Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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879
FXUS61 KPHI 222056
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
356 PM EST Wed Feb 22 2017

.SYNOPSIS...
High pressure will remain centered east of the Mid Atlantic coast
through the end of the week. A cold front will approach from the
west Thursday then stall at night, before lifting northward as a
warm front Friday. A strengthening low pressure system will develop
over the Plains and track across the Great Lakes Saturday, with a
strong cold front sweeping across our area Saturday afternoon. High
pressure then builds to our south later Sunday into Monday. The next
cold front arrives during Monday before stalling to our south, then
it should move northward as a warm front Wednesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
High pressure was centered off the southern Mid-Atlantic coast this
afternoon. Southwesterly return flow around continues to draw
unseasonably warm and moist air up the eastern seaboard. Many
locations struggled to mix out the nocturnal inversion with light
surface winds and denser cloud clover earlier in the day. However,
temperatures have increased steadily into the upper 50s and lower
60s this afternoon as the mid and high clouds thin out.

Expect favorable radiational cooling conditions this evening under
generally clearing skies and light winds. Models indicate stratus
forming to our south toward the mouth of the Chesapeake Bay and
lower Delmarva this evening, then advecting/expanding to the north-
northeast overnight across the coastal plain. Farther north into
eastern PA and NJ, a better setup for radiational fog looks to exist
since the stratus should arrive later in the night or possibly not
until early morning. While locally dense fog is a possibility toward
daybreak, the current thinking is the stratus will limit the extent
of it. Differences between models in timing and areal coverage of
the fog/stratus contribute to some uncertainty with this aspect of
the forecast.

Most of the region should remain dry tonight but a weak disturbance
tracking over Delmarva may be accompanied by an isolated shower or
patchy drizzle overnight.

Forecast low temperatures range from the upper 30s in the sheltered
valleys in northwestern NJ to the upper 40s in the the cities and
mid Delmarva region.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
High pressure will remain anchored off the Mid-Atlantic coast on
Thursday. Return flow around the high will allow the spring-like
warmth with build into tomorrow. Near-record high temperatures are
forecast from about the Philadelphia region southward. It is
important to note some uncertainty with the temperatures exists with
the forecast for tomorrow. How warm we will get will depend on how
quickly the morning fog and stratus mixes out. Stratus occasionally
is stubborn to erode this time of year due to the low sun angle.
Where stratus persists into the afternoon, highs will be in the 60s
vs the 70s. Forecast soundings and RH fields show lower clouds
eroding from south to north during the late morning and afternoon.
Accordingly, highs in the low to mid 70s are favored in Delmarva,
southeastern PA and southern NJ with 60s farther north.

A weakening cold front will approach from the west during the
afternoon. Models generally show enough lift to support isolated to
scattered convection in our far western zones of eastern PA and
northwestern NJ during the mid to late afternoon. Forecast soundings
show a weak mid-level cap in place. Added a slight chance for
thunderstorms out toward Reading and in the southern Poconos, where
a relative min in CIN indicates a low potential for deeper
convective to develop.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Summary...Potential for near record warmth through late week,
followed by a strong cold front Saturday afternoon and evening.

The synoptic setup is comprised of a digging trough across the
Plains and Midwest Thursday night and Friday, then this moves across
the Great Lakes Friday night and Saturday before lifting across
the Northeast and eastern Canada. The flow may then turn more
zonal for awhile allowing for some moderation in the temperatures
once again after a brief shot of cooler air. We used a model
blend for Thursday night through Friday night, then blended in
the 12z WPC Guidance thereafter. Some adjustments were then made
following additional collaboration with our neighboring
offices.

For Thursday night and Friday...A frontal boundary near our northern
zones Thursday night into Friday morning should shift northward as a
warm front. This is due to increasing flow and warm air advection
ahead of a organizing/strengthening storm in the Northern Plains.
There is not a lot of focused lift, however some showers could
result. Some fog and low clouds should develop each night as the low-
level moisture increases. Despite Friday starting out with more
clouds and some fog, enough clearing should take place to allow for
ample boundary layer warming and there is the potential for record
highs. It appears that any showers with the warm front Friday should
be shifting north of our area. Some thunder may occur near the
northward moving front especially to our west, however this may tend
to weaken with an eastward extent.

