Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 280902
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
402 AM EST Tue Feb 28 2017

.SYNOPSIS...
Low pressure organizing in the Central Plains today moves into the
Great Lakes tonight as it strengthens, then tracks across northern
New England later Wednesday. This storm may continue to strengthen
as it moves into the Canadian Maritimes Thursday. An associated warm
front lifts to our north today, then a strong cold front sweeps
through later Wednesday. A clipper system may move through Friday,
otherwise high pressure begins to build in from the west later
Friday before sliding to our south Saturday. Low pressure in the
Midwest Sunday moves into the Great Lakes Monday with a warm front
to our north.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Some light rain continues to be possible early this morning with WAA
underway and some mid-level shortwave energy passing north of the
area. Not expecting much more than a couple of hundredths of an inch
or so of QPF.

After that, dry and warm from late morning through early afternoon.
There may be some breaks in the clouds from time to time as deep mid-
level moisture approaches from the south and west ahead of
intensifying low pressure over the Midwest. Despite the clouds, deep
S-SW flow will push temps into the 50s in the Pocono Mountains,
otherwise, highs will be well into the 60s for much of NJ and SE PA,
and into the low 70s in the Delmarva.

Surface low pressure takes a northeast track from the Midwest to the
Great Lakes, and this will lift another warm front into the Mid-
Atlantic and Northeast by late afternoon. Precip chances ramp up
over the Lehigh Valley, SE PA, and western portions of the Delmarva
after 4 pm.

Will keep thunder out of the forecast for today due to marginal
instability.

Think record high temps should be safe, for the most part. However,
if more sunshine is able to break out today, then temps could be
warmer than forecast, resulting in some new high temperature
records. See Climate section below for high temperature records for
today and Wednesday.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Warm front continues to lift north tonight. Strong WAA will be
underway, and low level moisture will increase across region. As
upper trough digs through the Great Lakes, several shortwaves will
eject ahead of that trough and towards the Mid-Atlantic and
Northeast, and this will provide the forcing needed for showers to
develop.

Will carry categorical/likely PoPs early Wednesday night, as both
NAM and GFS have a slug of precip in the 00-06Z Wednesday timeframe.
PWATs during this time look to range from 1.25-1.4", so it is
possible for a brief period of locally heavy rain with the passage
of the warm front. Lingering showers possible from 06-12Z Wednesday,
but any additional rainfall will be light.

With abundant low-level moisture, will expect at least patchy fog to
develop, but would not rule out some areas, especially along the
coasts and areas where rainfall was heaviest, for locally dense fog
to develop.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Summary...Record warmth potential Wednesday afternoon, and severe
thunderstorms possible Wednesday afternoon and evening. Cooler
Thursday and then colder Friday and Saturday followed by another
warm up Sunday and Monday.

The synoptic setup is comprised of a sharpening upper-level trough
moving across the Midwest and Great Lakes Wednesday then across the
Northeast/Mid Atlantic Wednesday night and Thursday. This trough
looks to sharpen again Friday before shifting eastward with the flow
backing in its wake. Some ridging arrives early next week ahead of
the next sharpening trough in the Midwest and Great Lakes. This all
results in a temperature roller coaster. We used a model and
continuity blend for Wednesday into Friday, then blended in the 00z
WPC Guidance thereafter. Some adjustments were then made following
additional collaboration with our neighboring offices.

For Wednesday...A sharpening upper-level trough from the Great Lakes
region will drive a strengthening surface low to our northwest. Our
region will be within the warm sector, and with increasing flow and
instability the ingredients are coming together for the potential
for severe thunderstorm development. Record warmth possible for
several areas especially with enough sunshine.

The model guidance indicates an initial burst of showers (perhaps
some thunder) to start the morning as a lead impulse/warm air
advection burst moves east-northeastward. Once this feature clears
the area, at least partial clearing is anticipated with probably a
lull in showers. An inversion is forecast to be in place due to a
warm layer aloft, however this should erode with adequate boundary
layer heating and mixing. Farther south for Maryland and Delaware
and even into southern New Jersey, the inversion may hold until late
afternoon or early evening. Model guidance is in good agreement in
developing convection ahead of a cold front, potentially initially
tied to a pre-frontal trough however the overall convective
evolution may be complicated by the extent of some clearing. The
development will be within a modestly unstable airmass with CAPE
values on the order of 1000 j/kg. The airmass is characterized by
strengthening flow as a low-level jet of 60 knots moves across the
area and mid level flow of around 80 knots. This flow will allow for
strong deep-layer shear, however there is some veering noted in the
lowest levels of some of the soundings. Given the deep
unidirectional flow, storms should favor a linear mode with embedded
bowing segments. Given the strong shear however, transient rotation
cannot be ruled especially if the low-level flow backs a bit and
some circulations can develop within any QLCS areas. An elevated
mixed layer (EML) has been indicated at least in some of the
soundings, which would favor more instability and heighten the hail
and wind threat. If enough instability combined with a well mixed
boundary layer coincides, DCAPE would increase resulting in a higher
damaging wind threat.

