288
FXUS63 KUNR 220009
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
609 PM MDT MON SEP 21 2026
KEY MESSAGES
- SHOWERS TAPER OFF TONIGHT AS SOME LOW CLOUDS/FOG LINGER
- DRYING OUT FOR THE REST OF THE WEEK, BUT SOUTHWEST FLOW ALOFT
WILL BRING LIMITED CHANCES FOR SHRA/TS
- BREEZY/WINDY SPOTS TUESDAY/WEDNESDAY
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 00Z TAFS THROUGH 00Z TUESDAY EVENING)
ISSUED AT 553 PM MDT MON SEP 21 2026
THE CLOUDS IN WESTERN SD WILL LINGER THROUGH TONIGHT AND THE
CLEARER SKIES IN NE WY WILL SEE CLOUDS REDEVELOP LATE THIS
EVENING WITH THOSE CLOUDS LASTING THROUGH THE NIGHT. SOME OF THOSE
CLOUDS WILL LIKELY BE VERY LOW AT TIMES. ALSO, FOG IS A HIGH
POSSIBILITY IN AND AROUND THE EASTERN BLACK HILLS AND SD PLAINS.
SO FOR TONIGHT, MVFR TO IFR CONDITIONS ARE EXPECTED ACROSS THE SD
PLAINS, WITH SOME POTENTIAL BR AND IFR TO LIFR CEILINGS THROUGH EARLY
TUESDAY MORNING WITH CLOUDS SLOWLY IMPROVING TUESDAY AFTERNOON.
FAR NORTHEASTERN WY WILL ALSO SEE LOWERED CEILINGS THROUGH TUESDAY
MORNING BUT THEY SHOULD REMAIN MOSTLY MVFR/IFR AND BURN OFF TUESDAY
MORNING.
UNR WATCHES/WARNINGS/ADVISORIES
SD...NONE.
WY...NONE.
DISCUSSION...HELGESON
AVIATION...GS
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