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.
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...
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.
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.
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
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
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.
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.]
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.
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z