Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

056
FXUS61 KPHI 220603
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
103 AM EST Sun Dec 22 2024

.SYNOPSIS...
Deep low pressure will continue moving across Atlantic Canada
through tonight. Meanwhile, arctic high pressure centered near
the Great Lakes will track east through the rest of this
weekend, eventually building over New York State and the
Northeast Sunday night into Monday morning. This high departs
Monday night, and then weak low pressure passes through the
region on Christmas Eve. High pressure returns to close out the
week. A new system could approach by next Saturday.

&&

.NEAR TERM /THROUGH TODAY/...
Frigid conditions and very dry air through Sunday night.

For the 1 AM Update: No significant changes made. Forecast
remains on track with very cold and dry air advecting into the
region.

As of 935 PM, made some tweaks to the hourly temperature, dew
point and wind grids based on the latest observations and
trends. Not much stratocumulus left across the area now, however
a well aligned northwest flow of very cold air is resulting in
a narrow band of clouds down the length of Delaware Bay into the
adjacent Atlantic (bay effect). Not seeing anything on radar
therefore this may just be a shallow cloud band. Some narrow
streamers of snow that were aimed toward the Poconos have
weakened and therefore while some flurries or even a brief snow
shower cannot be ruled out there for a while longer, looks like
chances have decreased from earlier. Some high level clouds have
moved into our southern zones as these clouds are developing
from strong winds blowing over the mountains of western
Maryland.

Otherwise, deep low pressure will continue to move farther away
as high pressure advances across the Great Lakes. A strong
pressure gradient has been across the area with a wealth of cold
air advection on a North/Northwest wind. Mercury readings will
bottom out in the single digits for the southern Poconos and
teens for other areas. Winds will remain mostly 10 to 15 mph.
We`ll continue with the Cold Weather Advisory for the southern
Poconos given forecast wind chills as low as 12 degrees below
zero.

Sunday, the large high pressure system will edge closer to our
area so dry and cold weather will continue. The winds will be
less of a factor in the wind chill compared to today as the best
pressure gradient will be off to the east. Winds will still be
around 10 mph much of the time with some gusts to 20-25 mph.
Temperatures will be the coldest of the stretch with highs in
the 20s most areas and teens for the southern Poconos. So close
to twenty degrees below average for late December.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...
Expansive arctic high pressure crests over the east coast
Sunday night leading to ideal conditions for radiational cooling
with the light winds and mainly clear skies. Our colder spots
such as interior portions of NJ and much of eastern PA should
easily get down into the single digits with this set up while
warmer spots like the urban corridor from around wilmington
through Philly should see lows mainly in the 10 to 13 degree
range.

For Monday, the high starts to move off the coast with the flow
turning light southerly allowing the airmass to start
moderating somewhat. Temperatures will be a little warmer than
Sunday but it will still be quite cold with highs mainly in the
20s to low 30s except mid to upper 30s closer to the coast.

A clipper system approaches Monday night from the west while
generally weakening with time. Expect a little light snow to
move into the region from west to east after midnight and it
could produce upwards of a quarter to half inch of accumulation
but mainly for areas north of the urban corridor. The highest
chances for accumulation will be over the southern Poconos and
NW NJ. The system moves out through Christmas Eve morning with
precip possibly ending as a little light rain over southern
areas. High pressure will start to build back in by late day but
considerable cloud cover may linger until then. Generally
expect highs in the 30s to low 40s.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
High pressure builds back in over the area through Christmas
Day with highs warming back closer to average, but still a few
degrees shy of average for this time of the year. Although high
pressure will dominate the area into the end of the week, there
may be a few weak mid-level impulses that could produce some
light rain or snow on Thursday and Friday. PoPs will be capped
at slight chance for both days. Temperatures warm back to normal
with highs in the low to mid 40s.

Looking ahead to next weekend, chances for precip increase
somewhat by next Saturday as a stronger system approaches from
the west. However the ridging over the east coast will be quite
strong and this could delay it from arriving until later next
weekend or the following week. With highs by next Saturday
forecast to be generally in the 40s, rain would be the favored
precip type with this system.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through 12Z...VFR. Northwest winds 10-15 knots, with gusts up
to 20 knots possible, although gusts will not prevail. High
confidence.

Sunday...VFR. Northwest winds around 10-15 knots with some
gusts up to 20 knots early, with then some decrease by late
afternoon. High confidence.

Outlook...

Sunday night through Monday...VFR.

Monday night through Tuesday...MVFR or IFR conditions possible
in snow, possibly mixing with and changing to rain before ending
Tuesday afternoon.

Wednesday through Thursday...Mainly VFR.

&&

.MARINE...
Small Craft Advisory remains in effect through the early
morning hours on Sunday for Delaware Bay, and into early Sunday
afternoon for the Atlantic coastal waters. Northwest wind gusts
to around 30 knots this evening before a gradual decrease
overnight. Seas will also gradually subside into Sunday. Water
temperatures overall appear to be a bit to mild for freezing
spray therefore left out a mention.

Outlook...

Sunday night through Thursday...The conditions are expected to
be below Small Craft Advisory levels through this period.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Cold Weather Advisory until 9 AM EST this morning for PAZ054-
     055.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 4 AM EST early this morning for
     ANZ430-431.
     Small Craft Advisory until 1 PM EST this afternoon for
     ANZ450>455.

&&

$$

SYNOPSIS...Fitzsimmons/MPS
NEAR TERM...Gorse/OHara/Staarmann
SHORT TERM...Fitzsimmons/MPS
LONG TERM...Fitzsimmons/MPS
AVIATION...Fitzsimmons/Gorse/OHara/Staarmann
MARINE...Fitzsimmons/Gorse/OHara