Warm moist air advecting over a cold surface air mass resulted in widespread dense fog starting late on December 9th and continuing through much of the day December 10th.
Every event with a place to put it; 4 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS PHI wrote it up.
Warm moist air advecting over a cold surface air mass resulted in widespread dense fog starting late on December 9th and continuing through much of the day December 10th.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Dec 9, 6:44 PM EST, 2 hours 16 minutes earlier.
Area Forecast Discussion
National Weather Service Mount Holly NJ
644 PM EST Mon Dec 9 2024
.SYNOPSIS...
Low pressure and a warm front lifts north of the area this
evening. Another area of low pressure over the Great Lakes
tracks east and intensifies over the area on Wednesday. A strong
cold front passes through the area Wednesday night through
Thursday morning as the low departs. High pressure returns to
close out the week. Another system may affect the area early
next week.
&&
.NEAR TERM /THROUGH TUESDAY/...
As of 6:30PM...the first system has primarily departed the
area, leaving behind just some stratus to the north and west of
Philadelphia. Otherwise, most locales are encountering clear
skies with some patchy ground fog developing at this hour.
For the overnight hours through Tuesday, we'll remain in a
relatively benign pattern in between today's and the Wednesday
system. Southwest flow aloft and light onshore (southeast) flow
at the surface. A warm front and another shortwave trough will
approach by late Tuesday afternoon.
Generally expecting dry conditions tonight through Tuesday
morning outside of a stray shower. Mild temperatures tonight
with lows near 40-45 degrees. The thing to keep an eye on will
be the fog potential. With the surface flow more southerly than
easterly, the pattern favors more stratus than fog, although
guidance is notably aggressive with visibility reduction
overnight. With the warm advection and lack of dry air, low
clouds and possibly some fog will persist much of the daytime
Tuesday as well. Have carried a mention of fog through the
entire forecast period. As the warm front and some better
forcing arrive later in the day, some rain could develop (30-50%
chance), although the "main show" will hold off until Tuesday
night. High temperatures mainly in the upper 40s and 50s.
&&
.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
Low pressure over the Ohio Valley lifts to the northeast into
western Pennsylvania and western New York Tuesday evening, and
as it moves into northern New York and eastern Canada, it will
drag a warm front north through the region. A strong, closed, H5
low over central Canada will dig down into the Northern Plains,
and a deep upper trough will extend from that low down into the
Gulf Coast. This trough will take a negative tilt as it tracks
east through the region Wednesday through Wednesday night.
Strong shortwave energy associated with this system will lift
from the Gulf Coast and into the Mid-Atlantic and Northeast on
Wednesday, and strong surface low pressure will develop over the
area. An intense low level jet will lift along the coast with
80-90 kt winds on Wednesday. There should be enough of an
inversion up to around 925 mb that the strongest winds will
remain aloft, and southerly winds increase to 15 to 20 mph with
gusts up to 30 mph. Rain moves into the area Tuesday night and
continues through Wednesday, with rain becoming moderate to
heavy at times late Wednesday and Wednesday evening. A few
thunderstorms are possible, and those would be associated with
the low level jet that could result in enhanced locally heavy
rain as well as gusty winds.
Low pressure departs Wednesday evening and a strong cold front
passes through the region. Rain ends from west to east, although
may end as a period of snow in the southern Poconos with
minimal snow accumulations. A tight westerly pressure gradient
develops with winds 15 to 20 mph with 30 to 35 mph gusts
Wednesday night into Thursday.
In terms of temperatures, lows Tuesday night will occur in the
evening with temperatures rising into daybreak. Lows will be in
the 40s to low 50s, rising into the upper 50s by daybreak.
Strong warm air advection will be in place on Wednesday with
highs in the low to mid 60s. Surface dew points will even be in
the low 60s as well. Temperatures Wednesday night will crash
behind the front with lows in the upper 20s to low 30s. Highs on
Thursday will be well below normal, and generally in the 30s.
This will also result in the most rainfall the area has
received from one storm since probably early June. From Tuesday
afternoon through Wednesday night, 1.5 to 2 inches of rain is
possible for Delmarva and most of New Jersey, while as much as 2
to 3 inches of rain is possible for southeast Pennsylvania, the
southern Poconos, and northern New Jersey. The Weather
Prediction Center has the region in a Marginal Risk (1/4) for
Wednesday.
&&
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
The center of high pressure to the west moves into the Mid-
Atlantic Thursday night, then will be over the Northeast on
Friday before departing Friday night. Although the high will be
over the Canadian Maritimes on Saturday, it will be close
enough to continue to influence the region before departing
Saturday night.
Another frontal system approaches on Sunday and slowly works
its way through the region through Monday.
Temperatures remain well below normal Friday and Saturday
before moderating back to normal levels Sunday and Monday.
