Sep 21, 2018 1300 UTC Day 1 Convective Outlook
Updated: Fri Sep 21 12:57:03 UTC 2018 (20180921 1300Z Day 1 shapefile | 20180921 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180921 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
ENHANCED 42,180 8,909,220 Cleveland, OH...Buffalo, NY...Rochester, NY...Akron, OH...Syracuse, NY...
SLIGHT 55,123 10,761,226 Columbus, OH...Pittsburgh, PA...Toledo, OH...Cincinnati, OH...Dayton, OH...
MARGINAL 114,020 17,784,574 Detroit, MI...Indianapolis, IN...Louisville, KY...Lexington-Fayette, KY...Fort Wayne, IN...
Probabilistic Tornado Graphic
20180921 1300 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
5 % 29,419 4,294,427 Buffalo, NY...Rochester, NY...Erie, PA...Youngstown, OH...Cheektowaga, NY...
2 % 24,847 6,352,361 Cleveland, OH...Pittsburgh, PA...Akron, OH...Syracuse, NY...Parma, OH...
Probabilistic Damaging Wind Graphic
20180921 1300 UTC Day 1 Damaging Wind Probabilities Graphic
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.
Day 1 Wind Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
30 % 42,335 8,957,334 Cleveland, OH...Buffalo, NY...Rochester, NY...Akron, OH...Syracuse, NY...
15 % 55,130 10,659,505 Columbus, OH...Pittsburgh, PA...Toledo, OH...Cincinnati, OH...Dayton, OH...
5 % 114,594 17,954,667 Detroit, MI...Indianapolis, IN...Louisville, KY...Lexington-Fayette, KY...Fort Wayne, IN...
Probabilistic Large Hail Graphic
20180921 1300 UTC Day 1 Large Hail Probabilities Graphic
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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