NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Oct 29, 9:20 AM EST, 2 hours 40 minutes earlier.
Area forecast discussion, Oct 29, 9:20 AM EST, 2 hours 40 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
1020 AM EDT Fri Oct 29 2021
.SYNOPSIS...
A large and slow moving low pressure system over the Tennessee
Valley will move towards the Middle Atlantic region today and
remain across the area for much of the weekend. Later Sunday,
the low will begin to move away allowing high pressure to ridge
in from the Plains states. A cold front will arrive for Tuesday.
Another disturbance could affect the area towards the end of
the week.
&&
.NEAR TERM /THROUGH TONIGHT/...
Overview: The large closed low over the Tennessee Valley will
be progressing into the Middle Appalachian region tonight. The
occluded front associated with this low will cross our region
overnight.
What has changed: No big changes with the latest update.
Timing: Showers pushed northward through the region this morning
and have largely dissipated. The next round of precipitation is
starting to push into Delmarva and will continue to overspread
the region through the day. The main band of rain (and primary
risk for heavy rain), doesn't look to arrive in our region until
this afternoon into this evening. The period for the strongest
winds will be from early this afternoon through late this
evening. Progression will be from south to north, so northern
Delmarva and far southern NJ will see the strongest winds
through the afternoon, while the northern NJ coast won't see the
strongest winds until likely early this evening.
Hazards: The main hazards with this system remain coastal flooding
(this is discussed further in the tide and coastal flood section
below), strong winds, and still some risk for heavy rain. There is
also a small chance for tornadoes across portions of Delmarva.
Strong winds: This, along with coastal flooding, remains my biggest
concern with hazards today. As mentioned above, the wind advisory
was expanded to include New Castle County DE, Salem, SE Burlington,
Ocean, and Monmouth Counties in NJ. In these areas, model soundings
have wind gusts very close (but just below) 40 kt/46 mph. Given the
strength of the low level jet, erred on the side of caution and have
forecasted winds right at wind advisory criteria in these locations.
For the remainder of the zones where a wind advisory or high wind
warning was already in effect, no major change to the wind forecast.
Heavy rain: The threat for this continues to decrease, especially as
it now looks like the main band of heavy rain will progress quickly
north. There remains some chance for localized flooding, especially
in areas that had flooding with the Monday/Tuesday coastal low, or
in areas where heavy rain could exacerbate tidal flooding if it
arrives near the time of the high tide. However, the risk for
freshwater flooding seems too marginal to issue a flood or flash
flood watch at this time.
Severe thunderstorms/tornadoes: Instability will be quite limited.
However, even with a marginal instability, a few thunderstorms are
possible, primarily in Delmarva. Any thunderstorms in the vicinity
of the occluded front will be developing in an area with significant
wind shear and helicity. Thus, rotating storms will be possible in
this area. The higher threat for this looks to be further south from
our region near the front during the period of max mixing, but will
have to watch if the front progresses north faster than currently
forecast, the risk for rotating storms could develop as far north as
northern Delmarva.
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY/...
The surface low pressure system and upper low will still be ambling
across western Pennsylvania Sat morning and thru the day the upper
low will move little. One surface low across PA will be replaced
by another closer to the coast. This second low will move northward
close to our area. Since the systems will be moving from SW to NE,
higher pops will be across north NJ and the southern Poconos Sat
with categorical pops and decrease to chc pops for Srn Delmarva
and the ern shore of MD. The pops further decrease Sat night.
By Sunday, most of the surface system will be far enough away to
allow drier air to build across the region. The upper trough will
still have to cross the region, so we will keep a low chance for
a shower mostly across the northern areas. Skies will be mostly cloudy
early then change to partly sunny by afternoon. Highs will be near
normal with 50s for the Nrn areas and low 60s for the Philly metro
and Delmarva.
&&
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
The beginning of the long term will have dry weather as expansive
high pressure from the Plains will build up into the Middle Atlantic
region. A cold front will cross our area Tuesday allowing drier and
cooler air from the Great Lakes to settle in. Pops with the frontal
passage are in the slight chance or low chance range with little in
the way of significant moisture available. Temperatures will be near
normal Mon and the go below normal Tue. High temperatures Tue will
range from the low/mid 50s N/W to the low 60s for the shore areas
and Delmarva.
A fair degree of uncertainty with regards to the weather at the
middle of the week. The front will remain to the south and its
possible that a weak wave may move along it and bring clouds and
rains to the region. Yesterdays 18Z GFS was rather bullish with
that scenario bringing rains to much of the area. The newer 00Z
has backed off but still keeps some rains for Delmarva for Wed night.
I expect that we will see this go back and forth during the
weekend and early next week, so for now we will just go along
with a model blend for the later part of the week for now with
mostly chc pops for Wed night and Thu. It looks to be a cold
middle and later parts of next week with highs mostly in the 50s
and lows favoring the 40s for SE areas and mid/upper 30s N/W.
The weather grids have some -RW/-SW for the southern Poconos and
north NJ for Wed/Thu morning hours.
&&
.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Today...Ceilings should gradually lower through the morning. Expect
widespread MVFR conditions for most of the day. Brief IFR ceiling
and visibility restrictions are possible late in the day if the band
of heavy rain comes in early enough. Easterly winds will be
increasing through the day with the highest speeds expected to be 20
to 30 kt with gusts of 30 to 40 kt. The strongest winds are expected
at KACY, KMIV, and KILG, and should be centered within a few hours
around 21Z. Moderate confidence on the overall pattern, but low
confidence on the exact timing of flight category changes and when
we will see the band of heavy rain.