Much of the area has the potential for some severe thunderstorm
development mainly Wednesday afternoon and evening, however about
the southern half of the area may have a higher chance given the cap
holding longer allowing the instability to build further. A
challenge is the extent of cloud cover (early activity exiting soon
enough) as this would impact the amount of instability realized,
however it appears there will be enough boundary layer mixing to
allow for adequate heating. PoPs decrease for a time through the
morning, then ramp up in the afternoon and evening as convection
traverses the area from west to east. There could very well be
enough upscale growth and cold pool development to organize the
convection into a solid squall line as it shifts eastward. Given the
potential along with the new Day 2 SPC Outlook, damaging winds has
been added to the forecast for the afternoon and early evening.
Convection will produce a period of heavy rain as the PW values
are forecast to near 1.5 inches, however storm motion should be
quick enough and therefore limit the flood potential. Improving
conditions during Wednesday night as a strong cold front shifts
offshore, however cold air advection arrives in its wake
setting the stage for a return to colder conditions.

For Thursday and Friday...Cold air advection continues, and an upper-
level trough is forecast to sharpen from the west later Thursday
night and Friday into the Northeast. Most of the model guidance
indicates a clipper accompanying this with a reinforcing shot of
colder air in its wake. Moisture looks more limited with this
clipper, however a band of precipitation may accompany it especially
near and just north of the surface low track. This could be a
compact feature, however moisture may be the limiting factor. We
will indicate low chance PoPs for now later Thursday night and
Friday. Thermal profiles support mostly snow up north to a rain/snow
mix southward and possibly all rain close to the coast and far
southern areas. A gusty northwesterly wind will occur both days,
with peak gusts to about 35 mph possible. High pressure starts to
approach from the west late Friday.

For Saturday through Monday...High pressure slides to our south
Saturday before shifting offshore. The airmass will start to
moderate Saturday aloft, however a northwest flow will occur
although lighter. This should result in another chilly day, before a
return flow starts to take hold Sunday along with an increase in
warm air advection. As a result, a warming trend is expected once
again Sunday and Monday.

&&

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

Early this morning (Until 12Z)...VFR. Spotty light rain possible,
especially at KMIV/KACY. LGT/VRB winds.

This morning (12Z-18Z)...VFR. LGT/VRB winds.

This afternoon (18Z-00Z)...MVFR conditions possible in -SHRA, mainly
at KABE/KRDG/KPHL/KILG. Otherwise, VFR. S winds 5-10 KT.

Tonight (00Z-06Z Wednesday)...IFR and lower conditions in SHRA/BR. S
winds 5-10 KT. LLWS possible at KPHL after 06Z.

Outlook...
Wednesday...MVFR/IFR possible to start with some showers, then
conditions should improve to VFR for a time. A period of MVFR/IFR
conditions then as a strong cold front arrives in the afternoon with
showers and thunderstorms through the evening. Some of the
thunderstorms will be capable of producing gusts to 40 knots or
greater. Southwesterly winds increasing to 15-20 knots with gusts to
30 knots, becoming westerly at night in the wake of the cold front
and possibly diminish some. Conditions improve to VFR later
Wednesday night as the strong cold front moves offshore.

Thursday and Friday...VFR overall, however there is a low chance for
some snow/rain late Thursday night and Friday as a weak system moves
through. Northwesterly winds 15-20 knots with gusts up to 30 knots,
diminishing Thursday evening before increasing some again on Friday.

Saturday...VFR overall with less wind.

&&

.MARINE...
Today...Sub-SCA conditions. No headlines. S-SW winds 5-10 KT with
seas 2-4 feet.

Tonight...VSBY restrictions expected in showers and fog. Widespread
fog, possibly dense, expected late tonight on all waters. SW winds
increase to 15-20 KT with 25 KT gusts on the ocean waters, so a SCA
will be issued starting at 5 AM.

Outlook...
Wednesday...Small Craft Advisory issued through early Thursday.
Southwesterly wind gusts at or above 25 knots, however mixing may be
more limited away from the coast due to much warmer air moving over
the chilly ocean water. Some fog in the morning, however the extent
of this will depend on the strength of the winds. Showers and
thunderstorms especially late in the afternoon and at night, and
some thunderstorms with a possible squall line may produce wind
gusts to 40 knots. Winds shift to west and northwest at night as a
strong cold front moves through.

Thursday and Friday...Small Craft Advisory conditions are expected
within a cold air advection regime resulting in more efficient
mixing. There is the potential for periods of gale force gusts,
however held off on a watch at this time.

Saturday...The winds and seas dropping below Small Craft Advisory
criteria through the day.

&&

.CLIMATE...
Record high temperatures are listed below.

Location     Tuesday 2/28      Wednesday 3/1

ABE           67 in 1976        67 in 1972

ACY           70 in 2011        72 in 1972

GED           74 in 1997        73 in 1976

ILG           66 in 2011        75 in 1972

MPO           64 in 1997        67 in 1972

PHL           68 in 1976        76 in 1972

RDG           70 in 1976        74 in 1972

TTN           69 in 1903        74 in 1972

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 4 AM Wednesday to 6 AM EST Thursday
     for ANZ450>455.
     Small Craft Advisory from 10 AM Wednesday to 6 AM EST Thursday
     for ANZ430-431.

&&

$$
Synopsis...Gorse
Near Term...MPS
Short Term...MPS
Long Term...Gorse
Aviation...Gorse/MPS
Marine...Gorse/MPS
Climate...



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