Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 200527
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1227 AM EST Sat Jan 20 2018

.SYNOPSIS...
Surface high pressure will remain across the Southeast and adjacent
western Atlantic Ocean this weekend before moving east next week. A
surface low progressing across southeast Canada this weekend may
bring a slow-moving cold front into southern New England and the
northern Mid-Atlantic on Sunday and Sunday night before retreating
as a warm front on Monday. A cyclone in the central plains will race
northeastward through the Great Lakes Monday night to the Canadian
Maritimes on Tuesday, bringing a strong cold front through the
region late Monday night and Tuesday. High pressure will build into
the eastern United States for the rest of the week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Made adjustments to hourly grids based on latest surface obs.
Temps in northern NJ will fluctuate based on cloud cover, but
have been in the teens and may not drop much from there.

Otherwise, expect lows mostly 20s across the region.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
The small warming trend is expected to continue into tomorrow as the
low will move further away from the region, allowing 1000-500 mb
thicknesses to increase. Highs are expected to be about 10 degrees
above normal, ranging from the mid 40s to mid 50s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
A strong storm system will move through the region early next
week, bringing a good chance for rain to much of the area.
Otherwise, the pattern will be relatively benign during the
long-term period.

There are indications that a surface low moving across southeast
Canada this weekend will bring a slowing cold front into New
England and (perhaps) the northern Mid-Atlantic on Sunday. How
far south this progresses is somewhat in question, but I suspect
given the strength of the surface high encompassing the western
Atlantic and southeastern U.S. that the front will struggle to
make much progress into our area. Nevertheless, there is some
potential for a high-temperature bust on Sunday should the front
progress farther south than forecast. Currently, highs on Sunday
are forecast to be about 10-15 degrees above seasonal averages,
though a little bit lower in the northern CWA to at least
account for the potential for the front to progress a little
farther south. Saturday night and Sunday should be dry.

Operational models have trended slower with the cyclone in the
central plains Sunday and Monday, and this means the earlier-
progged isentropic ascent along the developing baroclinic zone
in the northern Mid-Atlantic during this period would be
strongest farther west. Previous shift reduced PoPs Sunday night
and Monday to adjust for this trend, and I have continued this
trend with today`s update. I also fine-tuned the precipitation
type forecasts based on somewhat increased confidence in hourly
temperatures during this period (though this should not be
assumed to be high confidence). The setup is favorable for
liquid precipitation, but temperatures will be close to or even
somewhat below freezing at times in the far northern CWA, so any
precipitation that falls in the southern Poconos and adjacent
northwest New Jersey may be freezing rain. Of course, given the
trends in the cyclone track to the west, the odds of
precipitation occurring are lower...too low to introduce ice
accumulations into the grids at this time. We will continue to
monitor this potential as the event approaches.

The cyclone makes the move northeast into the Great Lakes and
southeast Canada Monday afternoon through Tuesday, and a strong
cold front will sweep across the eastern half of the U.S. during
this period. Widespread frontal lift will be accompanied by
larger-scale ascent via warm advection and considerable
differential cyclonic vorticity advection downstream of a potent
midlevel trough. Models continue to generate widespread rain
near and ahead of the front, but the timing of the front still
has about a 6-12 hour difference between the faster GFS and
slower ECMWF. The CMC remains in the middle ground, but using it
as consensus is troubling because its QPF field makes little
sense in our area given the strong lift and moisture advection
preceding it. Here, the GFS and ECMWF are in stronger agreement
with widespread 0.50-1.00 inch totals in most of the CWA. As a
result, weighted the forecast toward an ECMWF/GFS blend, which
includes the timing of the frontal passage. This continues to
favor the 06Z-18Z Tuesday time window for highest PoPs.
Temperatures should be warm enough for all rain during this
period.

Most models keep the area dry Tuesday night through Friday with
strong high pressure developing into the region. However, there
continue to be indications of at least one perturbation moving
through the zonal to northwest flow upstream of the trough, the
timing of which would be Wednesday into Wednesday night. This
system would be moisture-starved, but the lift may be strong
enough to generate a few snow showers in the southern Poconos
and vicinity (though the ECMWF/CMC/GFS are not indicative of
this at the moment). No PoPs were included at the moment, and
given the surface high the GFS/ECMWF depict late this week, have
the forecast dry through the whole period.

The origin of the surface high is western/central Canada, so the
near-average temperatures MOS is giving for this period may be
too conservative (i.e., too warm). Euro MOS trends are down, and
MEX MOS is at least edging that way. Though I did not stray from
consensus too far this period, I have a suspicion the forecast
may end up being too warm Wednesday through Friday.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. SKC conditions at the TAF sites. VRB winds less
than 5 KT. LLWS with 35-40 KT winds at 2000 FT at KRDG/KABE.

Saturday...VFR/SKC. W-SW winds around 10 KT with gusts up to 20
KT in the afternoon.

OUTLOOK...

Saturday night and Sunday...Predominantly VFR, though clouds
will be increasing Sunday, potentially to sub-VFR late (and
especially north of Philly). Winds generally west or southwest
and light. Average confidence.

Sunday night and Monday...Periods of sub-VFR possible, with a
chance of rain primarily north of the Mason-Dixon Line. Winds
predominantly south or southeast below 10 kts. Below average
confidence.

Monday night and Tuesday...Periods of sub-VFR likely with a good
chance of rain, especially from around midnight to noon. South
winds 5 to 15 kts (potentially gusty at the coast) becoming west
10 to 20 kts (potentially gusty everywhere) subsequent to a
frontal passage some time during this period. Slightly below
average confidence, particularly regarding timing of
CIGs/VSBYs/wind shift.

Tuesday night and Wednesday...VFR with west to northwest winds
10 to 20 kts during the day and 5 to 15 kts at night. Some
potential for gusts Wednesday afternoon. High confidence.

&&

.MARINE...
For the Delaware Bay, Delaware and far southern NJ coastal waters:
winds and seas should stay below small craft advisory criteria
tonight and Saturday, though gusts near or just above 20 kt are
possible Saturday morning.

For the northern and central NJ coastal waters: winds will increase
late tonight, with gusts to 25 kt expected before sunrise Saturday
morning.

OUTLOOK...

Saturday night and Sunday...Sub-advisory conditions and fair
weather expected.

Sunday night and Monday...Sub-advisory conditions, though there
is a slight chance of rain off the New Jersey coast during this
period.

Monday night and Tuesday...Strong south winds and building seas
will likely generate at least advisory-level conditions, if not
marginal gales before frontal passage on Tuesday. Thereafter,
winds should become westerly and may again reach gale-force for
a time thereafter. A good chance of rain during this period,
with some visibility restrictions possible.

Tuesday night...Residual gale-force gusts possible with strong
northwest winds.

Wednesday...At least advisory-level conditions likely, though
the trend in winds and seas will be downward.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 4 AM early this morning to 6 PM EST
     this evening for ANZ450>452.

&&

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



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