Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 221342
AFDPHI

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

.SYNOPSIS...
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 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/...
For the 930 AM update, made some adjustments to the temperatures as
they are quickly rising so far with ample boundary layer heating
within a rather dry airmass. The surface dew points vary quite a bit
this morning due to not much mixing so far and earlier radiational
cooling. Rather dry low-level air looks to continue and therefore
the dew points were lowered especially near and north/west of the
fall line through this afternoon. To the south and east of there,
dew points should increase some by late afternoon as a sea breeze
makes some inland progress. This of course will have an impact on
temperatures as well, and the going forecast looks to have a good
depiction of these anticipated trends.

The high level clouds have thinned quite a bit across the southern
areas this morning, however some additional cloudiness is to the
west and heading east-southeastward. As a result, kept the most
clouds across the far southern areas although may need to expand
this farther north some if the incoming thicker cirrus holds
together. There is a stronger shortwave within a trough axis aloft
back to our west that swings east through the afternoon, therefore
this could provide for some more high cloudiness farther north.

Otherwise, the development and progress inland of a sea breeze will
be watched. 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 wind and a slight drop in
temperatures.

&&

.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 usher in an 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 /14Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR with some high clouds. Light and variable winds, locally
becoming west or northwest mainly below 10 knots. A sea/bay breeze
though should result in a quick wind shift to south or southeast
around 10 knots this afternoon at ACY, MIV, and ILG (maybe even PHL
by late afternoon).

Tonight...VFR. Winds quickly becoming light and variable to locally
calm.

Monday...VFR. There is a small chance of MVFR visibility due to
light fog at MIV and ACY early in the morning. Light and variable to
calms winds, becoming south-southeast 5-10 knots in the
afternoon.

Outlook...
Monday night...VFR. Winds mainly light and variable.

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...
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/Gorse
Short Term...CMS
Long Term...MPS
Aviation...CMS/Gorse/MPS
Marine...CMS/MPS



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