Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 250718
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
318 AM EDT Wed Apr 25 2018

.SYNOPSIS...
Low pressure will move across our area today then into New England
tonight and Thursday. The next low pressure system moves across the
East Coast during Friday, then a cold front moves through on
Saturday. High pressure builds into our region Monday before
gradually shifting just off the Mid-Atlantic coast Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Low pressure over western North Carolina will lift to the north and
east, and should be over the Delmarva Peninsula later this morning,
and continues its northern track towards NYC by early this evening.
Meanwhile, an upper trough over the Great Lakes tracks east through
the day and moves into western NY/PA by this evening as well.

Deep easterly flow ahead of the low will usher abundant low level
moisture into the region. PWATs will build to 1.3", and this will be
enough moisture to result in locally heavy rain mainly during the
morning commute. 00Z NAM has the bulk of the precip across the
Coastal Plain of NJ, while the 00Z RGEM has the slug of heavy precip
farther west, generally over southeast PA.

Heavy rain lifts to the north later in the morning as the low lifts
to the north.

Models indicate a dry slot lifting north through the region during
the afternoon, but it is during this time that the upper trough
begins to dig into the Northeast U.S. This results in scattered
showers for the afternoon and early evening. There may be enough
elevated instability for some isolated thunderstorms, mainly across
southeast NJ and into the Delmarva. Will go ahead and add isolated
thunderstorms for those areas, but not expecting much convection.

The rain and clouds will keep northern portions of the forecast area
relatively colder than the rest of the region, and highs will top
off in the 50s to low 60s north and west of the Fall Line. for
southern NJ, southeast PA, and the Delmarva, there should be a break
in the precip and possibly a break in the clouds that result in
warmer temps, and highs will get into the mid and upper 60s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Upper trough passes north of the region tonight, and several strong
shortwaves will lift through the Mid-Atlantic and Northeast. This
will keep some showers in the region, with the best chances in
northern NJ and the southern Poconos. Showers taper off after
midnight as this system departs. Winds shift to the northwest, and
conditions will dry out towards daybreak Thursday.

Lows range from the 40s to low 50s.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Summary...Some showers at times into the weekend, then a significant
warm-up probable starting early next week.

Synoptic Overview...An upper-level trough from the Plains to the
East Coast initially has several strong short wave troughs embedded
within, however with time this consolidates into one main trough in
the East. This is forecast to take place through the weekend, then
strong energy rolling through the western U.S. allows for a ridge to
build in the Plains during the weekend which then shifts eastward
early next week. Due to the initial multiple short wave troughs
within the larger trough, timing can be less certain as well as the
strength of the surface features which impacts the details. The
overall pattern favors milder air overall (Sunday the coolest day),
then as we transition to an incoming ridge early next week
the development of significant warmth is probable.

For Thursday...A strong short wave trough lifts up across the
Northeast with northwesterly flow in its wake across our area. The
main synoptic ascent is forecast to be to our northeast, therefore a
dry forecast is carried for during the daytime. There will be a
northwest breeze and with this downsloping especially into the
coastal plain results in mild temperatures. A period of dry air
advection should help to erode the clouds, however additional clouds
will be on the increase at night as a short wave trough rotating
around the main trough aloft approaches from the southwest. Some
showers with this may arrive into our southwestern areas late at
night.

For Friday...As the main upper-level trough becomes more
consolidated, a short wave trough tracks up its east side and across
the Mid-Atlantic and Northeast. This feature looks to open up and
start to shear out as it tracks northeastward, however plenty of
lift with it as it takes on some initial negative tilt should
produce showers especially through early afternoon. A weak surface
low may accompany it and this may help maintain some showers into
early Friday evening. It is not out of the question that enough
instability develops Friday afternoon for some thunder, however
opted to leave it out for now. Widespread low clouds accompanying
the showers may limit the overall heating potential, therefore
leaned toward the cooler guidance for high temperatures. There
should be some improving conditions at night.

For Saturday and Sunday...The upper-level trough sharpens some
across the Northeast and northern Mid-Atlantic region Saturday as a
ridge shifts eastward from the Plains. There may be enough
instability Saturday (especially the afternoon) with strong short
wave energy glancing the area and a cold front moving through to
produce some showers, especially across the northern half of the
area. Despite the incoming upper-level trough, mild air should hang
on during Saturday then cooling arrives at night and Sunday as the
trough axis crosses our area. Sunday is expected to be dry with a
northwest breeze as high pressure starts to build in from the west.

For Monday and Tuesday...As the pattern shifts to a trough out West,
a ridge shifts eastward and this drives surface high pressure over
the Mid-Atlantic region Monday then just offshore on Tuesday. The
presence of the building ridge will result in warm air advection and
therefore a probable significant warm-up starting during this time
frame. Given the presence of surface high pressure with a ridge
aloft, dry conditions are forecast.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Moderate to locally heavy rain this morning results in IFR
and possibly LIFR VSBYs. Rain tapers off from south to north by
early afternoon, and then additional showers are possible this
afternoon. Generally IFR CIGs, lifting to MVFR late this afternoon.
Cannot rule out a thunderstorm this afternoon, mainly at KACY. East
winds 10-15 kt, becoming LGT/VRB late this afternoon.

Tonight...Scattered showers through midnight or so. IFR CIGs lift to
VFR after midnight. LGT/VRB winds in the evening become NW 10 kt or
less by daybreak Thursday.

OUTLOOK...
Thursday...VFR during the day, then MVFR (locally IFR) ceilings may
develop as showers arrive by late night. West-northwest winds 10-15
knots, then diminishing in the evening. Low confidence with onset of
sub-VFR conditions.

Friday...MVFR (locally IFR) with showers, then conditions should
improve to VFR especially at night.

Saturday...Mostly VFR, with a few daytime showers possible.

Sunday...VFR. Northwest winds may gust up to 25 knots, then diminish
toward late day and in the evening.

&&

.MARINE...
Rain and patchy fog resulting in reduced VSBYs on the waters through
late morning, then scattered showers this afternoon. A few
thunderstorms are possible on the waters late this afternoon and
early this evening. On DE Bay, wind gusts of 25-30 kt will continue
through this morning. For the ocean, wind gusts of 25-30 kt through
this afternoon, but seas remain elevated through tonight.

OUTLOOK...
Thursday...Small Craft Advisory for the Atlantic coastal waters due
to elevated seas. The seas however should subside some at night.

Friday and Saturday...A Small Craft Advisory may be needed for a
time for the Atlantic coastal waters. The winds should be below
advisory criteria, however southeasterly flow ahead of a system may
be enough to build the seas to around 5 feet.

Sunday...The conditions are anticipated to be below Small Craft
Advisory criteria, although wind gusts to around 20 knots may
occur nearshore during the day.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT Thursday for ANZ450>455.
     Small Craft Advisory until 7 AM EDT this morning for ANZ430-
     431.

&&

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


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