Mesoscale Discussion 1812

Issued

Oct 9, 2021, 12:04 PM CDT (17:04 UTC)

Expired

Oct 9, 2021, 2:30 PM CDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch likely

Chance of a watch

80%

The Aberdeen radar (KABR), 122 km from the middle of the area, began a volume at 17:07 UTC, the scan nearest the moment this went out.

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• Severe Thunderstorm Watch 514Oct 9, 2:55 PM CDT

704 
ACUS11 KWNS 091704
SWOMCD
SPC MCD 091704 
MNZ000-SDZ000-091930-

Mesoscale Discussion 1812
NWS Storm Prediction Center Norman OK
1204 PM CDT Sat Oct 09 2021

Areas affected...Parts of central/eastern South Dakota and adjacent
portions of western Minnesota

Concerning...Severe potential...Watch likely 

Valid 091704Z - 091930Z

Probability of Watch Issuance...80 percent

SUMMARY...Increasing thunderstorm development, perhaps including a
few supercells with primarily a risk for severe hail, particularly
across and northeast of the Huron and Mitchell areas, by the 2-4 PM
time frame.

DISCUSSION...Considerable convective development, with increasing
embedded thunderstorm activity, is ongoing in a corridor
near/southeast of the Black Hills into portions of the upper Red
River Valley.  This is near the nose of a strong southwesterly
mid/upper jet, still mostly over the southern Great Basin, but with
one significant emerging speed maximum now progressing through the
Nebraska Panhandle vicinity.

Forcing with this speed maximum appears to be contributing to
recently increasing thunderstorms across and northeast of the Pine
Ridge and Rosebud Reservation vicinities, where 16Z surface
observations suggest that there may also be a developing surface
low.  Models indicate that increasingly divergent upper flow will
contribute to lower/mid tropospheric cyclogenesis today through
tonight, with more rapid deepening forecast east/north of the Huron
and Mitchell vicinities into the upper Red River Valley after 00Z
this evening.

In the nearer term, southeasterly near-surface winds are
contributing to gradual boundary-layer moistening to the
east/northeast of the developing low, across southeastern into
central South Dakota, where dew points are now rising into the lower
60s.  It is not clear that moisture will increase much further, but,
coupled with mid-level cooling accompanying the emerging speed
maximum, thermodynamic profiles may become increasing conducive to
severe hail.

Based on recent model output and observational data, it appears that
this may become focused across and northeast of the Huron/Mitchell
vicinity as early as 19-21z.  Thereafter, a continuing gradual
increase in forcing for ascent and southeasterly inflow of
destabilizing boundary-layer air probably will support further
upscale growth, intensification and organization in the presence of
strong deep-layer shear.  This may include a couple of supercells,
with perhaps low-level hodographs becoming supportive of a risk for
tornadoes, though stronger south/southeasterly 850 mb jet
strengthening is generally not forecast until after dark across
western Minnesota.

..Kerr/Hart.. 10/09/2021

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

ATTN...WFO...MPX...FSD...ABR...

LAT...LON   44169962 45079868 45469668 44579601 43659715 43599913
            44169962

Mesoscale Discussions in South Dakota, 2021Mesoscale Discussions in Minnesota, 2021Every mesoscale discussion in 2021

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.