Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220802
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
402 AM EDT Sun Apr 22 2018

.SYNOPSIS...
A large area of high pressure over eastern Canada and the
Northeast United States will slowly build east and move off the
New England coast on Monday. Meanwhile, low pressure over the
western Gulf Coast states will track to the north and east and
impact the Mid-Atlantic and Northeast for the middle of the new
week. Another low may affect the area at the end of the week,
and then a cold front may pass through the region next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
An upper-level jet streak will slowly move out of the northern
Mid- Atlantic today, and this will allow for the slow downward
trend in upper-level cloudiness during the day, even with a vort
max progged to move through the region this afternoon. The
region where this will take the longest to occur is the Delmarva
Peninsula. However, I suspect this will matter little regarding
maximum temperatures today, particularly as drier air advects
into the upper levels this afternoon.

GFS/NAM BUFKIT soundings suggest temperatures about 2-3 degrees
warmer than yesterday (mixing to around 800 mb). With a surface
ridge in place, winds should be light, which will inhibit
stronger surface warming to some degree, so I suspect the warmer
MAV MOS is probably a stretch. The past couple of weeks of
verification indicate MAV guidance has a general 3-4 degree warm
bias, whereas the MET guidance has very little bias (and
generally the lowest errors among the statistical suite).
Followed the MET closely today, and followed a mix of model-
simulated 2-m temps/dew points for hourly values, which seem to
mimic the diurnal evolution better.

One thing to watch will be the development and progress inland
of a sea breeze. According to high-resolution simulations,
sea/bay breezes will likely progress well into New Jersey by
late afternoon, and may reach Philadelphia by late in the day.
With the sea/bay breeze passage, may see a burst of stronger
winds and a slight drop in temperatures. Modified the grids to
account for this, especially in portions of eastern/southern NJ
and much of Delaware.&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
Main forecast concern tonight is the development of fog near
the coast as remnant (light) onshore flow from the sea breeze
combines with dropping temperatures to allow for low-level
saturation. Spread the mention of fog a little bit further
inland in southern/eastern NJ and coastal Delaware, though not
expecting widespread/dense fog at this point.

With the surface ridge in place, winds will likely decouple
fast, and with clear skies thanks to being upstream of passing
northwest- flow midlevel perturbations, temperatures will fall
readily. I went a little below guidance for lows tonight, and
would not be surprised to see the low-lying spots with a little
frost late. However, suspect this will be quite patchy at best,
so made no mention of this in the grids at this point.

Forecast lows are similar to tonight`s values.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Unsettled weather on tap for much of the new week as low
pressure lifts along the coast.

High pressure over the Northeast U.S. moves off the New England
coast on Monday, and then slowly tracks out to sea Monday night
and Tuesday. Onshore flow develops, and although temperatures
climb into the 60s for most inland areas, temperatures will be
colder along the coast with highs staying in the 50s.

Low pressure tracking along the Gulf Coast states will be over
the Southeast U.S. Monday evening, and then the low lifts along
the coast Monday night and Tuesday.

Onshore flow will user a increasingly moist airmass into the
region, and PWATs will increase to around 1.25" Tuesday night
and Wednesday morning. This may allow for a period of moderate
to locally heavy rain during this time.

The low will be off the Mid-Atlantic coast Wednesday morning,
and then the low tracks north during the day Wednesday and will
be north of the region by Wednesday night.

From there, conditions generally dry out Wednesday night and
Thursday, but there is the potential for unsettled weather to
continue into Friday as several weak upper level disturbances
pass through the region. There is the chance for another coastal
low on Friday, as the CMC-GDPS indicates this low, but the GFS
and the ECMWF do not. The GFS has a quick moving cold front that
the ECMWF holds off until Saturday. For now, will carry chance
PoPs Friday through Saturday due to low confidence.

Temperatures during this time will generally be near or below
normal levels for most of the week.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR with upper-level cloudiness gradually diminishing
with time. Winds generally west or northwest around or below 10
kts. However, combination of sea/bay breezes may allow for a
quick change to southerly winds around 10 kts during the
afternoon (maybe with an occasional gust) at ILG, ACY, and MIV
(maybe even PHL by late afternoon). Moderate confidence in sea
breeze formation/timing; high confidence in VFR.

Tonight...VFR with light/variable winds. High confidence.

Outlook...

Sunday night through Monday night...VFR. There is a small
chance of visibility reduced in fog at KMIV and KACY on Monday
morning. Winds will be light and variable for much of the
period. High confidence, except the development of fog, for
which confidence is low at this time.

Tuesday through Thursday...MVFR/IFR conditions expected in rain
and fog, especially Tuesday night through Wednesday afternoon.
Easterly wind gusts up to 20 kt are possible Tuesday night into
Wednesday. Otherwise, generally light winds (less than 10 kt)
are expected. Medium confidence on flight categories, but low
confidence on the timing of any flight category changes.

&&

.MARINE...
Winds/seas will remain below advisory levels through tonight.
Winds will become southerly during the afternoon, with speeds 10
to 15 kts, before becoming light and variable overnight. Seas
will likely remain below 3 feet through the period.

There is potential for at least patchy fog overnight, with
visibility restrictions possible. Chances are particularly high
in Delaware Bay and adjacent portions of the southern NJ/DE
Atlantic coastal waters, where light easterly/southeasterly
surface flow may remain for most of the night.

Outlook...

Sunday night through Monday night...Winds and seas are expected
to remain below SCA criteria.

Tuesday and Wednesday...East winds increase to 15-20 kt with
gusts up to 25 kt, and seas build to 8-10 feet on the ocean.
Wind gusts to 25 kt also expected on DE Bay.

Wednesday night and Thursday...Winds shift to the northwest and
subsequently decrease. The main question will be how long seas
remain above 5 feet on the coastal waters. Current forecast is
for elevated seas into late Thursday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...MPS
Near Term...CMS
Short Term...CMS
Long Term...MPS
Aviation...CMS/MPS
Marine...CMS/MPS



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