Dense Fog in Delaware, March 15 to 16, 2025

Area Forecast Discussion
National Weather Service Mount Holly NJ
304 PM EDT Sat Mar 15 2025

.SYNOPSIS...
A warm front slowly lifts across our region through Sunday, then a
strong cold front moves through Sunday night into early Monday
morning. High pressure builds in Monday night and Tuesday before
shifting offshore later Wednesday. A potentially strong cold front
then moves through later Thursday, followed by high pressure
extending into our area Saturday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
As of this afternoon, high pressure was situated off to our east
over the Atlantic while a large area of low pressure extended
from the upper Great Lakes southward towards the Gulf coast. A
warm front associated with this low currently extends from Ohio
into western PA and then southward into Virginia. North and east
of this front, our area continues to remain socked in with low
clouds and even a bit of spotty drizzle, mist, and fog. This has
kept temperatures cooler than initially forecast. By later this
afternoon, we may see a bit more in the way of some breaks in
the cloud cover developing but not expecting any significant
clearing occuring.

More in the way of clouds and some fog/drizzle may redevelop this
evening but then may break a bit overnight as a strong warm front
slowly but surely lifts northward. As a result, temperatures should
hold fairly steady or even rise some overnight as southerly winds
start to increase.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/...
Changing conditions as a strong cold front moves through later
Sunday. Strong gradient winds, a low severe thunderstorm risk
(damaging winds) and locally heavy rain by late Sunday and Sunday
night.

As a strong upper-level trough across the Midwest and Plains
gradually shifts eastward, deep surface low pressure tracks across
the Great Lakes into Ontario through Sunday. This will result in a
warm front lifting northward through Sunday ahead of the cold front
with warm air advection strengthening. Part of the upper level
energy associated with the system looks to lift up into Canada,
however there is trailing strong energy that maintains the
trough all the way to the Gulf Coast area. As this shifts into
the east by Sunday night, the wind field is forecast to increase
substantially. This along with the tightening of a pressure
gradient within the warm sector will result in an increasing
surface wind. This is expected to peak across the area Sunday
afternoon and evening ahead of the cold front as we see a little
more vertical mixing. Speaking of which, that continues to be
one of the most challenging parts of the forecast...the question
of how deep we'll be able to mix on Sunday as we'll still have
a low level inversion in place. Deeper mixing will allow for
stronger wind gusts. Think peak wind gusts look to be in the
35-45 mph range although cannot rule out some local gusts to 50
mph if we can mix a bit deeper. The best chances for these
higher gusts appear to be over Delmarva into SE PA where it
should be a bit warmer with more mixing. Temperatures will be
warm for this time of year but not as warm as they would be if
we were able to get some sunshine which is unlikely. Generally
expect highs in the 60s, except around 70 over most of Delmarva
except near the water. The surface dew points are forecast to
surge to near 60 degrees across much of the area through Sunday
evening ahead of the strong cold front.

As the cold front starts to arrive later Sunday afternoon, the
forcing for ascent looks to be tied close to or with the front
itself or a pre-frontal band. The main widespread shower activity,
from west to east, looks to be late Sunday afternoon through about
the first half of Sunday night. Given the robust wind field in place
and strong forcing arriving, cannot rule out a strengthening
convective line with or just ahead of the cold front. The deep-layer
shear profiles will be very strong however the overall instability
looks to be rather weak (low CAPE/high shear setup). Despite the
rather weak instability, especially given the timing at night,
strong forcing could still drive a low-topped convective line that
produces little or no lightning. It is within these environments
that some of the very strong winds from aloft could get transferred
down to the surface resulting in some damaging wind gusts. As a
result, there remains some risk of severe thunderstorms although
again the damaging wind threat from convection could occur with
little or no lightning. A plume of precipitable water is forecast to
reach or exceed 1.5 inches and therefore areas of heavier hourly
rainfall rates look probable. Given how dry it has been, not
expecting much in the way of flooding, however when hourly rainfall
rates peak and especially when over the urban areas then some
localized flooding is quite possible late Sunday afternoon and
Sunday night. The storm total rainfall amounts across the region
currently are between 0.75-1.25 inches (highest near the coast and
lowest in our north/west zones), with locally higher amounts
possible. SPC withdrew the Slight Risk with the update yesterday
afternoon, putting most of the region in a Marginal Risk (level
1 out of 5). This remains the case, which matches the setup of
high shear and little to no CAPE. Overall, while it can't be
ruled out, the threat for damaging winds from convection remains
low.

The trends in the forecast guidance are continuing to lean in the
direction of developing a new area of low pressure along the cold
front Sunday night as it reaches the coast. This is likely to keep
rain/showers going through Sunday night into the first part of
Monday near and S/E of the I-95 corridor while precip tapers off
farther north and west. In fact if this trend continues, much of
Monday could remain unsettled with lingering showers over the
eastern half of the forecast area (roughly from around Philly and
points east). However precip by Monday won't be as heavy and the
threat for storms will have passed as we'll be on the cool side of
the system. Generally expect highs in the 50s Monday with N/NW
breezes.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Summary...Temperatures above average, with the exception of Friday.
The next main system with shower chances are centered on Thursday.

Synoptic Overview...An upper-level trough axis is forecast to shift
offshore Monday night with the flow aloft turning zonal in its wake.
A mid level ridge axis then extends into our area Tuesday before it
shifts offshore Wednesday as a strong upper-level trough moves
through the Tennessee and Ohio Valleys. This trough then arrives
across the East Thursday into Friday, as it may take on a negative-
tilt, before lifting out into Saturday. At the surface, high
pressure initially across our area Wednesday weakens and shifts
offshore. A potentially strong cold front then arrives later
Thursday, with a trough or secondary cold front moving through on
Friday. High pressure should then extend into our area Saturday.

