Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KPHI 181834

Area Forecast Discussion
National Weather Service Mount Holly NJ
234 PM EDT Fri Aug 18 2017

A cold front will approach from the west and move through the region
tonight into early Saturday. High pressure returns for Sunday
through Tuesday. Another cold front will cross the region on
Wednesday followed by high pressure for the end of the week.


Severe Thunderstorm Watch #455 is in effect until 9:00 PM.

Surface dewpoints well into the 70s across the region, making
it feel quite oppressive. Some breaks in the clouds will help
destabilize the atmosphere. Owing to strong heating and
anomalously high moisture content in the boundary layer,
forecast models indicate a moderately unstable environment
(SBCAPE of 2000-3000 J/kg) to sustain strong convective updrafts
this afternoon. Storms are expected to initiate just ahead of
the cold front across the higher terrain in central and
northeastern PA during the mid afternoon. These storms are then
expected to move eastward into eastern PA late this afternoon
and the I-95 corridor this evening. Convection may linger into
the overnight near the coast. CAMs have come into much better
agreement with the above- mentioned timing of storms.

A Flash Flood Watch remains in effect for most of the CWA for
this afternoon and evening. The Watch does not include the
southern Poconos (higher FFG) and southeastern NJ into southern
DE (higher FFG and greater uncertainty if storms maintain
intensity to produce excessive rainfall rates by the time they
move into our far southeastern zones late this evening or
overnight). PWATs are forecast to increase to between 2.25-2.5,
which is +2 to +3SD above climo for this time of year. While
storm motion should be fast enough to preclude widespread heavy
rainfall, upstream corfidi vectors show a potential for
convective training that could lead to locally heavy rainfall
totals. With NWP support from CAMs, there is a potential for
isolated rainfall amounts of 3-6 inches. The flash flood threat
may peak after sunset this evening as convection (and associated
cold pools) starts to weaken with the loss of heating. Storm
motion may subsequently decrease with high rainfall rates (2+
inches per hour) still occurring.

Have added damaging winds to the weather grids with that being
the primary severe thunderstorm threat. However, isolated large
hail and even a tornado is possible this afternoon and evening.

Today will feel very muggy with dewpoints rising into the mid 70s
and even near 80F across Delmarva and far southern NJ. Heat indices
are expected to peak into the mid 90s across the urban I-95 corridor
and near 100F in Delmarva. Held off on a Heat Advisory but heat
indices will only be a few degrees below issuance criteria. Humid
conditions continue into tonight, especially along and east of I-95,
where forecast lows are in the mid 70s.


The cold front should be offshore by Saturday morning with all
precipitation having come to an end. As the upper trough pushes
through later Saturday, we should really start to see the dry air
filter into the area. Saturday will be a fairly nice day with the
sun shining across the region. Models are showing a slug of moisture
crossing the region as the upper trough moves through. However, it
appears to weaken as it moves over the mountains to our west and
peters out before reaching our area. We will continue to keep the
forecast mostly dry at this time, although an isolated shower or
thunderstorm can not be ruled out later in the day Saturday. Best
chance for anything to reach the ground looks to be across the
northwestern zones so have included a slight chance for showers
later Saturday.


High pressure will build in for Sunday and a period of relatively
nice weather should continue for the start of the week with the high
shifting offshore on Monday. Another cold front will approach the
region from the west on Tuesday, crossing the area on Wednesday.
Models continue to show a surface trough developing on Tuesday in
advance of the cold front. This trough, combined with a shortwave
moving through the mid levels, will spark off some convection on
Tuesday. Additional showers and thunderstorms will be accompany the
cold front on Wednesday, with the storms ending from west to east on
Wednesday evening. A few showers may linger early Thursday, mainly
along the coast. High pressure, accompanied by some cooler air, will
build down from Canada and across our area for the end of the


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

MVFR/patchy IFR conditions slow to scatter out, but should lift
to VFR early in the afternoon.

Showers and storms are expected to redevelop after 18Z this
afternoon mainly in vicinity of western terminals (ABE-RDG),
then move eastward through the I-95 terminals this evening
(between 23-03Z) and eventually our eastern terminals (MIV-ACY)
late in the evening. A period of IFR restrictions are likely
when these storms move through.

Winds will predominately be out of the south today and tonight.
Speeds will generally be 10 kt or less. However, they could increase
to above 10 kt for a period this afternoon with occasional gusts up
to 20 kt possible. Much stronger winds are possible with storms.


Saturday...Fog possible early. Otherwise, mainly VFR conditions
expected. Northwest to west winds around 10 to 15 knots.

Sunday...Mainly VFR conditions expected. Westerly winds around 10
knots or less.

Monday and Tuesday...Mainly VFR conditions expected. Southerly winds
around 10 knots or less.


A SCA was issued for the coastal Atlantic waters and Delaware Bay
from late this morning through this evening. Despite less than ideal
mixing profiles in southerly flow (warm air above the relatively
cooler waters), 25 kt winds are only 200-300 ft off the deck. Deeper
mixing on land could easily transport these higher winds down to the
surface in our nearshore waters. Seas in the coastal waters are
forecast to increase to around 5 ft late this afternoon and evening.

Strong thunderstorms mainly this evening could contain locally gusty
winds, requiring Special Marine Warnings.


Saturday through Tuesday...Sub-Small Craft Advisory conditions
expected on the area waters through Tuesday.

A moderate risk for the development of dangerous rip currents is
forecast for today for DE and NJ shore, given increasing southerly
winds and seas.


PA...Flash Flood Watch until midnight EDT tonight for PAZ060>062-
NJ...Flash Flood Watch until midnight EDT tonight for NJZ001-
DE...Flash Flood Watch until midnight EDT tonight for DEZ001-002.
MD...Flash Flood Watch until midnight EDT tonight for MDZ008-012-
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450>455.
     Small Craft Advisory until 10 PM EDT this evening for ANZ430-



Near Term...Klein/MPS
Short Term...Meola
Long Term...Meola
Marine...Klein/Meola/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.