Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 182211
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
611 PM EDT Sat Mar 18 2017

.SYNOPSIS...
Low pressure to our south and west this afternoon will redevelop off
the southern Delmarva Coast this evening and then strengthen as it
passes to the southeast of Cape Cod on Sunday. High pressure builds
in Sunday night and Monday. A weak disturbance is expected to move
through the area Monday night before a cold front arrives Tuesday
night. Expansive high pressure builds in from the northwest
Wednesday before the center of the high retreats off the Mid-
Atlantic coast Thursday night. A warm front lifts through the region
Friday as the next low pressure system tracks into the Upper
Midwest.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
600 pm update: Made quite a few adjustments to the grids.
Adjusted temperatures and dew points, as both are running a bit
lower than forecast at this time. Additionally, most of the
precipitation falling in the CWA is now either rain or snow, so
took out most mention of freezing rain/drizzle. Sussex County,
NJ, seeing accumulating snow at this time, per trained spotters.
This required adjusting the weather grids accordingly. Adjusted
PoPs considerably through the evening hours based on the latest
high-resolution model guidance, with the highest chances in
western CWA and Delmarva. However, most hi-res simulations are
handling the precipitation in northern/eastern CWA quite poorly
early this evening. Increased/extended PoPs in these areas
through the evening hours.

Storm total snow grids were modified to increase totals in the
southern Poconos a few tenths of an inch this evening and to
spread very light accumulations farther south/east overnight as
temperatures slowly fall. Confidence remains very low on
locations with highest amounts, but present indications suggest
these may occur in the southern Poconos as well as Berks/Chester
Counties in PA. More updates likely through the evening as the
details become clearer.

Previous discussion...

A dissipating front/trough and upper low moving across the area
tonight will keep low clouds and scattered showers over the area.
The best chc for precip will probably be across the wrn areas and
also Delmarva where the upper low will track closer. Confid in fcst
nor the best with models showing qpf possible just about anywhere
across CWA. We continue to mention light freezing rain/freezing
drizzle for the higher elevations in NW NJ and NE PA into the
evening with support from model soundings (which show the column
drying in the mid levels underneath the subsidence inversion).
We have received several ground truth reports of a light wintry
mix with a few spots of freezing drizzle near and north of I-80.
We cancelled the Winter Weather Advisory earlier in the day
since we do not expect icing to be widespread or impactful given
the above freezing temperatures reported in the valleys. We
currently plan to cover it with short-fused statements.

Lower temps arrive later tonight from NW/SE and light rains may
change to snow across wrn areas late. We do have small accums
for the srn Poconos southward into Chester county PA. Perhaps an
inch in these areas is psbl. Lows tonight will drop into the
upper 20s north, low 30s over metro Philadelphia and central NJ
and mid 30s over south NJ and Delmarva. Winds will be mostly NE
then switch to N late.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Any lingering showers across Delmarva will end shortly after
sunrise. Other than that, fair weather is expected Sunday as high
pressure builds into the area from the NW. Drier air will arrive on
a gusty Nrly flow with gusts over 20 mph for the morning. The winds
will trend lighter by evening. High temperatures Sunday will be a
little below normal, reaching the low 40s north and mid 40s over
southern NJ and Delmarva.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Shortwave ridging aloft builds downstream into our area Sunday night
and Monday as the closed upper low drifts farther off the Mid-
Atlantic coast. Breezy northerly winds will linger near the coast
Sunday evening before the pressure gradient weakens over the course
of the night with the surface high moving in from the west.
Accordingly, the coldest spots are forecast to be farther inland
(west of the Fall Line) where the radiational cooling
conditions will be most favorable. Lows range from the low/mid
20s in NE PA and NW NJ to low/mid 30s in the I-95 cities and
along the coast. Temperatures continue to trend toward normal on
Monday with highs in the 40s N/W of the Fall Line and low/mid
50s farther S/E.

A weak disturbance that is currently over California trek across the
CONUS along the northern periphery of the upper ridge, reaching the
Mid Atlantic Monday night. A brief (3-6 hr) period of light
precipitation may accompany the trough passage. Partial thicknesses
show the potential for some sleet to mix in north of I-80. No
accumulations are currently in the forecast.

Tuesday should remain dry in between systems. The big weather
story for Tuesday is the potential for temperatures to return to
climo if not a couple of degrees above for the first time since
March 9th.

