Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
714 am EST sun Dec 21 2014
high pressure over Pennsylvania will retreat into the Canadian
Maritimes early next week...as weak low pressure lifts up the
East Coast. A deep upper trough and associated surface low will lift
northeast through the Great Lakes during the middle of next week.
Near term /until 6 PM this evening/...
stratocu remains trapped beneath sharp inversion early this
morning across central PA. Deeper moisture and lift associated with
passage of short wave could produce seeder-feeder effect with
lower clouds...resulting in a few flurries across the NE counties between
Falling inversion heights in wake of shortwave should cause stratocu
to begin breaking up today east of the Appalachians. However...not
confident in clearing across the West Mountains...where model soundings
imply lingering low clouds.
Went close to consall guidance with maxes reaching the low 30s
over the alleghenies to the upper 30s across the lower susq
Short term /6 PM this evening through 6 PM Monday/...
combination of retreating high pressure over new eng and weak low
lifting up the East Coast will spread an increasingly moist southeasterly
flow into central PA on Monday. The increasing moisture...combined
with lg scale forcing ahead of shortwave lifting out the Ohio
Valley...could spread a bit of light rain into southern PA Monday
PM. Blend of latest model guidance supports chance probability of precipitation across the
southern counties Monday afternoon.
Main concern will be potential of some freezing rain given cold air damming
signature and implied low-level ageostrophic northerly flow
holding low-level cold air in. Oper model soundings and ens ptypes
suggest the best chance of late day -fzra would be over the Laurel
Highlands and higher terrain of the SC mountains
Long term /Monday night through Saturday/...
moist southeasterly flow off the Atlantic and associated orographic forcing of shallow
moisture layer will likely produce patchy light rain/drizzle Monday
night and Tuesday. Surface temperature fields of both oper and ensemble data
suggest there could be some lingering icing concerns Monday night
over the higher terrain of central PA.
The most significant forecast issue in the long term remains the
evolution of deep trough and associated surface low to our west 23-25
December. Strong southerly flow ahead of this system will bring
warm air into the region Tuesday and Wednesday. By Tuesday and
Wednesday many low-level temperature and moisture fields rise well
above their seasonal values. This implies very warm weather...especially
Wednesday...when the shallow cold air scours out east of the
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 4...but
given diminishing spread amongst model members and increasing
confidence in frontal timing...will maintain categorical probability of precipitation
Wednesday/Wednesday evening. The pattern...sharp north-south front to our
west...implies most of the rain should fall within 3-6 hours of
The cold front will likely push through the region Wednesday night...accompanied
by gusty winds and likely followed by orographic snow showers
across the West Mountains Christmas morning.
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 /12z Sunday through Thursday/...
though it sounds like a broken record...stratocu again remains
trapped beneath sharp inversion early this morning across central
PA bringing generally MVFR ceilings to central and northern mountains
even though surface high pressure is in place. There is an area
of clearing over SC mountains and lower susq which is providing VFR
conditions...with additional lower ceilings along the Mason/Dixon line
extending northward into kjst and also extending southward into
the middle Atlantic region.
Falling inversion heights in the wake of an exiting shortwave
should allow area of clearing to expand into kunv-kipt later this
morning. However...not confident in clearing across the West Mountains
/including kjst-kaoo/ where model soundings imply lingering low
clouds. Winds today will be light.
Combination of retreating high pressure over New England and weak
low lifting up the East Coast will bring a gradually increasing
moist southeasterly flow into central PA late tonight and especially Monday.
Anticipate that we/ll see redevelopment/reinforcement of the
stratocu deck over The Spine of the Appalachians /from kjst-kbfd/
tonight...poss spreading into central mountains...with most points east
The increasing moisture...combined with lg scale forcing ahead of
shortwave lifting out the Ohio Valley...could spread a bit of
light precipitation into southern PA Monday PM.
Monday...ceiling reductions west. Restrictions developing east. Light
rain poss south...freezing rain poss western higher terrain. Becoming
Tuesday...low ceilings with drizzle/light rain likely. Breezy.
Wednesday...periods of rain along with widespread reductions. Windy
with low level wind shear.
Thursday...windy and colder with scattered MVFR ceilings and -shsn. Low level wind shear.