Hail and Damaging Winds in South Dakota, May 1, 2012

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
352 PM CDT TUE MAY 1 2012

.DISCUSSION.../ISSUED AT 352 PM CDT/
SEVERE THREAT REMAINS THE MAIN FOCUS FOR THE LATE AFTERNOON AND
EVENING ACROSS THIS AREA. 19Z SURFACE ANALYSIS SHOWS LOW PRESSURE
NEAR/JUST NORTH OF KABR WITH A WARM FRONT INTO WEST-CENTRAL AND
SOUTH-CENTRAL MN /NEAR KETH-KMKT LINE/...PRE-FRONTAL TROUGH TO THE
SOUTH THROUGH THE MIDDLE JAMES VALLEY...AND COLD FRONT SOUTHWEST
TOWARD K9V9-KOGA. ONGOING LINE OF HIGH-BASED CONVECTION ALONG WEAK
WEST-SOUTHWEST TO EAST-NORTHEAST MID LEVEL THETA-E RIDGE AXIS NOT
EXPECTED TO PRODUCE SEVERE WEATHER...THOUGH WITH STEEP MID-LEVEL
LAPSE RATES...DRY LAYER ALOFT AND RELATIVELY LOW FREEZING LEVEL
AROUND 10KFT AGL...MARGINAL HAIL/STRONG WINDS WILL BE POSSIBLE.
EXPECT MORE ORGANIZED SEVERE THREAT TO DEVELOP TOWARD EVENING AS
UPPER LEVEL WAVE LIFTS INTO THE AREA FROM WESTERN NEBRASKA. MODEST
DEEP LAYER SHEAR WILL SUPPORT LARGE HAIL/ISOLATED DAMAGING WIND WITH
INITIAL DEVELOPMENT OF SURFACE BASED STORMS ALONG THE PRE-FRONTAL
TROUGH/COLD FRONT...WHICH IS EXPECTED TO BECOME NEARLY STATIONARY
NEAR KBKX-KYKN LINE AS THE WAVE LIFTS NORTHEAST THROUGH THE EARLY
EVENING. STORMS THEN SHOULD TRACK EAST OF THE BOUNDARY AFTER 01Z-02Z
AS THE WAVE CONTINUES TO MOVE NORTHEAST THROUGH THE AREA. SEVERE
THREAT EXPECTED TO WANE TOWARD LATE EVENING 03Z-06Z AS THE STORMS
MOVE EAST ACROSS SOUTHWEST MN/NORTHWEST IA. /JH

BY WEDNESDAY MORNING...ANY LINGERING PRECIPITATION FROM OVERNIGHT
CONVECTION SHOULD HAVE SHIFTED EAST OF THE FORECAST AREA...LEAVING
BEHIND CLEARING SKIES AND MOUNTING INSTABILITY. THE SURFACE BOUNDARY
SHOULD DRAPE SW TO NE NEAR THE SD/IA/MN BORDER DURING THE MORNING...
THEN MAY FLATTEN TO TAKE ON MORE OF AN WSW TO ENE ORIENTATION SOUTH
OF INTERSTATE 90 AS WEAK HIGH PRESSURE BUILDS IN NORTHEASTERN SD AND
WESTERN MN. SHOULD SEE A BROAD AREA OF 2000 TO 3000 J/KG DEVELOP IN
THE WARM SECTOR BY 21Z...AND MODEL SOUNDINGS INDICATE SIGNIFICANT
EROSION OF THE CAP WITH SURFACE HEATING IN PROXIMITY TO THE
BOUNDARY. THUNDERSTORM INITIATION WILL BE LIKELY BY 21Z-23Z...FIRST
IN NORTHWEST IOWA WHERE THE CAP WILL BE WEAKEST...THEN BUILDING WSW
ALONG THE FRONT.

WITH AMPLE INSTABILITY...FORCING FROM THE BOUNDARY...AND A MINIMAL
CAP...SHOULD SEE CONVECTION GO ALONG THE LENGTH OF THE BOUNDARY IN
NW IA AND SE SD WITHIN 1 TO 2 HOURS OF INITIATION. WITH BULK SHEAR
VECTORS AND RIGHT MOVING STORM MOTION DIRECTLY PARALLEL TO THE
BOUNDARY...COULD SEE AN ISOLATED THREAT FOR TORNADOES AS STORMS
TRACK ALONG THE FRONT AND INGEST ADDITIONAL LOW LEVEL VORTICITY.
HOWEVER...WITHOUT A DISCERNIBLE PREFERENTIAL LOCATION FOR CONVECTIVE
INITIATION...LOOKS AS THOUGH THIS THREAT WILL QUICKLY END AS STORMS
MELD INTO A MESSY COMPLEX. ANY STORMS THAT CAN DEVIATE NORTH OF THE
BOUNDARY WILL LIKELY BECOME ELEVATED...BUT MAY STILL PRODUCE LARGE
HAIL DUE TO SUPPORTIVE 2-6 KM BULK SHEAR VALUES UP TO 40 KTS AND
DECENT ELEVATED INSTABILITY. THE SEVERE WEATHER THREAT WILL LESSEN
AS STORM BECOME MORE DISORGANIZED AFTER SUNSET.