Phasing between a Pac NW disturbance and a piece of the polar vortex
over Baffin Bay is forecast to occur early in the week. This will
allow the polar jet to amplify and dig southward toward the
Northeast states late Tuesday-Wednesday. Meanwhile, high pressure
currently over Alaska will build southeastward behind the trough.
Northwesterly flow around the high will advect CP air into our
region Tuesday night-Wednesday. A potential interesting setup may
unfold during this time as a wave of low pressure is forecast to
develop along a W-E baroclinic zone on the southern periphery of
this polar airmass. Guidance is currently in very good agreement
with the track of the secondary low over the Carolinas. These models
also shows the northern edge of the precip shield expanding toward
the Delaware Bay, but the colder air arrives too late to allow for a
changeover to snow. It certainly bears watching.

High pressure moves into the Mid-Atlantic region Wednesday night-
Thursday. After a brief respite from the late-season cold early in
the week, temperatures return to below normal.

High pressure progress offshore by Friday. Southerly return flow
around the high will yield milder conditions to end out the week.
Low chance PoPs (just rain) were added to the forecast for NE PA and
NW NJ for late Friday when a disturbance moving atop the ridge
passes to our north.

Rain chances increase next weekend with a more organized low
approaching from the west. Timing is still uncertain with the more
progressive model solutions indicating Saturday is the wet day
while slower solutions hold off the rain until Sunday.

&&

.AVIATION /22Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Overall, conditions have shown slow downward trend in CIGS over
most areas today. VSBYS in fog have shown up at KTTN and KRDG also
as the lower layer humidity continues to increase. Winds have mostly
been E or SE at 5 to 10 knots. Mostly cloudy conditions will
continue tonight as an upper low and strengthening offshore sfc low
will maintain the Erly/NErly low level flow. Conditions will remain
mostly IFR into the evening and then improve from NW to SE around
and shortly after dawn Sunday. Mostly VFR expected Sunday with gusty
N winds 15 to 25 kts.

OUTLOOK...

Sunday night...MVFR CIGs w/ stratocu could possibly linger into the
evening. However, clearing will occur from west to east later in the
night. Nly winds 10-15 kt early will back slightly out of the NW and
diminish.

Monday..VFR. Light NW winds.

Monday night...Mainly VFR but MVFR is still a possibility with
a brief period of SHRA.

Tuesday through Thursday...Predominately VFR. Low pressure passes to
our south but still soon to rule out it bringing light precip to our
southern terminals late Tuesday-early Wednesday if the track of the
system trends northward. Winds N to NW for most of the period with
the strongest winds expected to occur on Wednesday (gusts around 25
kt possible).

&&

.MARINE...
Low pressure will move to the offshore waters this evening then
strengthen as it moves away to the NE. We have only made a few
changes to the marine hazards, with the speeding up of the times of
the SCA flags across the srn coastal and Delaware Bay waters. The
timing was reduced a few hours in both cases. The SCA flag is already
in effect for the northern NJ waters and will continue until
replaced by the Gale Sunday morning. The Gale will also take-over in
the other coastal waters Sunday morning. Scattered showers are
expected tonight and fair weather for Sunday.

OUTLOOK...

Sunday night...SCA will likely be needed once GLW headline is
dropped since northerly winds are forecast to gust 25-30 kt and
seas in our coastal waters are between 5-8 ft.

Monday...Winds weaken but seas may take some time to subside to
below 5 ft so a SCA may need to be extended for our coastal waters
through at least the morning, if not later. Eventually, conditions
fall to below SCA criteria.

Tuesday...No marine hazards anticipated.

Wednesday...NW winds increase in conjunction with a steady pressure
rises. A SCA may be needed with GFS soundings indicating the
potential for gusts right around 25 kt starting early in the morning
Wednesday and continuing into the evening.

Thursday...Wind and seas below SCA criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ452>455.
     Gale Warning from 6 AM to 2 PM EDT Sunday for ANZ450>455.
     Small Craft Advisory until 6 AM EDT Sunday for ANZ450-451.
     Small Craft Advisory from 11 PM this evening to 2 PM EDT
     Sunday for ANZ430-431.

&&

$$
Synopsis...Klein
Near Term...CMS/Klein/O`Hara
Short Term...O`Hara
Long Term...Klein
Aviation...Klein/O`Hara
Marine...Klein/O`Hara



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