861
FXUS63 KUNR 152003
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
203 PM MDT Tue Jul 15 2025

.KEY MESSAGES...

- Isolated severe thunderstorms capable of large hail and damaging
  winds are possible this afternoon through late this evening,
  mainly over far south central SD, southwestern SD, and
  northeastern WY.

- Unseasonably cool and unsettled conditions are expected tomorrow,
  with record low minima and maxima possible.

- Zonal flow promoting more seasonable temperatures and daily
  chances for showers and thunderstorms returns late this week.

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 203 PM MDT Tue Jul 15 2025

Water vapor imagery reveals an upper wave crossing the region at
this hour, supporting recent/ongoing convective initiation across
south central SD (along a cold front) and the central/southern
Black Hills. A cumulus field, with embedded towering cu, extends
from around the White River in Tripp County southwestward through
the Sandhills. More mature convection with plentiful lightning
activity is evident over the southern Black Hills. Another cu
field is present over northeastern WY east of the Bighorns.
Surface observations show mostly postfrontal northerly flow across
the area, with the exception of far south central SD (south of
the front) and in the vicinity of the southern Black Hills, where
flow is wrapping around terrain.

The cold front has effectively stalled at this hour across south
central SD; otherwise, the surface front has largely cleared our
area to the south, as reflected by cooler temperatures and northerly
flow. South of the surface front, MLCAPE of 2000+ J/kg and gradually
increasing effective bulk shear could support the development of
organized surface-based thunderstorms over the next few hours, some
of which could become severe. Given relatively modest low-level
shear and mean winds but reasonably high DCAPE (1000+ J/kg), hail
growth zone CAPE, effective storm-relative helicity, and proximity
to a corridor of enhanced vertical theta-E differences, suspect that
pulsey multicells or transient supercells will be the primary mode,
supportive of severe hail (up to around ping pong ball sized) and
robust downdrafts/downbursts will prevail along and ahead of the
front. Enhanced MUCAPE extends behind the front westward toward the
central and southern Black Hills and northwestward into northeastern
WY. Locally enhanced effective storm-relative helicity and effective
bulk shear should again be sufficient for multicell/supercell
organization over the central and southern Black Hills, and locally
higher dew point depressions aided by downslope flow may support
some downbursts--an assertion further supported by an extension of
uncapped surface-based CAPE per the SPC mesoanalysis. Farther west
over northeastern WY, a relatively lower CAPE but higher shear
environment is in place, with MLCAPE values of <1000 J/kg combined
with effective bulk shear magnitudes of 40-50 kt likely supporting
the development of organized convection later this afternoon into
the evening over the Bighorn Basin/Mountains. Much of this shear is
concentrated in low levels, with 0-3 km shear vector magnitudes of
30-40 kt across northwestern WY/south central MT at this hour. This
will further support multicellular organization and the potential
evolution into an MCS/QLCS late this afternoon into the evening.
Should this convection emerge off of the Bighorns and into
northeastern WY, it will pose a threat for primarily damaging winds.

The cold front, aided by convective outflow, will surge southward
overnight into early tomorrow. Behind the front, both fog and smoke
may spread into portions of the region. Meanwhile, more robust, deep
Q-vector convergence arrives from west to east tomorrow morning.
Coupled with developing isentropic ascent, we should see fairly
widespread shower/rain activity, especially over our northwestern
half. Embedded thunderstorm activity is also likely, and even a few
strong storms cannot be ruled out given strong low-level shear
coupled with lingering elevated CAPE. Rising heights and Q-vector
divergence coupled with surface high pressure building into the
region should bring an end to shower/storm activity tomorrow
evening/overnight. Lingering cloud cover may prevent optimal
radiational cooling tomorrow night, but if clear skies prevail,
record low temperatures for the date could certainly be broken.
Record low maximum temperatures are also at risk; for example, the
current forecast high of 59 degrees for Downtown Rapid City would
break the prior record for 7/16 by four degrees.

Beyond tomorrow, midlevel zonal flow prevails thru much of the
period as low-level thermal ridging gradually expands over the
western/central CONUS, which should bring more seasonable
temperatures coupled with daily chances for showers/storms.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1149 AM MDT Tue Jul 15 2025

Widespread MVFR/IFR CIGS are expected through the TAF period
spreading north to south through tonight. TSRA will develop this
afternoon/tonight with strong, gusty erratic winds and hail at
times, mainly over northeastern WY through far southern SD.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...Dye