&&
.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Tonight...For KRDG/KABE/KTTN, anticipate mainly IFR lowering to LIFR
conditions. For the KPNE/KPHL/KILG/KMIV, VFR early will
ultimately lower to LIFR by 04-06Z. For KACY, skies will
primarily remain near VFR with some lowering CIGs after 08Z.
Areas of fog possible at all terminals. Winds primarily 5 kt or
less, becoming variable/calm. Overall, low confidence on the
timing and evolution of restrictions.
Tuesday...Initially IFR/LIFR conditions improving to MVFR
conditions after 15Z. Some lower restrictions could linger to
18Z at KABE/KRDG. Southeast winds 5-10 kts. Moderate confidence
overall.
Outlook...
Tuesday night...MVFR/IFR conditions in RA/BR.
Wednesday...Continued MVFR/IFR conditions in RA/BR. Isolated
TSRA possible for KACY/KMIV late in the day. Potential for 50+
kt of LLWS in the afternoon and evening.
Wednesday night...Conditions improve to VFR after midnight.
Strong west winds develop.
Thursday...VFR. West winds gusting 25 to 35 kt.
Friday through Saturday...VFR.
&&
.MARINE...
Marginal SCA conditions possible for the ocean waters. The best
chances for 25 kt gusts will remain across the southern waters,
so will keep the previous SCA in effect for now. Gusts to 25
kt and seas building around 5 ft into the evening. SCA is set
to expire around midnight, although it may be canceled early.
Some light rain ending by early this evening. Areas of fog
possible.
For overnight tonight and Tuesday, no marine hazards are
currently anticipated. Southerly winds 5-10 kts. Areas of fog
and low clouds are expected to develop, especially Tuesday
afternoon. Visibility in fog could drop below 1 nautical mile at
times, which could prompt a Dense Fog Advisory if fog becomes
widespread.
Outlook...
Tuesday night...SCA conditions developing. S winds 15 to 20 kt
with 25 to 30 kt gusts and seas building to 4 to 6 ft. VSBY
restrictions in rain and fog.
Wednesday...Gales develop on the ocean waters with S winds 15
to 20 kt with gusts up to 35 kt. A Gale Watch is now in effect
starting Wednesday afternoon. Seas build to 9 to 11 ft. VSBY
restrictions in rain and fog. Isolated thunderstorms possible in
the afternoon.
Wednesday night...A Gale Watch remains in effect. Winds turn
west 15 to 25 with 35 to 40 kt gusts. VSBY restrictions in rain
and fog. Isolated thunderstorms possible in the evening.
Thursday...A Gale Warning may be needed. West winds 15 to 25 kt
with gusts up to 35 kt. Conditions subside to sub-SCA criteria
Thursday night.
Friday through Saturday...Sub-SCA conditions expected.
&&
.HYDROLOGY...
The Tuesday through Wednesday Night system will bring a
widespread 2 to 3 inches of rain with rainfall likely being
heavy at times. Given low stream levels, and most of the area
still being in moderate to extreme drought, flooding concerns
should mainly be confined to urban and poor drainage areas (make
sure to clear out any drains clogged with falling leaves!).
Even ensemble river forecasts show less than a 30% chance of
hitting action stage for nearly all forecast points.
NBM Probabilities of 2+ inches are around 80-90% regionwide for
the 48 hour period ending Thursday morning, meaning there is
high confidence in totals generally falling somewhere in the 2
to 3 inch range. Cannot rule out some higher amounts, especially
along the coast and in northern New Jersey, as the development
of a coastal front could enhance precipitation rates a touch.
Probability of 3+ inches of rain within the 48 hour period
ending Thursday morning is around 30-45% for the NJ coastal
plain, with around 15-30% for the rest of the area. A MARGINAL
(1/4) risk for excessive rainfall is in place, which is about
right given the limiting factors mentioned above.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Wednesday evening through late Wednesday night
for ANZ430-431.
Gale Watch from Wednesday afternoon through late Wednesday
night for ANZ450>455.
Small Craft Advisory until midnight EST tonight for ANZ451>455.
&&
SYNOPSIS...MPS
NEAR TERM...DeSilva/Staarmann
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...DeSilva/MPS/Staarmann
MARINE...DeSilva/MPS/Staarmann
HYDROLOGY...WFO PHIThe national radar mosaic for the moment the first event began.
4 dense fog. Local standard time, as filed.
Delaware Coastal Airport reported several hours of dense fog from late on December 9th into December 10th.
Delaware Coastal Airport reported several hours of dense fog from late on December 9th into December 10th.
Delaware Airpark reported several hours of dense fog from late on December 9th into December 10th.
Wilmington Airport reported several hours of dense fog from late on December 9th into December 10th.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Delaware in 2024Every state in 2024This day in weather historyWarnings in Delaware, December 2024
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1225880, 1225881, 1225882, 1225883.