815
FXUS63 KFSD 172353
AFDFSD
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
ISSUED BY NATIONAL WEATHER SERVICE RAPID CITY SD
653 PM CDT SUN MAY 17 2026
KEY MESSAGES
- THERE IS HIGH CONFIDENCE THAT SEVERE THUNDERSTORMS WILL
DEVELOP THIS AFTERNOON AND EVENING, PRODUCING LARGE HAIL,
DESTRUCTIVE STRAIGHT-LINE WINDS, AND A FEW TORNADOES. AN
ENHANCED RISK (LEVEL 3 OF 5) OF SEVERE STORMS IS IN PLACE.
- A TRANSITIONING STORM MODE WILL SHIFT HAZARDS FROM LARGE HAIL
AND TORNADOES INITIALLY, TO WIDESPREAD WIND DAMAGE AS AN
INTENSE LINE OF STORMS TRACKS EAST OF INTERSTATE 29 THIS
EVENING.
- A LOWER RISK OF SEVERE WEATHER EXISTS LATE MONDAY AFTERNOON
AND EVENING, MAINLY OVER NORTHWEST IA, BEFORE QUIETER WEATHER
MOVES IN FOR THE MID-WEEK PERIOD.
DISCUSSION
ISSUED AT 120 PM CDT SUN MAY 17 2026
THE SHORT-TERM CONVECTIVE OUTLOOK PRESENTS A HIGHLY VOLATILE
SPRING SEVERE WEATHER SETUP. LATEST SURFACE ANALYSIS SHOWS A
COMPLEX PATTERN, ANCHORED BY A STRONG 994 MB LOW OVER NORTHWEST
KS. AN INVERTED TROUGH EXTENDED NORTH OF THE LOW THROUGH
CENTRAL SD, WHILE A WELL-DEFINED WARM FRONT STRETCHED NORTHEAST
OF THE LOW INTO EAST CENTRAL NE. A WEAKER LOW WAS ALSO NOTED IN
NORTH CENTRAL NE. VISIBLE SATELLITE IMAGERY REVEALED STABLE
WAVE CLOUDS ADVANCING NORTHEAST OVER FAR NORTHEAST NE AND
EXTREME SE SD, AHEAD OF THE ADVANCING WARM FRONT. REGIONAL
RADAR IMAGERY SHOWED A COMPLEX OF STRONG TO SEVERE STORMS OVER
NORTH CENTRAL NE. THIS STORM COMPLEX IS FORECAST BY SHORT RANGE
GUIDANCE TO CONTINUE TRACKING EAST THIS AFTERNOON, INTENSIFYING
AS INTERACTS WITH THE RICHER LOW- LEVEL MOISTURE POOL NEAR AND
SOUTH OF THE WARM FRONT.
MODERATE TO STRONG INSTABILITY WILL BE IN PLACE OVER THE
SOUTHERN TO EASTERN CWA BY LATE AFTERNOON WITH MLCAPE NEARING
3000 J/KG. IN ADDITION, DEEP LAYER SHEAR WILL INCREASE TO AROUND
50 KT, COINCIDENT WITH A MID LEVEL SPEED MAX EJECTING FROM THE
DEEP UPPER TROUGH TO OUR WEST. AS THE SYSTEM EVOLVES OVER THE
NEXT SEVERAL HOURS, IT APPEARS THE INGREDIENTS FOR A SIGNIFICANT
SEVERE WEATHER EVENT WILL BE MOST FAVORED OVER THE SOUTHERN TO
EASTERN CWA. AHEAD OF THE MAIN CONVECTIVE LINE, SOME CAMS SHOW
SUPERCELL DEVELOPMENT ALONG THE NORTHWARD ADVANCING WARM FRONT.
THESE STORMS WOULD PRESENT A RISK OF VERY LARGE HAIL (2"+) AND
TORNADOES. GIVEN THE AMOUNT OF LOW LEVEL SRH/CURVED HODOGRAPHS
PRESENT, A COUPLE STRONG TORNADOES ARE POSSIBLE. THE CONVECTIVE
CLUSTER CURRENTLY ENTERING OUR FAR SOUTHWEST CWA SHOULD
STRENGTHEN AS IT SHIFTS EAST, WITH AN INITIAL HAIL AND TORNADO
THREAT TRANSITIONING TO A DAMAGING STRAIGHT LINE WIND THREAT
(70+ MPH) AS IT GROWS UPSCALE INTO A SQUALL LINE/QLCS. SOME
LINE-EMBEDDED QLCS TORNADOES ARE POSSIBLE, ESPECIALLY FOR ANY
LINE SEGMENTS THAT BECOME ORIENTED NORTH/SOUTH OR
NORTHWEST/SOUTHEAST, GIVEN SOUTHWESTERLY 0-3 KM SHEAR VECTORS.
