Upper Merion, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

204
FXUS61 KPHI 210917
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
417 AM EST Thu Nov 21 2024

.SYNOPSIS...
A new surface low develops off the coast this morning before
shifting westward and meandering in our region through Friday.
Continued unsettled weather as a result with some much needed
rain as well as higher elevation snow. The low then gradually
moves away through the weekend. Weak high pressure briefly
arrives for later Sunday and Monday, followed by a cold front
Monday night into Tuesday.

&&

.NEAR TERM /THROUGH FRIDAY/...
Latest analysis places a developing low pressure system just off the
Jersey shore. The surface cold front cleared the region hours ago
and widespread northwest winds have enveloped the region, though
they are stronger in southern areas where precipitation has already
mostly tapered off. Meanwhile, the upper level low helping to drive
this system remains way back near Chicago, but with the surface low
forecast to slow to a near stall just off the coast through today,
the upper low should have no trouble catching up to it over the next
24 hours. In fact, the surface low looks likely to turn northwest
and end up inland to our north 24 hours from now as the upper low
settles over Virginia. The surface and upper level lows will then
converge directly over our forecast area, or very close to it, by
late Friday afternoon.

Though today, the slowing of the surface low will cause the back
edge of precipitation to gradually grind to a halt over our region
today, so that while areas further southwest likely end up with some
sun later on, north and northeastern portions of the forecast area
will likely see steady precip continue thru the day. The grey area
is in between, as exactly where the precip grinds to a halt is yet
to be determined, and in fact, some guidance shows it retrograding
back to the south again after halting, so even if the precip ends
for some areas later on, it may restart towards evening. However,
the signal remains clear that the heaviest rain will fall across our
northern zones with considerably less to the south, but overall,
forecast precip has diminished slightly. Still a generous and much
needed precipitation event. With the low nearby, a gusty northwest
wind likely prevails through the day, advecting cooler air into the
region, resulting in a considerably less pleasant day even where
rain ends and the sun tries to emerge. Highs closer to 50 for most,
40 in the Poconos.

Speaking of the Poconos...such chilly temps for highs are certainly
the harbinger of things to come. With cold advection continuing and
the low remaining nearby, precip likely continues through tonight
into Friday and a change to snow is still expected. Forecast totals
have not changed significantly, and we`ve upgraded the winter storm
watch to a warning, while continuing the theme of much higher totals
at the higher elevations than in the lower valleys. With gusty winds
and a heavy wet nature to the snowfall, downed trees and powerlines
are a potential problem in addition to snow covered slippery roads.

Heaviest snow likely falls overnight, but continues through Friday.
In fact, as the surface low heads back south to converge with the
upper low moving overhead, enough cold air looks to accompany it to
change the rain showers to snow showers potentially all the way to
Philly proper and the I-95 corridor. Accumulations will be tough as
temps likely stay a couple degrees above freezing, but we are
forecasting light accumulations, especially on the higher hills,
reaching to just northwest of the city and I-95. So, certainly the
potential first appearance of snow across a significant portion of
the area.

Gusty winds and showers of rain mixed with snow will continue
through Friday, with highs struggling to rise back above 40. A
rather winter-like day, for sure.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
As the surface and upper lows begin moving further east,
precipitation is expected to begin slowly winding down Friday
evening with perception clearing the area by Saturday morning.
With temperatures mainly in the low 40s through the evening,
rain is the expected precip type for most. However, at higher
elevations, temperatures should be cold enough for snow or a
rain/snow mix to continue. Additional accumulations overnight
Friday will be minimal, with a few hundredths of rain or a
tenth of an inch of snow at best. Skies will begin to clear
after precipitation moves out and lows in the 30s are forecast
for most, upper 20s in the southern Poconos.

As the low pressure system that has been driving our weather
pulls further away to our northeast Saturday into Saturday night,
a gusty west-northwest wind will continue and some clouds will
linger, particularly for the northwestern portion of the area. A
rain/snow shower or two is possible in the Poconos, most likely
daytime Saturday, but overall PoPs are low (10-15% for the
Poconos, less than 10% most everywhere else). Temperatures will
be a bit warmer though, with highs on Saturday mostly in the
upper 40s to low 50s, and overnight lows in the upper 30s to low
40s Saturday night.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Summary...Temperatures should turn milder for the start of the
work week ahead of a cold front, then cooling takes place. Some
low shower chances late Monday into Tuesday with a cold front.

Synoptic Overview...A strong closed low is forecast to
gradually move across portions of the Canadian Maritimes through
Sunday night. As this occurs, the cyclonic flow across our
region eases Sunday with the flow aloft becoming more zonal
through early next week. A shortwave trough however may slide
across the Northeast Tuesday. At the surface, strong low
pressure is forecast to shift northeastward across portions of
the Canadian Maritimes in tandem with the closed low through
Sunday. Weak high pressure briefly builds in later Sunday into
Monday, then low pressure tracking to our north pulls a cold
front across our area Monday night into Tuesday. High pressure
then starts to extend eastward toward our area Wednesday.

