Mesoscale Discussion 1728

Issued

Sep 11, 2021, 11:20 PM CDT (04:20 UTC)

Expired

Sep 12, 2021, 1:15 AM CDT

In force

115 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

20%

Counties Olmsted County, MN, Wabasha County, MN, Winona County, MN, Buffalo County, WI, Clark County, WI, Jackson County, WI, Pepin County, WI, Trempealeau County, WI

The La Crosse radar (KARX), 55 km from the middle of the area, began a volume at 04:20 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.

574 
ACUS11 KWNS 120420
SWOMCD
SPC MCD 120420 
WIZ000-MNZ000-120615-

Mesoscale Discussion 1728
NWS Storm Prediction Center Norman OK
1120 PM CDT Sat Sep 11 2021

Areas affected...Parts of southeastern Minnesota into west central
Wisconsin

Concerning...Severe potential...Watch unlikely 

Valid 120420Z - 120615Z

Probability of Watch Issuance...20 percent

SUMMARY...The development of a few storms, with at least some
potential to produce large hail, appears possible by 1-3 AM CDT, if
not before.  While the need for a watch still appears unlikely,
trends will continue to be monitored.

DISCUSSION...Westerly to northwesterly mid-level flow across much of
the Upper Midwest is seasonably strong, but broadly confluent,
downstream of a substantive short wave trough straddling the
Montana/North Dakota international border area.  However, at least a
couple of much more subtle, lower amplitude perturbations are
progressing through this regime, and may contribute to a developing
area of strengthening large-scale ascent aided by lower/mid
tropospheric warm advection.  

During the next few hours, it appears that this may become focused
within a corridor generally north of the Rochester, MN and La Crosse
WI areas.  This is to the north/cool side of a slowly southward
advancing surface cold front, where low-level moisture overrunning
the front, beneath the northern periphery of very warm and capping
elevated mixed-layer, appears to be contributing to sizable CAPE up
to around 2000 J/kg.

Latest model output varies concerning the extent to which mid-level
inhibition may suppress thunderstorm development tonight, but given
the potential instability and at least subtle signal for increasing
lift in the mid-levels, there appears at least low probabilities for
 the initiation of storms.  This may occur as early as 05-06Z, but
it appears a bit more probable toward 07-08Z.

Of primary concern, if storms do initiate, cloud-bearing layer shear
profiles appear conducive to supercells which could pose a risk for
sizable (perhaps greater than golf ball) hail.

..Kerr/Grams.. 09/12/2021

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

ATTN...WFO...GRB...ARX...MPX...

LAT...LON   44559262 44779039 43999000 43949101 43719258 44559262

Mesoscale Discussions in Wisconsin, 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.