Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 301318

Area Forecast Discussion
National Weather Service Mount Holly NJ
918 AM EDT Thu Mar 30 2017

High pressure will move offshore today. Low pressure moving
through the Ohio Valley on Friday will redevelop near the New
Jersey coast Friday night and move offshore on Saturday. Canadian
high pressure will move into the area for Sunday and Monday.
Low pressure over the Ohio Valley on Tuesday will move through
the middle Atlantic Tuesday night and offshore on Wednesday.


Forecast on track, so will only make minor adjustments to hourly
grids based on latest surface obs.

High pressure remains just south of Hudson`s Bay, centered over
NW Quebec, and ridging southward down the eastern side of the
Appalachians and into the Mid Atlantic, characteristic of a
cold-air damming (CAD) pattern. Meanwhile, a 1000 mb surface low
was positioned over eastern KS. The low will track eastward
into the Midwest today.

Expect precipitation to hold off during the period with low
pressure still distant to our west and downstream ridge building
occurring over the eastern seaboard. Mid and high level clouds
are expected to overspread the area throughout the day. Based on
latest NAM/GFS RH cross-sections, trended faster with the
arrival of the denser cloud cover than previous forecast,
yielding mostly cloudy skies late this morning in our far
western zones, first half of the afternoon for DE Valley and
late in the afternoon toward the coast.

Forecast high temperatures today about 4-8F lower than what was
observed yesterday, which is near climo for the end of March.
Favored the warmer MAV solution with models exhibiting a slight
cool bias in recent days although we did not want to go much
warmer with clouds increasing during peak heating.

Light north-northeasterly winds 5-15 mph early this morning
will veer out the east late this morning and then south-
southeast this afternoon as the ridge axis progresses gradually
eastward and just off the northern Mid-Atlantic coast.


Low pressure will continue its track eastward tonight along a warm
front positioned over the Midwest. The nose of a southerly low-level
jet ahead of this system will be positioned over the Mid Atlantic
tonight. Forcing for ascent over the region will predominately be
provided by isentropic lift as warm, moist air overruns the low-
level baroclinic zone situated over the Ohio Valley-Delmarva. PoPs
increase from NW to SE this eve. Did not ramp up PoPs quite as fast
as I would normally like to in this close to the event due to some
notable timing differences with respect to onset of precip this eve.
Did have enough confidence to increase to PoPs to likely for
southern Poconos this evening and then eventually the remainder of
the CWA overnight. QPF amounts will generally be light tonight
(under one-quarter inch) with the deeper/stronger lift still
upstream of the region.

Despite the center of the high retreating to our northeast, CAD
wedge will be reinforced in-situ owing to the effect of evaporative
cooling when precip initially falls into an relatively dry boundary
layer. Temperatures were populated using a blend with much of the
weighting from the NAM 12km/NAM 3km nest as this model has shown to
handle complex thermal profiles in CAD setups like this. Freezing
rain is likely overnight tonight in the mountains of Carbon and
Monroe Counties in PA and Sussex County, NJ as temperatures fall
into the lower 30s. Held off on issuing a late second period
Freezing Rain Advisory since areal coverage looks to be limited to
the highest elevations (approximately above 1300 ft). However,
places like Mount Pocono and High Point may see a few hundredths of
an inch of ice accretion late tonight, especially on elevated
surfaces such as trees and wires.


The primary players during the long term continue to be two southern
stream systems that are expected to impact our region in the Friday-
Saturday and Monday-Tuesday time frames.

The first system will produce a wide swath of rainfall across the
region, with localized amounts in excess of 1.5 inches, especially
across the I-95 corridor. While urban and poor drainage flooding are
possible, and have been mentioned in the HWO, we do not expect major
impacts on rivers and streams.

Freezing rain looks to persist into at least the morning on
Friday along the higher elevations of Carbon, Monroe and Sussex
(NJ) counties. Currently, our weather grids indicate a change
over to just plain rain with temperatures rising above freezing
in these high elevations by the afternoon. However, some of the
colder model solutions indicate the potential for freezing rain
to persist into Friday afternoon and perhaps Friday night. The
latest storm total ice graphic was populating running the FRAM
off our forecast database and represented an increase in the
icing amounts ranging from a few hundredths of an inch in these
counties at around 1000 ft in elevation to almost a quarter inch
at above 1800 ft in elevation (isolated areas in Monroe
County). These amounts may need to be increased if trends toward
a prolonged icing event (i.e., freezing rain persisting into
Friday afternoon/night) continue.