For Sunday...A closed low gradually shifts across parts of the
Canadian Maritimes and therefore the cyclonic flow eases Sunday
with the flow turning more zonal thereafter. Strong surface low
pressure will follow the closed low, however a tightened
pressure gradient remains across our area through much of
Sunday as high pressure approaches from the west and south. A
notable breeze therefore will continue, however this should
start to ease some Sunday and especially Sunday night.
Temperatures may edge upward a little Sunday given heights aloft
starting to rise. Drier air is expected to continue to
overspread the area with PoPs less than 15 percent during this
timeframe. Some more sunshine should also occur especially
during Sunday as the influence of the closed low wanes, then
clouds may start to increase some again Sunday night ahead of
the next cold front.

For Monday through Wednesday...The flow is forecast to be more
zonal with weak high pressure cresting briefly across our area
Monday. This will result in much less wind. A fairly quick
moving system crossing much of eastern Canada may sharpen some
across the Northeast Monday night and especially Tuesday. This
should drive surface low pressure to our north with a cold front
crossing our area Monday night into Tuesday. As of now with
moisture potentially being more limited, continued with slight
chance PoPs (20 percent) with this system. The front may then
settle to our south Tuesday into Wednesday. High pressure
centered across the Midwest may then start to extend toward our
area during Wednesday.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Mostly MVFR thru the TAF period and into Friday as a strong low
pressure lingers in the area, bringing intervals of rain, with
potential intervals of IFR on occasion. Rain may mix with or even
change to snow across terminals near and especially northwest of I-
95 later tonight into Friday, though any accumulations would be
extremely light. Winds will remain gusty, generally westerly though
with more of a northerly component today and a bit more southerly on
Friday, with gusts of 30 kts at times. Moderate confidence
overall.

Outlook...

Friday night...Sub-VFR, with potential IFR, with light
rainfall. Winds will also be gusty from the northwest.

Saturday and Sunday...VFR. West or west-northwest wind gusts up
to 25 knots, diminishing Sunday night.

Monday...VFR overall with no significant weather anticipated.

&&

.MARINE...
Low pressure lingering nearby will result in gust at or near gale
force through Friday, so while it won`t be galeing the entire
time, decided to extend and expand gale warning for ocean waters
through Friday. Will still allow bay to drop this afternoon as
winds on the more confined waters may drop below gale force more
consistently. Showers will linger through Friday as well.

Outlook...

Friday night through Sunday...Small Craft Advisory conditions
probable. Some low- end gale force gusts possible at times
through Saturday night.

Sunday night and Monday...The conditions are anticipated to be
below Small Craft Advisory criteria.

&&

.HYDROLOGY...
Significant rainfall is forecast across the region for the
first time in months, with widespread totals in excess of 2
inches expected across northern parts of the forecast area. With
the ground relatively dry and hard, we could end up with some
minor flooding issues due to excessive runoff, despite how low
water levels are presently. These would mainly be for the
flashier small streams. In addition, accumulation of leaves
around storm drains may result in localized roadway issues. That
all having been said, given the excessive dryness across the
region presently, there are no plans for a flood watch at this
time.

&&

.CLIMATE...
Driest autumn (Sep-Nov) on record, and any 3-calendar month
period on record, plus current status:

                   9/1-11/20   Driest          Driest 3         Year/
Site              2024 precip  Autumn Year  Calendar months    months
Allentown (ABE)       1.96      3.81  1922      3.58        Oct-Dec 1928
A.C. Airport (ACY)    1.01      3.34  2001      2.35        Oct-Dec 1946
A.C. Marina (55N)     0.79      2.89  1941      2.52        Aug-Oct 1895
Georgetown (GED)      1.17      2.67  2001      2.20        Aug-Oct 2024
Mount Pocono (MPO)    3.35      4.21  1931      3.36        Oct-Dec 1928
Philadelphia (PHL)    1.66      2.37  1922      2.37        Sep-Nov 1922
Reading (RDG)         1.96      2.89  1922      2.89        Sep-Nov 1922
Trenton (TTN)         0.79      3.18  1922      2.66        Jun-Aug 1966
Wilmington (ILG)      1.49      3.17  1922      3.17        Sep-Nov 1922

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Warning from 4 PM this afternoon to 4 PM EST
     Friday for PAZ054-055.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning until noon EST today for ANZ430-431.
     Gale Warning until 6 PM EST Friday for ANZ450>455.

&&

$$

SYNOPSIS...AKL/Gorse/RCM
NEAR TERM...RCM
SHORT TERM...AKL
LONG TERM...AKL/Gorse
AVIATION...AKL/RCM
MARINE...AKL/RCM
HYDROLOGY...RCM
CLIMATE...RCM