SPC AC 180548
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1148 PM CST Fri Nov 17 2017
Valid 181200Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE LOWER
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
GREATER OHIO VALLEY...
Thunderstorms capable of severe wind gusts, marginal hail, or a
brief tornado will be possible Saturday mainly over the Ohio Valley
A vigorous mid-level trough will quickly move east from the Plains
toward the East Coast on Saturday. As this occurs, a surface cyclone
near St. Louis will deepen and race to the northeast, reaching
upstate New York by Sunday morning. Strong mid-level winds (nearing
100 knots at 500 millibars) will overspread much of the Ohio Valley
as a surface cold front pushes southeast.
...Lower Ohio Valley...
Strong forcing for ascent along the southeastward-advancing cold
front should allow for thunderstorms to develop in the 18-21Z time
frame. Despite widespread cloud cover, most-unstable CAPE values
should still increase to between 500-1000 J/kg given the degree of
theta-e advection associated with the 50-60 knot low-level jet. The
modest buoyancy and lower-tropospheric flow largely parallel to the
front should result in thunderstorms organizing into one or more
small, bowing segments. Damaging wind gusts will be possible with
the strongest clusters/segments. Long hodographs with small loops in
the lowest three kilometers suggest that a brief tornado cannot be
ruled out within the larger thunderstorm clusters/segments. With
time and eastward extent, thunderstorms should outrun the greatest
buoyancy, diminishing the overall severe threat.
...Northeast Missouri eastward into Ohio...
Strong, moist isentropic ascent on the nose of a 40-60 knot
low-level jet should allow for elevated thunderstorms to occur along
and north of the track of the surface low/warm front. Here,
most-unstable CAPE values up to 1000 J/kg and mid-level-lapse rates
between 7 and 8 C/km will support a marginal hail risk.
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