939
FXUS63 KUNR 300536
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1136 PM MDT Tue Jul 29 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms expected overnight with
  some possibly becoming severe with heavy rain also possible.

- More unsettled weather expected for the heading into the
  weekend

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 111 PM MDT Tue Jul 29 2025

Current water vapor analysis shows deeply entrenched 500hpa high
pressure over the southern CONUS with anti-cyclonic upper flow
across much of the upper midwest. Embedded minor 500hpa short wave
seen moving into western WY which will be impacting our weather a
bit later on during the evening and overnight hours. At the
surface, ill defined surface low is located across south central
WY with an inverted trough poking to the north into central MT.
Much of SD is experiencing NE-E surface winds resulting in pockets
of low clouds/stratus, especially across the SW part of the
state. Surface dewpoints across the area remain in the 60s to near
70. Elevated convection ongoing across parts of SW SoDak already.

The main focus is for convective activity tonight. As the 500hpa
wave moves into the Wyoming part of the CWA, it will encounter an
airmass favorable for thunderstorm development. 700-500hpa lapse
rates by 00z Wednesday at 8+ C/km, especially across the SW part
of the state. MUCAPE values are 2500-3000 J/kg along with PWAT
values across much of the CWA in excess of 1.40. 0-6km mean bulk
shear values are on the order of 45-50 kts. Not only will severe
weather (hail and gusty winds) be a threat, but also concerned
about locally heavy rain.

Models indicate that overnight convective activity will most
likely linger into Wednesday morning across portions of the
eastern/southeastern CWA as upper 500hpa flow begins to turn a bit
more NW. Surface flow looks to become more light and variable,
tending southeasterly toward the evening. 850Hpa RH values are
also high through the day Wednesday, so envision a day where lower
clouds will be tough to scatter out. After the morning leftover
convection exits to the east/southeast, not really anything to
latch onto for further pcpn during the day. Cloud cover and 850hpa
temps suggest another below normal day as far as high temps go.

High pressure builds into the northern plains through Thursday
evening with a 500hpa ridge axis passing through the area. This
helps to keep most of the pcpn focused west of our area. By Friday
models are showing the region moving under quick moving zonal
flow, turning SW by the end of the weekend. Quick moving
disturbances in this flow pattern will bring back near daily
chances for pcpn along with a return to near seasonable temps.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 1129 PM MDT Tue Jul 29 2025

Thunderstorms will continue overnight across western South
Dakota...particularly along the I-90 corridor and perhaps into
south central SD by sunrise. There could be areas of MVFR/IFR
clouds early this morning across NE WY/western SD, but these
clouds should dissipate shortly after sunrise. After a quiet
morning and much of the afternoon, more storms are expected
Wednesday evening.

&&

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

$$

DISCUSSION/KEY MESSAGES...Hintz
AVIATION...13