move along it. A strong low will move through the Great Lakes and
southeast Canada next Wednesday and Thursday, which will pull
another strong cold front through the region by late in the
week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure will continue to remain anchored over the western
Atlantic Ocean. Persistent return flow around the backside of
the high has resulted in another unseasonably warm day across
the region. Temperatures this afternoon are in the mid/upper 60s
in northeastern PA and northwestern NJ, mid 70s in central
Delmarva and low 70s in between (except much cooler in upper
50s-mid 60s at the beaches due to the influence of the colder
ocean waters). High temperature records were set today at
several of our climate sites (see RERs and climate section below
for more information).

A cold front was analyzed well to our west across the eastern Great
Lakes and Midwest region this afternoon. Recent visible satellite
imagery shows a CU field developing out to our west in the pre-
frontal warm sector as well as across the higher terrain in the
southern Poconos. 20Z mesoanalysis from SPC shows 250-500 J/kg
of SBCAPE over the area, but LAPS soundings show that the
atmosphere is capped by a mid-level subsidence inversion. Hi-res
CAM models are in good agreement, showing pre-frontal
convection becoming more robust just upstream of us after sunset
this evening across the higher terrain as lift strengthens and
the cap is temporarily broken. Showers may become increasingly
organized along a pre-frontal trough in our far northwestern
zones of the CWA (RDG-ABE-MPO-FWN north and west) during the mid
to late evening hours. A slight chance for thunderstorms was
also mentioned for these far northwestern zones given the
potential for slightly elevated convection to tap into 400-700
J/kg of MUCAPE. This activity will weaken overnight.

Debris clouds from this evening`s convection contribute to forecast
uncertainty regarding the development of stratus/fog overnight. This
cloud cover would likely be more extensive north and west of I-95
(closer to the showers), which will like at least delay stratus/fog
development. However, the clouds may dissipate by early morning,
allowing for at least patchy dense fog to develop closer to sunrise,
particularly where it rains. The S-SWly wind field at the top of the
boundary layer looks slightly stronger tonight compared to last
night, which favors more stratus vs. fog compared to last
night. Nonetheless, there is a potential for dense fog again
late tonight in most of the region, but confidence in it
becoming more extensive than just patchy is not high enough to
issue a Dense Fog Advisory this far out.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
The cold front will dissipate inland by the morning. Similar to
today, the day will start with low clouds and fog. The low-level
flow is forecast to be ever-so-slightly backed tomorrow (Sly)
compared to today (S-SWly), which may allow the stratus to hang on
an hour or two later into the day than today. Regardless, there
looks to be an opportunity for temperatures to spike to near record
levels in the mid to late afternoon hours with clearing skies during
peak heating. Trended warmer for tomorrow (closer to the RGEM),
yielding highs in the low to mid 70s south of I-78 and mid to upper
60s farther north.

&&

.LONG TERM /FRIDAY THROUGH THURSDAY/...
The long term period looks active, as several systems are likely to
affect the area through the middle part of next week.

At 00Z Saturday, a strong vorticity maximum will approach the Great
Lakes and Ohio Valley, with a 140-kt 250-mb jet streak positioned
across much of the southern U.S.  As the nose of this jet enters
difluent flow near the Atlantic Coast, considerable upper-level
divergence will collocate with differential cyclonic vorticity
advection downstream of the aforementioned vort max.  Strongly
forced convection will develop along an associated cold front in the
Midwest moving east into New York and Pennsylvania by late
Saturday morning. A downstream jet streak will develop in the
high-amplitude ridging nosing into eastern Canada, providing
additional lift via jet coupling to produce widespread
precipitation in Quebec, New York, and adjacent New England
Saturday afternoon. As the vort max impinges upon the high-
amplitude ridging, it will make a shift northward, leaving our
region on the southern fringes of the stronger dynamics/lift and
well within the warm sector of the system.

