023
FXUS63 KFSD 170902
AFDFSD
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
402 AM CDT SUN MAY 17 2026
KEY MESSAGES
- A SIGNIFICANT SEVERE WEATHER EPISODE IS POSSIBLE SUNDAY
AFTERNOON INTO SUNDAY NIGHT. VERY LARGE HAIL, STRAIGHT LINE
WINDS, AND SEVERAL TORNADOES, POSSIBLY STRONG, WILL BE
POSSIBLE. THE MAIN TIMEFRAME FOR SEVERE STORMS IS BETWEEN 2
PM TO MIDNIGHT ON SUNDAY.
- IF YOU HAVE OUTDOOR PLANS SUNDAY AFTERNOON THROUGH MONDAY, THE
TIME IS NOW TO PREPARE AND HAVE ALTERNATIVE PLANS IN PLACE.
- TUESDAY-THURSDAY LOOK TO BE DRY, BUT COOLER WITH HIGHS IN THE
50S TUESDAY, AND 60S WEDNESDAY AND THURSDAY.
UPDATE
ISSUED AT 358 AM CDT SUN MAY 17 2026
THE FORECAST REMAINS LARGELY UNCHANGED FOR TODAY. ONGOING RAIN
SHOWERS OVER NORTHWEST IOWA WILL CONTINUE TO TAPER OFF THROUGH MID-
MORNING. A FEW ISOLATED LIGHT SHOWERS MAY LINGER INTO THE LATE
MORNING, BUT FOR THE MOST PART THE REST OF THE MORNING AND EARLY
AFTERNOON WILL BE DRY. CLOUDS WILL BE SLOW TO CLEAR OUT THIS
AFTERNOON, WHICH MAY LIMIT SURFACE HEATING AND THEREFORE
DESTABILIZATION. THIS COULD IMPACT WHEN AND WHERE CONVECTION
INITIATION OCCURS. HOWEVER, A VERY STRONG PUSH NORTHWARD OF A
THETA E RIDGE IN THE MID-AFTERNOON MAY RAPIDLY DESTABILIZE THE
LOWER ATMOSPHERE, OVERCOMING THE LIMITATIONS OF THE CLOUDS.
MUCAPE INCREASES WITH THE THETA E RIDGE, CLIMBING AS HIGH AS
2500-3500 J/KG. MID-LEVEL LAPSE RATES OF 8+ DEG C/KM AND 0-6 KM
BULK SHEAR OF 40-50 KTS AT A ROUGHLY 45 DEGREE ANGLE TO THE COLD
FRONT ROUND OUT AN ENVIRONMENT RIPE FOR SUPERCELLS. ONE LAST
HICCUP TO CONVECTION INITIATION IS A MODERATELY STRONG CAP.
SOUNDINGS INDICATE CONVECTIVE TEMPERATURES ARE REACHED IN THE
EARLY TO MID AFTERNOON, BREAKING THE CAP AND RESULTING IN
EXPLOSIVE CONVECTIVE GROWTH. AT THAT POINT THE STRONGEST
SUPERCELLS WILL FORM, AS WELL AS THE HIGHEST RISK FOR
TORNADOGENESIS.
ALONG WITH THE QUESTION OF WHEN CONVECTION WILL INITIATE IS HOW FAR
WEST? THE 19.07Z HRRR AND RAP INDICATE INITIATION IN THE EARLY
AFTERNOON OVER CENTRAL SOUTH DAKOTA. WHILE THE NAMNEST, ARW, AND FV3
HOLD BACK ON CONVECTION UNTIL EARLY EVENING OVER SOUTHEAST SOUTH
DAKOTA AND NORTHWEST NEBRASKA. THAT BEING SAID, THE MOST LIKELY
TIMING OF THE SEVERE THREAT WILL BE BETWEEN 2 PM AND 12 AM CDT. ALL
HAZARDS ARE POSSIBLE WITH THESE STORMS: HAIL OF 2-3 INCHES, DAMAGING
WIND GUSTS OF 60-70 MPH, AND A FEW TORNADOES.
