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FXUS61 KCTP 151109
Area Forecast Discussion
National Weather Service State College PA
709 AM EDT Tue May 15 2018
Severe thunderstorms capable of significant damaging wind are
forecast Tuesday afternoon and evening across parts of central
Pennsylvania. A wet and muggy weather pattern will persist
through late week into the weekend with areas of locally heavy
rain and risk of flash flooding.
Elevated convection crossing Lake Erie early this morning and
brushing the NW zones with brief heavy downpours and perhaps
some pea sized hail. Spotty shower/T-storm could still develop
over the eastern 1/2 of the area as WAA zone continues to lift
northeast. Locally dense fog has improved and should dissipate
by 9AM.
In the wake of isolated early morning showers/convection and
associated mid level cloud debris, diabatic heating and advection
of low to mid 60s dewpoints is expected to contribute to moderate
instability. Vort max currently over IL along with southeastward
advancing cold front are likely catalysts for organized storm
initiation moving into a very favorable thermodynamic and
kinematic environment.
The individual CAMs along with the HREF mean and experimental HRRR-
TLE and HRRRE are very bullish on an intense T-storm complex
with significant damaging wind threat -- particularly moving across
north-central into northeast PA this afternoon/evening. The
convective complex should evolve into line segments/bows with
high probability of 50+kt damaging winds.
SPC increased the ENH risk area a bit while adding a sig severe
hatched area to indicate 10% or greater probability of wind
gusts 65kt or greater within 25 miles of point.
The tail end of the convection appears to hang up across far
south-central PA this evening, with potential for repeat storms
along merging boundaries. High moisture availability/PW should
spur heavy downpours with a risk of localized flash flooding.
While we would have preferred to use the flash flood watch in a
more short-fused fashion, collaborated with PBZ/LWX/PHI to pull
trigger on FF watch this cycle.
A wet and humid weather pattern is expected to last through the
end of the week as tropical moisture flowing northward collides
with a stalled frontal zone across the Mid Atlantic region. This
should result in some relatively cooler days (vs. mid-May
climo) and warmer nights given moist/humid airmass. Highest
precip probs are over south-central PA.
A persistent flow of subtropical moisture interacting with a
quasi-stationary frontal system will keep conditions soggy over
the region for the foreseeable future. The front will wag north
and south for the next several days bringing periods of showers
and thunderstorms with very little chance of any 12 hour period
remaining dry in any one location. Proximity of drier air mass
and high pressure to the north over southern Canada means that
drier air will filter southward into far northern PA from time
to time, giving at least some chance of periodic drier periods
this week, but the main story is for daily chances of showers
and thunderstorms until further notice. The Thursday night into
Friday timeframe looks particularly wet as deep layer moisture
surges (PW near 2.0") thanks to amplification of the offshore
upper ridge coupled with a nearly stationary upper trough over
the central plains. This will direct copious subtropical
moisture northward into the eastern GLAKS and set the stage for
locally heavy rains and potential flooding given what will have
been a very wet week by that time.
Temperatures should be above seasonal climate averages this week
with a muggy summertime/humid feel given dewpoints in the 60s.
07Z regional radar loop showing a few isolated
showers/thunderstorms popping up along approaching warm front,
running from northeast Ohio to northern Virginia. As the front
moves northeast, there could be a a few showers or thunderstorms
across central Pa overnight, mainly over the northwest part of
the state. Any shower or storm could be accompanied by a brief
vis reduction. Model soundings and SREF prob charts also
indicate the possibility (<50pct) for a brief period of IFR cigs
in the vicinity of KBFD early Tuesday morning associated with
passage of the front.
Perhaps the biggest concern overnight will be the possibility of
fog across eastern Pa. Dewpoint depressions already low in this
area, and when skies clear expect fog to develop between 05Z-08Z.
Latest SREF prob charts and NAMNest indicate the Susquehanna
Valley (especially KIPT/KLNS) is the area most likely to
experience significant vis reductions early Tuesday morning.
Any fog should mix out by 13Z-14Z over eastern Pa, followed by
widespread VFR conditions for late morning. A cold front will
push south across the region Tuesday afternoon, accompanied in
many locations by a round of strong to potentially severe
thunderstorms, with briefly reduced cigs/vsbys and possible strong
wind gusts.
Wed-Thu...Rain/low cigs possible, mainly southern Pa.
Fri...Rain/low cigs likely.
Sat...Rain/low cigs possible.
Some repeat/training convection (on the tail end of fcst
damaging Tstm wind complex) is possible across the southern tier
of central PA mainly this evening. Collaborated with adjacent
offices on FF watch across srn PA although would have preferred
to issue a more short-fused watch. 1-3hr FFGs as low as 1-1.5
inches suggest some runoff problems are possible.
Deep moisture feed advecting northward along stalled frontal
boundary suggests potential for heavy rain/FF risk through late
week. Current WPC EROs favor areas just south of PA for marginal
to slight risk of flash flooding.
Flash Flood Watch from 2 PM EDT this afternoon through this
evening for PAZ033>036-056-057-059-063>066.
NEAR TERM...Steinbugl
SHORT TERM...Steinbugl

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