Area Forecast Discussion
Issued by NWS State College, PA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KCTP 251153
Area Forecast Discussion
National Weather Service State College PA
753 AM EDT Sat Mar 25 2017
A cold front will sag southward across Pennsylvania today and
tonight, before returning north as a warm front late Sunday and
Sunday night. Very mild conditions are expected early today and
again Sunday night into Monday as we enjoy the warm sector
south of the frontal boundary. However...thickening clouds and
southeast flow are expected on Sunday as the boundary sags
southward as a backdoor cold front...bringing cooler conditions
and eventually some patchy drizzle or light rain. Unsettled
conditions will last from late Sunday through Tuesday as a
series of disturbances interact with the front. No heavy rain is
expected during this timeframe however.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Weak surface frontal boundary is along the Pennsylvania...New
York border early this morning, and will be drifting slowly to
the south through this afternoon. The cold front should make it
to northern Virgina and West Virginia before stalling out once
Warm temps will persist this morning. However, the warmup will
be short-lived, as high pressure building into New Eng pushes a
backdoor cold front through central Pa during the mid
afternoon and evening hours. A good deal of cirrus will hold
temps down a bit, but even just mixing to 925mb supports max
temps in the upper 60s and low 70s south of I-80.
A earlier arrival of the front by just a few hours will result
in a cooler and cloudy day across the northern tier counties,
where southeast flow and nearly saturated soundings in the sfc-
850mb layer suggest patchy drizzle is possible. Further south,
there could be scattered afternoon showers in the increasingly
warm and unstable air over southern Pa...but most will stay dry.
Scattered light showers and areas of drizzle will be along and
just to the north of this boundary today into this evening.
The locations most likely to receive some rain today and tonight
will be the northern mountains of the state. Expect less than
one tenth of an inch of rain today in most places as short
range/high res models indicate that there will be a shallow dry
layer between the southward advancing stratus and stratocu...and
the alto cu and cirrus deck. SREF`s prob for LIFR stratus
ceilings behind the front will be limited to primarily the
mountains to the North and Northeast of KIPT. Areas farther
south stand to see the least cloud cover.
Temps around sunrise today should range from the upper 30s to
mid 40s in the east, to the very mild mid 50s to lower 60s
across the region near and to the west of the RT 99 corridor.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Frontal boundary sags back against the Alleghenies of western
PA, pushed by a high pressure area sliding SE across S Ontario
on Sunday as a low approaches from the Ohio Valley. This will
advect cooler and more moist llvl air into the region. Light
showers...with areas of drizzle/ridge top fog are likely
through much of the day Sunday.
Local experience causes us to lean greatly away from blended
model guidance for max temps in such llvl cold air damming
situations, as warmer outliers and the mean temp displayed by
such blends/ensembles typically masks the amount of shallow
cold air that gets funneled southwestward through the deep
valleys of Central PA.
Went primarily with the latest...25/06Z NAM blended about 25
percent of the way with the Nat`l Blend of Models. Forecast NDFD
Temps Sunday will be on average about 8-10 deg F below NBM
guidance (which also collaborates well with neighboring offices
.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
-- Changed Discussion --The main precip event impacts central PA Sun night into early
Mon as low moves through. QPF looks to be around 0.50 inch.
Above average temperatures (especially nighttime) appear very
likely through early next week, as GEFS indicates PA remains
beneath a mean upper level ridge and plume of anomalous PWATs.
The chance of showers again spikes with passage of next
shortwave Tuesday into Wednesday and another one to start the
U.S. 25/00Z and 06Z GFS and GEFS differs drastically with the
25/00Z EC and its ensembles during the second half of the week,
as the EC guidance is dominated by stronger southern stream
troughs and associated northward advection of warmer air and
higher 500 mb heights. In contrast, the GFS and its ensemble
mean appears to maintain a several decameter deeper and more
persistent upper low over nrn New England and the Canadian
Maritimes, while also latching onto a piece of northern stream
energy that breaks away from a potent upper low that will be
digging SE across the Four Corners area Tuesday, before heading
east across the High Plains of E New Mexico and West Texas
later Wednesday and Wed evening.
The presence of this aforementioned northern stream short wave
has big implications for maintaining/reinforcing colder NW flow
aloft across southern Canada and the NE U.S., while shearing its
initially potent southern stream "parent" low harmlessly east
(for us here in Central PA) across the Carolinas.
Considering the several-run persistence in the GFS, along with
the amount of snow cover across intact parts of the nrn Mid
Atlantic and New England States, (not to mention the persistence
of cold air there and points north across Ont and Quebec), I
slightly favor the GFS/GEFS solution at this point in time with
a more extended period of drier and cooler conditions for Wed-
Sat. The northern edge of the precip shield associated with the
shearing srn stream wave and mdtly strong forcing via the R.E.
region of a 100-120 kt upper jet across the Lower Great Lakes
and Mid Atlantic region may support a period of cold
rain...mix...or even a light accumulation of snow across the
southern half of PA late in the week.
The "Less likely" EC solution (at this point) would help to
develop a stronger upper ridge across the eastern U.S. for the
second half of the week with the upper low lifting NNE to the
upper midwest by 00Z Sat with warmer air at the surface and
aloft surging northward through the Ohio River Valley and
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.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --Frontal boundary just north of the PA/NY border early this
morning will be pushed southward through the airspace toward the
PA/MD line by 25/00z as strong high pressure builds over eastern
Canada. Increasing low level moisture and wind shift to the east
should translate into a lower trend in ceilings and visibility
from north to south with widespread MVFR to IFR restrictions
likely by tonight into Sunday. Scattered light rain showers will
be possible today especially near the southward-moving frontal
zone with pockets of fog/drizzle overnight into early Sunday
morning. The front should lift back to the north on Sunday which
should allow for at least some marginal improvement especially
over portions of the southern airspace.
Sun...VFR-MVFR western 1/4 with low risk for thunderstorm in the
evening. MVFR-IFR central and east with drizzle/fog.
Mon-Tue...Sub-VFR with periods of rain.
Wed...Becoming VFR. No sig wx.
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