193
ACUS01 KWNS 191953
SWODY1
SPC AC 191952

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0252 PM CDT Thu Sep 19 2024

Valid 192000Z - 201200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MIDWEST INTO EASTERN KANSAS...WESTERN MISSOURI...AND FAR
NORTHEAST OKLAHOMA...

...SUMMARY...
Scattered severe thunderstorms are possible this afternoon and
evening across central/eastern Minnesota, Iowa, and western
Wisconsin. A couple of tornadoes, large hail, and damaging winds may
occur across this region. Farther south, an area of locally severe
wind gusts and hail may also develop over parts of eastern Kansas,
southwest Missouri, and northeast Oklahoma.

...20Z Update...
The 18Z observed Minneapolis sounding showed steep mid-level lapse
rates in addition to a long and predominantly straight hodograph.
The expectation remains that a few supercells may initially develop
from north-central Iowa into east-central Minnesota over the next
2-3 hours. These storms would pose the greatest risk for
large/very-large hail. Given weak low-level shear evident on area
VAD data in combination with linear forcing from the cold front,
upscale growth should still occur quickly with a transition to more
of a damaging wind threat. As this is in line with the thinking of
the previous forecast, no changes have been made other than minor
changes to the general thunderstorm area.

See the previous discussion for further details.

..Wendt.. 09/19/2024

.PREV DISCUSSION... /ISSUED 1126 AM CDT Thu Sep 19 2024/

...Minnesota/Wisconsin/Iowa...
Widely spaced and mainly elevated thunderstorms ongoing late this
morning across parts of IA into eastern MN and western WI should
continue to diminish in coverage over the next couple of hours. In
their wake, daytime heating of a seasonably moist low-level airmass
(surface dewpoints generally in the 60s) and somewhat steep
mid-level lapse rates will contribute to at least moderate
instability by mid afternoon along/ahead of a cold front extending
north to south from western MN into eastern NE and western IA. An
upper low over northern ND and southeast SK/southwest MB will
continue to advance slowly northeastward across south-central Canada
through the period. A belt of 35-50 kt mid-level west-southwesterly
winds on the southern periphery of the upper low will spread
eastward from the northern Plains into the Upper Midwest through
this evening. A veering/strengthening wind profile with height
through mid levels will support around 30-40 kt of deep-layer shear
across the warm sector. This shear will be sufficient for organized
thunderstorms, including the potential for a few supercells.

Current expectations are that scattered thunderstorms will develop
by mid to late afternoon across central/eastern MN into
northern/central IA along or just ahead of the eastward-advancing
cold front. A rather favorable thermodynamic environment will foster
robust updrafts, and any initial supercells should pose a threat for
large to very large hail (around 1.5-2.5 inches in diameter). But, a
fairly quick transition to more linear structures should occur given
the presence of the cold front, with a potentially greater damaging
wind threat evolving with eastward extent through the evening before
convection eventually weakens. South-southwesterly low-level flow
and related shear ahead of the front is not forecast to be overly
strong by most guidance this afternoon. But, some threat for a
couple of tornadoes remains apparent, especially with any sustained
supercells across east-central/southern MN into northern IA late
this afternoon/early evening.

...Eastern Kansas into Missouri and Northeast Oklahoma...
An upper ridge, with rather warm mid-level temperatures, will remain
centered over the southern Plains today. The surface cold front is
forecast to continue moving east-southeastward across the mid MO
Valley and central Plains through late this afternoon. A surface
trough extending southward from this front across the southern High
Plains should also mix eastward through peak daytime heating. Given
weak large-scale ascent and rather warm mid-level temperatures
across this area, there is still a fair amount of uncertainty
regarding how many additional thunderstorms will develop later
today, notwithstanding the ongoing (mainly elevated) convection over
eastern KS and far northwest MO. Still, other surface-based
thunderstorms may develop later this afternoon along/near the
front/dryline intersection in south-central KS/north-central OK, and
along the cold front in eastern KS. While low/mid-level flow should
remain fairly modest, steepened low-level lapse rates and moderate
instability should support a risk for mainly severe/damaging
downdrafts as convection spreads east-southeastward into western MO
and northeast OK through this evening.

$$