Area Forecast Discussion
Issued by NWS State College, PA-- Remove Highlighting --
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FXUS61 KCTP 281135
Area Forecast Discussion
National Weather Service State College PA
735 AM EDT Wed Sep 28 2016
A multi-day heavy rain event is probable across far south-central
Pennsylvania beginning later today and continuing tonight through
Thursday. Flooding impacts are likely across the southern half of
central PA...but impacts may be lessened to some extent by dry
antecedent conditions. Unsettled weather with occasional lighter
rain showers will persist into the weekend with a gradual drying
trend by early next week.
.NEAR TERM /THROUGH TONIGHT/...
IR satellite trends continue to show low clouds advancing
northward across south-central PA. The IR loop also reveals
patchy fog in the river/stream valleys across the northern
Alleghenies. Therefore will continue to make upward adjustments
to sky cover and speed up northward cloud expansion early today.
Low POPs along the PA/MD line still look good to account for
shower activity shifting northward from Northern VA.
Backing low to mid level flow downstream of southward moving
closed upper low over the OH valley will begin the large scale
set-up for what should be a multi-day/36+hour heavy precipitation
event focused over far south-central PA into Northern VA/MD. Precip
probs will increase through the day with moderate showers likely
reaching the Lower Susq. Valley by 29/00Z. Model guidance still
shows a surge in rain coverage and intensity later tonight into
Thursday which corresponds with arrival of stronger dynamical
forcing and strengthening E-SE upslope low level flow. Max QPF
amounts derived from WPC blend are located over the Lower Susq.
Valley along/south of I-81.
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
*Flood Watch in effect for the Laurel Highlands, South Central
Mountains and southern portions of the Central Mountains midnight
tonight through Friday morning.
*Heavy rainfall event probable over far south-central
Pennsylvania on Thursday
*Flooding is likely however high impacts may be mitigated by dry
Heavy rain event is expected to be ongoing over south-central PA
on Thursday. An enhanced convergence axis/frontal zone is progged
to develop on the east side of an upper low meandering over the
OH/TN Valley, tapping into deep moisture/high PW air off the
Atlantic Gulf Stream via anomalous east/southeast low level jet.
The heavy rain axis is progged to gradually pivot to the southwest
with time oriented within moderate excessive rain outlook area from
southwest PA near the Laurel Highlands southeast into Northern
VA/D.C. area. The strength of the upslope flow and high PW
transport along with some elevated instability should provide an
environment for efficient rainfall processes.
WPC QPF blend again favored a slight shift in QPF to the
southwest but maintained a broad area of 2-4" with localized 4-6"
possible. Keep in mind these amounts are over a 36+ hour period
with preceding dry conditions and near to below normal
streamflows. Therefore the dry antecedent conditions may
limit more significant flooding/runoff issues. Still expect
flooding to result from long duration event...and have issued
Areal Flood Watch for Laurel Highlands, South Central Mountains
and southern portions of the Central Mountains for late tonight
through Friday morning. Will continue to highlight potential flood
risk in HWO elsewhere.
The upper low is fcst to drift back to the west/NW into the OH
Valley on Friday before slowly moving to the ENE across the Lower
Great Lakes through the weekend. We should continue to see periods
of showers on Friday but rainfall amounts will be much lower and
.LONG TERM /SATURDAY THROUGH TUESDAY/...
Model guidance still suggests unsettled conditions over the
weekend with a gradual decrease in the chance for showerThe most likely watch area would be from
Cambria/Somerset southeast to York County.s. Temps still holding
close to seasonal normals as we turn the calendar to October.
.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Expect VFR conds to continue overnight across most of the
airspace. Exception will be far southeast airfields, KLNS/KMDT,
where low clouds/IFR cigs have crept in ahead of developing
weather system over the MidAtlc region. Sub VFR cigs should
dissipate after sunrise.
Wednesday should be the last good VFR day before more showery and
cloudy conditions develop later Wednesday Night into Thursday with
widespread reduced conditions likely.
Thu-Fri...Widespread showers/cig reductions.
Sat...Reductions possible with a chance of showers.
Flood Watch from late tonight through Friday morning for