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Office: PHI

FXUS61 KPHI 250552 PAA
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1252 AM EST Sun Feb 25 2018

.SYNOPSIS...
A warm front will slowly lift northward into southern New Jersey
Sunday morning, where low pressure will develop on this front and
move to eastern Long Island by evening. The trailing cold front may
stall in the Carolinas on Monday awaiting the passage of another
wave of low pressure to our southeast. Strengthening high pressure
builds into the Mid-Atlantic states Tuesday. A cold front will drop
down through the region on 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 Friday night before heading only slowly out sea
over the weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
A band of rain is moving through Delaware Bay and southern New
Jersey at this time, with fairly decent rain rates with the
embedded convective showers. While traversing Delmarva, a
lightning strike occurred near Dover. Think chances of thunder
are low in general for the next few hours, but the look of the
HRRR with the precipitation moving through later tonight
suggests that a few lightning strikes cannot be ruled out
entirely. Model soundings from southern New Jersey and vicinity
show elevated CAPE, albeit quite marginal. Should a few more
strikes be observed upstream, will likely add mention of thunder
to the grids.

The latest hi-res output seems to be handling this initial batch
of rain fairly well. It looks like the main show rolls in
between 08Z and 10Z, with most of the precipitation north of a
Stevensville, MD, to Atlantic City, NJ, line. The best chances
of embedded convection (and perhaps a lightning strike or two)
will be on the south edge and the downstream side of the
precipitation shield, where instability will be maximized thanks
to enhanced frontal/isentropic ascent atop the near-surface
stable layer. The parcels here will be relatively warm/moist and
will be least likely influenced by stabilizing precipitation to
the north and west. Some fairly decent rain rates may occur as
the depth of the above-freezing air is large, precipitable
waters are seasonally very high, and a low-level jet of 40+ kts
will provide considerable moisture transport into the region (as
evidenced by substantially increasing 850-mb values during the
past few hours).

Main changes to the forecast overnight were fine-tuning timing
of PoPs given the rain moving through the region now and the
improved consensus of hi-res guidance later tonight. Fog/drizzle
has/have been slow to materialize across the area, given a
deeper stable layer near the surface than progged this time last
night. Nevertheless, the trends in cloud bases and visibility
are downward, so included mention through the rest of the night
across the area. This will be especially probable near the ocean
and in Delmarva, in closer proximity to the surface warm front.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
The warm front will make progress north late in the morning
and into the afternoon hours, roughly from Chester County
PA to Ocean County NJ. Fog will likely linger into the
morning hours, and may be dense, especially over portions
of Delmarva.Precipitation will gradually taper off during the
afternoon, especially to the south of the front, where the sun
may make an appearance from time to time. There is some weak
instability in the warm sector, but not enough to include
thunder in the forecast, which is in agreement with SPC.
Temperatures south of the front into the 60s, with some lower
70s possible across southern Delmarva. Further north, mid-50s
to mid-60s are expected. Southwest winds could gust up to
around 20 mph in the warm sector.

&&

.LONG TERM /TONIGHT THROUGH SATURDAY/...
Hazards: None explicitly attm and maybe there wont be as many as
currently possible in the Thursday-Saturday time frame but potential
exists for a powerful storm.  A small stream hydro hazard may
develop somewhere in the CWA Friday, and possible winter wx hazard
for elevations of ne PA and nw NJ, a coastal flood hazard appears
likely but we just dont have any confidence on severity.  For now
minor tidal inundation flooding expected for at Thursday and Friday
morning high tide cycles, with at this time the higher departures
(flooding potential) expected in NJ, but always depending on the
position of the western Atlantic cyclone as well as its intensity. A
Gale event, the first 2 hour or longer gale since Feb 5, is expected
but we just dont know when. It will again be dependent on the
latitude of the cyclone. Its even possible storm force gusts could
occur.

