NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Sep 18, 10:29 AM EST, 1 hour 1 minute earlier.
Area forecast discussion, Sep 18, 10:29 AM EST, 1 hour 1 minute before
Area Forecast Discussion
National Weather Service Mount Holly NJ
1129 AM EDT Sat Sep 18 2021
.SYNOPSIS...
Tropical Storm Odette will continue to progress northeastward
towards the North Central Atlantic, moving further away from our
region. A cold front approaching from the northwest is expected to
pass through our region on Saturday night, followed by high pressure
for the period from Sunday through Tuesday. Another cold front may
arrive Wednesday night or Thursday.
&&
.NEAR TERM /THROUGH TONIGHT/...
Ongoing forecast working out well with only some minor
adjustments to dew points and wind. Current fcst for max temp
looks good attm.
Surface high pressure over the area slides offshore later
today. A cold front approaches from the west and passes through
the region late this afternoon. There is some weak shortwave
energy associated with it, but not enough for widespread or
organized convection. There is some surface moisture over the
region with dew points in the mid to upper 60s, but without that
upper level support, any convection is limited. Will carry
slight chance PoPs for showers and thunderstorms, but only for
several hours this afternoon.
Tropical Storm Odette remains well to the east of the mainland,
and will continue to track north and east away from the coast.
There will not be direct impacts to the local area from Odette,
but the surf will be rough, and there will be a high risk of rip
currents due to swells from Odette.
&&
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Surface ridge and a high amplitude mid level ridge will gradually
build over our region Sunday and Monday. Plenty of subsidence should
keep tranquil conditions through this period. The main question
through this period is that as the low level flow becomes
southeasterly on Monday, will that set the stage for either low
stratus or fog development Monday night. With the depth
(southeasterly winds from the surface through at least 800 mb) and
speed (5 to 20 kt through the layer) of the southeasterly flow, this
is more likely to be a low stratus set up. Thus, don't have any
mention of fog, but do increase cloud cover Monday night.
In the wake of the cold front tonight, day time temperatures Sunday
and Monday will be lower than Saturday, but still slightly above
normal, with highs in the 70s to lower 80s. The onshore flow will
limit radiational cooling at night, so overnight lows for the most
part should be 5 to 10 degrees above normal, generally in the 50s
and 60s.
&&
.LONG TERM /TUESDAY THROUGH FRIDAY/...
Overview: The main focus for this period is the cold front which
should arrive sometime between Wednesday and Thursday night. This
will bring the best chance for widespread rain to the region.
Details:
Tuesday...Another day under the surface and mid level ridge axis
will mean conditions similar to Monday. Consequently, the biggest
question will be if we will have either low clouds or fog. As with
Monday night, this still appears to be a more favorable set up for
low clouds instead of fog.
Wednesday through Thursday night...As mentioned above, the cold
front arriving during this period has the best chance for widespread
rain across the region. However, there is a large spread in guidance
on the timing of this front. The GFS is by far the fastest,
depicting the front arriving in our region Wednesday afternoon into
evening. Most of the other major operational models, most notably
the ECMWF and CMC depict a slower progression, with the front not
arriving until Thursday or even Thursday night. Even for a day 5-6
period, this large of a difference is unusual and concerning. Given
the more meridional mid level pattern and the fact that the front
could be occluding as it approaches our region, I have favored a
slower timing in the forecast, keeping the highest PoPs during the
day on Thursday. Although there is a chance for showers in the
current forecast from Wednesday morning through Thursday evening, it
won't be raining through that entire period - that is more an
artifact of the timing uncertainty. More likely we will see a 12
hour window centered just ahead of the front, when we will have the
potential for widespread rain. Potential for thunderstorms or
hazards like heavy rain will be highly dependent on the timing, and
thus remains quite uncertain for now.
Friday...Once the front finally does clear the region, should have a
return to tranquil and seasonable conditions.
&&
.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...VFR has returned to most TAF sites and should continue
into the afternoon. A short period of MVFR CIGS and a stray
shower possible this afternoon. N winds around 10 kts, becoming
NW. Moderate confidence.
Tonight...VFR. LGT/VRB winds. High confidence.
Outlook...
Sunday through Tuesday...Mostly VFR conditions expected during the
day time hours. There is a chance for either low stratus or fog both
Monday night and Tuesday night, but low confidence in the details of
that at this time. Northerly winds 5 to 10 kt on Sunday, becoming
easterly and even southeasterly at 5 to 10 kt Monday continuing into
Tuesday. Moderate confidence on the overall pattern, but low
confidence on the potential for overnight stratus or fog.
Wednesday...If we get low stratus and/or fog Tuesday night, it may
linger through mid day Wednesday. Later in the day on Wednesday,
there is a chance for showers to move in, and with it, additional
ceiling and visibility restrictions. However, guidance is trending
slower, so showers may not arrive in our region until Thursday.
Southeasterly winds 5 to 10 kt. Low confidence on most aspects of
the forecast, but moderate confidence on the wind forecast.
&&
.MARINE...
Swells from Tropical Storm Odette will result in 5 to 8 ft seas
across the coastal ocean waters. A Small Craft Advisory remains in
effect, but was extended until 8 pm. Conditions subside to sub-SCA
criteria tonight. N winds 15-20 kt this morning will shift to the W,
then SW at 10-15 kt this morning. Winds become NW 10-20 kt tonight.
Outlook...
Winds and seas should stay below SCA criteria through at least
Tuesday. On Wednesday, there is a chance that seas on the Atlantic
coastal waters could build to 5 feet.
Rip currents...
Swells from Tropical Storm Odette will result in increased seas and
rough conditions in the surf zone today with breaking waves of 2 to
4 feet. There is a HIGH risk for the development of dangerous rip
currents today. Conditions begin to abate tonight, and then there is
a MODERATE risk for the development of dangerous rip currents on
Sunday.
&&
.TIDES/COASTAL FLOODING...
Spotty minor tidal flooding is possible primarily on the
Delaware Bay and tidal Delaware river centered around the
highest astronomical tides through Monday. The combination of
increased swells from the departing Tropical Storm Odette,
higher astronomical tides with the full moon on Monday, and
developing on shore flow will all contribute to this. The
highest astronomical tides, and thus the higher risk for spotty
minor flooding, will be the evening/overnight high tides. At
this point, we are forecasting the tidal levels to remain below
advisory criteria. However, if the onshore flow develops faster
than what is currently forecast, there could be an increased
risk for widespread minor flooding especially with the Sunday
evening/night high tide.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk until 8 PM EDT this evening for NJZ014-
024>026.
DE...High Rip Current Risk until 8 PM EDT this evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
ANZ450>455.
&&
Synopsis...Johnson
Near Term...MPS/PO
Short Term...Johnson
Long Term...Johnson
Aviation...Johnson/MPS
Marine...Johnson/MPS
Tides/Coastal Flooding...WFO PHI