AS THE STORMS PROGRESS EAST DEEP LAYER SHEAR BECOMES MORE PARALLEL
TO THE FRONT, WHICH WILL RESULT IN DISCRETE CELLS CONGEALING INTO A
LINE. SOME BOWING SEGMENTS ARE POSSIBLE. AT THIS POINT THE MAIN
THREATS WILL SHIFT TO DAMAGING WIND AND HAIL, BUT TORNADOES IN
EMBEDDED SUPERCELLS CANNOT BE RULED OUT. STORMS ARE EXPECTED TO PUSH
EAST OF THE REGION AROUND OR SHORTLY AFTER MIDNIGHT. THERE ARE
ADDITIONAL CHANCES FOR THUNDERSTORMS MONDAY AFTERNOON. FOR MORE
DETAILS, PLEASE SEE THE DISCUSSION BELOW.
AS ALWAYS, PLEASE BE WEATHER AWARE, ESPECIALLY IF YOU HAVE OUTDOOR
PLANS. KNOW WHERE YOUR NEAREST SHELTER IS AND TAKE ACTION
IMMEDIATELY IF YOUR AREA COMES UNDER A SEVERE WEATHER WARNING.
DISCUSSION
ISSUED AT 101 PM CDT SAT MAY 16 2026
THIS AFTERNOON: WE CONTINUE TO SEE A PLEASANT SATURDAY AFTERNOON
ACROSS THE REGION WITH A LIGHT TO EASTERLY WIND PREVAILING.
TEMPERATURES ARE GRADUALLY RISING TOWARDS THE 80S, BUT HUMIDITY
REMAINS VERY LOW IN THE 15 TO 20% RANGE. WEAK WARM ADVECTION TO OUR
SOUTH WILL BEGIN TO STREAM ELEVATED ACCAS OVERHEAD, PERHAPS WITH A
SPRINKLE OR TWO BY EARLY EVENING. THIS ACTIVITY IS BASED WELL
ALOFT AOA 600 MB, WITH SOUNDINGS SHOWING MINIMAL POTENTIAL FOR
DEEPENING. ANY SURFACE BASED CONVECTION WILL REMAIN WELL SOUTH
OF THE CWA WITH THE FRONTAL BOUNDARY STUCK NORTH OF I-80.
TONIGHT: EASTERLY LOW-LVL FLOW WILL KEEP THE RISK FOR CONVECTION
LOWER THROUGH THE EVENING HOURS AS A GRADUAL NORTHWARD ADVANCEMENT
OF THE SURFACE FRONT BEGINS TO OUR SOUTH. WE'LL BEGIN TO SEE
CONVECTION FORM ALONG AND SOUTH OF THIS BOUNDARY EXTENDING FROM
WESTERN NEBRASKA INTO WESTERN IOWA LATE THIS EVENING. THIS
ACTIVITY WILL BE PULLED NORTHWARD BY A STRONGER SHORTWAVE
LIFTING OUT OF THE CENTRAL ROCKIES INTO THE PLAINS. THE
ADVANCEMENT OF A LOW- LVL FRONT AND STRONG THETA-E ADVECTION
WILL BRING THE RISK FOR ELEVATED THUNDERSTORMS TO THE TRI-STATE
AREA AFTER 10PM WITH ONE MORE CLUSTERS OF STORMS (OR POTENTIALLY
AN MCV) THAT FORM TO OUR SOUTHWEST TRACKING TOWARDS THE AREA
THROUGH DAYBREAK. MUCAPE PLOTS AND SOUNDINGS AHEAD OF THIS
CONVECTION CONTINUE TO LOOK MARGINAL, WITH MID-LVL LAPSE RATES
AROUND 7 C/KM AND STORMS LIKELY TO BE STRONGLY ELEVATED OR
MARGINALLY SEVERE. AT THIS POINT THE GREATEST SEVERE WEATHER
RISK WOULD BE SOUTH OF I-90 AND PERHAPS MORE FOCUSED ALONG THE
MO RIVER AND HIGHWAY 20 CORRIDORS. ONE THING TO WATCH WOULD BE
THE POTENTIAL FOR ANY MCV THAT DEVELOPS TO BRING WAKE LOW
CONDITIONS, OR STRONG EVAPORATIVE COOLING WINDS NORTHEAST AS
CONVECTION WANES. SOME HIGH-RES GUIDANCE SUPPORTS POCKETS OF 50+
MPH WINDS MOVING NORTHEAST WITH THE WEAKENING CONVECTION.
