Tredyffrin, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

263
FXUS61 KPHI 140259
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
959 PM EST Wed Nov 13 2024

.SYNOPSIS...
High pressure over the area presently lifts to the northeast by
Thursday night. Low pressure develops over the Mid- Atlantic on
Thursday and moves offshore Thursday night through Friday. High
pressure returns for the weekend. A weak frontal system passes
through the region on Monday, followed by a return to high
pressure for the middle of next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
10 PM update... made some tweaks to temps based on latest
readings. Otherwise, no changes.

Earlier discussion...A narrowing mid to upper level ridge will
cross our area later tonight before weakening during Thursday.
Some blocking sets up on the synoptic scale through Thursday,
with chilly air remaining in place.

An approaching upper-level trough should close off while
another closed low retrogrades, slides northwest or west, to
near Nova Scotia Canada through Thursday. Both of these support
surface low pressures with the first developing off the
southeast Virginia/North Carolina coast while the latter puts
low pressure closer to the Nova Scotia area. Given the
retrograding to our northeast, surface high pressure while it
weakens over our area does still remain overhead for much of our
region. While we need the rain, this pattern offers decreasing
chances due to the incoming system hitting a brick wall and
being deflected to our south.

Regarding more of the details, strong surface high pressure
centered well to our north this afternoon and tonight will shift
northward and weaken with time during Thursday. This will
result in light to near calm winds tonight and with a very dry
air mass the temperatures will drop quickly this evening. Some
cirrus will continue to crest the ridge axis as it crosses our
area, however this should not significantly impact the
radiational cooling for most of tonight. As a result, many areas
are forecast to drop below freezing once again overnight. It
will be the warmest along the coast due to a continued
northeasterly wind.

As we go through Thursday, clouds will quickly increase/thicken
with this initially being high to mid level clouds and then
some lower clouds should also develop within the easterly low-
level flow. The air mass will be undergoing some moistening and
this looks to be the most pronounced across our southern zones
where surface high pressure weakens the most (driest across
northern New Jersey into northeast Pennsylvania). While some
sprinkles or a few light showers cannot be ruled out across our
eastern Maryland Shore zones north to western Chester and far
western Berks counties late in the afternoon, it looks like the
dry air will win out overall and therefore lowered the PoPs some
more for the late afternoon hours. The increased cloudiness
will result in another chilly day with highs only in the 40s to
lower 50s with a mostly light northeast or east breeze.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
High pressure will be centered over eastern Canada with the
base of the high down through New Jersey. A closed H5 low will
move through the Ohio and Tennessee Valleys and surface high
pressure out ahead of it will be shunted south towards the Mid-
Atlantic and Southeast Thursday night through Friday morning.
Best chances for rain will be over Delmarva where likely to
categorical PoPs are forecast, but there may be some showers up
to the southern Poconos. Generally 1/10 to 1/4 inch QPF possible
for Delmarva, but only a few hundredths to less than 1/10 inch
QPF possible elsewhere. There may even be a few wet snowflakes
in the southern Poconos.

Low pressure departs on Friday. Canadian high pressure builds
down into the Great Lakes. A milder airmass spreads into the
region Friday and Saturday, with highs in the upper 50s Friday
and then in the lower 60s on Saturday. A drier airmass also
spreads into the region with surface dew points generally in the
20s to low 30s. A brisk northwest flow develops on Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
High pressure over the Ohio Valley Saturday night will slide
off the Mid-Atlantic and Southeast coasts Sunday, and this high
will remain off the Southeast coast. Seasonably chilly Saturday
night, and then mild on Sunday with highs in the low 60s.

A weak frontal system will pass through the region late Sunday
night through Monday morning. Although NBM showing this will
pass through dry, CONSALL guidance showing slight chance to low
chance PoPs. 12Z/13 GFS does have some strong shortwave energy
with its passage, and does have a wetting rain with its passage.
Although it seems to be an outlier, will go ahead and carry
slight chance PoPs from after midnight Sunday night through
around daybreak Monday morning. Based on recent trends, however,
if there is any QPF that falls, it will likely be minimal at
best.

High pressure builds in from the west on Monday and moves off
the Mid-Atlantic coast on Tuesday. Another weakening frontal
system will pass through the region Tuesday night through
Wednesday. Will carry slight chance PoPs for now.

A, relatively speaking, warm airmass spreads into the region
Monday with highs in the mid to upper 60s. These highs are 10+
degrees above normal for this time of year. Still mild, but
several degree cooler on Tuesday and Wednesday. The airmass will
be dry, but not overly so, as dew points look to generally be
in the upper 30s to low 40s.