TIMING FROM LATEST HIGH-RES GUIDANCE SUGGESTS THIS ACTIVITY
EXITS TO THE EAST OF OUR CWA LATE THIS EVENING.
PRECIPITATION AMOUNTS WILL VARY SIGNIFICANTLY BASED ON CONVECTIVE
TRACKS, THOUGH NBM GUIDANCE AND HREF PROBABILITY MATCH MEAN
FIELDS PROJECT A WIDESPREAD 0.50 TO 1.50 INCHES OF RAINFALL,
PARTICULARLY ACROSS NORTHWEST IOWA AND SOUTHWEST MINNESOTA. ON
MONDAY, THE COLD FRONT FINALLY PUSHES INTO THE AREA. INSTABILITY
AND SHEAR ARE FORECAST TO BE MUCH LOWER THAN TODAY, HOWEVER,
THERE WILL BE A CONDITIONAL RISK OF SEVERE STORMS OVER THE
EASTERN CWA DURING THE LATE AFTERNOON TO EVENING, IF THE AIRMASS
CAN DESTABILIZE SUFFICIENTLY AHEAD OF THE FRONT. SPC'S DAY 2
OUTLOOK INDICATES AS MARGINAL RISK (LEVEL 1 OF 5), FOR OUR MN
AND IA COUNTIES.
BEHIND THE DEPARTING UPPER WAVE, QUASI-ZONAL FLOW SETS UP FOR
TUESDAY AND WEDNESDAY, YIELDING QUIET CONDITIONS AND SEASONALLY
COOL HIGH TEMPERATURES MODERATING FROM THE UPPER 50S TUESDAY TO
MID-UPPER 60S WEDNESDAY.
THE NEXT UPPER WAVE MOVES IN FOR THE THURSDAY TO FRIDAY PERIOD,
HOWEVER GULF MOISTURE REMAINS CUTOFF AHEAD OF THIS SYSTEM.
EVALUATING THE ECMWF EXTREME FORECAST INDEX (EFI) REVEALS NO
SIGNIFICANT CLIMATOLOGICAL ANOMALIES FOR PRECIPITATION OR WIND
ACROSS THE UPPER MIDWEST DURING THIS PERIOD, REINFORCING THE
THE IDEA OF A LOW-IMPACT SYSTEM FOR OUR AREA. THE CURRENT
FORECAST STICKS CLOSELY TO THE NBM CONSENSUS, MAINTAINING BROAD
20-30 PERCENT PROBABILITIES FOR LIGHT RAIN FOR LATE THURSDAY
INTO SATURDAY.
BY LATE THIS WEEK AND ESPECIALLY THIS WEEKEND, TEMPERATURES
WILL EMBARK ON A STEADY UPWARD TRAJECTORY, AS THE MID LEVEL FLOW
BACKS WEST, THEN SOUTHWEST. THE CLIMATE PREDICTION CENTER (CPC)
6-10 DAY OUTLOOK FAVORS ABOVE-NORMAL TEMPERATURES ACROSS THE
NORTHERN PLAINS AND UPPER MIDWEST, WHICH SUPPORTS BLENDED
GUIDANCE HIGH TEMPERATURES REACHING WELL INTO THE 80S ON
SUNDAY.
AVIATION /00Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 650 PM CDT SUN MAY 17 2026
SHOWERS AND STORMS WILL CONTINUE TO MOVE EASTWARD INTO MN THROUGH
THE EVENING HOURS, THE FORECAST AREA SHOULD BE LARGELY CLEAR OF
SHOWER/STORM ACTIVITY AFTER ~05Z. INTERMITTENT MVFR/IFR CONDITIONS
IN HEAVY RAINFALL AS WELL AS HAIL AND STRONG WINDS WILL ACCOMPANY
THE STRONGEST STORMS. WIDESPREAD MVFR/IFR CIGS WILL SET IN OVERNIGHT
AND PERSIST THROUGH MUCH OF MONDAY.
FSD WATCHES/WARNINGS/ADVISORIES
SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
DISCUSSION...ILX/25
AVIATION...WONG
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