A MIDLEVEL WAVE WILL MOVE NORTHEAST OUT OF CENTRAL NEBRASKA BETWEEN
00Z AND 06Z...ENHANCING ONGOING CONVECTION AND LIGHTING UP ANY OF
THE BOUNDARY IF IT HAS NOT ALREADY INITIATED. WITH A STRENGTHENING
LOW LEVEL JET AND TRAINING CELLS ALONG A SLOW MOVING BOUNDARY...A
FLASH FLOOD THREAT WILL INCREASE DURING THIS TIME FRAME. THE GFS
PAINTS A BROAD AREA OF MORE THAN AN 1 INCH/6 HOURS...BUT AMOUNTS
COULD BE LOCALLY HIGHER UNDER CONVECTIVE CELLS.

BY THURSDAY MORNING...MOST MODEL SOLUTIONS SHOW THE SURFACE BOUNDARY
CLEARING THE FORECAST AREA TO SOUTHEAST...ALLOWING ALL BUT NORTHWEST
IOWA TO CLEAR OUT BY EARLY TO MID MORNING. ONGOING CONVECTION AND/OR
CLOUD COVER IN NORTHWEST IOWA WILL KEEP THE AREA CAPPED TO SURFACE
BASED CONVECTION THROUGHOUT THE DAY...SO THE SEVERE POTENTIAL SHOULD
STAY FAIRLY LOW. THESE THUNDERSTORMS ARE EXPECTED TO SINK SOUTH OF
THE FORECAST AREA BY 06Z. WITH A WEAK MIDLEVEL WAVE MOVING INTO THE
NORTHERN HIGH PLAINS ON THURSDAY EVENING...ADDITIONAL CONVECTION
MAY FORM IN WESTERN SOUTH DAKOTA LATE THURSDAY NIGHT AND CLIP THE
NORTHWESTERN CWA OVERNIGHT...HOWEVER THE LATEST MODEL RUNS ARE QUITE
A BIT WEAKER WITH THIS FEATURE THAN PREVIOUSLY.

WITH SOUTHWEST FLOW ALOFT AND ANY NUMBER OF SHORTWAVES TRAVELING FROM
THE BROAD TROUGH OVER THE WEST...IS NEARLY IMPOSSIBLE TO PIN DOWN
PRECIPITATION TIMING IN THE EXTENDED. WHILE THERE WILL CERTAINLY BE
DRY PERIODS IN DAYS 4-7...DIFFERENCES IN MODEL SOLUTIONS AND POOR
TEMPORAL CONTINUITY BETWEEN SIMULATIONS NECESSITATE LOW TO MODERATE
CHANCES OF PRECIPITATION IN VIRTUALLY EVERY PERIOD. TEMPERATURES
SHOULD COOL SLIGHTLY INTO NEXT WEEK...BUT IS NOT EXPECTED TO SETTLE
MORE THAN A FEW DEGREES BELOW NORMAL. /LAFLIN

&&

.AVIATION.../FOR THE 18Z TAF ISSUANCE/
WITH STRATUS AND ANY FOG BURNED OFF...OUTLOOK IS FOR VFR UNTIL
STORMS DEVELOP AFTER 21Z. STORMS WILL OF COURSE PRESENT THEIR LOCAL
ENVIRONMENTS OF HIGHLY VARIABLE CONDITIONS DOWN TO LOCAL AND BRIEF
IFR. STORMS WILL BE MAINLY OVER SOUTHEAST TWO THIRDS OF FORECAST
AREA AND SHOULD BE DISSIPATED OR MOVED OUT BY 02/12Z. VFR SHOULD
REMAIN GENERAL OUTSIDE STORM AREAS...EXCEPT OVER SOUTHEAST PART OF
AREA...ROUGHLY KSUX TO KMJQ AND SOUTHEAST OR MUCH OF NORTHWEST
IA...WHERE GENERAL CEILINGS AOB 1K FEET AND VISIBILITIES 3-5SM BR
MAY REDEVELOP AND AFFECT THIS AREA 02/08Z-15Z. OCCASIONAL SURFACE
GUSTS ABOVE 25 KNOTS THROUGH 02/01Z OVER AREA. OVER SOUTHWEST MN
SUSTAINED WINDS MAY BE CLOSE TO 25 KNOTS WITH GUSTS TO 35 KNOTS
THROUGH THE SAME TIME.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
MN...NONE.
NE...NONE.
SD...NONE.
&&

