Coastal Flood in Delaware, April 18 to 19, 2022

Area Forecast Discussion
National Weather Service Mount Holly NJ
921 PM EDT Mon Apr 18 2022

.SYNOPSIS...
Low pressure along the Virginia coast this evening will track
northward tonight and strengthen. The low will then move to our
north early Tuesday morning. High pressure will then gradually
build into our area Wednesday, followed by a weak cold front
moving through later Thursday. High pressure returns Friday into
the weekend before shifting offshore, however a warm front will
approach our region from the southwest over the weekend into
early next week.

&&

.NEAR TERM /THROUGH TUESDAY/...
A wet and raw evening and overnight as a quick-moving deepening
surface low moves along the triple point this afternoon directly
over our region tonight. As of 9 PM the surface low, which has
deepened to around 1003 mb., is across Delmarva, south of our
region. It continues to move along the coast at a quick pace and
will be NE of the region by daybreak.

A shield of rain (which had some sleet and snow at onset) has
overspread the area and is heavy at times along the fall line.
As forecast earlier, rainfall amounts were trended upwards a
bit, with much of the hi- res guidance indicating potential for
a stripe of 2 inch rain somewhere over the region, with the best
agreement being mostly along and northwest of I-95 over central
and northern New Jersey. The flood watch issues earlier will
remain in place. Any heavier bands of rain that drop over a
quarter to a half inch of rain for an hour or two could push
some streams into action or even minor flooding in the Raritan
and Passaic basins. Extended the watch into Philadelphia and
southeastern Pennsylvania where flash flood guidance is still
locally lower.

While this will be a rainfall event for the vast majority of us,
the precip has been all snow so far across the Pocono plateau
where local totals in excess of 8 inches is possible. You can
expect highly variable totals however with the totals highly
elevation- dependent, with some portions of Carbon and Monroe
Counties on the southeastern sides receiving lesser totals or
possibly no snow where warm air advects northward enough. The
quick forward motion of this system and lack of any strong southerly
wind component over this area leads to think some places may stay
all snow above 1800 feet.

Winds have so far performed as expected with sustained winds 20
to 30 mph and gusts around 50 mph at the shore and lesser
speeds inland. The Wind Advisory for coastal New Jersey and
Delaware will continue overnight.

Lows tonight will range from the upper 20s over the Poconos to the
upper 30s and lower 40s elsewhere.

Tuesday looks much quieter as cold air advects into the region
and high pressure builds directly to our south. The pressure
gradient between this surface high over Virginia and the surface
low over northern New England and southern Quebec will produce
gusty winds up to 35 mph through the afternoon. It should remain
partly to mostly cloudy. Highs will range from the upper 40s to
lower 50s.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
A closed low will be traversing southeastern Canada Tuesday night
with its axis crossing our region during the evening. This feature
also captures surface low pressure that will be occluding and
weakening with time. Our region will remain within cyclonic flow
aloft for much of this time. Cold air advection and a clearing sky
will occur Tuesday night. Surface high pressure is forecast to
remain to our south and southwest, therefore while it will be cold
and winds diminish, at least some wind should hang on through the
night. Low temperatures drop back into the 30s more much of the
area. A freeze is not expected, and unless the wind can completely
decouple the frost potential looks to be on the low side as of now.

As we go through Wednesday, a mid level ridge slides overhead
resulting in surface high pressure building in. As heights rise
some, temperatures will be a little milder however still remain
below average. There will be much less wind given high pressure
arriving. The surface high quickly shifts offshore into Wednesday
night and the winds will then become light and variable as the
boundary layer decouples. Low temperatures Wednesday night will be
higher as some high level clouds should start to arrive. As a
result, lows are currently forecast to be around 40 degrees across
the majority of the region (mid 30s however in the highest
elevations).

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Summary...A warmer pattern overall; A weak cold front arrives
later Thursday then stalls to our south; A warm front should
then slowly approach over the weekend and early next week.

