Greenville, Delaware 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

864
FXUS61 KPHI 180720
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
220 AM EST Sat Jan 18 2025

.SYNOPSIS...
Surface high pressure is ushered offshore tonight as a warm
front approaches. A strong cold front then crosses our region
Saturday night. Low pressure looks to bring a round of wintry
precipitation to the region Sunday into Sunday night.
Thereafter, a bitterly cold airmass builds in through the
middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As an expansive upper-level trough continues to become carved out
from about central Canada to the southern Plains through tonight, a
leading shortwave trough will lift up across the Mid-Atlantic and
Northeast later this morning and into this afternoon. This feature
will be accompanied by a south-southwest low-level jet of around 40
knots at 925 mb. This will assist in increasing the warm air
advection for a time today ahead of a strong cold front that will
arrive later tonight. A very weak wave of low pressure arrives by
midday along with a warm front also advancing northward at least
some. The main warming looks to get into the coastal plain areas
with high temperatures this afternoon climbing through the 40s with
even 50 degrees possible in southern Delaware. The overall forcing
for ascent with this feature looks to be on the weak side, however
some guidance is showing a brief period of somewhat stronger lift
northward especially across northern New Jersey back into northeast
Pennsylvania. This should result in some rain showers developing
from south to north during the mid to late morning hours.

The model forecast soundings show some initial moistening in the
lowest levels with some lingering dry air aloft for a time, and this
can sometimes lead to some drizzle (freezing drizzle where
temperatures are at or below freezing). However, the overall air
mass initially looks to dry to support much of anything at the
surface and therefore will need to wait until some deeper moistening
occurs later in the morning. By this time, temperatures for most
areas are forecast to be above freezing along with increasing dew
points. As a result, mainly some rain showers. Some snow showers are
expected though across mostly the Pocono area given colder air
remaining longer, with even some sleet possible for a time. Any
snow/sleet accumulations there are expected to be less than an inch.
The warming layer aloft looks weak enough plus with some initial
column cooling due to drier air should keep the risk of any light
freezing rain rather low. Elsewhere, rainfall amounts look to be
less than a tenth of an inch with the greatest PoPs peaking for a
time across the northern half or so of the region. Any showers
should start to end by mid to late afternoon.

As we go through tonight, a strong cold front is forecast to
gradually slide across our region however lift looks rather weak and
therefore no precipitation is currently forecast with it. More
cloudiness however will linger and a light wind will eventually
start to shift to the west and northwest overnight as the front
moves through. Some drier and colder air will then start to seep
south and eastward through the early morning hours on Sunday.
Temperatures are forecast to drop below freezing across the region
overnight, however most surfaces should be drying off by then to
limit any icy spots. We then quickly look back to the west and
southwest as a strong shortwave shifts across the Tennessee Valley
region late tonight. This will begin to activate some lift
approaching from the southwest toward daybreak. This may start to
bring in some light rain/snow (possible sleet) toward some of our
southwest counties close to daybreak Sunday. The main show though
holds off until during the day Sunday and therefore PoPs are on the
very low side until then.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY/...
The strong arctic cold front will cross through the region
early on Sunday morning. With a substantial cold air advection
push behind the front, this will usher in a much colder airmass.
As it does so, an area of low pressure will be developing over
the Deep South along the tail-end of the arctic front. This area
of low pressure will ride along the frontal boundary and deepen
as it moves off the coast near the Virginia/North Carolina
border by Sunday afternoon.

From here, forecast guidance has come into better agreement this
afternoon and has shown an overall northwestern shift to the track
of the surface low by most global and ensemble guidance. This will
place the surface low just off the New Jersey coast on Sunday
afternoon. Still expect that the 12Z Canadian/GEM guidance is
underdoing the cold air advection regime on the northwest side of
the low, so suspect that the low will remain just off our coastline
instead of tracking directly over southern New Jersey. Heading into
Sunday night as the low exits, it appears that the deepening surface
low will pass right over or just off to the northwest of the 40N/70W
benchmark. Also, available guidance suggests an overall uptick in
forecast QPF values by nearly 0.1-0.2", especially north and west of
the I-95 corridor. Factoring in rather high snow-to-liquid ratios to
the north and west, there again has been an increase in forecaster
confidence in plowable snowfall for areas northwest of the I-95
corridor. Thus, our forecast this afternoon generally incorporates a
blend of the situation discussed above. Still, this does not mean
that areas south and east, including the Philadelphia metro will see
snow through the entire duration of the event. Some short-range
guidance does depict a subtle warm layer at 850mb so there may be at
least a brief rain/snow mix for areas directly along the I-95
corridor at the onset. For this reason, have kept a rain/snow mix
for these areas through Sunday morning and early afternoon. However,
as the cold air begins to pour into the region as the low departs,
we should see an eventual changeover from rain to rain/snow mix to
all snow even down to the coast by Sunday night.

