Jun 18, 2021 1300 UTC Day 1 Convective Outlook
Updated: Fri Jun 18 12:56:37 UTC 2021 (20210618 1300Z Day 1 shapefile | 20210618 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20210618 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
ENHANCED 56,208 12,634,187 Indianapolis, IN...Columbus, OH...Cincinnati, OH...Akron, OH...Dayton, OH...
SLIGHT 96,367 11,482,810 Cleveland, OH...Pittsburgh, PA...Toledo, OH...Fort Wayne, IN...Peoria, IL...
MARGINAL 172,346 30,365,495 Detroit, MI...Denver, CO...Kansas City, MO...New Orleans, LA...Omaha, NE...
Probabilistic Tornado Graphic
20210618 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 % 29,510 8,879,587 Columbus, OH...Cleveland, OH...Akron, OH...Dayton, OH...Parma, OH...
2 % 98,950 12,678,294 Indianapolis, IN...New Orleans, LA...Toledo, OH...Fort Wayne, IN...Mobile, AL...
Probabilistic Damaging Wind Graphic
20210618 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
SIG SEVERE 45,145 9,489,065 Indianapolis, IN...Columbus, OH...Cincinnati, OH...Dayton, OH...Springfield, IL...
30 % 56,402 12,697,900 Indianapolis, IN...Columbus, OH...Cincinnati, OH...Fort Wayne, IN...Akron, OH...
15 % 96,321 11,467,473 Cleveland, OH...Pittsburgh, PA...Toledo, OH...Peoria, IL...Erie, PA...
5 % 171,158 30,114,120 Detroit, MI...Denver, CO...Kansas City, MO...New Orleans, LA...Omaha, NE...
Probabilistic Large Hail Graphic
20210618 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
SIG SEVERE 44,447 2,441,651 Springfield, IL...Peoria, IL...Decatur, IL...Champaign, IL...Bloomington, IL...
15 % 113,886 17,556,641 Indianapolis, IN...Columbus, OH...Cleveland, OH...Toledo, OH...Cincinnati, OH...
5 % 117,845 25,148,728 Detroit, MI...Denver, CO...Kansas City, MO...Omaha, NE...St. Louis, MO...
   SPC AC 181256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Fri Jun 18 2021

   Valid 181300Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   ILLINOIS...INDIANA AND OHIO...

   ...SUMMARY...
   Scattered severe storms will pose a threat of damaging wind, large
   hail and perhaps a couple tornadoes this afternoon and tonight over
   parts of the Ohio Valley region and mid Mississippi Valley.

   ...Synopsis...
   In mid/upper levels, the northern-stream flow belt will be aligned
   generally from west-northwest to east-southeast over the northern
   CONUS.  The highest-amplitude shortwave trough is apparent in
   moisture-channel imagery over southern SK, and forecast to move to
   WI by the end of the period.  A more-subtle, but still apparent,
   perturbation over NE will move eastward toward the IL/IN border
   through 12Z tomorrow.  Meanwhile, a weak, slowly retrograding trough
   over eastern/central TX and northeastern MX -- to the east of a
   major anticyclone over the Four Corners region -- will affect the
   motion of a tropical disturbance discussed below. 

   At the surface, 11Z analysis showed a cold front from northern WI
   across western IA and south-central NE, to a low near GLD, then
   across central Co.  The low is forecast to weaken and move roughly
   eastward to northeastern KS by 12Z.  Another frontal-wave low may
   develop tonight over WI and move to Lake Michigan, ahead of the
   strongest mid/upper-level shortwave trough, with the front
   connecting those lows, and extending across southeastern CO by the
   end of the period. 

   ...Midwest corridor...
   This outlook area is a spatial composite of two most-probable
   regimes that ultimately may merge overnight.  In both, ultimate
   upscale growth will foster a dominant damaging-wind threat, with
   some significant (65+ kt) gusts possible.  Enough low-level shear
   may develop to support some tornado potential (especially the
   eastern/first area).  

