Don't sleep on today in C/N IA
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0739 AM CDT Thu Jun 17 2021
Valid 171300Z - 181200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON INTO TONIGHT FOR PARTS OF THE UPPER MS VALLEY...
Severe storms are expected late this afternoon into tonight for
parts of the upper Mississippi Valley, from northern Iowa/southern
Minnesota into southern Wisconsin and northern Illinois. Wind gusts
near or exceeding 75 mph, hail up to the size of baseballs, and a
couple of tornadoes will be possible.
...Upper MS Valley this afternoon into tonight...
A midlevel shortwave trough over the northern Plains/southern
Prairie Provinces has crested the ridge, and heights will be
suppressed east-southeastward toward the upper MS Valley and upper
Great lakes by tonight. An associated, weak surface cold
front/trough will likewise move slowly southeastward across
NE/IA/MN/WI through tonight. The front and any lingering outflow
boundaries from overnight convection will likely focus afternoon and
evening thunderstorm development across southeast MN and along the
IA/MN border, in phase with an embedded speed max moving eastward
from SD (as evidence by the weak midlevel convection over SD this
The thermodynamic environment will feature steep midlevel lapse
rates with a warm elevated mixed layer extending eastward from the
Rockies, atop boundary-layer dewpoints in the 65-70 F range. Strong
surface heating in the wake of morning elevated convection will
drive large buoyancy (MLCAPE in excess of 4000 J/kg) and weakening
convective inhibition as surface temperatures warm into the 90s.
Effective bulk shear of 40-50 kt and the large CAPE will support a
threat for supercells initially, with very large hail (up to
baseball size) and damaging winds. Any semi-discrete storms in the
zone of stronger low-level shear/warm advection closer to the MS
River will also pose a threat for a couple of tornadoes. Otherwise,
storms are expected to coalesce and grow upscale into one or more
bowing clusters with the potential to produce significant damaging
winds (greater than 75 mph) prior to the convection spreading east
of the larger buoyancy late tonight.