FXUS61 KPHI 210816

Area Forecast Discussion
National Weather Service Mount Holly NJ
416 AM EDT Tue Mar 21 2023

The center of high pressure will slide to our south today then off
the Mid-Atlantic coast tonight into Wednesday. A warm front lifts to
our north later Wednesday. A cold front settles across our area
later Thursday night and Friday then stalls to our south Saturday as
a couple of low pressure systems track along it. High pressure then
briefly returns Sunday, followed by low pressure on Monday.


Broad surface high pressure centered over the Appalachians will
continue to slide eastward today, slipping off the Mid-Atlantic
coast tonight. Mid-level ridging will shift to our east as well.
This will favor light southwest flow both at the surface and
aloft, with warm air advection. Additionally, a very dry airmass
and fairly well-mixed boundary layer through 900 mb today,
combined with widespread sunshine, will favor high temperatures
jumping by 10 degrees or so from Monday`s values. Did not sway
from the previous forecast for low 60s across much of our region
this afternoon. That will be accompanied by a light southwest
breeze. These conditions will combine for arguably the most
pleasant day of the week.

Meanwhile this morning, we have clear skies and calm or very
light southwest winds allowing for good radiational cooling. The
usual cold spots like the Poconos and the NJ pine barrens will
bottom out in the low 20s or upper teens early this morning.
Otherwise, temperatures overnight have been running about 4 to 8
degrees milder than last night, with most of the area set to
see a slightly milder start to Tuesday than the previous day.

Some high clouds will stream quickly northeastward from the
Tennessee Valley this morning, reaching our area by the late
afternoon or early evening. Those clouds will thicken tonight
with increasing west to southwest flow aloft advecting moisture
into our region. That will result in a noticeably milder night
ahead, with clouds limiting radiational cooling.


Warmth builds across the region, peaking on Thursday, with an
increase in shower chances.

An upper-level trough is forecast to be shifting from the Midwest
eastward across adjacent Canada. This will take surface low pressure
with it. As this occurs, strengthening warm air advection will
propel a warm front northward during Wednesday. This warm front
crosses our area Wednesday night, with our entire area in the warm
sector Thursday before a cold front settles into our area later
Thursday night.

A subtle shortwave with minimal forcing for ascent moves through
Wednesday afternoon and evening, with perhaps a few showers mainly
across the coastal plain. Showers are expected to be on the increase
Wednesday night with the warm front and also as the flow aloft
increases in advance of a cold front. The showers may diminish
during Thursday afternoon especially across our southern areas as
the best forcing translates northward. The air mass is forecast to
be warm and if more sunshine occurs then temperatures certainly
could be even warmer. Looks like temperatures peaking right around
60 degrees for many Wednesday, then widespread mid/upper 60s
Thursday (some places reaching 70 degrees, especially near and south
of Philadelphia). A southerly wind component though will tend to
keep temperatures cooler along portions of especially the New Jersey
coast due to the chilly ocean water. It will be rather mild Thursday
night with temperatures particularly from I-95 south and east only
dropping into the 50s as we await the arrival of a cold front
settling southward.


Summary...Cooling across much of the area Friday and Saturday along
with showers or a period of rain, then a brief break followed by
another system arriving early next week.

Synoptic Overview...A ridge centered off the Southeast coast to the
far western Caribbean builds northward some across the Mid-Atlantic
Friday into Saturday. An upper-level trough moving out of the Plains
Friday lifts across the Great Lakes and Northeast during Saturday.
Some upper-level ridging arrives later Sunday into Monday, then
another upper-level trough tracks northeastward Monday similar to
the previous one. The pattern offers an increase in precipitation
into the start of the weekend, and then again early next week.

For Friday and Saturday...As an upper-level trough lifts north and
east of northern New England Friday taking surface low pressure with
it, a trailing cold front will continue to settle southward across
our area. This front should be south of our region by Saturday.
Surface high pressure slides to our north, however energy ejecting
out across the southern Plains will assist stronger warm air
advection up the Ohio Valley. This should result in an area of rain
to our west, however some of this in the form of showers should move
across at least a portion of our region during Friday. As high
pressure slides across northern New England Friday night into
Saturday, a low-level onshore flow will develop. The front to our
south should move toward our area some as a warm front, however
surface low pressure tracks up the Ohio Valley and eastern Great
Lakes during Saturday. A new low may develop along the occluding
front and move across our area. The associated forcing for ascent
will result in a period of rain Saturday before drier air works in
Saturday night following a cold front. The southbound front Friday
into Saturday will determine temperatures as clouds and
precipitation will keep it cooler, however the low-level flow turns
onshore for a period of time and that will also keep it cooler.
Rainfall amounts could be as much as an inch especially across the
central to northern zones given ample moisture and lift in place for
a time.

