Alert archive / Iowa / Mesoscale Discussions in 2021

Mesoscale Discussion 1741

September 13, 202131 counties in Iowa; 5 counties in Nebraska

The Storm Prediction Center issued this mesoscale discussion at Sep 13, 2021, 9:23 PM CDT for 31 counties in Iowa; 5 counties in Nebraska. It was in force for 97 min.

The area

The outline as issued.

The facts

Issued: Sep 13, 2021, 9:23 PM CDT (02:23 UTC)
Expired: Sep 13, 2021, 11:00 PM CDT
In force: 97 min
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch unlikely
Chance of a watch: 20%

All 36 counties

Audubon County, IA, Boone County, IA, Buena Vista County, IA, Butler County, IA, Calhoun County, IA, Carroll County, IA, Cerro Gordo County, IA, Crawford County, IA, Dallas County, IA, Franklin County, IA, Greene County, IA, Grundy County, IA, Guthrie County, IA, Hamilton County, IA, Hancock County, IA, Hardin County, IA, Harrison County, IA, Humboldt County, IA, Ida County, IA, Kossuth County, IA, Monona County, IA, Palo Alto County, IA, Pocahontas County, IA, Polk County, IA, Pottawattamie County, IA, Sac County, IA, Shelby County, IA, Story County, IA, Webster County, IA, Woodbury County, IA, Wright County, IA, Burt County, NE, Cuming County, NE, Dodge County, NE, Thurston County, NE, Washington County, NE

The radar

The Des Moines radar (KDMX), 129 km from the middle of the area, began a volume at 02:26 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

The nearest sounding before it

Omaha, NE (KOAX), launched 00:00Z, 2.4 hours before the alert went out and 141 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 1361 J/kg
Mixed-layer CIN: -168 J/kg
Mixed-layer LCL: 1731 m above ground
0-6 km bulk shear: 42 kt
0-1 km storm-relative helicity: 98 m²/s²
0-3 km storm-relative helicity: 213 m²/s²
Significant tornado parameter: 0.1
Supercell composite: 6.1

Open this sounding in Storm Lab

The text

As transmitted.

688 
ACUS11 KWNS 140223
SWOMCD
SPC MCD 140223 
IAZ000-NEZ000-140400-

Mesoscale Discussion 1741
NWS Storm Prediction Center Norman OK
0923 PM CDT Mon Sep 13 2021

Areas affected...Far eastern Nebraska into western and north-central
Iowa.

Concerning...Severe potential...Watch unlikely 

Valid 140223Z - 140400Z

Probability of Watch Issuance...20 percent

SUMMARY...An isolated strong to severe storm may occur with a threat
of large hail and damaging winds.

DISCUSSION...Elevated convection has developed in far eastern
Nebraska and western Iowa. However, so far, these storms have been
quite weak. The 00Z KOAX RAOB can provide a likely explanation.
MUCAPE is over 3000 J/kg, driven by a capped surface parcel.
However, based on the moisture profile, these storms are likely
forming in the moist layer just below 500mb. From this layer, CAPE
is quite low and thus, likely explains the weak nature of the
convection despite plan-view SPC mesoanalysis plots which show over
2000 J/kg MUCAPE in the region. RAP forecast soundings show
continued moistening below this layer which may gradually increase
the depth of the instability and support more robust updraft
development. Additionally, any storms rooted above 800mb will likely
have minimal effective shear with mostly uniform southwesterly flow
between 45 and 55 knots per RAP forecast soundings overnight. Given
the multiple factors against sustained strong updraft development, a
watch is unlikely, but convective trends will continue to be
monitored.

..Bentley/Edwards.. 09/14/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...ARX...DMX...FSD...OAX...

LAT...LON   42139704 42819545 43299427 43149291 42719251 41539356
            41329550 41369637 41619685 42139704

Nearby

Mesoscale Discussions in Iowa, 2021 Mesoscale Discussions in Nebraska, 2021 Every mesoscale discussion in 2021

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.