Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
148 am EDT Sat Mar 15 2014
a weak cold front will pass through the state early this morning...followed
by a secondary cold front late today. An area of low pressure will
track south of PA Sunday night and Monday. Another low pressure
system will likely track west of Pennsylvania in the middle of
Near term /until 8 am this morning/...
regional radar showing a band of rain showers across the northwest mountains at 0530z
in associated with cold front. Still expect band of rain showers to dissipate as
they pass east of the Allegheny plateau...mainly the result of
downsloping westerly flow. Latest rap and nampara simulated
reflectivity support this idea...so will mention high probability of precipitation across
the West Mountains and just a chance of a -shra across the susq valley
during the pre-dawn hours.
An examination upstream observation and latest model soundings suggests any
changeover to -shsn over the northwest mountains will occur after the bulk of
precipitation has passed...so removed overnight snow accums entirely.
Temperatures on track to bottom out by dawn from around 32f across the northwest
mountains...to the l40s across the susq valley.
Short term /8 am this morning through 6 PM Sunday/...
secondary cold front trailing south-southwest from clipper over S Quebec will
push southeast across the County Warning Area late today...but with little in the way of
precipitation /except for a few snow showers across the northwest mtns/.
Sky cover during much of Saturday will range from partly to mostly
sunny in the southeast...to mostly cloudy in the kbfd vicinity.
West-northwest winds will gust to the middle-upper 20kt range during
the day Saturday...making it feel noticeably cooler than the
otherwise slightly above normal temperatures in the 30s northwest...to lower and
middle 50s in the susq valley.
Tricky temperature forecast for Saturday night as high pressure builds east
from the upper Great Lakes and much colder air filters south in to
the state. Main question will be just how much middle and high clouds
will spread east from the Ohio Valley along the axis of a
strengthening east/west jet core.
For now...will maintain the fairly large north-south temperature spread
from the lower teens across the north...to the upper 20s in the
Long term /Sunday night through Friday/...
surface anticyclone...with origins over the northwest territories...will dive
southeast across the Great Lakes late in the weekend...reinforcing the cold air
across central PA. Meanwhile...a southern stream shortwave and associated
surface low will be gathering Gulf of mex moisture over the lower MS
valley/Delta Region with a baroclinic zone setting up from Missouri to
Virginia. Models are coming into better agreement on southward surge of
Canadian air associated with surface high into the northeast states. Gefs
mean 925 temperatures of 1-2sd blw average support high temperatures Sunday of only
the m20s north mountains and perhaps around 40f in southern valleys.
Upper level troughing remains a fixture across southeast Canada and
much of the eastern Continental U.S....which should support near to below normal
temperatures in central PA through most of the period.
There are two systems to focus on for this cycle.
1) surface wave or waves migrating northeastward from Gulf Coast states across
the southeast and off the Outer Banks of NC Mon-Tue.
2) plains surface low and trailing cold front moving east/NE through the Great
Lakes and applchns Wed-Thu.
The key feature of interest /system #1/ remains the main forecast
challenge in the medium range. Have increased probability of precipitation across
the region Sun night-Monday based on blend of 00z guidance...all
of which now track surface low close enough to bring snow to at least
southern PA. A poor-mans ensemble of the operational
models/ensemble/HPC guidance...suggest a moderate snowfall is
possible across the southern part of the state sun nite-Monday.
Concerning system #2...it looks as though a surface ridge should
separate the departing system #1 and surface low lifting northeastward from the
plains into the Great Lakes. There is a large spread in the timing
of this system...with the GFS/gefs trending slower into midweek.
Confidence is low with this system...and stayed close to guidance
for days 6-8.
Aviation /06z Saturday through Wednesday/...
sattelite shows middle level clouds moving into northwest mountains
with showers becoming possible over the next few hours. Currently
VFR conditions as the cold front will push into the region
by 07z. Gusty winds continue over bfd...jst and ipt. These winds
could subside briefly...by 06z...before picking up Post
frontal....between 10 to 14z.
The surface cold front will push into the northwest mountains with showers of
rain...changing to shsn early tomorrow morning. Ceiling reductions to MVFR
and tempo IFR will accompany this cfropa. Ceiling heights will likely
go no lower than MVFR across the Central Ridge and valley region
Saturday morning. IFR until 15z is expected at jst and bfd.
One main impact overnight to flight operations is in the form of a
gusty SW surface winds /and widespread llws/...as an axis of strong
winds aloft /associated with a low level jet/ slides east. Low level wind shear
is increasing as 850mb jet increases to 50-60 kts across all taf
sites. Low level wind shear will move off as the jet moves northeast by middle
MVFR ceilings may linger into Sat at kbfd...but improving conditions
should be the rule. Breezy west-northwest winds will persist however.
VFR will follow for late Sat and Sat night as cold high pressure
moves east from the upper glakes.
Another storm tracking south of PA late Sunday into Monday could
bring some snow close to southeast PA.
Sat...am -shrasn/reduced ceilings poss West Mountains breezy.
Sun...no sig weather expected.
Sun night-Mon...snow with restrictions possible southern PA.
Tuesday night and Wednesday...periods of MVFR to IFR along with scattered
rain/snow showers possible as a warm front lifts northward across the