Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
242 PM EST Sat Dec 20 2014
high pressure sliding eastward through Quebec with a second high
pressure cell over the eastern Great Lakes and Ohio Valley will
keep maintain dry weather across central Pennsylvania for the rest
of this weekend. An amplifying upper ridge moving into the region
will bring increasing amounts of sunshine on Sunday...but will be
quickly replaced by a large storm system tracking across the
central United States. Moisture associated with that storm and
increasing east flow off of the western Atlantic will bring
increasing clouds on Monday...with quite mild but unsettled
conditions prevailing through middle week. A vigorous storm system
will sweep across the commonwealth on Christmas evening bringing
widespread showers and a chance for thunderstorms...followed by
windy and colder conditions with scattered snow showers and
flurries across the west and north for Christmas day. Milder
conditions will arrive once again by late this week.
Near term /through tonight/...
subsidence and downsloping over much of the Laurel Highlands and
parts of the central and south central mountains is bringing some
much needed sunshine to those areas. Visible imagery indicates the
northward extent pushing towards central Clearfield County and
southern Centre County...and it appears likely to reach a little
farther north before sunset.
Expect clouds to fill back in overnight...as 500 mb wave tracks across
the Ohio Valley and into southwest PA by 12z sun. Mins will be
fairly uniform and range from the lower 20s north to the upper 20s
Short term /Sunday/...
deep layer drying behind the aforementioned departing 500 mb wave and
upper level ridging has the potential to bring sunshine to most if
not all of central PA on Monday. The stubborn stratus that has
plagued US for more than a week will have a chance to mix
out...but transitory surface high pressure may still make this a slow
process over the north.
Prior to that...some areas could perhaps see a flurry in the pre
dawn hours Sunday before the wave quickly exits eastward. Went
close to consall guidance again with maxes reaching the middle to
upper 30s north to the lower 40s south.
Long term /Sunday night through Friday/...
The most significant forecast issue in the long term is the
evolution of the massive cyclone to our west 23-25 December.
Strong southerly flow ahead of this system will bring warm air
into the region late Monday into Tuesday. By Tuesday and Wednesday
many low- level temperature and moisture fields will be well above
their seasonal values. This implies very warm weather Tuesday and
Wednesday...when the shallow cold air scours out east of the
A cold front with strong winds will produce colder
conditions...likely late Christmas evening into Christmas day. There
will be strong westerly winds and strong wind gusts. Snow showers
are likely behind the system in western areas.
The changeable details...Monday the ensemble forecast shows a
wave passing to our south which could produce light snow or a
mix...mainly in southeastern areas. The implied low-level boundary
is the retreating cold air to our northeast as the strong cyclone
to our west begins to take shape. Implied low-level ageostrophic
northerly flow holding low-level cold air in Monday into Tuesday.
Tuesday the cold air should continue to erode. Had to keep some
freeze rain/drizzle in northeastern areas overnight Monday into
Tuesday based on implied ageostrophic flow and ensemble
precipitation types. But during the day Tuesday the warm air
should win out. Though all systems show some hints of cold air
damming....daytime highs east/northeast areas will be well below
what one might expect with 6-10c 850 mb temperatures.
Any precipitation Tuesday and Wednesday should be rain much after
noon Tuesday in the northeast. Precipitation on Tuesday should be
light...ensembles show probabilities in the 20-30 range and light
amounts. The pattern...sharp north-south front to our
west...implies some isolated showers but the main rain event
should within 3-6 hours of the front.
Tuesday the strong cyclone...with some of the lowest surface
pressures in the eastern US on 24 December in the past 30 years
will move into the Great Lakes. Pressure anomalies are -3 to -4
sigma below normal with a return period on the order of 30 years
in some forecast systems.
The front associated with the cyclone pushes into our region
Wednesday...at this time the 850 mb southerly winds are forecast
to be about +3 sigma above normal. The strong southerly flow...the
high precipitable water...the strong front all imply a frontal
rain event and perhaps some rare late December (elevated)
convection. If this were autumn or Spring we might be thinking
qlcs. But surface cape is virtually non-existent but showery rain
and perhaps isolated thunder possible.
Do not like having probability of precipitation above 60 to 66 percent at day 5 in 12 hour
periods. Timing will clearly change but have high probability of precipitation for
consistency with other offices.
Christmas evening and early Christmas is the area of high uncertainty
as to when the cold air comes in. This will dictate when the rain
comes and when the gusty winds set in. Gradients are where
forecasts are their poorest. But by Christmas morning most if not
all of the region should be below 0c at 850 and the strong
westerly winds should be mixing down behind the frontal boundary.
Windy overnight Christmas evening into Christmas day. Any residual
precipitation should be snow/snow showers and flurries.
Temperatures should rebound Friday ahead of another northern stream
wave moving into the mid-west. This should raise our heights and
temperatures Friday into early Saturday. It appears a series of
waves may eventually carve out a trough over the east-central US. If
so...we could end December a bit on the chilly side.
Aviation /20z Saturday through Thursday/...
shallow stratocu deck remains trapped beneath subsidence
inversion this morning...with MVFR ceilings. Lower ceilings will persist
in the northern mountains...with improving ceilings central and south. Winds
will be light from the west.
Ceiling restrictions in expanding stratocu deck likely return again
tonight...though low clouds should not be quite as expansive as
previous nights. A shortwave will track across the region bringing
a thickening middle-level deck as well. Surface winds will continue to
be light...with SW flow aloft beginning to increase as a large low
pressure system begins to deepen over the central U.S.
Sun...no sig weather expected.
Monday...low ceilings with light rain/snow possible. Becoming breezy.
Tuesday...low ceilings with rain likely. Breezy.
Wednesday...mild with periods of rain of varying intensity...leading to
MVFR to IFR. Also...windy with low level wind shear.
Thursday...windy and colder with scattered MVFR ceilings and -shsn.