Given sufficient diabatic heating, marginal boundary-layer based
instability is expected by afternoon in our area, which may act to
compensate for the somewhat weaker large-scale ascent.  Vertical
wind profiles will be quite strong, with deep-layer shear 40-60
knots. Unidirectional wind profile and orientation parallel to the
initiating boundary will support a line of convection moving through
the area during the afternoon. Strength of the wind profile and
steep lapse rates near the surface suggest isolated damaging wind
gusts are possible as the line moves through the region.  The key to
the extent of this threat will be the degree to which the boundary
layer destabilizes during the day and the ability for updrafts to
withstand the strong vertical wind profile -- the usual players in a
high CAPE-low shear environment.  Today`s simulations suggest there
will be a window of at least partial surface heating just downstream
of the cold front, with MUCAPE near 500 J/kg during peak heating.
Maintained a threat of thunderstorms in the grids for Saturday
afternoon. Models have sped up the timing of frontal passage a
tad, and reflected this in the grids somewhat by increasing PoPs
a bit earlier in the day and decreasing them a bit more
Saturday evening/night.

Temperature forecast on Saturday is a big challenge, as sky cover,
precipitation, and timing of the frontal passage are all highly
complicating issues.  I went pretty close to statistical guidance,
which was generally in good agreement despite the complex scenario.
Nevertheless, there is large uncertainty and potential for error for
highs Saturday, which currently are around five degrees lower than
the values in place for Friday.

A sprawling but progressive high will move in behind the front for
Sunday and Monday, so a period of dry weather and much cooler
(though still near to slightly above average) temperatures are
forecast during this period.

By Monday afternoon, model agreement becomes quite poor.  These
large discrepancies exist owing to an unusually high degree of
spread associated with an upper-level low digging off the California
coast this weekend and progressing eastward early next week.  The
GFS is very quick to move a perturbation from this upper low into
the central plains by 00Z Monday and to the Mid-Atlantic States by
00Z Tuesday.  Widespread precipitation breaks out on the East Coast
Monday night in advance of this system.  The ECMWF, meanwhile, is
much weaker with this perturbation and basically shows no
precipitation at all with its passage, but is more aggressive in
breaking out scattered/light precipitation on Tuesday
afternoon/evening as isentropic ascent increases downstream of a
stronger system that develops in the central plains into the Great
Lakes by midweek.  As the low rapidly develops and lifts into
southeast Canada by Thursday, a cold front sweeps through the East
Coast Wednesday night and Thursday, bringing widespread
precipitation through the area. Meanwhile, the GFS moves this
low/associated precipitation mostly to the north of the area.  Other
guidance provides little insight, as the CMC becomes a strong
outlier with the depth/orientation of the longwave trough beyond
this weekend, and the ensemble guidance shows that the GFS/ECMWF
solutions are nearly equally plausible.

As a result, broadbrushed PoPs/sky cover/temps/winds Monday night
through Thursday, with generally slight chance to chance PoPs during
this period.  There is at least some chance for a mix of rain and
snow in the southern Poconos Monday night if the GFS solution pans
out.  Then again, if the ECMWF solution is closer to reality, there
may be no precipitation at all... Temps during this period should be
well above average.

&&

.AVIATION /22Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions thru the remainder of the afternoon and into
this evening for most locations. However, convective showers and
perhaps an isolated thunderstorm may organize ahead of a cold
front this afternoon, possibly reaching as far southeast as RDG-ABE
during the late evening or overnight (02Z-06Z). We may need add
a TEMPO group in there for RDG and ABE (currently advertised as
PROB30).

Another round of stratus and fog is likely late tonight (after 06Z),
with the potential for LIFR conditions with 2-3 hours on either side
of sunrise. Confidence is low for the formation of stratus and fog
but low-medium for coverage and magnitude of the fog. It could
certainly become dense with vsby near 1/4SM early in the morning.

S-SW winds around 10 kt this afternoon will become light tonight and
Friday morning, before increasing again to around 10 kt with gusts
to 15 kt during the afternoon.

OUTLOOK..

Friday night...Sub-VFR conditions possible with areas of low clouds
and fog.

Saturday and Saturday night...Line of showers/storms likely will
move through the area during the day, with gusty/erratic winds and
sub-VFR conditions likely during passage.  South winds may gust at
times above 20 kts before frontal passage.  Winds will rapidly
switch to west or northwest after the front passes, with gusts above
20 kts likely. Rapid improvement to VFR after frontal passage.

