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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 101034
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
634 AM EDT Fri Apr 10 2020

.SYNOPSIS...
An area of low pressure will drift through eastern Canada
through the weekend, while high pressure builds across the deep
south today and Saturday, before moving offshore Saturday night.
Our area will remain under a tight pressure gradient between
these two features today through the weekend. A strengthening
low pressure system will move across the Mid West states Sunday
night, then into the Great Lakes region Monday and into southern
Canada Monday night. This will lift an occluding frontal system
across the Mid Atlantic region Sunday night into Monday.
Another system could affect portions of the Mid Atlantic region
around Wednesday, with a clipper type system around Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
630 am update: No updates needed to the forecast at this time.

Previous discussion...

Way behind with the forecast overnight owing to a high-impact storm
that produced a swath of significant wind damage from the
Harrisburg, PA, area east-southeast to Wilmington, DE, and adjacent
portions of southern New Jersey. A wind gust of 69 mph was reported
at KILG and of 62 mph was reported at KACY. In addition, areas of
snow developed in northern portions of the forecast area, with
several spotters reporting brief wind gusts of 40 to 50 mph with
some of these showers. Several reports of graupel/small hail were
received from many of these showers, as well.

Most of the shower activity for the CWA is coming to an end early
this morning. The exception will be the southern Poconos and
adjacent areas, as northwest-flow showers will occasionally reach
these areas today. Temperatures will likely remain cold enough for
snow or a rain/snow mix this morning, with potential for light
accumulations in the highest elevations. However, chances will
slowly dwindle tonight as a reinforcing shortwave trough digs
southeast into the Atlantic during the evening, and upstream
descent takes over. PoPs are generally negligible after 00z Saturday.

Clouds will thicken fairly quickly during the day, as mixing and
diurnal heating generate saturated parcels readily after sunrise.
This should keep temperatures from warming (especially given the
ambient cold advection). Nudged highs slightly lower today given
these factors.

The main story today will be the winds. Model forecast soundings
indicate another day of mixing to 850-800 mb, with winds of 35-45 kt
at these levels likely mixing to the surface rather efficiently in
the cold-advection regime in place. Wind gusts of 40 to 50 mph will
be likely at times today, especially near/south of Philadelphia in
closer proximity to stronger winds aloft. However, the threat of
advisory-level gusts persists across the area, and the wind advisory
was unchanged with this forecast update. With the elevated winds and
strong mixing, fire-weather concerns are elevated today. Please see
the Fire Weather section for more details.

Attention tonight turns to temperatures, as the strong cold
advection occurring today will lead to a temperature plunge tonight.
Model guidance is generally in agreement that areas northwest of the
Philadelphia metropolitan area will reach or drop below freezing
overnight. However, confidence is not particularly high, since winds
will likely remain elevated through the night. In such regimes,
models tend to be too aggressive with the temperature drop,
especially in more urbanized areas. Nevertheless, with the growing
season in full swing southeast of the Poconos and with consistent
guidance showing temperatures falling below freezing in Berks
County, the Lehigh Valley, the far west suburbs of Philadelphia, and
northern New Jersey, hoisted a freeze watch for the overnight hours
in these areas. Will need to watch the Pine Barrens as well, as
temperatures often drop precipitously, even in otherwise elevated
wind conditions.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY/...
Saturday will remain rather breezy, though considerably less so than
today. Winds will likely diminish by late afternoon as high pressure
approaches the area. With the upper-level trough axis moving
offshore, should see thicknesses increase, allowing for a slightly
warmer day compared to Friday. Used a statistical blend for highs,
resulting in readings in the mid to upper 40s in the Poconos to the
mid 50s southeast of the I-95 corridor. Unfortunately, the breezy
conditions and resultant mixing will allow for another day of
elevated fire weather concerns. As high pressure shifts offshore
Saturday night, winds will become light and back to a more southerly
or southeasterly direction. Subtle warm advection should prevent a
collapse in temperatures, but I suspect the rural/valley locations
will see another cold night, with potential for more frost/freeze
products.

By Sunday, the Mid-Atlantic will begin to see downstream midlevel
ridging as a very potent shortwave trough ejects into the southern
Plains. Warm advection will increase in earnest during the day, with
an associated increase in cloud cover as well. Despite the dampened
insolation, the height rises and warm advection will result in a
considerable warmup. Highs may approach 70 in much of the area. The
region will be well downstream of large-scale lift with the
approaching system, so have unmentionable PoPs through the day. The
sensible weather of interest will once again be the winds, as they
increase from the south during the day. Could see some gusts of 25
to 35 mph by the afternoon, but fortunately, the warming
temperatures will be compensatory to most.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
The main focus for the long term forecast will be Sunday night
into Monday. An area of low pressure will move across the Mid-
West states and into the Great Lakes area Sunday night, before
lifting into southern Canada on Monday. This will lift an
occluding frontal system across the Mid-Atlantic and
northeastern states. Rainfall is expected to begin overnight
Sunday, then continue into Monday, before ending later Monday
afternoon into Monday evening as the cold front pushes offshore.
Several concerns are beginning to take shape with this system.
The first will be the potential for moderate to heavy rainfall
overnight Sunday into Monday morning. 1-2 inches of rain will be
possible, with the heaviest rainfall expected for portions of
eastern Pennsylvania and northern/central New Jersey. Latest
model guidance indicates that there will be some instability
developing during day ahead of the approaching cold front, so
there will be a chance of thunderstorms. Thunderstorms would
mostly be confined to daytime hours Monday. This leads to the
next concern, which may be the greatest threat. Low level winds
with this system are forecast to increase significantly with
925-850 mb winds potentially increasing to 50-70+ knots. Not all
of these winds would mix down to the surface, however, we will
likely have some decent mixing overnight Sunday into Monday, so
we do expect a very windy overnight Sunday into Monday period.
We can expect at least 40-50 mph winds, especially for south and
eastern New Jersey, and central and southern Delaware and
Maryland where the strongest winds are expected.

