788
FXUS63 KUNR 202302
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
502 PM MDT Sat Jul 20 2024

.KEY MESSAGES...

- Isolated/scattered thunderstorms possible through Monday
- Hazy skies due to Canadian fires
- Increasingly hot/dry middle/end of next week with
  elevated/critical fire danger

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 127 PM MDT Sat Jul 20 2024

19z surface analysis had weak through from central ND through the
Black Hills into southern WY. Water vapour matinee had upper
ridge from the southwest CONUS into southwest Canada. Northerly
flow aloft noted over the northern Plains with a parade of weak
shortwaves rotating southward into the CWA. SPC mesoanalysis had
1-1.5KJ/kg MLCAPE along/east of surface trough with minimal cap.
Shear profiles are weak, except for a small area of 25kts
effective shear over southwest SD on the lee side convergence zone
area given northeast boundary layer flow. Geocolor satellite loop
clearly shows smoke pouring into the region courtesy of (mostly)
Canada. Forecast concern revolves around upper ridge.

Late this afternoon through Monday afternoon, thunderstorm
chances continue as shortwaves rotate through. Enough
buoyancy/shear through this evening for isolated strong/severe
thunderstorms, mainly over far southwestern SD. Weaker
buoyancy/shear expected Sunday with perhaps a small uptick for
Monday. Organized severe thunderstorm chances low. HRRR integrated
smoke guidance depicts hazy skies through Sunday night.
Temperatures will be near guidance at night, but possibly a bit
below for highs depending on smoke aloft thickness.

Monday night through Friday, upper/thermal ridge builds into the
northern Plains. Increasingly hot/dry weather expected with
chances for 100F+ readings 50%+ Thursday most areas and Friday for
western SD. Upper ridge should flatten late Friday, allowing a
cold front to move through the area. Deep boundary layer mixing
will promote gusty winds during peak insolation, especially
Thursday/Friday. While precipitation chances will be very low, an
unmentionable stray thunderstorm cannot be ruled out. Mentionable
PoPs return late Friday with potential cold front.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 500 PM MDT Sat Jul 20 2024

VFR conditions are generally expected through the period.
Isolated to scattered showers and thunderstorms will continue
through this evening, especially across the southern Black Hills
area. Brief MVFR conditions can be expected near any stronger
storms. Otherwise, hazy skies from Canadian forest fires will
continue. Isolated to scattered showers and thunderstorms again
expected Sunday afternoon.

&&

.FIRE WEATHER...
Issued At 127 PM MDT Sat Jul 20 2024

Warm conditions continue through Monday with chances for
thunderstorms, but coverage will be limited. An upper level ridge
will bring increasingly hot/dry temperatures for Tuesday into
Friday. An upper ridge breaks down Friday with cold front with
some uncertainty as to the timing of the cold front. Pre-frontal
environment will be hot/dry with locally gusty winds possible.
Temperatures ahead of the front will be in the 90s to around 100.
LREF is giving 50-90% probability of relative humidity less than
15% across northeastern Wyoming and far western South Dakota. The
result will be elevated to critical fire weather conditions,
especially across northeastern Wyoming and far western South
Dakota.

&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Johnson
FIRE WEATHER...Helgeson