500 mb: a short wave in the central plains Sunday night will cross
the Mid Atlantic coast Monday evening with east coast ridging to
follow during mid week. Then a weakening closed low in the desert
southwest Tuesday night starts re-energizing in the Ohio Valley
Thursday, becoming a large 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. Calendar day averages should be nearly
10 degrees above normal Monday, Tuesday and Wednesday, 10 to 15
degrees above normal Thursday, before cooling to between 5 and
10 above normal Friday and 5 degrees or less above normal next
Saturday.

Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/24
GFS/NAM MOS will be used Sunday night - Monday night, the 12z/24 GFS
MEX MOS for Tuesday, and the 15z/24 WPC 12 hr elements of max/min
temp and pop, as well as 6 hourly dewpoint/wind/sky for Wednesday-
Saturday. I did modify WPC using 1/3 WPC, 1/3 EC and 1/3 GGEM
max temps Thursday and Saturday.

The dailies...

Sunday night...Variable cloudiness. There may be some leftover
evening stratus near and N of I78 during the evening, then that
should clear to reveal cirrus but there could be some patchy fog
later at night due to an expected decoupled northwest wind.
Confidence: Average.

Monday...Northwest wind gusts 15 mph during the day and
possibly 15 to 20 MPH at night as instability transfer deepens
with cold air advection aloft. Expecting considerable mid and
high cloudiness into early afternoon Monday then clearing late
in the day and at night. The short wave may spread a little rain
northward into far southern DE for a time during the day.
Confidence: Average.

Tuesday...Sunny and beautiful! Light wind. 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: Above
Average.

Wednesday night-Thursday...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 Thursday. It may become heavy in the
afternoon. Winds becoming east. Confidence: Below average on any
details.

Friday into Saturday...Probably gusty northerly winds and periods of
precipitation. As of now, rain most of Friday and thereafter
unknown, but potential exists for some elevation snow.
Confidence: below average

Please see the WPC Day 7 winter wx outlook graphic and their day 4-7
qpf graphics issued this Sunday afternoon to gain further
appreciation for the upcoming modeled-outlooked potential.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Rest of tonight...Conditions currently deteriorating to MVFR/IFR
CIGs and should become even more prevalent with time as rain
moves in around/after 08Z (with occasional showers possible
through that time). Once the rain sets in, VSBYs should lower to
at least MVFR (probably lower), especially if fog becomes more
widespread near daybreak. However, fairly frequent variations in
categories with both CIGs/VSBYs are expected through the night.
Winds will remain east-northeast around 5-10 kts. Moderate
confidence.

Sunday...Prolonged sub-VFR expected through most of the day,
with IFR or worse conditions most likely during the morning
hours. Rain should begin to diminish during the afternoon from
west to east, but sub-VFR CIGs/VSBYs may linger longer, perhaps
through most of the daylight hours. Winds will become variable
during the afternoon but should remain below 10 kts. Moderate
confidence.

Sunday night...Conditions should improve to VFR during the
evening hours from west to east with any remaining
precipitation ending. There is some potential for patchy fog at
the susceptible sites (RDG, ABE, TTN, MIV), but confidence is
too low for mention in the TAFs at this point, especially as
drier (albeit light) northwest surface winds commence during
this period. Moderate confidence.

Outlook...
Sunday night...Conditions quickly improving to VFR by midnight. Any
rain showers will quickly end by 9 PM. Patchy IFR fog is possible
late at night vicinity RDG, ABE, TTN and MIV. Northwest wind.
Confidence: Average.

Monday...Probably VFR bkn clouds aoa 8000 ft, clearing late.
Northwest wind gusting 15-20 kt, especially late day.
Confidence: above average. A little concerned about low pressure
forming vcnty N Carolina and spreading rain into southern DE
for a time during the morning.

Tuesday...VFR virtually clear. Light wind. Confidence: well above
average.

Wednesday...VFR cirrus. 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. Confidence: Average.

&&

.MARINE...
Dense fog appears to be in a lull on Delaware Bay and the
adjacent Atlantic waters, but models are adamant that
visibilities will drop precipitously late tonight, perhaps
spreading northward through much of the New Jersey coastal
waters as well. Should these trends be observed, the advisory
may need to be expanded northward. Will keep a close eye on
it for the rest of the night.

