Mesoscale Discussion 1567

Issued

Aug 20, 2021, 1:51 PM CDT (18:51 UTC)

Expired

Aug 20, 2021, 3:15 PM CDT

In force

84 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

The Omaha radar (KOAX), 62 km from the middle of the area, began a volume at 18:51 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.

• Severe Thunderstorm Watch 452Aug 20, 4:00 PM CDT
• Severe Thunderstorm Watch 453Aug 20, 4:40 PM CDT

170 
ACUS11 KWNS 201851
SWOMCD
SPC MCD 201851 
IAZ000-MOZ000-NEZ000-SDZ000-202015-

Mesoscale Discussion 1567
NWS Storm Prediction Center Norman OK
0151 PM CDT Fri Aug 20 2021

Areas affected...Eastern Nebraska western Iowa and far southeastern
South Dakota

Concerning...Severe potential...Watch possible 

Valid 201851Z - 202015Z

Probability of Watch Issuance...40 percent

SUMMARY...Convective development is expected ahead of a cold front
by early afternoon across northeastern Nebraska. Bands of storms
capable of damaging winds/hail are expected and a weather watch is
likely needed later this afternoon.

DISCUSSION...As of 1840 UTC, regional WV imagery showed a large
negatively tilted upper low and developing 40 to 50 kt jet streak
across the western High Plains. At the surface, a 1004 mb low was
located along the ND/SD border with a cold front trailing south
through east-central Nebraska. There, latest visible imagery showed
agitated cumulus and weak convective cells ongoing along and ahead
of the front. 

As surface heating continues ahead of lift from the advancing
trough, remaining inhibition observed on the 18z OAX sounding will
erode and showers/thunderstorms will continue to form along the
front early this afternoon. With dewpoints in the low 70s F, SPC
mesoanalysis shows MLCAPE of 2000-3000 J/kg supportive of strong
updrafts. Regional soundings and VADS show 30-40 kts of effective
shear near and behind the front supportive of organized
lines/clusters and initial supercell structures. Strong forcing
along the fast moving front and will favor rapid upscale growth of
any storms that do develop suggesting the main threats will be
damaging winds and some hail. 

Convective trends and Hi-res guidance suggest initiation of
near-surface based storms is likely by as early as 1930-20z but,
uncertainty remains relatively high on the initial organization of
any storms that do develop. Stronger mid-level flow is somewhat
lagging behind the fast moving surface features suggesting storms
may initially be slow to organize. While the potential for severe
winds and hail does appear probable given the favorable parameter
space, the exact timing for a convective watch is uncertain.

..Lyons/Guyer.. 08/20/2021

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

ATTN...WFO...DMX...EAX...FSD...OAX...GID...

LAT...LON   42869758 43289717 43349674 43349620 43379569 43229529
            42989510 42589498 42159491 41749490 41279501 41009519
            40739551 40419593 40059663 40149732 40429752 40769774
            41219786 41759776 42869758

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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.