&&

.AVIATION /03Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Overnight...VFR, with increasing mainly high clouds especiallly
overnight. Northeast winds around 5 knots to locally light and
variable. High confidence.

Thursday...VFR ceilings, but lowering to less than 10kft with
scattered clouds down around 3kft possible. Northeast to east-
northeast winds 5-10 knots. Moderate confidence.

Outlook...

Thursday night...Overall, VFR, but cannot rule out some MVFR
CIGs in light SHRA. Best chances for SHRA KRDG-KPHL-KILG.

Friday through Saturday...VFR. NW winds with gusts up to 25 kt
expected on Saturday.

Sunday through Monday...Generally VFR, but some brief MVFR CIGs
possible from 06Z-12Z Monday in light SHRA.

&&

.MARINE...
A Small Craft Advisory remains in effect for most of the New
Jersey and Delaware Atlantic coastal waters through at least
Thursday mostly due to elevated seas. The north to northeasterly
wind gusts should be near or less than 25 knots, however this
flow will keep the sea heights elevated. The conditions are
anticipated to stay below Small Craft Advisory criteria on
Delaware Bay through Thursday.

Outlook...

Thursday night...SCA remains in effect for the NJ ocean waters
south of Manasquan Inlet for seas around 5 feet. Sub-SCA
conditions elsewhere.

Friday...The SCA may have to be extended through Friday as seas
will remain around 5 feet.

Friday night through Saturday...SCA conditions expected. for NW
winds gusting 25 to 30 kt and seas 4 to 6 feet.

Sunday through Monday...Sub-SCA conditions expected.

&&

.FIRE WEATHER...
For Thursday, the low-level moisture increases especially
across southern New Jersey, Delmarva into southeastern
Pennsylvania as a system approaches from the west. Minimum RHs
are forecast to generally between 35-60 percent (lowest across
northern New Jersey) and light northeast to east-northeast
winds (lightest also across northern New Jersey). Given this,
not expecting any concerns regarding wildfire spread.

A weak system will pass south of the region Thursday night,
possibly resulting in a wetting rain for portions of Delmarva
with 1/10 to 1/4 inch QPF for the eastern shores of Maryland and
Delaware. Less than 1/10 inch QPF likely elsewhere, with
majority of the area especially from Philadelphia north and east
likely seeing no measurable precipitation.

High pressure returns for the end of the week and weekend.
Friday will feature northwest winds 10 to 15 mph with gusts up
to 20 mph and MinRH values 25 to 45 percent.

Stronger winds with lower humidity values on Saturday with
northwest winds 15 to 20 mph with gusts up to 25 mph and MinRH
values 25 to 30 percent. Sunday will once again feature MinRH
values 25 to 35 percent with northwest winds 10 to 15 mph with
gusts up to 20 mph.

&&

.TIDES/COASTAL FLOODING...
Confidence remains high that a combination of persistent shore
parallel winds and increasing astronomical tides with a full
moon on Friday will lead to coastal flooding for the back half
of the week. Widespread minor tidal flooding is expected along
the Atlantic Ocean coasts of New Jersey and Delaware and the
associated backbays beginning with the Thursday morning high
tide. The threat for moderate tidal flooding has continued to
decrease so the previous Coastal Flood Watch has been converted
to a Coastal Flood Advisory. Cannot completely rule out some
spotty moderate flooding though, particularly with the Friday
morning high tide, most likely along the back bays in Cape May,
NJ and Sussex, DE counties.

The Coastal Flood Advisory also includes all counties along the
Delaware Bay in both New Jersey and Delaware for both the
Thursday and Friday morning high tides.

By Friday, tidal piling into the Delaware Bay looks to result
in minor tidal flooding occuring further north, along the tidal
Delaware River. However, confidence is a little bit lower in the
extent of widespread minor coastal flooding along the Delaware
River so have held off on issuing an advisory for this area for
now.

Coastal flooding is not expected to occur along the Chesapeake
Bay in the Eastern Shore of Maryland.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 7 AM Thursday to noon EST Friday
     for NJZ016.
     Coastal Flood Advisory from 4 AM Thursday to noon EST Friday
     for NJZ012>014-020>027.
DE...Coastal Flood Advisory from 7 AM Thursday to noon EST Friday
     for DEZ001.
     Coastal Flood Advisory from 4 AM Thursday to noon EST Friday
     for DEZ002>004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST Friday for ANZ451>455.

&&

$$

SYNOPSIS...MPS/RCM
NEAR TERM...Gorse/RCM
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...Gorse/MPS/RCM
MARINE...Gorse/MPS/RCM
FIRE WEATHER...
TIDES/COASTAL FLOODING...