Scientific Forecaster Discussion
fxus61 kctp 100453
Area forecast discussion
National Weather Service State College PA
1153 PM EST Mon Dec 9 2019
temperatures will be above normal overnight as a mild southwest
flow persists across the region. The brief warm- up will be
accompanied by periods of rain which may end as light snow late
Tuesday night. Sharply colder conditions will follow for
Wednesday into Thursday. A mid to late week drying trend will
likely reverse by next weekend with another complex storm system
on the horizon.
Near term /until 6 am Tuesday morning/...
dry slot with sparser coverage of light showers has overspread
the southern half of central PA this evening, while best
overrunning and focus for steady rainfal has lifted over the northwest
and NC mountains. Strong warm advection has pushed sfc temps
into the mid to upper 40s throughout all but far northeast
sections late this evening. It will be a mild night with broad
southwest flow continuing well ahead of the sfc cold front which
will push into the northwest by 12z Tue. Another upper wave sliding in
from the west will keep a steady light to moderate rain ongoing
over western sections, where rainfall totals will be around an
Short term /6 am Tuesday morning through 6 PM Tuesday/...
sfc cold front pushes east across PA Tuesday morning, before
slowing as it reaches the Delmarva/Virginia coast.
A secondary and moderately tight 850-700 mb thermal gradient and
fgen forcing will occur across the southeast 1/3 to 1/2 of the cwa, as
the thermally direct r.E. Region of a strong upper level jet
approaches from the mid Ohio Valley and accentuates upward
Boundary layer temps slip to critical changeover levels for wet
snow beginning in the mid evening Tuesday across the central
mtns and mid susq valley, then across our southeast zones tues night.
A second, Post-frontal period of rain Tuesday afternoon/evening
will likely change to a several to 6-8 hour period of light to
moderate wet snow with 1-3 inches of accumulation looking
increasingly likely southeast of a kaoo to kseg and kmpo line with the
highest amounts occurring across the ridges. Could see a few
higher amounts on the ridges. Depending on the temperature and
snowfall rates, some travel impacts are possible for the morning
commute to work and school Wednesday.
Long term /Tuesday night through Monday/...
*rain to snow transition Tuesday night to bring light/slushy
accumulation in south-central PA with limited travel impacts
*cold/dry Wed PM-Thu PM before stormy pattern resumes Fri-Sat
*temperature swings: sharply colder through midweek before
gradual warming trend into the weekend
A strong cold front will push southeast across the commonwealth
Tuesday evening-Wednesday morning. Model guidance continues to
indicate anafrontal precip type setup with rain to snow
transition due to combined fgen and dynamic cooling/wetbulb
effects. The warmness of the antecedent airmass including Road
temperatures will likely initial accumulations to some extent.
Elevation will also be a factor with snow covered roads more
likely in the higher terrain. In general, expect wet/slushy
light accums on the order of a coating to an inch or two. Timing
would suggest that am commute impacts are probable across the
lower Susquehanna valley Metro areas and I-76/81/83 corridors
but could vary depending on location. Rain/snow line shifts to
the east later Wednesday morning with precipitation ending over
the far southeast zones.
The rest of Wednesday-Wednesday night will be blustery and much
colder with some lake effect snow showers over the far northwest
mtns. Wind chills will drop into the single digits and teens
Modified Arctic high pressure 1040mb will migrate eastward from
the upper Midwest over central PA Thursday morning before
departing northern New England into the Canadian Maritimes by
the end of the week. This should maintain dry weather across the
area Thursday through Thursday night.
Models are in pretty good agreement in tracking low pressure
from the Gomex up the eastern Seaboard Fri-Sat. Strong moisture
transport and large scale lift will likely bring significant
precipitation to central PA in this time range. The onset and
overall timing of precipitation will be important given
potential for low level cold air to offer potential for snow/ice.
Forecast temperatures remain marginal for mixed precipitation
with wet-bulb effects likely in-play into Friday night before
warming aloft changes ptypes to plain rain into Saturday am.
Cyclonic flow in the wake of the weekend storm should Foster a
colder trend with periods of snow/showers possible over the
Aviation /06z Tuesday through Saturday/...
rain falling across central PA this evening will gradually taper
off tonight, as low pressure tracks north across the eastern
grt lks. A moist southerly flow ascending the northern mountains
should result in persistent IFR/LIFR cigs over that part of the
state tonight, while more favorable conditions are expected
across the southern half of the state. Strong southwest winds
aloft, combined with much lighter winds at ground level, will
continue to result in low level wind shear into the overnight
hours across much of central PA.
A cold front will push southeast across the area Tuesday,
accompanied by additional rain showers. Model soundings indicate
morning IFR cigs are possible across the western mountains
(bfd/jst) associated with upslope flow behind the front. Elsewhere,
tempo MVFR conditions could accompany showers associated with
the cold front. A gusty westerly wind will push drier air into
the region during the afternoon, likely returning most of the
region to VFR conditions. However, upslope flow could result in
lingering MVFR cigs from bfd south through jst into Tuesday
Wed...am light snow possible southeast PA.
Thu...no sig wx expected.
Fri...rain/low cigs likely, especially late.
Sat...rain/low cigs possible, especially early.