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000
FXUS61 KPHI 170052
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
852 PM EDT Sun May 16 2021

.SYNOPSIS...
High pressure will continue to build across the eastern U.S.
through the week, leading to quiet but summer-like pattern.
There is a chance for a few showers and thunderstorms as we head
into the week as the center of the highs gradually shifts to
our south and west.

&&

.NEAR TERM /THROUGH MONDAY/...
The last of the showers were dissipating quickly this evening
with the loss of daytime heating.

A short wave trough traveling in the northwesterly mid level
flow passed off our coast early this evening. A secondary
impulse is expected to follow in its wake overnight. Once it
passes, we are anticipating a decrease in cloud cover over much
of our region.

The wind is forecast to be light and variable overnight, with
lows mostly in the 40s and lower 50s.

For Monday, our region is forecast to be on the western side of
the slowly departing shortwave trough. Some additional
shortwave energy moves through within the lingering
northwesterly flow aloft during the day. The guidance overall is
not all that excited about shower chances, and this is probably
due to a bit more subsidence arriving in the wake of the main
shortwave trough which would add to at least some mid level
capping. In addition, the flow aloft is forecast to start taking
on a little more anticyclonic curvature. Given this plus with
the moisture still on the low side for this time of the year,
carried just some slight PoPs during the afternoon for the
inland areas. While a rumble of thunder cannot be ruled out,
PoPs are so low and therefore opted to leave a mention out for
now. Temperatures are expected to rise into the lower to mid 70s
across the area except cooler in the higher terrain, and along
the coast (south to southeast wind increases some through the
day).

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
A large upper-level ridge of high pressure will begin to build
eastward across the Mid-Atlantic Tuesday as a weak shortwave
trough shifts further off the New England Coast. At the surface,
high pressure will broaden to encompass a large portion of the
eastern half of the CONUS. This will lead to a warm May Day with
highs climbing into the upper 70s to near 80. With relative
humidity values in the 30-40 percent range, this will feel quite
comfortable compared to the sultry summer days we know all too
well. Light winds and clearing skies will make it even that much
more pleasant.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
The upper-level ridge, somewhat reminiscent of an Omega Block,
will gradually shift eastward, with a closed 500-mb anticyclone
developing across the Ohio and Tennessee Valleys Wednesday into
Thursday. This will lead to some very warm, dare we say, hot
weather by late week with highs nearing 90 degrees. The
paramount question of this forecast is how far east the mid-
level ridge will build before eroding to the southwest. If it
builds further eastward, then we are more likely to see highs
well into the low 90s Thursday and Friday. The Canadian appears
to favor this scenario more than the GFS with the EC being
somewhat of an average of the two. Being this far out, went with
a highly blended approach to the highs, favoring a warm bias of
a degree or two, especially Thursday and Friday as my instinct
is that the GFS is suppressing the ridge too much.

By Friday, the ridge begins to deamplify a touch and shift to
our south and west as a series of shortwave troughs ride around
the circumference of the ridge. This leads to anticyclolysis
across our region and opens the potential for each subsequent
shortwave and associated surface low/cold front to push further
south across New England. There is still decent disagreement on
when this occurs, whether it be early in the weekend or into
next week. If the ridge ends up retreating southwest much
faster, any one of these fronts could penetrate our region as a
backdoor cold front, leading to some forecast bust potential.
I`m not fully on board this yet, but there is some potential for
this to occur Friday, but moreso into next week outside of this
forecast timeframe.

The good news is that relative humidity values will stay on the
low side with the very dry forecast, so heat indices don`t
climb too far above air temperatures. Should we see an expanded
coverage of shower and thunderstorm with any of these fronts
pushing further southward towards our region, this could elevate
RH values and raise heat indices into the weekend.

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Overnight...VFR. Wind light and variable to calm. High
confidence.

Monday...VFR. A few isolated showers possible in the afternoon.
Light and variable winds becoming south to southwest mainly
5-10 knots, however becoming southeast mainly at MIV and ACY.
Moderate confidence.

Outlook...

Tuesday and Wednesday...VFR with light west-southwesterly winds
in the morning increasing to 5 to 10 knots each afternoon. High
confidence.

Thursday...VFR with a very slight chance for an afternoon
shower or thunderstorm, especially at KRDG and KABE. Southwest
winds from 5 to 10 knots turning more southerly into the
afternoon. Moderate confidence.

Friday...VFR with another slight chance for a brief afternoon
shower or thunderstorm. Southwest winds from 5 to 10 knots.
Moderate confidence.

&&

.MARINE...
The conditions are expected to remain below Small Craft
Advisory criteria through Monday. Southerly winds will prevail.
Our friends at the USCG like to remind everyone enjoying the
water in the Spring that the water temperatures are only in the
upper 50s to lower 60s.

Outlook...

Tuesday through Friday...winds and seas are expected to remain
below advisory thresholds with high pressure building overhead.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$

Synopsis...Davis
Near Term...Gorse/Iovino
Short Term...Davis
Long Term...Davis
Aviation...Davis/Gorse
Marine...Davis/Gorse