Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
648 am EST Thursday Dec 19 2013
increasingly meridional flow expected over the next several
days...as trough deepens over the plains and ridge builds off the East
Coast. At the surface...a quasi-stationary front will remain
draped across the eastern Great Lakes through the weekend. This
boundary will finally push southeast through Pennsylvania late
Sunday or early Monday.
Near term /until 9 am this morning/...
combo of decoupled blyr...M/clear skies and snow cover has allowed
temperatures to drop into the low-middle teens in some of the normally
colder valleys across east-central PA. Some radiational fog/haze has
even developed at across the middle-lower susq valley. Meanwhile...higher
elevations sites along/west of the Allegheny Front are seeing a
modest non-diurnal trend with a steady SW breeze allowing temperatures to
tick upward a few degrees since midnight. Have updated sky cover
forecast to mclear early this morning based on latest infrared satellite trends.
Increasing middle-high cloud cover still expected later today...especially over
the northern 1/3 of the County Warning Area.
Short term /9 am this morning through 6 am Friday/...
a broad southwesterly return flow will prevail through the period on the
northwest side of surface high sliding off the Carolina coast into the western
Atlantic. Shortwave/surface low moving east-northeastward from the upper Great Lakes through
the St. Lawrence vally into northern New England should have little to
no affect on the area...but will extend a trailing frontal zone
southwestward into the plains/middle MS valley. This boundary will provide a
conduit for the first in a series of waves to quickly skirt
across the upper Ohio Valley into the lower lakes region by 12z Friday.
500 mb trough amplifying over the western states with developing closed
low over the southwest/northern Mexico will promote a backing middle-
upper level flow regime upstream over the Great Lakes/Ohio Valley...with
the development of a relatively strong 40-50kt SW low level jet transporting
higher moisture/precipitable waters around the northwest periphery of the departing surface
high...focusing the best moisture flux anomalies into northwest PA by the
end of the period. Model quantitative precipitation forecast/S are in good agreement in keeping the
bulk of warm air advection precipitation mainly to the north/west of the County Warning Area through early
Friday...however a consensus blend of probability of precipitation suggests schc/S are
justified across the entire northern tier zones mainly after midnight.
While the probability is quite low...any light precipitation reaching the
northern/northestern mountains could fall as -fzra as surface temperatures will be near 32f.
The aforementioned SW flow will advect mild air into central PA
with highs averaging ~ 5 degrees above middle/late December normals
approaching 40f in the north and surging well into the 40s across
the southern counties. Clouds will be more prevalent across the
north...while mostly sunny conditions are likely in the southern tier.
Departures from climatology will be more pronounced overnight with mins
averaging from 5 to as much as 15 degrees above normal...with the
largest spreads over the western mountains
Long term /Friday through Wednesday/...
little spread amongst medium range models through the upcoming weekend...all
of which show increasingly meridional flow as trough deepens over
the plains and ridge builds off of the southeast coast. At interface of
these systems...a deep southerly flow and associated plume of anomalously high
precipitable waters will extend along quasi-stationary front draped from the
western Gulf of mex to the east Great Lakes.
Waves of low pressure riding west of PA will bring the chance of
rain to mainly the northwest counties and keep the entire County Warning Area on the warm
side of the boundary. By Sunday...the strongest of the waves is
prognosticated to track across northwest PA. Gefs precipitable waters on the order of 4sd above
normal will accompany the associated ll jet and will likely produce heavy
rain just west of low track. Latest model blend indicates this heavy
rain threat will be just west of our area. However...the northwest
counties could potentially see enough rain...combined with
snowmelt...to produce some flooding concerns. Will highlight this
possibility in the severe weather potential statement. In the warm sector...record high temperatures are
possible Sunday...as gefs mean 925/850 temperatures surge to between 2-3sd
above normal. 00z European model (ecmwf) MOS numbers already indicating near record
temperatures in the 60s across most of central PA.
00z CMC appears to be an outlier by stalling trailing cold front
out along the East Coast early next week. 00z gefs and European model (ecmwf)
ensemble data strongly imply a more progressive scenario with a
return of drier weather and more December-like temperatures early next week
with surface high building into PA by xmas evening.
Aviation /12z Thursday through Monday/...
19/12z...surface high pressure will slide off the NC coast today while a
weak clipper low tracks east-northeastward through the St. Lawrence Valley and northern
New England. Cold front trailing the weak low draped across the lower
lakes will connect with the first of two developing frontal waves in
the middle MS/Ohio Valley. The lead system will track northeastward into the
northeast states by Saturday. A second...more significant frontal
wave is prognosticated to develop over the lower MS valley by later Saturday
and track northeastward through the Ohio Valley and eastern Great Lakes Sunday into early
Monday. AVN impacts/precipitation will more widespread with the second
system later this weekend.
Initial AVN concern involves low level wind shear through the late morning with a
40-50kt low level jet and relatively tight surface pressure gradient betwn low to the
north and departing high to the south of central PA. Surface winds are
decoupled over eastern sites with calm winds/clear skies resulting in
some local MVFR radiational fog. Further west...the blyr will remain
well-mixed with 10-15kt sustained at bfd/jst and 20-25kt gusts.
Kept VFR ceilings at all sites through tonight...although bfd has the
best shot at seeing reduced conds. Also can/T rule out some light
rain in vicinity of bfd by daybreak Friday but again low probability at this
Friday...MVFR ceilings/periods of rain likely north & west.
Sat-sun...widespread reductions likely in rain and low ceilings. Low level wind shear. Strong
frontal passage late sun into early Monday.
Monday...Post-frontal low ceilings and occasional shsn west/central. VFR east.
Gusty west winds.