Regarding the small craft advisory through Sunday night, the
wind forecast is trending downward, and I am becoming far less
confident winds will mix to advisory levels in much of the
advisory area (especially for the southern NJ and DE coastal
waters). Will leave the advisory up for now and monitor trends
through the next few hours to determine if the advisory can be
trimmed for portions of the area.

Rain is expected periodically through this evening, with
attendant visibility restrictions expected. Fog may linger for
much of the day on the waters, and an extension of the advisory
in time is possible. Additionally, cannot rule out the
possibility of a lightning strike with the stronger/embedded
convective showers within the main shield of rain. Should this
be observed upstream, may add a mention of this to the forecast.

Outlook...
Sunday night...Lingering SCA seas possible, mainly the NJ
Atlantic coast. Have reduced the duration of the SCa.
Confidence: Average.

Monday through Wednesday...Sub-advisory conditions are expected on
the area waters with northwest winds gusting 15-20 kt Monday
becoming light Tuesday and southwest on Wednesday.
Confidence: Average.

Thursday...an easterly SCA likely with a possible Gale for the
NNJ waters. Confidence: Average.

&&

.HYDROLOGY...
Periods of rain through Sunday could result in minor/poor drainage
flooding. The heaviest rain is expected tonight and Sunday across
portions of eastern PA and northern NJ, when up to additional inch
of rain will fall. By the time the rain ends Sunday evening, the
region will have received between 1 and 2 inches of rain since
Thursday, with the highest amounts falling over the northern third
of the forecast area. By Sunday night and Monday, there will be
rises on area rivers and streams, with some possibly approaching
bankful.

Of note will be how much rain falls over the Susquehanna, as
there is the potential for inconvenience flooding below the
Conowingo Dam on Monday.

We may need to think about a new hydrology section for late this
coming week to cover NJ/PA but just too far in advance with
plenty of uncertainty to add any confidently stated information,
except that a fairly potent weather pattern appears to evolving
for a portion of our forecast area.

&&

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

February projected climate ranking as of Noon today-Saturday Feb 24,
based on our mid shift fcst through the 28th and mins this Saturday
morning; and for rainfall, amounts 1201 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 42.7 or 7.4F warmer than the 35.3 norm.
43.0 -2017
42.7 -2018
40.6 -1954

ILG #3 warmest 41.3 or 6.3F warmer than the 35.0 norm.
43.1-2017
42.3-1903
41.3-2018
41.2-1976

PHL #3 warmest 42.5 or 6.8F warmer than the 35.7 norm.
44.2-2017
42.5-2018
42.2-1925
41.8-1998
41.4-1890

Water equivalent February pcpn as of 1201 AM today.

PHL ranked #10 with 5.30". An additional .50 would raise to #5.
All time 6.87-1896

ILG ranked #8 with 5.48". An additional .50 would raise to #5
All time 7.02-1979


ABE ranked #13 with 4.47". An additional .50 would raise to #5.
All time 7.62-2008

ACY ranked #2 with 6.12". Wettest is 6.50-2010. Our expectation
still is that ACY will exceed the previous all time record rainfall
for February, by the end of the weekend.  It could be a close
though.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 7 AM this morning to 2 AM EST Monday
     for ANZ452-453.
     Dense Fog Advisory until 10 AM EST this morning for ANZ431-
     453>455.
     Small Craft Advisory from 4 AM early this morning to 7 AM EST
     Monday for ANZ450-451.
     Small Craft Advisory from 7 AM this morning to 6 PM EST this

FXUS61 KPHI 250552 PZB
AFDPHI
     evening for ANZ454-455.

&&

$$
Synopsis...Drag
Near Term...CMS
Short Term...Franck
Long Term...Drag
Aviation...CMS/Drag
Marine...CMS/Drag
Hydrology...Staff
Climate...Drag