The front will have moved offshore by Monday afternoon/Monday
evening. Gusty winds will continue night into Tuesday, before
beginning to relax overnight Tuesday. There will be a slight
chance of showers Tuesday as a couple of short waves/vorticity
impulse move across the area.

There continues to be some disagreement with the long term
models in how they handle a potential coastal storm developing
along the old cold front offshore of the east coast Tuesday
night into Wednesday.. The ECMWF is much more aggressive
strengthening the low moving it offshore of the Mid- Atlantic
and bringing rain or even snow to portions of the area. The GFS
and Canadian suppresses the system and keep rain away from the
area. We will have a chance of showers due the uncertainty, but
no more than 40 to 50 percent across southern New Jersey and
southern Delaware and Maryland.

There is better agreement with a possible clipper type system
affecting the area around Wednesday night into Thursday. This
could bring a chance of rain, or even snow showers to portions
of the area Wednesday night into Thursday. High pressure is
forecast to briefly build into the area late Thursday, before
another potential storm at the end of the week.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Mainly VFR, though CIGs of 3500-6000 feet will likely
encompass much of the area by midday. West to northwest winds
15 to 25 kt with gusts 30-40+ kt likely. Moderate confidence.

Tonight...VFR with west to northwest winds 10 to 20 kt with
gusts up to 30 kt, especially during the evening. High
confidence.

Outlook...

Saturday...VFR with west winds 10 to 20 kt with gusts up to 30
kt, likely diminishing slowly by late afternoon. High
confidence.

Saturday night...Mainly VFR with winds becoming light southerly
or southeasterly. Moderate confidence.

Sunday...Increasing cloudiness with south winds increasing to
10 to 20 kt with gusts up to 30 kt by afternoon. Mainly VFR, but
conditions may approach MVFR late. Low confidence.

Sunday night-Monday...A frontal system will move across the
area leading to periods of moderate to heavy rain, along with
MVFR to IFR conditions likely, improving during the day Monday.
Very wind conditions are likely to develop overnight Sunday into
Monday, with gusts potentially reaching 35-45 knots. Moderate
confidence.

Monday night-Tuesday...VFR conditions expected. West to
northwest winds gusting to 20-30 knots. Moderate confidence.

&&

.MARINE...
Gales continue on the Atlantic waters and Delaware Bay today.
West to northwest winds of 20 to 35 kt with gusts to 45 kt are
likely. Conditions should slowly improve tonight but will likely
exceed gale criteria through most of the night. The gale
warning continues through 6 am Saturday.

Seas will generally range from 3 to 6 feet.

Outlook...

Saturday...Advisory conditions likely with west winds 10 to 20
kt with gusts to 30 kt. However, conditions will improve with
time and possibly become sub-advisory by evening.

Saturday night...Sub-advisory winds/seas anticipated.

Sunday...South winds will increase rapidly, likely reaching
advisory criteria by afternoon. Seas are expected to
approach/exceed 5 feet late in the day as well.

Sunday night-Monday...A frontal system will move across the
area, leading to very strong winds across the waters. At least
Gale force gusts are expected, with potentially Storm force
winds possible late Sunday night into Monday.

Monday night-Tuesday...Gale force winds could continue into
early Monday night, before lowering to Small Craft Advisory
levels overnight into Tuesday.

&&

.FIRE WEATHER...
Windy conditions will occur again today. This will allow the
low levels to mix readily, with dew points likely dropping to
the upper teens to lower 20s. This will result in relative
humidity values dropping to near or below 30 percent in most
locations outside the southern Poconos. As a result, the threat
for the rapid spread of fires is quite high today, and a Special
Weather Statement has been issued in coordination with our
fire-weather partners.

Conditions will not improve on Saturday. Although winds will
likely be lower, relative humidity values will also likely be
lower than values seen today. As a result, the threat for the
rapid spread of fires will be high again.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Freeze Watch from late tonight through Saturday morning for
     PAZ060>062-101-103-105.
     Wind Advisory until 7 PM EDT this evening for PAZ054-055-
     060>062-070-071-101>106.
NJ...Freeze Watch from late tonight through Saturday morning for
     NJZ001-007>010.
     Wind Advisory until 7 PM EDT this evening for NJZ001-007>010-
     012>027.
DE...Wind Advisory until 7 PM EDT this evening for DEZ001>004.
MD...Wind Advisory until 7 PM EDT this evening for MDZ008-012-015-
     019-020.
MARINE...Gale Warning until 6 AM EDT Saturday for ANZ430-431-450>455.

&&

$$
Synopsis...Robertson
Near Term...CMS
Short Term...CMS
Long Term...Robertson
Aviation...CMS/Robertson
Marine...CMS/Robertson
Fire Weather...CMS