SUNDAY-SUNDAY NIGHT:
THE KEY TAKEAWAYS FOR SUNDAY CONTINUE TO BE:
1. CONVECTIVE DEBRIS, STRATUS, AND AN EML MAY BE PROBLEMATIC AND
COULD MAKE THIS FORECAST CONDITIONAL.
2. IF STORMS GET ROOTED INTO THE BOUNDARY LAYER, THERE IS NO
SHORTAGE OF SPIN POTENTIAL INTO THE EARLY OVERNIGHT AND TORNADIC
ACTIVITY BECOMES AN INCREASED CONCERN.
3. IF DISCRETE MODE IS SHORT LIVED AND STORMS TURN LINEAR, THEN
DURATION OF THREAT IN ANY ONE AREA MAY BE INTENSE BUT BRIEF.
THE MESOSCALE PICTURE MAY BE QUITE CLOUDY TO START SUNDAY.
CONSIDERABLE STRATUS WILL BE PRESENT IN THE AREA AND WE'LL ONLY
GET A BETTER PICTURES OF WHERE THE COLD FRONT/INVERTED TROUGH
AND ASSOCIATED WARM FRONT WILL BE POSITIONED THROUGH THE
MORNING. MOST OF THE MORNING IS LIKELY TO REMAIN QUIET, WITH
SOME RENEWED CONVECTION POSSIBLE IN SW SOUTH DAKOTA/WESTERN
NEBRASKA ON THE NOSE OF THE UPPER JET ENTERING THE PLAINS. SOME
CAMS DO HAVE THIS MOSTLY ELEVATED CONVECTION BRUSHING THE NW
EDGE OF THE CWA BY 18Z. FURTHER EAST, IT MAY TAKE INTO THE MID-
AFTERNOON HOURS FOR CLOUDS TO THIN AND THE WARM FRONT TO JUMP
NORTHWARD (IF IT DOES). A STUBBORN EML WILL ALSO REMAIN WITHIN
THE 600-700 MB. GUIDANCE IS STILL VERY BULLISH ON THE RAPID
NORTHWARD ADVECTION OF 2000-3000 MLCAPE NORTHWARDS TOWARDS
HIGHWAY 14 AND WEST TO HIGHWAY 281 BY 21-00Z. THE INCREASING
CAPE WILL BE AIDED BY THE ARRIVAL OF 8.5-9 C/KM 700:500 LAPSE
RATES, AND ONCE THE NEXT WAVE OF MID- LVL VORTICITY ARRIVES,
SCATTERED CONVECTION MAY QUICKLY DEVELOP ALONG THE FRONTAL
BOUNDARY STRETCHING NEAR HWY 281 AFTER 20Z. A SECOND AREA OF
CONVECTION MAY FORM FURTHER SOUTH ALONG THE DRY LINE
INTERSECTION IN EASTWARD NEBRASKA AND TRACK NORTHEAST INTO THE
EVENING. SOME UNCERTAINTY PERSISTS ON HOW DISCRETE STORMS WILL
REMAIN OR IF THEY WILL QUICKLY TURN LINEAR AS THEY EXPAND AND
SLIDE EAST/NORTHEAST INTO THE EVENING AND EARLY OVERNIGHT.
SUPERCELLS WITH VERY LARGE HAIL (2-3") AND AN INCREASED TORNADIC
RISK WILL BE POSSIBLE INITIALLY, BUT IF LINEAR TRENDS DEVELOP,
LINE SEGMENTS WITH STRAIGHT LINE WINDS/EMBEDDED TORNADOES MAY
BECOME A POSSIBILITY AS STORMS TRACK EAST NORTHEAST TOWARDS I-29
AND MN/IA. THE DEGREE OF LOW- LVL HELICITY/SHEAR COMBINED WITH
INCREASING 0-1KM CAPE APPROACHING 100-200 J/KG AND CRITICAL LCL
VALUES ALONG AND SLIGHTLY EAST OF I-29 IS PARTICULARITY
CONCERNING HEADING INTO THE EVENING AND CONTINUES TO SUPPORT THE
10-15% TORNADO PROBABILITIES BY SPC. IN FACT, STP VALUES PUSH
5+ ALONG PORTIONS OF I-29 BETWEEN SF/SC BY 00Z. MOST OF THE
RECENT CAMS SUPPORT THE LINEAR LINES MOVING EAST OF THE CWA BY
MIDNIGHT WITH LOWER RISKS INTO DAYBREAK MONDAY.