Synoptic Overview...An upper-level trough shifts eastward from the
Midwest, adjacent Canada and the Great Lakes Thursday. This trough
however is forecast to mostly track to our north Thursday into
Thursday night as a ridge across the Gulf Coast states builds and
expands some northward. This ridge may then dominate right through
the weekend as a potentially strong upper-level trough ejects into
the Plains. At the surface, high pressure is offshore as a weak cold
front moves through later Thursday, then high pressure moves back in
Friday before settled eastward some Saturday through Monday. A
baroclinic zone in the form of a surface warm front is forecast to
be draped just to our south and southwest Friday right into early
next week.

For Thursday and Friday...A strong upper-level trough shifts
eastward across Canada, the Midwest and Great Lakes. This feature is
forecast to track mostly to our north, however a weak cold front
moves through later Thursday. Given that the trough lifts to our
north, the bulk of the forcing is also removed from our area. While
a few showers are possible mainly across the northwest zones with
the frontal passage, overall it looks like a dry front. The front
then stalls to our south Friday due to a strengthening ridge to the
south. Some warmer air circulating around the ridge will result in
an uptick in our temperatures. High temperatures are therefore
forecast to be in the 60s for most areas Thursday, then into the
lower 70s for many on Friday.

For Saturday and Sunday...A southeastern U.S. ridge looks to mostly
dominate this time frame with northwesterly flow aloft across the
Northeast and northern Mid-Atlantic. A potentially strong upper-
level trough across the Plains helps to sustain the downstream
ridge, and a strengthening surface warm front is forecast to be
positioned near Delmarva. Surface high pressure slides across
eastern Canada and should link up with a high across the Carolinas.
The aforementioned warm front at the surface separates these two
features. Some ripples of energy however embedded within the
northwesterly flow aloft may result in a few weak surface
reflections tracking along or near the warm front. This may result
in a small convective potential combined with warming temperatures.
This is of low confidence as it is more difficult to time any
smaller scale features this far in advance. Given the presence of
high pressure to the north, an onshore wind will be present across
our area. This will keep it cooler especially along the coast, and
may result in cooler air arriving from the north with a decent
temperature gradient setting up from north to south. Saturday is now
cooler than before, especially for eastern and northern areas, with
more warmth building for Sunday.

For Monday...As an upper-level trough lifts across the Midwest and
Great Lakes, surface low pressure will track with it. In the East,
looks like not much movement given the Southeast U.S. ridge and this
may keep a warm front near Delmarva from moving much northward. Some
showers may start to occur near and north of the warm front, however
given the parent storm tracking across the western Great Lakes it is
less clear how much forcing for ascent works its way this far east.
We carried some slight chance to low chance PoPs for now, with
daytime highs in the 60s to 70s.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...MVFR conditions reducing to IFR by 02Z as heavy rain
overspreads the terminals through the night. CIGs look to lift
to MVFR near or after 10Z. Winds from the east from 10 to 15
knots with gusts from 25 to 30 knots, then turning to the west-
northwest from 15 to 20 knots with gusts to 25 knots quickly as
the low moves north of the region after 06Z. Moderate confidence.

Tuesday...VFR conditions return with winds from the west at 15 to 20
knots with gusts to 30 knots. High confidence.

Outlook...

Tuesday night...VFR. West-northwest winds diminish to around 10
knots. High confidence.

Wednesday...VFR. West-northwest winds around 10 knots, becoming
light and variable at night. High confidence.

Thursday...VFR. Southerly winds 10-15 knots, diminishing and
becoming west to northwest at night. Moderate confidence.

Friday...VFR. Northwesterly winds around 10 knots, becoming light
and variable at night. Moderate confidence.

Saturday...Mainly VFR. East to southeast winds 5-10 knots. Moderate
confidence.

&&

.MARINE...

