182
FXUS63 KUNR 201120
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
520 AM MDT Sun Jul 20 2025

.KEY MESSAGES...

- Marginal risk (level 1 of 5) to a sliver of slight risk (level
  2 of 5) for severe thunderstorms late this afternoon/evening
- Daily chances for at least isolated thunderstorms, some of
  which may be strong to severe, are expected next week
- Seasonably warm temperatures next week with sultry dew points
  potentially pushing heat indices into the lower 100s over south-
  central SD Monday/Tuesday

&&

.DISCUSSION...(Today Through Saturday)
Issued at 228 AM MDT Sun Jul 20 2025

08z surface analysis had weak low over northeastern WY, which was
along a wavering frontal boundary from ID through the ND/SD
border area. Dry line extended from low into southeast WY. Water
vapour loop had weak shortwave trough moving across eastern MT
which is rustling the elevated CAPE in place to produce transient
shra/TS early this morning. Convection is the main forecast
concern yet again.

Today/tonight, weakly rising heights behind shortwave this
morning will be replaced by weak shortwave moving across MT/ND.
This will give the northeast WY low a gentle nudge pushing dry
line into far southwest SD and supporting the development of a
diffuse warm front into eastern SD. 1-3KJ/kg mean MUCAPE develops
with 35-45kt 0-6km bulk shear, but decent SBCIN per amplifying
mid-level thermal ridge. Not much forcing to organize convection
today, which is reflected by amorphous CAM output. However,
juice/shear sufficient for isolated severe thunderstorms if
thunderstorms develop. Most likely place would be from northwest
into south-central SD where a smattering of updraft helicity
tracks are noted. SPC paints marginal (level 1 of 5) for these
areas with a sliver of slight (level 2 of 5) over the far
northeastern edge of the CWA. Best guess at timing is about 4pm-
midnight. Temperatures will be near guidance over the southwest
half and slightly below for the northeast half of the CWA per
higher dew points.

Monday, upper ridge amplifies over the MO river valley as flow
aloft turns southwest over the CWA. There is sure to be a wave
that moves through during peak heating. >3KJ/kg MUCAPE looks to be
available north/east of the Black Hills with sufficient shear for
severe thunderstorms. CAMs a bit more sure with convective
initiation over the higher terrain of WY, then moving into
moisture rich air mass over the eastern half of the CWA. Isolated
severe thunderstorms are expected with SPC marginal (level 1 of
5) risk. Temperatures should be near guidance with T/Td
combination probably (HREF 40-70% chance) pushing heat indices to
100-105F over south-central SD.

Broad upper level ridge builds into the southern to eastern half
of the CONUS for the rest of next week with attendant low level
thermal ridge building across the central/northern plains. This
will bring seasonable temperatures (80s/90s) across the region.
Southwest flow over the CWA will spew periodic shortwaves through
the area with bouts of convection, potentially severe from time-
to-time. Heat indices Tuesday could pop into the lower 100s (LREF
30-60% chance) over south-central SD. Typical late July weather.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Monday Morning)
Issued At 517 AM MDT Sun Jul 20 2025

Areas IFR conditions due to stratus/fog over northeastern
WY/southwestern SD will become VFR later this morning. More
widespread IFR conditions due to stratus/fog over northwest into
south-central SD will stick around most of the morning, but then
improve to VFR this afternoon. After 21z, isolated/scattered TSRA
will develop, mainly from K2WX-KICR with local IFR conditions,
gusty erratic winds, and hail.

&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Helgeson