   1.  Potential re-invigoration of convection is possible this
   afternoon associated with the UVV field now supporting the ongoing
   non-severe convection over the upper Great Lakes.  This swath of
   ascent should encounter a diabatically destabilizing and advectively
   moistening plume of boundary-layer air over the OH/southeastern
   MI/northern WV region, with some backbuilding possible this evening
   westward across IN and perhaps eastern IL.  A corridor of rich
   moisture now within reasonably short distance upstream (southeastern
   MO, southern IL, western KY) will spread up the Ohio Valley region,
   underlying sufficiently steep midlevel lapse rates to support a
   plume of 2500-3500 MLCAPE (locally/briefly higher).  

   The same relatively veered surface winds that will enable that moist
   advection also will limit vertical shear, with forecast soundings
   reasonably showing effective-shear magnitudes of 25-40 kt.  However,
   enough low-level speed shear will exist to enlarge hodographs and
   present effective SRH in the 150-250 J/kg range, supporting a blend
   of supercell and multicell structures, with bowing segments
   possible.  Cold-pool aggregation will transition the threat to
   mainly wind with time, though the eastern and southern bounds of the
   threat will be regulated by decreasing inflow-layer instability.

   2.  Upshear, mostly later development this evening and tonight,
   across the southern IA/northern MO to central/eastern IL/western IN
   corridor.  An initially blended hail/wind threat (some significant/
   2+ inch diameter hail possible with relatively discrete cells) will
   transition toward wind with time, as upscale growth occurs with
   east-southeastward extent.  This activity will be supported by a
   combination of frontal forcing and isentropic lift to LFC, near the
   nose of a 40-50-kt southwesterly LLJ, with MLCAPE 2000-4000 J/kg and
   (despite modest near-surface flow) 35-45-kt effective-shear
   magnitudes.  A conditional, very cap-dependent potential also exists
   for late-afternoon supercell formation along the boundary near the
   IA/MO border, in which case significant hail would be possible early
   in the convective cycle.  Given these areas of hail potential, a
   corridor of 10% significant-hail threat has been introduced within
   (and not affecting) existing categorical bounds, which still appear
   valid for this outlook cycle.

   ...Central High Plains...
   Isolated to widely scattered thunderstorms should develop this
   afternoon and last into part of the evening, moving eastward across
   the outlook area.  Isolated severe wind and hail will be the main
   concerns.

   After the frontal passage, northeasterly flow will veer gradually to
   easterly, imparting a substantial near-surface upslope component
   that will aid in enhancing deep shear, as well as in moist advection
   and storm-relative winds in the inflow layer.  Associated lift,
   along with diabatic heating of elevated terrain, will promote
   deepening afternoon convection beneath steep midlevel lapse rates,
   with MLCAPE to near 1000 J/kg, atop a well-mixed and deep subcloud
   layer suitable for strong to locally severe gusts.  35-45-kt
   effective-shear vectors will support mostly multicells, but with at
   least transient supercell characteristics possible.  

   ...Gulf Coast...
   A tropical storm is forecast to develop from an area of low pressure
   now over the west-central Gulf, its center making landfall on the
   south-central to southeastern LA coastline late tonight.  [See the
   latest NHC advisories for specific details on forecast track and
   intensity, as well as all tropical-related watches and warnings.] 
   Given:
   1.  The latest track forecast with eastward-expanded wind radii, 
   2.  The strongly sheared and somewhat hybrid tropical/subtropical
   character of this system, and related highly asymmetric, eastward-
   skewed convective distribution expected (per NHC discussion)... 
   The outlook is shifted eastward, but with similar shape, maintaining
   marginal tornado potential generally north through east of center
   through the end of the period.  Inland spread of the juxtaposition
   between favorably moist/unstable marine air and enlarged hodographs
   should be limited this afternoon and overnight, before expanding
   day-2 (per that outlook) with the added influences of diurnal
   heating and additional inland translation of the system.

   ..Edwards/Broyles.. 06/18/2021

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