For Saturday and Sunday...A strong upper-level trough will move
across the Great Lakes and Northeast Saturday and Saturday night,
sweeping a strong cold front across our area. Some low clouds
and fog may be around to start Saturday. We should be within
the warm sector and the synoptic pattern is favorable for at
least some convection to move through the area ahead of or with
the cold front. Forecast soundings show a strengthening wind
field ahead of the cold front Saturday with some instability
(high shear-low CAPE scenario). While the instability may be on
the low side, frontal/synoptic forcing and moisture advection
could result in a low-topped convective line. As a result, a
mention of thunder is maintained in the forecast ahead of the
cold front however the strength of the convection is less
certain at this time. While it will be another warm day on
Saturday, it appears that high temperatures fall short of
records. Once the cold front sweeps through later Saturday, cold
air advection and much drier air moves in for Saturday night
and Sunday. Given the front offshore Sunday, dry and much cooler
on Sunday along with a gusty northwesterly wind.

For Monday through Wednesday...High pressure shifts off the
Southeast coast to start Monday, with a cold front moving
through our area. This looks to be on the weaker side however
cannot rule out a few rain/snow showers. As another storm system
ramps up across the Plains, the front should stall just to our
south into Tuesday before starting to move northward as a warm
front Wednesday. Low pressure looks to track once again across
the Great Lakes with a driving warm sector northbound Wednesday.
We went with a chance of showers (rain/snow showers Monday
night) Tuesday ahead of the warm front then with some higher
PoPs on Wednesday as the cold front nears.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR through this evening with clearing skies. Stratus is anticipated
to develop to our south and expand northward overnight/early morning
into the region. Expect IFR CIGs once the low clouds arrive. There
may also be reduced VSBY where fog develops prior to the arrival of
the stratus (more likely to be an impact from PHL north). Confidence
in moderately high that low clouds/fog will develop but timing of
the onset and density of the fog is below normal. Light S-SW winds
tonight.

Conditions improve to MVFR and then VFR from south to north during
the 15-19Z period Thursday as low clouds/fog mix out. Light SW winds
in the morning increase to around 10 kt with gusts 15-20 kt during
the afternoon.

OUTLOOK...
Thursday night and Friday...VFR ceilings may lower to MVFR/IFR
Thursday night into Friday morning along with MVFR/IFR visibility
due to fog. Conditions should improve during Friday, however some
low clouds and fog may then redevelop Friday night. Some showers are
possible mainly Thursday night and Friday morning with a warm front
lifting northward.

Saturday...There is the potential for MVFR/IFR conditions at times
with showers and potential thunderstorms with a cold front.
Conditions should improve at night in the wake of the cold front.
Southerly winds ahead of the cold front potentially gusty, turn west
and northwest at night.

Sunday and Monday...VFR overall. West-northwest wind gusts up to 30
knots possible Sunday, then diminishing at night and Monday.

&&

.MARINE...
Winds and seas below SCA levels tonight and Thursday. Expect fog to
develop tonight and may be locally dense in spots late tonight and
Thursday morning as dewpoints rise into the mid/upper 40s and maybe
near 50F south.

OUTLOOK...
A frontal zone Thursday night will lift north as a warm front
through Friday, which should become located to our north during
Friday. An increase in warmth and dew points may result in some
dense fog developing on the waters especially Friday through early
Saturday. A strong cold front arrives later Saturday, with gusty
southerly winds becoming west-northwest in its wake. Small Craft
Advisory conditions may develop mainly Saturday and Sunday. Much
warmer air over cooler waters though should limit the mixing
Saturday, then improved mixing Saturday night and Sunday as cooler
air arrives. There is a chance for a period of low-end gale force
gusts Sunday. The conditions should then be below Small Craft
Advisory criteria Monday.

&&

.CLIMATE...
For reference, here are the high temperature records for Thursday,
Friday and Saturday.

Location    Thursday 2/23   Friday 2/24   Saturday 2/25

ACY         72-1985         75-1985       77-1930

PHL         75-1874         74-1985       79-1930

ILG         72-1985         78-1985       78-1930

ABE         71-1985         76-1985       74-1930

TTN         74-1874         74-1985       76-1930

GED         67-1985/1990    72-1961       76-1975

RDG         72/1932/1922    77-1985       77-1930

MPO         60-1977         60-1984       70-1930

&&

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

&&

$$
Synopsis...Gorse
Near Term...Klein
Short Term...Klein
Long Term...Franck/Gorse
Aviation...Gorse/Klein
Marine...Gorse/Klein
Climate...



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