Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Probability of hail 1" or larger within 25 miles of a point. Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Day 1 Hail Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
SPC AC 211257
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Fri Sep 21 2018
Valid 211300Z - 221200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER
NORTHEASTERN OHIO...NORTHWESTERN PENNSYLVANIA AND WESTERN NEW
YORK...
...SUMMARY...
Severe thunderstorm winds and a few tornadoes are possible across
parts of the lower Great Lakes region this afternoon and evening.
...Synopsis...
In mid/upper levels, a progressive northern-stream pattern will
prevail from the Pacific Northwest across the Great Lakes, while an
eastward-drifting anticyclone, centered over the Carolinas, slowly
weakens. The main perturbation pertinent to this forecast is a
strong shortwave trough, now evident in moisture-channel imagery
from northwestern ON across northwestern MN to central NE. This
feature is forecast to move eastward across the Great Lakes through
the period, accelerating and deamplifying over southeastern Canada
tonight as a strong cyclone digs southeastward across Hudson Bay. A
trailing area of enhanced vorticity, in weak mid/upper-level flow,
will drift across the Ozarks and southern Plains and contribute to
general thunderstorm potential there.
At the surface, 11Z analysis showed a cold front from eastern Lake
Superior across western Lake Michigan, central IL, southwestern MO,
southwestern OK, and southeastern NM. The associated surface
cyclone, initially near the northern end of Lake Superior, will
eject northeastward across northern parts of ON and QC through the
period. By 00Z, the cold front should extend across Lake Ontario,
OH, western KY, northern/western AR, and north-central TX to far
west TX. By 12Z, the front should reach Cape Cod, northern VA,
middle TN, and south-central TX.
...Lower Great Lakes region...
The bulk of the threat over the outlook area will arise in the form
of a quasi-linear convective system (QLCS) forecast to develop this
afternoon along/ahead of the cold front, across portions of southern
ON, southwestward over southeastern Lower MI, western Lake Erie, and
western OH to southern IN. Given the geometry of the deep-layer
winds, quick evolution of any discrete frontal cells to linear is
expected, and the resulting band of convection should sweep eastward
across the lower Great Lakes region through this evening before
weakening over eastern parts of the outlook area. Damaging gusts,
some severe, will be the main threat, though a few tornadoes are
possible, associated with embedded LEWP/misovortex features.
Forecast soundings show the most favorable buoyancy/shear parameter
space in and near the enhanced-risk area. Strong low-level and deep
shear will prevail just ahead of the line over parts of western NY
and northwestern PA, with 200-300 J/kg effective SRH and 50-55-kt
effective-shear magnitudes. Strong antecedent low-level moisture
transport and warm advection associated with a 45-55-kt
southwesterly LLJ will boost prefrontal theta-e, with surface
dewpoints mid-60s to near 70 F being common. Diurnal heating of
this air mass should offset weak mid/upper-level lapse rates enough
to boost MLCAPE to 1000-1500 J/kg. Any relatively discrete cells
that may develop in the warm sector will be in the most favorable
environment for tornadoes, with flow more backed and larger
hodographs compared to the immediate inflow region of the QLCS.
Development of any such prefrontal convection remains very uncertain
at this time, in the absence of evidence for substantial low-level
forcing or boundaries for such activity.
Severe potential will become more disorganized, isolated and sparse
in coverage with southwestward extent in the line and near the
front, as shear weakens. Instability also will decrease with time
and eastward extent this evening over the Mid-Atlantic and New
England, leading to diminishing severe probabilities with proximity
to the Atlantic Coast.
..Edwards/Peters.. 09/21/2018
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