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318
FXUS61 KPHI 201903
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
303 PM EDT Sun Aug 20 2017

.SYNOPSIS...
High pressure over the area moves offshore tonight, then slowly
drifts out to sea through Tuesday. A cold front approaches from
the west on Tuesday, then works its way across the region on
Wednesday. Canadian high pressure builds into the region late in
the week and into next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Relatively tranquil weather conditions will persist through
tonight as high pressure remains over the area. Scant cloud
cover and light and variable winds will lead to favorable
radiational cooling tonight, with temperatures falling to near
or within a few degrees of climatological normals. There may be
some patchy fog development towards daybreak, but with a
relatively dry air mass expect any development to be more
limited in nature.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
As high pressure shifts offshore, flow will return to a more
southerly direction allowing warm/moist advection through the
afternoon. Temperatures will be a few degrees higher than
Sunday, while humidity will markedly increase towards the
evening. There may be a chance for some showers and
thunderstorms as a short wave trough over the Midwest moves
eastward, reaching the area by afternoon/evening. One limiting
factor will be rather diffuse surface boundaries - with little
to no forecast convergence expected to focus or enhance
activity. As such, the best chance for any development will be
along higher terrain, mainly to the west of Philadelphia.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Some mid-level shortwave energy will pass through the region
Monday night. Onshore flow will usher a warm and humid airmass
into the region, as surface dewpoints will climb into the upper
60s to low 70s, and into the mid 70s across the Delmarva and
extreme southern NJ. This will result in isolated to scattered
showers and thunderstorms in the evening, gradually tapering off
after midnight. Patchy fog may form in the pre-dawn hours going
into Tuesday morning.

On Tuesday, low pressure over the Great Lakes will lift to the
northeast towards eastern Canada. This will drag a cold front
through the Great Lakes and into the Ohio Valley late Tuesday.
Pre-frontal trough sets up ahead of the front, and that looks
to trigger afternoon and evening showers and thunderstorms.
Abundant low level moisture will be in place with dewpoints in
the low to mid 70s, and with several strong shortwaves out ahead
of the upper trough/surface cold front, can expect some fairly
strong thunderstorms during the overnight hours Tuesday night
into Wednesday morning. Cold front works its way to the south
and east Wednesday through Wednesday evening, then becomes
nearly stationary over the Mid-Atlantic and Delmarva for the end
of the week. Quite warm and humid for the first half of the
long term period, with highs in the upper 80s to low 90s on
Tuesday, then in the mid to upper 80s on Wednesday. Max heat
index values on Tuesday look to be in the upper 90s for areas
along and south of the I-95 corridor, and around 100 for the
Delmarva.

Several waves of low pressure may develop on this boundary, but
latest model guidance keeps those systems well offshore.

Canadian high pressure then builds in from the north and west
to close out the work week, with the center of the high
gradually passing north of the region on Saturday before moving
offshore on Sunday. Dry conditions expected for the end of the
week and into the weekend with temperatures running several
degrees below normal. Highs will top off in the upper 70s to
around 80 from Thursday through Sunday, and lows will drop into
the upper 50s to low 60s, except cooler up north.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions. There is a low chance of patchy fog development
towards daybreak tonight but with drier conditions expect any
development to be limited. Westerly winds through this afternoon
will gradually become light southwesterly then light and
variable as high pressure shifts across the area.

By Monday, high pressure will shift offshore allowing southerly
flow to strengthen to around 5-10 kt through the afternoon.
Showers and thunderstorms will be possible by afternoon-evening,
mainly to the south and west of KPHI.

OUTLOOK...

Monday night...Sub-VFR conditions possible in scattered
SHRA/TSRA. Fog/stratus also possible with sub-VFR conditions.

Tuesday...Morning fog/stratus burns off, then predominantly
VFR. Scattered SHRA/TSRA develop late in the day and at night
with sub-VFR conditions.

Wednesday...Sub-VFR conditions in showers/thunderstorms, which
taper off late in the day. A wind shift from S to NW likely late
in the day with the passage of a cold front.

Thursday through Friday...VFR and light winds.

&&

.MARINE...
Winds and seas remain tranquil across the waters through Monday
as high pressure gradually shifts offshore. Seas will remain
two feet or less through Monday afternoon, with a few 3 foot
seas possible off the northern NJ coast by evening as southerly
winds begin to strengthen.

OUTLOOK...

Monday night...Winds and seas below SCA thresholds.

Tuesday...SCA possible late afternoon and night with S winds
increasing to around 25 kt ahead of cold front. Building seas
to 4-6 ft are forecast for the Atlantic coastal waters.

Wednesday...Winds should decrease to below 25 kt but SCA
conditions may continue into the morning as it may take longer
for seas in the coastal waters to drop below 5 ft.

Thursday through Friday...No marine hazards expected.

.RIP CURRENTS...
Winds will be northwest this afternoon before turning to the
southwest this evening. The overall risk for today looks to be
low at both the Delaware beaches and the New Jersey shore but
with a more southwesterly flow developing a few of the more
prone locations, like LBI, may see a slightly increased risk
through this evening.

&&

.TIDES/COASTAL FLOODING...
Minor coastal flooding is possible at the high tide Monday
evening. With high pressure offshore, onshore will winds
develop, and this will coincide with relatively high
astronomical tides due to a new moon. Latest guidance is showing
water levels just touching minor coastal flooding thresholds.

&&

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

&&

$$
Synopsis...MPS
Near Term...MMD
Short Term...MMD
Long Term...MPS
Aviation...MPS/MMD
Marine...MPS/MMD
Tides/Coastal Flooding...MPS