Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 202310

Area Forecast Discussion
National Weather Service Mount Holly NJ
710 PM EDT Fri Oct 20 2017

High pressure centered over the upper Ohio River Valley this
afternoon will slide to the east. The center of the air mass is
forecast to pass off the coast on Saturday before moving out to
sea. The high should continue to influence our weather into
Monday. A cold front approaching from the west is expected to
arrive on Tuesday night. Low pressure is anticipated to develop
over New England and its coastal waters late in the week as
another area of high pressure noses into our region from the
southwest and south.


High pressure centered over the central Appalachians early this
evening will build eastward toward the mid-Atlantic coast

Winds have already started to diminish around sunset in
response to the loss of daytime mixing and to the pressure
gradient weakening over the region as the high builds into the
area. There is still enough of a residual gradient over the area
to maintain a light breeze this evening, particularly outside
of sheltered locations. Accordingly, the rate of cooling over
the next few hours will be considerably different across the
region (i.e., cool much faster in sheltered areas owing to
boundary-layer decoupling), resulting in a relatively large
spread in temperatures from hour to hour and from location to

There will be a low floor for temperatures to bottom out at
given the ideal radiational cooling conditions (especially
overnight as the high shifts overhead) and the dry airmass in
place. Under this setup, felt it was prudent to adjust min
temperatures downward from stat guidance. Forecast lows range from
the upper 30s in the the valleys of NE PA/NW NJ to the lower 50s
in downtown Philadelphia and along the coast.

The dry airmass should inhibit radiational fog outside of the
typical sheltered river valleys in rural NE PA and NW NJ.


With high pressure over the region another sunny day is
expected. Temperatures will warm fairly rapidly into the 70`s
across the region with a few spots pushing 80. With a dry
ground, it should be another day where temperatures slightly
outperform MET/MAV guidance. Winds in the afternoon are likely
to pick up a bit, staying under 10 mph from the southeast as the
high pressure system moves offshore. Hope you can get outside
and enjoy it!


The axis of the mid level ridge is forecast to pass overhead on
Saturday night. Meanwhile, a sharp trough is expected to be
located over the middle part of the country during the weekend.
A mid level low is anticipated to develop over the south central
states on Sunday.

A re-enforcing mid level trough is expected to drop over the
eastern states for the mid week period. The trough is forecast
to pull the mid level low east then northeastward and over our
region on Tuesday. The long wave trough should begin to lift to
our northeast late in the week.

At the surface, high pressure moving out to sea should maintain
some influence over our weather for Sunday and Monday with no
precipitation expected. However, there should be an increase in
cloud cover during the period. A return southerly flow may
result in the development of low clouds and perhaps some fog
from late Sunday night into Monday morning.

The end of our current dry spell is anticipated to arrive on
Monday night. Showers ahead of an approaching cold front should
begin to move into our region at that time. We are expecting an
increasing chance of showers on Tuesday. As the surface cold
front nears our area and as the remnants of the mid level low
pass overhead there should be added lift. As a result, there may
be a period of moderate and perhaps briefly heavy rain along
with isolated thunderstorms on Tuesday afternoon and into
Tuesday evening. There is the potential for a half inch to an
inch of rain in spots.

The cold front and the precipitation associated with it should
pass off the coast on Tuesday night.

Low pressure is forecast to develop along the departing front
off the Middle Atlantic coast. The low is expected to lift over
New England and its coastal waters late in the week. The system
may push clouds and showers back into our region from the
northeast for the period from Wednesday night into Thursday
night. Otherwise, surface high pressure will likely try to build
into our region from the southwest and south.

Daytime temperatures will remain about 10 degrees above normal
on Sunday, Monday and Tuesday. Readings are forecast to drop
back to near normal levels for late October on Thursday, Friday
and Saturday.


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

VFR tonight and Saturday. Light W-NW winds under 10 kt early
this evening will become light/variable by midnight and persist
thru the morning. These light winds will eventually acquire a
more southerly direction (3-6 kt) during the afternoon.

Saturday night and Sunday...Mainly VFR.

Sunday night and Monday...Low clouds and fog are possible from
late Sunday night into Monday morning. Otherwise...mainly VFR.

Monday night through Tuesday night...Conditions lowering to
MVFR and IFR in showers. Isolated thunderstorms and moderate to
heavy rain are possible on Tuesday afternoon and Tuesday
evening, along with a south wind gusting around 25 to 30 MPH.

Wednesday...Conditions improving to VFR.


Winds and seas well below SCA criteria tonight and Saturday.

Light W-NW winds 5-10 kt over the waters this evening will
become light and variable later tonight. Winds will eventually
become more S-SEly and increase to 5-10 kt late in the day
Saturday. Seas in our coastal waters of 2-3 ft this evening will
settle closer 2 ft overnight and Saturday.

Saturday night through Monday night...No marine headlines are

Tuesday and Tuesday evening...A Small Craft Advisory may be
need for southerly wind gusts of 25 to 30 knots. Waves on our
ocean waters may build to 6 to 8 feet.

Late Tuesday night and Wednesday...A Small Craft Advisory may
be needed on our ocean waters for wave heights of 5 to 6 feet,
even as the wind becomes westerly and diminishes to 10 to 15


A precipitous rise in RH have occurred early this evening as
temperatures drop and dewpoints rise. There is no fire weather
concerns for tonight.

RH values are likely to drop to around 30% again Saturday
afternoon but light winds will limit the risk for the spread of




Near Term...Klein
Short Term...Gaines
Long Term...Iovino
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