Looking ahead to early next week, the next system is expected to
bring more precip to our region, especially Monday night into
Tuesday. This setup continues to look warmer compared to the
previous system, given the absence of high latitude blocking,
and would favor predominantly rain.

Temperature-wise, daily means are expected to average near normal
Friday thru Sunday, then around 5 to 10 plus degrees above normal
Monday thru Thursday.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR today. High clouds around 25 kft lower to 8-12 kft from
west to east this afternoon. N-NE 10 kt or less early this
morning veer more Ely late this morning and then out of the S-SE
this afternoon.

CIGs lower from W to E tonight as precip arrives. Held off MVFR
CIGS/VSBYs until closer to midnight EDT for ABE/RDG and until
07-09Z farther east toward I-95 terminals. However, there is a
low chance that MVFR begins 2-4 hours earlier than that if
precip moves in faster than currently forecast. PROB30 group
covers this potential. SE winds 6-12 kt are expected tonight.

MVFR possible in low clouds Friday and Friday night, then again
Monday night into Tuesday. Otherwise, predominantly VFR. Southeast
winds may gust 20 to 25 knots on Friday, and northwest winds may
gust around 20 knots on Saturday.


Winds and seas below SCA today and tonight. NE winds 5-15 kt this
morning will become SEly this afternoon and tonight. These SE winds
will initially be light this afternoon but increase to 15-20 kt by
late tonight.

We have issued a Gale Watch from 18Z Friday to 09Z Saturday for the
northern NJ coastal waters (ANZ450-451) for southeasterly wind
gusts up to 35 knots during this time frame.

Outside of the Gale Watch, a Small Craft Advisory is in effect
for Friday and Friday night for the southern New Jersey and
Delaware coastal waters. On lower Delaware Bay, a Small Craft
Advisory is also in effect, but only for Friday.

A Small Craft Advisory may be needed on Saturday for northwest wind
gust of 25 to 30 knots and seas of 5-8 FT, especially across the NJ
coastal waters.

Another SCA may be needed Monday night into Tuesday, as
southeasterly winds and seas increase.


Though astronomical tides will be gradually diminishing through the
week now that we are past the new moon, the threat of minor tidal
flooding along the NJ and DE Atlantic coasts increases by
Friday. Tidal anomalies are currently around 0.5 feet above
astronomical this morning. We expect this to continue today.
Beginning tonight, a low pressure system will bring a prolonged
period of onshore flow, further increasing the surge.

Forecast tidal levels from the ETSS have trended slightly
upward for tonight, indicating Reedy Point, Lewes and Sandy
Hook could reach their minor coastal flood stage with the high
tide cycle late this evening. These ETSS forecasts would fall
just shy of threshold for Coastal Flood Advisory with spotty minor
coastal flooding tonight. OFS guidance for tonight is
significantly lower than ETSS. It seems the ETSS may be a bit
overdone since with the expected SE wind direction and lighter
wind speeds, we should not see a surge any higher than what we
currently have tonight.

The high tide of most concern is still Friday evening and
Friday night, particularly along the northern and central New
Jersey Shore. By this tide cycle, it will take a surge of 0.8 to
1.0 feet to reach minor flooding thresholds, which is likely.
However, not sure yet if we will have another 0.3 feet surge to
reach advisory thresholds. The exact magnitude of the surge will
be dependent on how quickly the onshore flow develops and how
strong it will be by Friday.

Given the fetch and duration of southeasterly flow, some of the
surge guidance is also raising water levels along the eastern
shore of Maryland, particularly for the Friday evening high
tide. At this time, none of the guidance reaches the minor
flooding threshold, but this will bear watching.


MARINE...Small Craft Advisory from 8 AM Friday to 5 AM EDT Saturday for
     Gale Watch from Friday afternoon through late Friday night for
     Small Craft Advisory from 8 AM to 8 PM EDT Friday for ANZ431.


Near Term...Klein
Short Term...Klein
Long Term...Franck
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