HAZARDOUS WEATHER OUTLOOK...CORRECTED FIRST PARAGRAPH
NATIONAL WEATHER SERVICE SIOUX FALLS SD
216 PM CDT TUE MAY 01 2012

SDZ038>040-050-052>071-MNZ071-072-080-081-089-090-097-098-
IAZ001>003-012>014-020>022-031-032-NEZ013-014-021200-
AURORA-BEADLE-BON HOMME-BROOKINGS-BRULE-BUENA VISTA IA-
CHARLES MIX-CHEROKEE IA-CLAY-CLAY IA-COTTONWOOD MN-DAKOTA NE-
DAVISON-DICKINSON IA-DIXON NE-DOUGLAS-GREGORY-HANSON-HUTCHINSON-
IDA IA-JACKSON MN-JERAULD-KINGSBURY-LAKE-LINCOLN-LINCOLN MN-
LYON IA-LYON MN-MCCOOK-MINER-MINNEHAHA-MOODY-MURRAY MN-NOBLES MN-
OBRIEN IA-OSCEOLA IA-PIPESTONE MN-PLYMOUTH IA-ROCK MN-SANBORN-
SIOUX IA-TURNER-UNION-WOODBURY IA-YANKTON-
216 PM CDT TUE MAY 01 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHEAST SOUTH DAKOTA...
SOUTHWEST MINNESOTA...NORTHWEST IOWA...AND NORTHEAST NEBRASKA.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS...
ISOLATED HIGH-BASED SHOWERS OR THUNDERSTORMS WILL BE POSSIBLE IN
PARTS OF SOUTHEAST SOUTH DAKOTA AND SOUTHWEST MINNESOTA THROUGH
THE MID AFTERNOON. THESE STORMS ARE NOT EXPECTED TO BE SEVERE...
HOWEVER SMALL HAIL AND WIND GUSTS OF 45 TO 50 MPH WILL BE POSSIBLE
WITH THE STRONGER STORMS.

DISCRETE SURFACE-BASED THUNDERSTORMS ARE THEN EXPECTED TO DEVELOP
AFTER 5 PM THIS AFTERNOON...BETWEEN THE JAMES RIVER VALLEY AND
INTERSTATE 29. THESE SCATTERED STORMS WILL DEVELOP RAPIDLY...AND
WILL BE CAPABLE OF PRODUCING HAIL TO AROUND GOLF BALL SIZE AND
DAMAGING WIND GUSTS OF 65 MPH. THESE STORMS MAY TRANSITION TO
SMALLER BOWING SEGMENTS WITH POTENTIAL FOR QUARTER TO HALF DOLLAR
HAIL AND WIND GUSTS TO 70 MPH...AS THEY MOVE EAST INTO SOUTHWEST
MINNESOTA AND NORTHWEST IOWA AFTER 8 PM. THE SEVERE WEATHER
THREAT SHOULD END BY MIDNIGHT.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

THE POTENTIAL FOR SEVERE THUNDERSTORMS APPEARS TO BE A BIT BETTER
LATE WEDNESDAY AFTERNOON AND EVENING. SCATTERED THUNDERSTORMS
WILL DEVELOP FROM SOUTHEAST SOUTH DAKOTA AND NORTHEAST
NEBRASKA...THROUGH NORTHWEST IOWA BY LATE AFTERNOON. THESE STORMS
WILL GRADUALLY LIFT NORTHEAST THROUGH THE EVENING. THUNDERSTORMS
SOUTHEAST OF A LINE FROM TRACY MINNESOTA...TO PICKSTOWN SOUTH
DAKOTA...MAY BECOME SEVERE WITH LARGE HAIL UP TO TWO INCHES IN
DIAMETER AND DAMAGING WINDS IN EXCESS OF 60 MPH. A COUPLE OF
TORNADOES ARE A THREAT FROM LATE AFTERNOON INTO EARLY EVENING
SOUTH OF A LINE FROM JACKSON MINNESOTA TO VERMILLION SOUTH DAKOTA.

THUNDERSTORMS WILL CONTINUE AT TIMES THROUGH EARLY NEXT WEEK.
WHILE DIFFICULT TO PINPOINT THE RISK AT THIS RANGE...IT WOULD
APPEAR THAT THE RISK FOR A FEW SEVERE STORMS WILL REMAIN LATER
FRIDAY INTO FRIDAY NIGHT ACROSS THE AREA.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL BE POSSIBLE AFTER 5 PM CDT BETWEEN THE
JAMES RIVER VALLEY AND INTERSTATE 29...AND AFTER 8 PM EAST OF
INTERSTATE 29...ENDING BY MIDNIGHT.

&&

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE AT
WWW.WEATHER.GOV/SIOUXFALLS. THIS INCLUDES THE GRAPHICAL HAZARDOUS
WEATHER OUTLOOK.