A Gale Warning is in effect through 6 am for the upper Delaware
Bay and from 6 pm through 9 am for the rest of the waters.
Winds from the east this evening gusting from 35 to 40 knots
then turning to the west or northwest as low pressure moves
over and north of the area. Seas have built to around 10 ft on
the ocean already and will remain mostly 9 to 12 ft overnight.

Small Craft Advisory conditions are expected tomorrow with possible
Gale force winds in the afternoon with westerly winds gusting from
25 to 30 knots and seas from 4 to 6 feet.

Outlook...

Tuesday night and Wednesday...Small Craft Advisory conditions,
however conditions will improve on Wednesday.

Thursday through Saturday...The conditions are anticipated to below
Small Craft Advisory criteria.

&&

.TIDES/COASTAL FLOODING...
A coastal storm will continue to work its way up the coast tonight
before departing on Tuesday morning. Onshore flow associated with
the storm is resulting in tide levels at least 1 to 1.5 feet above
astronomical tides. With the full moon on April 16, astronomical
high tides are less than 1 ft below minor flood stage. Thus, this
amount of surge will likely be enough for widespread minor coastal
flooding along the Atlantic coast of New Jersey and Delaware, as
well as along Delaware Bay and up the tidal Delaware River. The main
tide affected will be the high tide this evening/overnight.

A Coastal Flood Warning remains in effect for Cape May, Sussex (DE)
and Kent (DE) Counties. Model guidance and observed tidal departures
continue to suggest the potential for moderate coastal flooding late
this evening in those areas. While modeled and so far observed
conditions are marginal for moderate coastal flooding, past setups
like this often result in abrupt surges of water at short notice,
which can place tidal levels on the higher side of guidance. Thus,
the Coastal Flood Warning appears to remain prudent.

Widespread minor coastal flooding is likely for all of our remaining
coastal areas with the exception of the eastern Maryland shore, and
a Coastal Flood Advisory has been issued. While flooding in these
remaining areas should mainly be in the minor category, we can't
rule out some localized instances of moderate flooding.

There will still be a surge at the time of the Tuesday morning/day
high tide, but that is the lower of the astronomical tides, so
coastal flooding is not expected. With the flow becoming offshore
early in the day on Tuesday, tide level should be close to
astronomical tides, by the time of the Tuesday evening/night high
tide.

On the northeastern Chesapeake Bay, tide levels are expected to
remain below minor flooding thresholds.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch until 6 AM EDT Tuesday for PAZ061-062-071-103>106.
     Coastal Flood Advisory from 1 AM to 6 AM EDT Tuesday for
     PAZ070-071-106.
     Winter Weather Advisory until 10 AM EDT Tuesday for PAZ054-055.
NJ...Flood Watch until 6 AM EDT Tuesday for NJZ007>010-012-015.
     Wind Advisory until 1 AM EDT Tuesday for NJZ021-023-024.
     Coastal Flood Advisory until 2 AM EDT Tuesday for NJZ012>014-
     020>022-025>027.
     Wind Advisory until 5 AM EDT Tuesday for NJZ013-014-020-022-
     025>027.
     Coastal Flood Warning until 3 AM EDT Tuesday for NJZ023-024.
     Coastal Flood Advisory until 3 AM EDT Tuesday for NJZ016.
     Coastal Flood Advisory from 1 AM to 6 AM EDT Tuesday for
     NJZ017>019.
DE...Wind Advisory until 1 AM EDT Tuesday for DEZ002>004.
     Coastal Flood Warning until 3 AM EDT Tuesday for DEZ002>004.
     Coastal Flood Advisory until 3 AM EDT Tuesday for DEZ001.
MD...None.
MARINE...Gale Warning until 10 AM EDT Tuesday for ANZ431-450>455.
     Gale Warning until 6 AM EDT Tuesday for ANZ430.

&&

Synopsis...Gorse
Near Term...Davis/po
Short Term...Gorse
Long Term...Gorse
Aviation...Davis/Gorse/po
Marine...Davis/Gorse
Tides/Coastal Flooding...WFO PHI

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