Sunday...VFR with winds northwest 10-20 kts with gusts to 30 kts
possible.

Sunday night and Monday...VFR with winds near/below 10 kts becoming
southerly by Monday.  Conditions may become sub-VFR late in the day
as another system approaches, but this is very low confidence.

Monday night and Tuesday...Potential for sub-VFR conditions with
precipitation, low ceilings, and reduced visibilities possible at
times. Very low confidence.

&&

.MARINE...
No marine hazards expected this afternoon and evening. A S-SW wind
around 10-15 kt this afternoon and evening will weaken overnight and
into the morning.

Another round of dense fog may occur over the waters either late
tonight or early Friday morning. After coordination with neighboring
offices, we held off on a marine dense fog advisory due to
uncertainty in when the dense fog forms and how widespread it will
become. There is a potential for the dense fog to persist into the
daytime a few miles off the Atlantic coast.

OUTLOOK...

Friday night through Saturday night...Fog may again develop on the
coastal waters, with at least some potential for visibilities less
than one mile, as very warm/moist air remains over the relatively
cold waters.  A line of storms will likely move through Saturday
afternoon and evening, with any fog rapidly dissipating as this
occurs.  Seas are expected to build to small craft advisory (SCA)
thresholds on Saturday, and winds will become westerly and become
gusty after cold frontal passage.  Gusts may approach gale levels
late Saturday night.

Sunday and Sunday night...SCA northwest winds continue but should
decrease below advisory thresholds Sunday evening.

Monday through Tuesday...Generally sub-SCA conditions expected,
though winds may approach advisory levels for a brief period Monday
afternoon.

&&

.CLIMATE...
An update to this section will be issued by 5 pm.

As of 5 PM...records were set at GED 75, MPO 63, RDG 73, ABE 72
As of 5 PM...records were tied ACY 72, ILG 73

For reference, here are/were the high temperature records for
today, Friday and Saturday (not including the new records
so far today).

Location    Thursday 2/23   Friday 2/24   Saturday 2/25

ACY         72-1985         75-1985       77-1930

PHL         75-1874         74-1985       79-1930

ILG         72-1985         78-1985       78-1930

ABE         71-1985         76-1985       74-1930

TTN         74-1874         74-1985       76-1930

GED         67-1985/1990    72-1961       76-1975

RDG         72/1932/1922    77-1985       77-1930

MPO         60-1977         60-1984       70-1930

Todays February rankings were updated at 515 PM. We`ll finish
the seasonal AT 6 pm.

The following are the monthly and seasonal expectations.

It is virtually certain that these values will be at or below
reality and that our forecast area is experiencing a record warm
February and a top 10 warmest winter.

Records date back to the late 19th century. Details below.

February:

PHL 43.9 #1  Normal 35.7 Record 42.2-1925 POR 1874

ABE 38.9 #1  Normal 30.7 Record 38.6-1998 POR 1922

ACY 42.9 #1  Normal 35.3 Record 41.6-1890 POR 1874

ILG 43.1 #1  Normal 35.1 Record 42.3-1903 POR 1895

Note for ABE: There is a pretty good chance ABE will end up
warmer and possibly very close to their monthly temp record.


Winter (DJF)

PHL 40.1 #7

ABE 35.8 #5 and solid. It wont slip.

ACY 39.6 #10

ILG 39.1 #6 tie

Past two years of monthly average temperatures through February
2017, a summary of above normal months listed below:

For ABE: 23 consecutive months of above normal temps!

FOR PHL: 22 of the past 23 months have been above normal.

For ACY: 19 of the past 23 months have been above normal. For ILG:
17 of the past 23 months have been above normal.

(Jan Feb March 2015 was the last time we had significant and
persistent below normal monthly temps.)


Snow:

Atlantic City should/could end up tied for 5th least snowiest
February on record.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...AMC
Near Term...Klein
Short Term...Klein
Long Term...CMS
Aviation...CMS/Klein
Marine...CMS/Klein
Climate...531P updated ABE record, and projected overall
February values and rankings.



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