For Monday night through Wednesday...In the wake of an upper-level
trough Monday night, the flow aloft turns zonal through Tuesday.
This zonal flow then backs as a strong upper-level trough migrates
across the Tennessee and Ohio valleys through Wednesday. This will
take weak surface high pressure over our area offshore by later
Wednesday night. The presence of the ridge aloft on Tuesday should
result in a milder air mass in place, although some onshore wind
component as the flow lessens will keep it cooler closer to the
coast. Some showers could persist Monday evening along the coast
given the slow moving trough axis, it then looks to be dry overall.
However, there could be just enough of an onshore flow coupled with
some warm air advection aloft Wednesday to result in a marine layer
developing. This may result in cooler temperatures, and at least
some model guidance has trended cooler for Wednesday. Given light
enough flow and a more onshore component to the surface winds, it
will be several degrees cooler closer to the coasts both days.

For Thursday and Friday...A potentially strong upper-level
trough is forecast to pivot across the East during this time frame.
This trough may take on a negative-tilt as strong energy rotates
through the base of the trough and then its east side. As deep low
pressure moves across the Great Lakes and adjacent Canada, its
strong cold front should arrive in our area later Thursday then be
offshore by Friday morning. This timing of course could change,
however as of now a plume of moisture should extend into our area
and given the strengthening wind field and strong trough and cold
front arriving, some heavier showers are possible. Given the parent
low currently forecast to track well north of our area and likely
occlude though, there could be a disconnect in the forcing across
our area and therefore a lower chance for showers with the arrival
of the front. There could also be some lingering stratus and if this
were to occur then temperatures will be cooler and any instability
will be more limited. In the wake of the cold front, another setup
of a tightened pressure gradient which would translate to a gusty
northwesterly breeze Friday. Given the trough aloft overhead Friday,
a much cooler air mass is forecast to be in place.

For Saturday...The aforementioned upper-level trough is forecast to
be lifting to our northeast, allowing surface high pressure to our
south to extend into our area with less wind. The next system
however may be moving into the Ohio Valley, and therefore any
sunshine may give way to increasing clouds. Given the trough moving
out, daytime temperatures should be several degrees milder than
Friday.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of this afternoon...IFR ceilings should slowly improve some.
Mostly VFR visibility. Southeasterly winds mainly 5-10 knots. Low
confidence on how much improvement occurs.

Tonight...Pending any improvements early, IFR/LIFR ceilings then
redevelop along with some fog. The conditions may then start to
improve from the south at some terminals overnight or closer to
daybreak Sunday. Southeasterly surface winds mainly 4-8 knots. A
strong southerly low-level jet develops overnight resulting in low-
level wind shear. Low confidence.

Sunday...Areas of IFR ceilings should improve to MVFR to possibly a
period of low-end VFR. Local visibility restrictions in the morning
due to some fog. While a few showers may develop, the main shower
activity looks to hold off until Sunday night. Southerly winds
increase to 10-20 knots with gusts to around 35 knots, with the
highest winds in the afternoon. Low confidence.

AMD NOT SKED at KMIV due to a comms issue.

Outlook...

Sunday night...MVFR/IFR conditions along with showers from west to
east. Some of the showers can be heavy. A line or broken line of low-
topped convection (mostly showers) could move through and contain a
few thunderstorms along with locally brief stronger wind gusts.
South to southwest winds diminishing to 10 knots or less and
becoming west-northwest late.

Monday...MVFR ceilings with some lingering showers possible in the
morning, then improving to VFR with decreasing clouds. Northwest
wind gusts up to 25 knots.

Tuesday...VFR.

Wednesday...Some MVFR ceilings possible.

Thursday...A period of sub-VFR conditions and some showers
possible.

&&

.MARINE...
A Marine Dense Fog Advisory remains in effect until 5 PM this
afternoon. Otherwise, as we head into tonight a Small Craft Advisory
in effect as seas build and winds increase from south to north.
continuing visibility restrictions will be possible due to areas of
fog.

Sunday through Sunday night...Gale Watch remains in place for
all coastal waters as wind gusts will near 34 kt out of the
south, primarily in the afternoon and evening. Seas 7-10 feet
expected.

The conditions then start to subside for a while Monday although
seas remain elevated on the ocean zones therefore a Small Craft
Advisory looks probable.

Outlook...

Monday night...Small Craft Advisory conditions for the ocean zones
especially for seas above 5 feet. The conditions should be below
Small Craft Advisory criteria on Delaware Bay.

Tuesday...Small Craft Advisory possible for the ocean zones as seas
gradually subside to and below 5 feet.

Wednesday...The conditions are anticipated to be below Small Craft
Advisory criteria.

Thursday...Small Craft Advisory conditions possible.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 10 PM this evening to 6 AM EDT
     Sunday for ANZ430-431-450>455.
     Gale Watch from Sunday morning through Sunday evening for
     ANZ430-431-450>455.
     Dense Fog Advisory until 5 PM EDT this afternoon for
     ANZ450>455.

&&

SYNOPSIS...Gorse
NEAR TERM...Fitzsimmons
SHORT TERM...Fitzsimmons/Gorse
LONG TERM...Gorse
AVIATION...Gorse
MARINE...Fitzsimmons/Gorse