Tonight...Starting off MVFR or even IFR across the region with both
ceiling and visibility restrictions. There is quite a bit of
difference between guidance on how quickly we may see improvement
back to VFR. Many models show a dry slot edging in to KPHL and
locations south by 06Z, resulting in a quick return to VFR
conditions, but there is still some guidance that keeps MVFR
ceilings through the night, even if the rain ends by 06Z. For now,
have favored the more optimistic solution as most of the high res
guidance appears to be favoring this solution. Easterly and
southeasterly winds will continue through this period, but should
drop off quickly, with speeds around 10 to 20 kt with gusts up to 25
kt by 03Z, dropping to generally 10 to 15 kt after 06Z. Moderate
confidence on the wind forecast, but low confidence on the flight
category forecast.
Outlook...
Sat/Sat night...IFR/MVFR CIGS with occasional showers and lower
VSBYS possible.
Sun...Isolated morning showers, but mostly VFR expected.
Mon/Tue...VFR expected.
&&
.MARINE...
Gale and storm force winds are expected to develop later today. The
strongest winds are expected during a period from early afternoon to
late this evening (will be progressing from south to north, so
Delaware coastal waters will likely see the strongest winds through
the afternoon, while the northern NJ coastal waters will likely see
the strongest winds this evening). No changes were made to the
marine headlines with this forecast update. Winds should decrease
quickly late tonight. The Delaware Bay may even be below SCA
criteria before dawn. For the rest of the waters, SCA conditions
will continue after winds subside below gale or storm force for the
rest of the night.
Outlook...
SCA conditions for the ocean Sat/Sun (seas) then sub-SCA for
Mon/Tue. Mostly sub-SCA for Delaware Bay. Showers Sat and
isolated showers Sun morning. Fair Mon/Tue.
&&
.TIDES/COASTAL FLOODING...
There is increasing concern for significant tidal and coastal
flooding with high tide cycles today. In many of these cases, the
areas with the highest risk will be back bays and tidal tributaries
where water won't have a chance to drain between tides, so tidal
levels will compound with multiple high tide cycles.
For the Tidal Delware River and adjacent tributaries: Major tidal
flooding is now expected with the high tide late this afternoon into
this evening. This is a change from the previous forecast which was
for moderate flooding with this high tide cycle. However, tide
levels continue to track higher than several models, so have
increased the forecast above most models. With the high tide cycle
this morning, widespread minor tidal flooding, with some areas
reaching moderate, is expected. The start time of the coastal
flood warning has been adjusted to cover the earlier high tide,
though the primary concern remains with the high tide this
evening.
For the tidal and coastal areas of Atlantic, Ocean, and southeastern
Burlington Counties: Moderate tidal flooding is now expected with
the high tide this afternoon. This will be different than most tidal
flooding scenarios we see, as I actually expect the oceanfront
to have more minor impacts, but back bays, including and
especially Barnegat Bay will likely have significant impacts (we
already saw significant flooding with the high tide late
yesterday), and could experience tidal flooding for a longer
duration than what we typically see (a few hours centered around
the time of high tide).
For the Chesapeake Bay: The high tide this afternoon could touch
moderate flooding levels, though the tide of biggest concern remains
the high tide tonight when widespread moderate tidal flooding is
expected. Current forecast is just slightly below major tidal
flooding, so will need to watch the tidal departures of the earlier
high tide today to see if any adjustments will be needed.
For the Delaware Bay, Delaware Coast, Cape May County: No changes
from the previous forecast. Still expecting moderate tidal flooding
with the high tide today. As with other areas, back bays, especially
Little Assawoman Bay in DE will likely see the most significant
impacts.
For tidal areas of Middlesex and Monmouth Counties: Due to a later
arrival of the strongest onshore flow, still expecting widespread
minor tidal flooding with the high tide today.
After today, we should see some improvement, though it will likely
take at least a few high tide cycles for back bays to fully drain,
so additional tidal and coastal flooding will be possible going into
Saturday.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Coastal Flood Warning until 1 AM EDT Saturday for PAZ070-071-
106.
NJ...Coastal Flood Warning until midnight EDT tonight for NJZ020-
026-027.
Wind Advisory from 2 PM this afternoon to 2 AM EDT Saturday
for NJZ013-014-020-026-027.
High Wind Warning until 11 PM EDT this evening for NJZ024.
Coastal Flood Warning until 9 PM EDT this evening for
NJZ021>025.
Wind Advisory until 11 PM EDT this evening for NJZ016-021>023-
025.
Coastal Flood Warning from 3 PM this afternoon to 9 PM EDT
this evening for NJZ016.
Coastal Flood Warning until 1 AM EDT Saturday for NJZ015-
017>019.
Coastal Flood Advisory until 8 PM EDT this evening for
NJZ012>014.
DE...High Wind Warning until 11 PM EDT this evening for DEZ004.
Coastal Flood Warning until 9 PM EDT this evening for
DEZ002>004.
Wind Advisory until 11 PM EDT this evening for DEZ001>003.
Coastal Flood Warning from 3 PM this afternoon to 9 PM EDT
this evening for DEZ001.
MD...Wind Advisory until 11 PM EDT this evening for MDZ012-015-019-
020.
Coastal Flood Warning until 6 AM EDT Saturday for MDZ012-015-
019-020.
MARINE...Gale Warning until 5 AM EDT Saturday for ANZ450>452.
Storm Warning until 6 PM EDT this evening for ANZ453>455.
Gale Warning until 11 PM EDT this evening for ANZ430-431.
&&
Synopsis...O'Hara
Near Term...Johnson/Meola
Short Term...O'Hara
Long Term...O'Hara
Aviation...Johnson/O'Hara
Marine...Johnson/O'Hara
Tides/Coastal Flooding...WFO PHI