799
FXUS63 KUNR 221120
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
520 AM MDT TUE SEP 22 2026
KEY MESSAGES
- SHOWERS HAVE TAPERED OFF WITH LINGERING LOW CLOUDS/FOG
DEVELOPING.
- OVERALL DRIER WEATHER EXPECTED FOR THE REST OF THE WEEK, BUT
SOUTHWEST FLOW ALOFT WILL BRING SOME CHANCES FOR SHOWERS AND
THUNDERSTORMS.
- BREEZY/WINDY SPOTS TUESDAY/WEDNESDAY
UPDATE
ISSUED AT 917 PM MDT MON SEP 21 2026
NOT MUCH CHANGE IN THE SHORT TERM FORECAST. BASED ON LATEST RADAR
AND SATELLITE IMAGERY, REMOVED LINGERING POPS AROUND THE BLACK
HILLS FOR TONIGHT AND INCREASED THE FOG POTENTIAL. STILL EXPECTING
A ROUND OF RAIN TO PUSH NORTHWESTWARD THROUGH THE SOUTHERN SD
PLAINS BEGINNING EARLY TUESDAY WITH ANOTHER ROUND OF ISOLATED TO
SCATTERED SHOWERS AND STORMS LATE TUESDAY INTO WEDNESDAY.
WINDS WILL BE A BIT ELEVATED TUESDAY INTO WEDNESDAY AS WELL,
ESPECIALLY FOR THE SD PLAINS WERE GUSTS UP TO 35 TO 45 MPH ARE
POSSIBLE.
FOR NOW, TUESDAY'S FORECASTED HIGHS ARE IN THE 70S FOR NE WY AND
60S TO LOW 70S FOR THE SD PLAINS. BUT IF CLOUDS CLEAR OUT QUICKER
THAN EXPECTED, ACTUAL AFTERNOON TEMPERATURES WILL BE WARMER.
DISCUSSION
(THIS EVENING THROUGH MONDAY)
ISSUED AT 109 PM MDT MON SEP 21 2026
19Z SURFACE ANALYSIS HAD SPRAWLING HIGH PRESSURE OVER THE GREAT
LAKES INTO THE NORTHERN PLAINS BRINGING LOW-LEVEL EASTERLY
COMPONENT TO BOUNDARY LAYER FLOW. WATER VAPOUR LOOP HAD UPPER LOW
SPINNING OVER THE SD/NE/IA/MN BORDER AREA WITH BACKWASH MOISTURE
SPILLING INTO THE CWA. THIS MOISTURE COMBINED WITH WEAK UPSLOPE
SUPPORTING A FEW SHOWERS/DRIZZLE OVER THE AREA THIS AFTERNOON,
PARTICULARLY IN/NEAR BLACK HILLS PER LATEST KUDX RADAR LOOP.
EFFECTS OF UPPER LOW EXITING WILL BE THE MAIN FORECAST CONCERN
MID-WEEK.
TONIGHT THROUGH WEDNESDAY, UPPER LOW SHIFTS AWAY WITH UPPER RIDGE
EXPANDING INTO THE NORTHERN PLAINS. SOUTHWEST FLOW ALOFT DEVELOPS
OVER THE AREA WITH A FEW WEAK SHORTWAVES EXPECTED TO SAUNTER
THROUGH THE CWA. FIRST ONE ARRIVES EARLY TUESDAY, HELPING INCREASE
GRADIENT BETWEEN DEVELOPING LEE TROUGH AND DEPARTING SURFACE
HIGH. TWO EFFECTS WILL OCCUR. BREEZY/WINDY SPOTS WILL DEVELOP
TUESDAY WITH INCREASING SOUTHEASTERLY WINDS. THIS WILL ALSO
SUPPORT SOME FOG/LOW STRATUS, ESPECIALLY EAST OF THE BLACK HILLS.
EARLIER NBM GUIDANCE HAD BONKERS WIND GUSTS, BUT LATEST SPC
HREF/NBM SUPPORT A FEW GUSTS TO 35-45MPH FROM NORTHWEST INTO
SOUTH-CENTRAL SD WITH <20% CHANCE OF >45MPH WIND GUSTS. OTHER
EFFECT WILL BE SMALL CLUSTERS OF POPS SOUTHEAST OF THE BLACK HILLS
GIVEN WEAK THETA-E ADVECTION. STRONGER WAVE MOVES NORTHEAST FROM
THE CENTRAL ROCKIES INTO MT TUESDAY NIGHT/WEDNESDAY. EFFECT OF
THIS WAVE MORE PRONOUNCED WITH STRONGER LOW-LEVEL JET/THETA-E
ADVECTION. THIS SHOULD PROMOTE SOME SHRA/TS MOVING SOUTHWEST TO
NORTHEAST OVER THE CWA AS MUCAPE SLOWLY INCREASES, ESPECIALLY OVER
NORTHEASTERN WY. TEMPERATURES WILL BE NEAR GUIDANCE AND WARMER
OVERALL.
WARMER/DRIER WEATHER EXPECTED THE REST OF THE FORECAST PERIOD PER
SOUTHWEST FLOW ALOFT AND PERIODIC LOW LEVEL DOWNSLOPE FLOW. THE
CHANCE OF PRECIPITATION IS NOT ZERO, HOWEVER.
AVIATION
(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)
ISSUED AT 519 AM MDT TUE SEP 22 2026
AREAS OF FOG AND LOW CLOUDS WILL PERSIST ACROSS PORTIONS OF
THE WESTERN SD PLAINS, WITH TRANSIENT IFR/LIFR CONDITIONS. LOW
CIGS WILL CONTINUE THROUGH MUCH OF TUESDAY ACROSS WESTERN SD. SOME
IMPROVEMENT TO VFR IS POSSIBLE ACROSS PORTIONS OF NE WY, BUT
CLOUDS WILL INCREASE AGAIN TOWARD EVENING AS THE NEXT SYSTEM
APPROACHES FROM THE SOUTHWEST. SOUTHEASTERLY WINDS WILL BECOME
GUSTY TUESDAY AFTERNOON ACROSS THE WESTERN SD PLAINS.
UNR WATCHES/WARNINGS/ADVISORIES
SD...NONE.
WY...NONE.
UPDATE...GS
DISCUSSION...HELGESON
AVIATION...SE
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