Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 212040
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
340 PM EST Sun Jan 21 2018

.SYNOPSIS...
A low pressure system will propagate from the southern Plains to the
Great Lakes region into Tuesday. A warm front will be gradually
moving north across our area later tonight and Monday. As the low
moves into southeastern Canada, a cold front is expected across our
area Tuesday. High pressure will then build across the region for
the end of the week. Another low pressure system may move into the
area late next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
A zonal flow aloft continues through much of tonight before starting
to back to the southwest. A powerful closed low is forecast to move
into the southern and central Plains through tonight, driving a
strengthening surface low northeastward overnight. As high pressure
continues to shift offshore of the southeast states through tonight,
a light southerly surface flow will occur across our region. As the
flow strengthens some aloft tonight and becomes more southwest to
south, warm air advection is expected to increase.

The combination of warm air advection coupled with low-level
moisture advection will maintain cloudiness across much if not all
of the area. The ongoing area of lower cloud cover will thin some
more into this evening, however this should tend to thicken and
lower some through the night especially across the northern half of
the area. This will be ahead of a northward moving warm front, which
is actually more pronounced in the Ohio Valley. The combination of
warm air and moisture advections in the lower levels will result in
some isentropic lift through the night. This lift is not forecast to
be all that strong, however some spotty light rain may occur
especially near and north of the Philadelphia metro. Any rainfall
amounts will be light. Given weak lift, lower level moistening with
some drier air aloft, patchy drizzle will be possible overnight
especially the farther north one goes across our area. Since clouds
are expected along with warm air advection and some increase in the
dew points, we are not anticipating any light freezing rain/drizzle
across the far northern areas.

As the warmer air advects northward initially above some cooler air
at the surface, some fog should form with this most widespread
across portions of the northern areas. Across Delmarva and even into
southern New Jersey, any precipitation and fog may be more limited
especially if the warm sector becomes more established sooner. Low
temperatures are mostly a blend of MOS and continuity.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
An potent closed low gradually moves toward the Great Lakes during
Monday. This will continue to drive strengthening low pressure also
closer to the Great Lakes toward late Monday afternoon. This system
will also be driving a warm front northward.

The eastward extent of the warm front is forecast to be shifting
northward during the day Monday, and with a lack of true cold air
damming this boundary should be able to steadily shift northward. In
addition, flow above the surface will be southerly and strengthen
through the day also helping to push the warm front north. There
should be low clouds and some fog north of the surface warm front,
and this however could slow its northward progress to some extent.
Any light rain and/or drizzle north of the warm front should be
ending in the morning. The warm front though may actually be more
pronounced aloft given the current airmass in place is not all that
cold.

The movement of the warm front and also some clearing will determine
the amount of warming that is realized. For now, we used a model
blend which placed the warmest temperatures near and south/east of I-
95.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Monday night and Tuesday...The main changes through this period were
a slightly later arrival of the front and a further east placement
of the southerly jet ahead of the front. This southerly jet should
be propagating east through the morning, off shore by late morning.

With the later arrival of the cold front, expect a longer period of
low level warm air advection in the morning, allowing for a brief
period of elevated instability to develop for much of the area. Have
expanded the mention of thunderstorms across more of the area. There
remains uncertainty if instability will develop before the jet is
further off shore. If we do have convection at the same time we have
the low level jet, gusty winds are possible with any storms that
develop. However, it would have to be very efficient mixing to get
severe gusts at this surface. This seems very unlikely to happen, so
will not mention any potential for severe storms in the HWO at this
time.

Precipitable water values could be over one inch, which is certainly
above normal for late January, but still considerably less than the
1.46 inches which was observed in the KIAD sounding on the 12th.
Additionally, very fast storm motions should limit any flooding
threat.

The low (and consequently the warm conveyor belt) is expected to be
far enough to the NW that we shouldn`t see any precipitation behind
the front.

Wednesday through Saturday...A large surface high slowly builds east
through this period, over our region and eventually off shore. We
should start the period below normal thanks to the Tuesday cold
front, but could see temperatures moderate as low level flow becomes
southwesterly late in the week.

Sunday...Another low pressure system lifting into the great lakes
region looks to bring the next chance for rain and next cold front
either Sunday or Monday. Through this time, it looks like most, if
not all of the precip should be ahead of or with the front, so
expect all rain at this time.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Rest of this afternoon...VFR ceilings, with localized MVFR ceilings
at times (cloud bases between 2,500-5,000 feet). Southwesterly winds
around 5 knots.

Tonight...VFR (marginal MVFR) ceilings lower to MVFR, then to near
IFR overnight. The lowest ceilings are anticipated for RDG, ABE and
TTN where especially some light rain, drizzle and fog will also be
possible. Low confidence however regarding the ceilings. Southerly
winds less than 5 knots or light and variable.

Monday...Sub-VFR conditions improve to VFR from south to north
through early afternoon. Some light rain, drizzle and/or fog is
possible in the morning, especially north and west of PHL. Lower
confidence regarding the ceilings. Light and variable to southerly
winds less than 10 knots.

Outlook...

Monday night...conditions lowering to IFR or less with low ceilings
and visibility restrictions due to rain. Southerly winds of 10 to 15
kt are expected. A period of low level wind shear (LLWS) is likely
to develop late Monday night as a strong low level southerly jet
develops, though exact timing and location of the jet is uncertain
for now.

Tuesday...Starting IFR or lower Tuesday morning, but improving to
VFR conditions behind a cold front expected during the day Tuesday.
Also expect an abrupt wind shift from southerly to westerly winds
with the cold front. Wind speeds of 10 to 20 kt with higher gusts
will be possible especially just behind the cold front. LLWS
conditions may linger into Tuesday morning along the coastal plains.
Timing of the front, and consequently improving conditions is
uncertain at this time.

Tuesday night through Friday...VFR conditions expected. Westerly or
northwesterly winds less than 15 kt. Gusts above 20 kt possible
during the day Wednesday and Thursday. High confidence.

&&

.MARINE...
The conditions are expected to remain below Small Craft Advisory
criteria through Monday. The winds continue to shift from a more
southerly direction this afternoon and this will continue through
Monday. This southerly flow will increase some though later tonight
and Monday from south to north, however any gusts are expected to
remain below 25 knots.

Outlook...

Monday night into Tuesday morning...Southerly winds will be
increasing late Monday night. By sunrise, expect southerly winds
gusting to at least 30 KT. Gale force winds are likely on the
New Jersey and Delaware coastal waters primarily after sunrise
Tuesday morning.

Late Tuesday through Thursday...An abrupt shift to westerly winds
with gusts above 25 kt is expected behind a cold front moving
through late Tuesday. There is a small chance that gale force
conditions develop again on Wednesday on the coastal waters.
SCA conditions are likely to continue into Thursday.

Friday...Winds and seas are expected to stay below SCA criteria.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Tuesday morning through Tuesday afternoon for
     ANZ450>455.
     Small Craft Advisory from 4 AM to 1 PM EST Tuesday for ANZ430-
     431.

&&

$$
Synopsis...Johnson
Near Term...Gorse
Short Term...Gorse
Long Term...Johnson
Aviation...Gorse/Johnson
Marine...Gorse/Johnson


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