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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KCTP 150703
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
203 AM EST Sat Dec 15 2018

.SYNOPSIS...
A slow moving storm system over the Gulf Coast states will
track northeast to the Mid Atlantic region this weekend,
bringing periods of rain tonight through Sunday. Cooler and
drier conditions are expected Monday and Tuesday with lake
effect snow showers and flurries over the west and north.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Visibilities across the southeast have continued to dip below
1SM at times this evening so extended patchy fog wording through
the overnight hours. Rain continues to advance to the north.

KCCX radar is undergoing ongoing maintenance for an issue that
developed earlier this evening. We hope to have radar operation
back to normal as soon as possible.


Earlier discussion below.

Northern edge of large baroclinic leaf associated with
sprawling storm system over the lower Mississippi River Valley
is overspreading the southern third of PA this afternoon. Areas
of rain have lifted north of the Mason Dixon line and will
increase in coverage through late afternoon. Steady rain not
expected to arrive until evening over my southern tier, but will
then spread across the southern third of central PA through late
tonight. Sharp deep layer moisture gradient across central and
northern PA means the northern edge of measurable rainfall will
be the most problematic part of this forecast tonight through
much of Saturday as surging and waning upper level forcing nudge
or retract that line northward and southward. A compromise in
solutions brings the steady rain to the central PA mountains
close to State College late tonight through the pre dawn hours
Saturday before being sheared off to the east as the first of
several upper level waves moves off. Latest NAM12 actually shows
drying extending into the southern tier for several hours from
Saturday morning through Saturday afternoon, which means that a
good part of Saturday could be just cloudy with patchy drizzle
or spotty light rain showers. Much wetter GFS supports higher
POPS but European supports the idea of a break between rainfall
tonight into Saturday morning and another higher likelihood of
measurable rainfall late Saturday afternoon and night. Have
tried to reflect this in POP/wx grids overall.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
The southern stream closed low will slowly lift NE on Saturday,
focusing best chances of rainfall along and south of the Mason
Dixon line for most of the day as dry high pressure tracks
across southeast Ontario into Southern Quebec Saturday afternoon
and Saturday night. Next best chance of steady precipitation
over central and southern PA arrives late Saturday afternoon
and continues Saturday night.

As noted above, much of Saturday may end up being overcast and
drizzly across central and southern areas, while northern areas
may even see some breaks as drier air entrains from the north
under the influence of aforementioned SFC ridge over srn
Ontario. PW gradient is extremely tight, with sub 0.25" across
my northern tier ranging to over 1 inch over the southern tier
Sat aftn and evening. Determining the northern extent of
measurable rainfall during this time is the forecast challenge,
and will continue to be the focus tonight and Saturday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
As a southern stream upper low begins to open up and move
through the Mid-Atlantic for the latter part of the weekend, wet
weather will continue. The best jet support and moisture inflow
for steadier rain still looks to occur late Saturday night and
early Sunday, particularly over south-central PA. Although
rainfall of 0.75-1.5" may occur on a localized basis, such
rainfall amounts will likely be spread out over a long enough
period of time (generally 18-24 hours) to preclude any
significant hydro issues.

Across north-central PA, the two main issues later this weekend
are how far north steadier precipitation gets, and also how much
low-level cold air is able to bleed down into the region. At
this time, we think that enough light rain and boundary layer
cold advection will each prevail for potentially some light
freezing rain late Saturday night into early Sunday morning.
Thus, we continue to highlight this possibility in our
Hazardous Weather Outlook.

Sunday afternoon, steadier precipitation should taper off from
west to east, as the aforementioned upper system and associated
surface cyclone push well off the east coast. It remains
questionable whether cold air on the back side of the low
deepens quickly enough for a period of wet snow later Sunday
over parts of Central PA, before precipitation tapers off/ends.
Even if this were to occur, it would very likely be short-
lived.

Monday-Tuesday, it still appears that a drier, colder northwest
flow regime will settle in. The best potential for lake enhanced
snow showers in the mountains appears to be Monday into Monday
night. Relatively shallow moisture should keep accumulations on
the light side.

As ridging briefly pushes in mid-week, Wednesday looks dry and
seasonable.

Late in the week, still plenty of uncertainty, as model
divergence is large with respect to how short-wave energy
progressing towards/through the eastern CONUS behaves. For now,
we`ll indicate at least somewhat higher chances for light rain
or snow for Central PA.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
Rain will continue across the southern airspace today with
widespread MVFR/IFR. High pressure drifting eastward across
southern Ontario into the St. Lawrence Valley will maintain dry
wx over the northern airspace with MVFR to VFR conditions. This
keeps continuity from the previous thinking that rain would be
more suppressed and confined mainly to the southern 1/3 of the
airspace later today. Precipitation will gradually spread
northward later tonight into Sunday with an icy mix possible
over the the northern tier. Expect precipitation to taper off as
a rain/snow mix Sunday afternoon.

.Outlook...

Sun...Light rain/snow and MVFR/IFR cigs.

Mon...MVFR cigs/snow showers north/west airspace. Gusty winds.

Tue-Wed....No sig wx.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...DeVoir/Martin
NEAR TERM...Martin/Tyburski
SHORT TERM...DeVoir/Martin
LONG TERM...
AVIATION...Steinbugl