Mesoscale Discussion 1675

Issued

Sep 1, 2021, 4:39 PM MDT (22:39 UTC)

Expired

Sep 1, 2021, 6:45 PM MDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

60%

Counties Box Butte County, NE, Dawes County, NE, Morrill County, NE, Sheridan County, NE, Bennett County, SD, Dewey County, SD, Haakon County, SD, Hughes County, SD, Jackson County, SD, Jones County, SD, Mellette County, SD, Oglala Lakota County, SD, Potter County, SD, Stanley County, SD, Sully County, SD, Todd County, SD

• Severe Thunderstorm Watch 485Sep 1, 5:25 PM MDT

304 
ACUS11 KWNS 012240
SWOMCD
SPC MCD 012239 
SDZ000-NEZ000-020045-

Mesoscale Discussion 1675
NWS Storm Prediction Center Norman OK
0539 PM CDT Wed Sep 01 2021

Areas affected...western Nebraska into central South Dakota

Concerning...Severe potential...Watch possible 

Valid 012239Z - 020045Z

Probability of Watch Issuance...60 percent

SUMMARY...Showers and thunderstorms will develop and increase in
coverage this afternoon and evening. Large hail and damaging
thunderstorm winds will be the primary hazard, although a tornado or
two will be possible through mid-evening. A watch may become
necessary and conditions will be monitored.

DISCUSSION...A moist, upslope regime persists across the region to
the north of a lee cyclone across northeast Colorado. To the north
of this lee cyclone, an inverted trough stretches from northeast
Colorado into southwest South Dakota. Area point-forecast soundings
indicate a pronounced warm nose in the 600-700 millibar layer, which
has provided a lid on convective initiation this afternoon. However,
continued heating and increasing large-scale ascent associated with
differential vorticity advection tied to the mid-level remnants of
tropical cyclone Nora should help to cool this warm layer through
the afternoon and evening. Additional, lower-level, forcing for
ascent will increase in response to increasingly warm, moist
advection on the nose of a strengthening southeasterly low-level jet
to the northeast of the lee cyclone across northeast Colorado. The
result should be an increase in showers and thunderstorms this
afternoon and evening within an environment characterized by
most-unstable CAPE values between 1500-2500 J/kg and effective-layer
shear of 35-40 knots. The most likely severe hazard from these
storms would be large hail and strong, gusty thunderstorm winds,
although a tornado or two cannot be ruled out this afternoon and
early evening, especially in the presence of low-level boundaries
that may provide an enhanced vorticity reservoir. 

A watch may become needed later this afternoon and conditions will
continue to be monitored.

..Marsh/Guyer.. 09/01/2021

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

ATTN...WFO...ABR...LBF...UNR...CYS...

LAT...LON   42790364 44820165 45340078 45239970 44399957 43210043
            41670264 41690369 42790364

Mesoscale Discussions in South Dakota, 2021Mesoscale Discussions in Nebraska, 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.