For Sunday...We get a bit of a break as one trough lifts out and
surface high pressure briefly builds in later in the day and at
night. Given a tightened pressure gradient in place, a west to
northwest wind may be gusty at times before diminishing at night.
The flow aloft is more zonal again for a time and despite some cold
air advection, temperatures should top out near 60 degrees in the
afternoon especially for the coastal plain. Clouds will start to
increase at night as we look to our west for the next system.

For Monday...An upper-level trough across the Plains and Midwest is
forecast to lift east and northeastward. The associated surface low
is forecast to track similar to the previous one, with it headed up
across the Great Lakes with potentially a new low developing in our
vicinity. There may be ample moisture surging northward ahead of
this system, and with increasing forcing for ascent with time some
heavier rain may occur. If we are able to get more into the warm
sector for a time with higher temperatures and dew points, some
thunderstorms cannot be ruled out.


The following discussion is for KPHL, KPNE, KTTN, KABE,
KRDG, KILG, KMIV, KACY and surrounding areas.

Today...VFR/SKC. SW winds increasing to around 10 kt in the
afternoon. High confidence.

Tonight. VFR with just some increasing high cloudiness. Winds
very light southwest or just calm. High confidence.


Wednesday...VFR ceilings lowering during the day, then a period of
MVFR ceilings possible at night. A few showers possible mainly near
KMIV/KACY later in the afternoon or evening, then some showers
possible again overnight. South-southwest winds 8-12 knots during
the day, then around 5 knots at night. Low confidence regarding sub-
VFR conditions details/timing.

Thursday...Times of MVFR conditions possible, with some showers
around especially in the morning. South-southwest winds increasing
to 10-15 knots with gusts up to 20 knots, then diminishing some at
night and becoming west late. Low confidence.

Friday...MVFR or lower probable with showers. West to northwest
winds around 10 knots, becoming north to northeast. Low confidence.

Saturday...MVFR/IFR conditions with rain probable. East or northeast
winds 10-15 knots with gusts to around 20 knots, shifting around to
southwest and west in the afternoon and night. Low confidence.


High pressure builds in from the south and west, shifting off
the coast this afternoon into tonight. As that occurs, light
westerly winds this morning will back so the SW and increase to
10 to 15 kt later today through tonight. Some gusts over 20 kt
will be possible this afternoon and this evening, mainly north
of Barnegat Light. Seas there may increase to around 3 ft or so,
with short-period waves mixing with a weak easterly swell.
Elsewhere, seas should stay less than 3 ft.


Wednesday...The conditions are anticipated to be below Small Craft
Advisory criteria.

Thursday...Small Craft Advisory conditions probable.

Friday and Saturday...Small Craft Advisory conditions possible.


Light or calm winds this morning with clear skies have allowed for
good radiational cooling with better humidity recovery for most of
the region. Most of Delaware, eastern MD and southern NJ will end up
over 80 percent to start the day Tuesday, while areas farther north
and west will be more around 60 to 70 percent. These values will
vary quite a bit locally, though. Dewpoints are still in the upper
teens to mid 20s, and will settle down to the lower end of that
range as we head into this afternoon. Combined with warmer afternoon
temperatures mainly in the upper 50s to low 60s, RH values will be
slightly lower than Monday, with minimum RH near 20 percent, if not
peaking briefly lower in some spots. 10-hour fuel moisture will
likely be in the 7 to 10 percent range as well. However, winds this
afternoon will be similar to Monday, from the southwest around 10
mph with perhaps brief gusts as high as 15 to 20 mph in the most
open, exposed areas. The relatively light winds will keep us out of
Red Flag Warning criteria, but the next shift may want to consider
putting out an SPS to cover the elevated fire weather conditions
once again.

This should be the last day for that though, as `higher` humidity
will start to arrive for Wednesday, and only increase through the
end of the week. Some low chance of marginally wetting rain will
arrive for Thursday, with better chances on Friday and Saturday.





NWS PHI Office Area Forecast Discussion