MONDAY: THERE IS BETTER AGREEMENT IN TODAY'S GUIDANCE PUSHING THE
EFFECTIVE BOUNDARY EAST OF THE CWA BY MONDAY MORNING. THAT SAID,
INCOMING SHORTWAVE ENERGY MAY PULL ELEVATED INSTABILITY AND
INCREASED LAPSE RATES BACK WEST OF THE BOUNDARY BY MID-LATE
AFTERNOON. THIS COULD PLACE AREAS OF NW IOWA UNDER THE RISK OF
ELEVATED HAILERS FOR A PERIOD OF TIME. FOR NW IOWA, THIS COULD
ALSO BE THE TIME WHERE THE MOST BENEFICIAL RAINFALL MAY ALSO
TAKE PLACE. FURTHER WEST, SIGNIFICANT COOLDOWN IS EXPECTED AS
TEMPERATURES STRUGGLE TO CLIMB THROUGH THE 50S.
TUESDAY-FRIDAY: QUIET AND COOLER THAN NORMAL TEMPERATURES ARE
EXPECTED THROUGH THE MIDDLE OF THE WEEK AS HIGHS CLIMB BACK THROUGH
THE 60S BY THURSDAY. ANOTHER TROUGH MOVES THROUGH THE PLAINS BY THE
END OF THE WEEK HOWEVER A LACK OF DEEP MOISTURE PREVENTS ANY
SIGNIFICANT SEVERE WEATHER RISK. THE LATEST AI LEARNING GUIDANCE
SUGGESTS LESS THAN 5% PROBABILITIES OF ANY SEVERE STORM AT THIS
TIME.
AVIATION /06Z TAFS THROUGH 06Z MONDAY/
ISSUED AT 1108 PM CDT SAT MAY 16 2026
FOCUS ON THE AVIATION FORECAST IS FOR CONVECTION INITIALLY
TONIGHT, THEN LOW CLOUDS WITH MVFR CEILINGS DEVELOPING ON
SUNDAY, THEN ADDITIONAL THUNDERSTORMS DURING THE AFTERNOON
HOURS. ISOLATED SHOWERS AND THUNDERSTORM ARE EXPECTED TO THE
SOUTH OF I-90 DURING THE OVERNIGHT HOURS ON THE NORTH SIDE OF
THE MUCH STRONGER AREA OF THUNDERSTORMS WELL TO THE SOUTH IN
NEBRASKA. GENERALLY LOWER CONFIDENCE ON THESE
SHOWERS/THUNDERSTORMS HITTING KFSD/KSUX, SO HAVE HANDLED WITH
PROB30. WITH THE PRIMARY CONVECTION BEING FARTHER TO THE SOUTH,
DOES LOOK TO BE A LULL IN THE PRECIPITATION ON SUNDAY MORNING,
AS THE LOW STRATUS DEVELOPS OVER THE AREA. THE FOCUS THEN TURNS
TO THE POTENTIAL FOR SEVERE THUNDERSTORMS TO DEVELOP DURING THE
AFTERNOON AND CONTINUE THROUGH EARLY EVENING. CONFIDENCE IS
LOWER ON EXACT LOCATIONS, ESPECIALLY FARTHER NORTHWEST INTO LESS
INSTABILITY, SO HANDLED WITH PROB30, EXCEPT AT KSUX, WHERE
GREATER CONFIDENCE IS IN PLACE FOR STORMS EVOLVING INTO A LINE
IN THE LATE AFTERNOON.
FSD WATCHES/WARNINGS/ADVISORIES
SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
UPDATE...AJP
DISCUSSION...DUX
AVIATION...MEYERS
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