Damaging Winds in South Dakota, June 23, 2023

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
446 PM MDT Fri Jun 23 2023

.DISCUSSION...(This Evening Through Friday)
Issued at 114 PM MDT Fri Jun 23 2023

Main concern over the forecast period will be the potential for
severe weather today into tonight. Upper low over the Great Basin
will approach the region this afternoon. Associated increase in
large scale forcing coupled with gradual daytime destabilization
along/south of a northward-moving frontal zone will promote the
development of convection. Initial convection may be discrete or in
clusters, but as broader ascent overspreads the region, gradual
upscale growth into one or more complexes/MCSs will become
increasingly likely. All hazards look possible.

WV imagery this afternoon indicates an upper low over the Great
Basin approaching the Rockies. Broad ascent is evident surrounding
the low. Vis/IR imagery reveals persistent stratus over portions of
northeastern WY and western SD transitioning to cu/stratocu with
daytime heating. Billow clouds indicate that a stable layer persists
over much of southwestern SD and extending southward into
northwestern NE in the wake of morning convection. Farther east,
skies are relatively clear owing to subsidence behind this
convection. With daytime heating, the position of a nearly
stationary synoptic front stretching from central SD southward into
northern NE and northwestward into northeastern WY is becoming
increasingly well-defined thru surface obs, but it is still somewhat
nebulous--partially due to modification from this morning's
convection. A warm, moist air mass is in place over much of western
SD, with dew points mostly in the 60s and temperatures in the upper
60s to mid/upper 70s. Slightly cooler over northeastern WY, with
temperatures in the 60s and dew points largely in the upper 50s.

Some northward retreat of the aforementioned front as a warm front
is anticipated this afternoon and evening. However, northward extent
of the surface unstable sector--particularly across western SD--is a
question mark. Within the unstable sector, SB/MLCAPE will likely
increase to 1-2 kJ/kg by 21z, potentially exceeding 2000 J/kg in
spots. Meanwhile, effective bulk shear magnitudes are broadly
anticipated to increase to 30-40 kt. Stronger deep-layer and low-
level shear are anticipated in the vicinity of the frontal zone and
potentially near the Black Hills owing to terrain-driven flow
modifications. This is reflected quite well in the 12z HREF mean
fields, which show a corridor of ~200 m2/s2 of 0-3-km SRH extending
from Fall River Co. eastward thru Jackson Co. by 22z, with second
localized maximum >200 m2/s2 in the vicinity of the northern Black
Hills. Collectively, the base state environment and localized
perturbations owing to terrain and the frontal zone will likely also
support a few areas of locally enhanced STP values of 1-2. Outside
of these areas of locally backed low-level flow and correspondingly
favorable hodographs for dominant right-moving supercells (at least
initially), hodographs this afternoon are relatively straight, which
could support splitting supercells and/or initially dominant left
movers. Combination of steepening midlevel lapse rates, fairly
strong deep-layer shear, and potentially strongly rotating updrafts,
could support some large/significant hail with discrete cells early
in the event. Upscale growth owing to increasing large-scale ascent
(as noted previously) and constructive cell interference with time
should promote a transition to primarily a damaging wind threat
later in the event (03z onward, if not earlier). Depending on the
northward extent of appreciable low-level buoyancy, a QLCS tornado
threat may evolve with time as 0-3-km shear vector magnitudes ahead
of any maturing QLCS increase to 30-40 kt. Given PWATs exceeding
130% of normal regionwide per 12z RAOBs, locally heavy rainfall and
flash flooding is also a concern--particularly in any locations
where training can occur and/or that have recently experienced heavy
rainfall (e.g., northern Crook County, Ziebach County, southeastern
Tripp County). Widespread issues are not currently anticipated given
that storms/complexes will be fairly progressive on the whole.

Forward speed of the upper low slows somewhat tomorrow as it
progresses eastward over the Dakotas. Lingering moisture and a
tightening pressure/height gradient on the backside of the system
will support another round of showers/storms--along with breezy
conditions--tomorrow. However, severe chances will decrease
significantly as surface buoyancy is largely shunted east of our
area and more stable midlevel lapse rates overspread the region. As
the upper low departs late in the weekend, dirty shortwave ridging
builds over the High Plains. Though PWATs are projected to briefly
drop below average on Sunday, model consensus suggests they will
return to near/above average by Monday. Thus, despite a somewhat
drier/warmer period owing to increased midlevel ridging, daily
chances for storms will continue. Troughing over the western CONUS
will likely expand eastward in the Tuesday/Wednesday time frame per
model consensus, which could support the return of a more active
pattern.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 440 PM MDT Fri Jun 23 2023

Scattered to numerous thunderstorms, some strong to severe with
large hail and damaging wind gusts, will spread eastward across
northeastern Wyoming and western South Dakota during the rest of
afternoon and tonight. Low stratus will return behind the
thunderstorms with areas of IFR CIGS. Showers and thunderstorms
are expected during the day on Friday with AREAS MVFR/LCL IFR
conditions.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

DISCUSSION...Sherburn
AVIATION...10

Hazardous Weather Outlook
National Weather Service Rapid City SD
1011 AM MDT Fri Jun 23 2023

SDZ012>014-024>032-041>044-046-047-049-072>074-WYZ054>058-071-
241300-
Butte-Northern Meade Co Plains-Ziebach-Northern Black Hills-
Northern Foot Hills-Rapid City-Southern Foot Hills-
Central Black Hills-Southern Black Hills-Custer Co Plains-
Pennington Co Plains-Haakon-Fall River-Oglala Lakota-Jackson-
Bennett-Mellette-Todd-Tripp-Sturgis/Piedmont Foot Hills-
Southern Meade Co Plains-Hermosa Foot Hills-Northern Campbell-
Southern Campbell-Western Crook-Wyoming Black Hills-Weston-
Northeastern Crook-
1011 AM MDT Fri Jun 23 2023 /1111 AM CDT Fri Jun 23 2023/

This Hazardous Weather Outlook is for portions of western South
Dakota and northeastern Wyoming.

.DAY ONE...Today and Tonight

There is an enhanced risk (level 3 of 5) of severe thunderstorms
today, meaning that numerous severe storms are possible. Primary
hazards include hail greater than two inches in diameter and wind
gusts over 75 mph. Tornadoes are also possible. Heavy rain may
result in localized flooding.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

There is a chance of thunderstorms on Saturday and Monday.
Organized severe thunderstorms are not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

SDZ001-002-241300-
Harding-Perkins-
1011 AM MDT Fri Jun 23 2023

This Hazardous Weather Outlook is for portions of northwestern
South Dakota.

.DAY ONE...Today and Tonight

There is a slight risk (level 2 of 5) of severe thunderstorms
today, meaning that scattered severe storms are possible. Primary
hazards include large hail and damaging wind gusts. Heavy rain
may result in localized flooding.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

There is a chance of thunderstorms on Saturday and Monday.
Organized severe thunderstorms are not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

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