In coordination with neighboring forecast offices and WPC, have opted
to issue Winter Storm Watches for Sunday morning into Sunday evening
for all of our Pennsylvania Counties except for Delaware,
Philadelphia and Lower Bucks. Also, have issued Winter Storm Watches
for Hunterdon, Warren, Sussex, and Morris Counties in New Jersey.
These are the areas which are likely to observe warning level
snowfall with lesser amounts expected elsewhere. In terms of
amounts, generally expecting a widespread 5-8" snowfall in the Watch
area with localized amounts up to 10" possible (especially across
the higher terrain). Stepping southeast into and along the immediate
I-95 corridor, expecting a 3-5" snowfall event due to potential
mixing at the onset. Of course, these totals can be higher or lower
depending on the duration of mixed precipitation. Closer to the
coast where prolonged periods of mixing and at times plain rain is
to occur before the changeover, a 1-3" event is expected. As always,
the forecast can vary over the next 36-48 hours as more hi-res
guidance becomes available. So stay tuned for the latest updates to
the forecast through the holiday weekend!

In wake of the system on Sunday night, we`ll start to see the
beginning of the cold air mass take aim at the area. Skies will
begin to gradually clear late Sunday night into Monday morning, and
with a fresh snowpack in place, this should result in low
temperatures falling into the single digits/teens areawide. With a
stiff northwest wind filling in on the backside of the departing
low, should see wind chills in the single digits for most. Mostly
clear skies will be on tap for Monday as strong Canadian high
pressure builds over the Northern Plains. High temperatures will be
limited to the teens/20s with wind chills primarily in the teens.
Enhanced northwest winds could pose a blowing snow in areas where
heavier snowfall occurs.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
A strong artic airmass will dominate the region for the majority of
the long term. Dangerously cold temperatures will be the main focus
of the forecast. These dangerously cold temperatures are anticipated
at least through the first half of the long term (i.e., Monday night
through Wednesday night). Gradual improvements in temperatures are
possible by Thursday/Friday.

Cold weather headlines are nearly certain for several periods of the
long term. Dangerously cold temperatures could result in impacts to
people, animals, and infrastructure. Dangerously cold conditions for
people and animals are possible resulting in an increased risk of
hypothermia and frost bite. For infrastructure, impacts could
include freezing water pipes and high demand for heating energy.
Stay tuned to the latest forecast for additional information as we
get closer.

While strong surface high pressure will influence and dominate over
the region for the majority of the term, deterministic guidance
continues to support strong baroclinic instability over the Gulf of
Mexico and offshore of the eastern CONUS Tuesday/Wednesday. This
will likely lead to the development a strong cyclone over or in the
vicinity of the aforementioned areas. However, guidance continues to
indicate this cyclone will remain mostly offshore of the northeastern
CONUS as it tracks northeastwards Tuesday night through Wednesday
night. It is worth mentioning this synoptic pattern and watching
for changes as we go forward in time; a slight shift in track could
lead to another snowfall event for the region during the Tuesday
night through Wednesday timeframe.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through today...VFR ceilings lower to a period of mainly MVFR
from about mid-morning onward along with some rain showers.
Some IFR ceilings cannot be ruled out. Visibilities lower to
MVFR at times with the rain showers. Light and variable winds
becoming south or southeast 5-10 knots, then becoming southwest
this afternoon. Low-level wind shear through about early this
afternoon as winds near 2,000 feet AGL increase out of the
south-southwest around 40 knots. Low confidence regarding timing
details.

Tonight...VFR ceilings overall should start to lower to MVFR late at
night and more toward daybreak Sunday. Southwest winds around 5
knots becoming light and variable, then becoming west to northwest
around 5 knots late. Low confidence regarding ceiling heights.

Outlook...

Sunday and Sunday night...Sub-VFR conditions. For KRDG, KABE,
KTTN, KPNE, KPHL, and KILG, snow likely. For KMIV and KACY, a
mix of rain of snow is likely before a changeover to rain.

Monday through Tuesday...VFR. Northwest wind gusts up to 30
knots.

Tuesday night through Wednesday...Sub-VFR conditions possible.
Watching low pressure development, which could winter precipitation
to the region.

&&

.MARINE...
Small Craft Advisory remains in effect through early this afternoon
from Little Egg Inlet, NJ northward as southerly winds gust to 25
knots for a time. Otherwise, the conditions are anticipated to
remain below Small Craft Advisory criteria through tonight.

Outlook...

Sunday...No marine headlines expected. Rain and/or snow likely.

Sunday night through Monday...SCA conditions likely due to wind
gusts up to 30 kt and seas around 5 feet.

Monday night...SCA conditions likely. Due to the combination of cold
air/water temperatures, gusty winds, and seas, freezing spray
accretion is likely.

Tuesday through Wednesday...SCA conditions not currently
anticipated. Watching low pressure development offshore. Some
freezing spray accretion possible at times due to combination of
cold air/water temperatures, gusty winds, seas.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Watch from Sunday morning through late Sunday
     night for PAZ054-055-062.
     Winter Storm Watch from Sunday morning through Sunday evening
     for PAZ060-061-101>105.
NJ...Winter Storm Watch from Sunday morning through late Sunday
     night for NJZ001-007>009.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ450-
     451.

&&

$$

SYNOPSIS...Wunderlin
NEAR TERM...Gorse
SHORT TERM...DeSilva
LONG TERM...Wunderlin
AVIATION...DeSilva/Gorse
MARINE...DeSilva/Gorse/Wunderlin