Public Information Statement
Issued by NWS

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NOUS41 KPBZ 211609

Public Information Statement
National Weather Service Pittsburgh PA
1109 AM EST Tue Nov 21 2017


Location...Plum to Murrysville in Allegheny and Westmoreland
County western Pennsylvania
Date...Nov 19 2017
Estimated Time...1211 AM - 1215 AM EST
Maximum EF-Scale Rating...EF1
Estimated Maximum Wind Speed...90 mph
Maximum Path Width...125 yd
Path Length...2.5 mi
Beginning Lat/Lon...40.4206N/79.7277W
Ending Lat/Lon...40.4910N/79.6829W
* Fatalities...0
* Injuries...0

* The information in this statement is preliminary and subject to
change pending final review of the event(s) and publication in
NWS Storm Data.

The National Weather Service in Pittsburgh PA has confirmed that
storm damage on Nov 19, 2017 that occurred from Plum to
Murrysville in Allegheny and Westmoreland Counties in western
Pennsylvania was the result of a combination of tornadic and
straight-line wind.

The storm system that produced the damaging wind is known as a
quasi-linear convective system (QLCS). The line of storms on
Saturday night formed in conditions that were favorable for the
development of small, short-lived damaging circulations (called
mesovortices) along the edge of the storm outflow. Such
circulations occasionally can produce tornadoes in addition to
extreme straight-line wind.

On Sunday and Monday, a team from the National Weather Service in
Pittsburgh surveyed damage along and adjacent to Saltsburg Road
from Plum to Murrysville.

In summary: damage along the entire 3+ mile path of the storm was
caused by a combination of tornadic and straight-line wind. At its
worst, the tornadic damage appears consistent with peak wind of 90
mph, with a rating of EF1.

Climatologically, tornadoes in the Ohio Valley are rare in
November. Since 1950, this is only the seventh November tornado
documented, but the second November tornado this year. Despite its
small size and short duration, the combination of the time of
year, intensity and time of day (overnight) makes this tornado
very unusual.

The survey team is immensely appreciative to local and county
emergency management/fire department personnel and the public for
their invaluable assistance in this two-day effort to document,
assess and provide additional details of the storm damage.

A detailed analysis of the evaluated storm damage is provided
below for interested readers.


A focused swath of damage appears to have begun in Boyce Park
near the entrance on New Texas Road, where trees were uprooted and
limbs were snapped. This swath of focused, but sporadic tree
damage continued along the storm`s path generally parallel to
Saltsburg Road. Based on the radar appearance of the storm, this
damage likely was the result of straight-line wind on the southern
flank of the intensifying mesovortex.

Damage became more concentrated and higher-end as the circulation
approached Kocher Road and Saltsburg Road, At this point, damage
suggests that a narrow tornado began, where large hardwood trees
were uprooted or snapped. At a senior residential community along
Serenity Lane, a vehicle was flipped by the wind and dragged
westward and air condenser units on both the upwind and downwind
sides of buildings were moved. Buildings suffered minor loss of
rooftop shingles and downwind loss of siding, and substantial
tree damage was observed.

As the storm progressed eastward, less-concentrated tree damage
was noted until it approached Abers Creek Road, where focused
areas of snapped trees were discovered. Trees at the base of a
hill on the northern end of the Clover Commons residential
community were hit particularly hard, with substantial snappage

Of special consideration: a home at the top of this small hill
suffered threshold damage and lost the roof of an attached
outbuilding owing to an open window in the structure. A properly-
mounted, high-quality wind sensor at this home measured a peak
wind gust of 82 mph from the west during the storm on the outer
periphery of the mesovortex circulation.

The tornado/parent mesovortex appear to have weakened briefly
east of this location, as only threshold wind damage in the form
of low-end tree debris was noted along Havana Road (although it
is possible that such damage was hidden from view of the survey

A reintensification of damage was found near the county line
between Westmoreland and Allegheny Counties, near and east from
Logan Ferry Road. Concentrated tree damage appears to have begun
in earnest near a Lutheran Church at the intersection of Route 286
and Logan Ferry Road. Damage continued into the adjacent
neighborhoods on Old Logan Ferry Road, Lott Court, Calla Drive and
Margaretta Drive, where trees were uprooted and limbs were

As the storm emerged from Lott Court toward Sardis Road and
Saltsburg Road, the tornado reintensified, producing the most-
substantial tree damage noted along its path. In this location, a
focused, convergent, 125 yd-wide path was cut through several
clusters of hardwood and softwood trees. Full-leaf trees were
snapped at the trunks, and other trees with no leaves were
uprooted. Even considering the pre-conditioned wet ground, wind
was quite strong in this area, as sheltered homes saw minor
rooftop damage from the wind in addition to the loss of siding
(and damage after being pelted by airborne debris), power poles
suffered minor damage and tree damage was considerable.

Along Sardis Road and Saltsburg Road, a classic indication of a
QLCS tornadic mesovortex was discovered, with a convergent debris
field noted to the north of a proximate divergent debris field.
The width of the entire debris field of note spanned perhaps
300-400 yd along Sardis Road.

The circulation weakened quickly after crossing Saltsburg Road,
with only sporadic snappage of tree limbs observed from afar in a
heavily-wooded area and no focused path continuing thereafter.

The damage near Logan Ferry Road likely was the result of a
developing, second surge of thunderstorm outflow (straight-line
wind) as the mesovortex restrengthened. Upon approaching/exiting
Lott Court, the earlier tornado strengthened to damaging level once
again before weakening as it crossed Saltsburg Road.

Despite the detail provided above, the entire swath of damage from
Plum to Murrysville appears to have occurred over a five-minute
period from 1210-1215 am Sunday morning. The tornado likely was in
progress from 1211-1215 am.

At many times during this tornado`s existence, the strongest wind
appears to have occurred at rooftop/treetop level, perhaps 30-50
ft above ground. This conclusion is most evident toward the end of
the path, where children`s plastic playhouses and lightweight
yard furniture on the ground were unaffected, while shingles and
chimney stacks were removed and trees were snapped at the trunk.

One of the tallest softwood trees remained standing while all
others around it were snapped. The very top of this tree retained
its evergreen branches while all lower branches were stripped from
the tree by the wind.

This information also can be found on our website at

For reference: the Enhanced Fujita Scale classifies tornadoes
into the following categories:

EF0...Wind speeds 65 to 85 mph
EF1...Wind speeds 86 to 110 mph
EF2...Wind speeds 111 to 135 mph
EF3...Wind speeds 136 to 165 mph
EF4...Wind speeds 166 to 200 mph
EF5...Wind speeds greater than 200 mph


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