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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 230508
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
108 AM EDT Wed Jun 23 2021

.SYNOPSIS...
A cold front will continue moving offshore tonight. High pressure
then builds in for Wednesday and Thursday before moving offshore. A
slow moving frontal boundary will cross the Great Lakes region from
this weekend into early next week. The mid-Atlantic will then be in
between the front to the west and offshore high pressure for several
days.

&&

.NEAR TERM /THROUGH TODAY/...
-- Changed Discussion --
Cooler and drier air continues to filter southeastward into the
region, with temperatures feeling chilly for this time of year.
Made a few minor adjustments to temps and dew points for the
early morning update.

Some patchy fog is possible overnight, but
it is not expected to become too widespread or dense with the
drier air arriving. Lows will be chilly for late June with
readings in the 40s in many areas with some low 50s across
Delmarva and metro Philadelphia. Winds will be from the north or
northwest settling to under 5 mph.

Wednesday: High pressure will build across the area with plenty of
great weather. Sunny skies and cool temperatures are expected. Highs
will top out in the upper 60s to low 70s for the NW areas and mid
70s for Philadelphia and much of Delmarva. Winds will be from the
northwest around 10 mph early then becoming light and variable
late.
-- End Changed Discussion --

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...
Mainly tranquil weather is expected this period. We remain in a post-
frontal air mass by Wednesday night with surface high pressure
overhead. Clear, calm, and rather cool conditions are expected with
lows dropping well down into the 50s in most areas. Another mostly
sunny and pleasant day is expected Thursday. High pressure will
shift off to the north and turn our flow onshore. This will keep
highs only in the upper 70s to near 80 despite some height rises
aloft, and a little cooler at the coast with the easterly breeze.
For Thursday night, most model guidance is indicating the
development of a weak surface low pressure off the Outer Banks,
likely along the remains of an offshore cold front, and moving in
the general direction of the New Jersey coast. This could
potentially bring some light showers to eastern and southeastern
areas by later Thursday night, where slgt chc PoPs were added. But
overall, should be a mostly dry and quiet night with lows a few
degrees warmer than the prior night.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Overview...

A typical summer pattern develops for the long term as a Bermuda
High becomes established offshore and a slow moving trough
develops over the Great Lakes. This is a classic southwest flow
regime which is likely to be in place for at least several days
beginning the end of this week and heading into next week. The
result will be a transition from the dry and cooler weather
during the middle of this week to a sustained warm and humid
pattern, especially by the time we get to the weekend. From
Saturday onward, we will mostly be looking at highs in the mid
80s to near 90. This is nothing atypical for the season, but
combined with the increasing humidity, we may flirt with Heat
Advisory levels at times during the period. Rain chances in this
pattern are quite uncertain at the moment. It will depend
heavily on how fast a cold front to the west approaches as Great
Lakes troughing slowly moves eastward and/or starts to lift
out. Model guidance has a spread of several days on when this
front will move through, or if it even will at all as opposed to
just washing out. My general thinking is that rain chances will
gradually increase from Saturday onward, especially to the
west, but that overall there will be a lot of dry hours through
the period.

Dailies...

Friday-Friday night... The weak wave of low pressure mentioned for
Thursday night will likely still be in the vicinity on Friday. Have
added some slgt chc PoPs to southeastern areas for the daytime
Friday. With rising heights aloft and a lack of any dynamic support,
this surface low should remain very weak, and any shower activity
should be limited. Otherwise, as the flow turns more southerly
combined with the height rises, we should warm up a bit on Friday,
with highs rising into the low 80s in most areas. By overnight, dry
weather is expected, but it will be a warmer and stuffier night as
humidity continues to increase.

Saturday-Sunday... We enter the southwest flow regime in earnest
over the weekend. Highs of 85 to 90 combined with dew points near 70
will yield heat index values of 90 to 95 degrees on both days, with
Sunday being a little warmer than Saturday. As alluded to above,
rain chances will heavily depend on the progress of a cold front to
our west. Am inclined to think the weekend will be mostly dry due to
the lack of forcing and weak shear. PoPs range from slgt chc to chc,
with the highest values to the west (closer to the front) and PoP
values on Sunday higher than on Saturday since the front should be a
bit closer. Coastal areas will see slightly cooler temperatures
and should have the lowest shower/storm chances, so current
indications are for a good weekend at the beaches.

Monday-Tuesday... Continued warm and humid weather is expected with
similar temperatures to over the weekend. The cold front should
continue to crawl eastward, and as a result my inclination is that
shower and storm chances should generally be higher for these days
than over the weekend. But much remains to be determined owing to
uncertainty over the timing of the front.

&&

.AVIATION /05Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight... Mostly clear with some patchy fog at rural sites
possible through sunrise. Otherwise VFR. Light N/NW winds.
Moderate confidence.

Wednesday... VFR with a few daytime Cu/Sc. Winds N to NW around
10 knots during the morning then light and variable by late
afternoon. High confidence.

Outlook...

Thursday-Thursday night... VFR. Winds east or southeast at 5 to
10 kt, possibly becoming light and variable overnight. High
confidence.

Friday-Friday night... Mainly VFR. Southeast wind 5 to 10 kt,
becoming more southerly overnight. Moderate confidence.

Saturday-Sunday... Mainly VFR. Scattered afternoon showers and
thunderstorms possible mainly west of PHL on both days, with a
slightly higher chance on Sunday. Southwest wind 5 to 15 kt.
Low to moderate confidence.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Tonight: SCA conditions continue on the ocean waters with gusts
around 25 knots expected and seas up to 5 ft. Wind gusts have
dropped below advisory thresholds over the bay with seas from 2
to 4 ft, thus the SCA has been allowed to expire here.

Tuesday: Sub-SCA conditions expected as high pressure crests
over the waters by late afternoon. NW winds 10 to 15 knots
during the morning then decreasing by afternoon.

Outlook...

Wednesday night-Sunday... No marine headlines are anticipated.
A couple of wind shifts through the period but gusts mostly 20
kt or less. Mainly dry weather expected. Seas 2 to 4 ft.

Rip Currents...

An underlying long period swell of 13 to 15 seconds, while not the
dominant wave period, is expected to result in a moderate risk for
the development of rip currents for Wednesday and Thursday.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
Astronomical tides will increase over the next couple of days as we
approach Thursday`s full moon. Offshore flow will prevail through
Wednesday which should allow the decreasing surge values to
partially cancel out the increasing astro tides. However some sites
on the Atlantic and DE Bay could approach minor flood stage (but
likely remaining below advisory) with Wednesday evening`s high tide
cycle. The flow will shift onshore for a time starting Thursday and
consequently this period may need to be monitored for additional
minor coastal flooding concerns.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT early this morning for
     ANZ450>455.
-- End Changed Discussion --

&&

$$
Synopsis...O`Brien
Near Term...Davis/Fitzsimmons/O`Hara
Short Term...O`Brien
Long Term...O`Brien
Aviation...Davis/Fitzsimmons/O`Brien/O`Hara
Marine...Davis/O`Brien/O`Hara
Tides/Coastal Flooding...WFO PHI