Flash Flooding and Hail in South Dakota, June 1, 2010

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
518 AM CDT TUE JUN 1 2010

.DISCUSSION...
PATTERN LOOKS TO REMAIN ACTIVE THROUGH THE NEXT SEVERAL DAYS WITH
CONFLUENT ZONAL FLOW FEATURING NUMEROUS WAVES...DIFFICULT TO TIME
AND DIFFICULT TO TRACK...MAKING FOR A CHALLENGING FCST. STRONGEST
TROUGHING EARLY THIS MORNING THRU CANADA WITH SECONDARY MAIN TROUGH
BACK THRU NV...WITH A COUPLE EMBEDDED SMALLER WAVES IN WSW FLOW INTO
NRN PLAINS. ONE SUCH WAVE LIFTING FROM WY/CO INTO WRN SD...AND
ANOTHER BACK IN UT. LLJ INCREASING THRU CNTRL PLAINS 88D VWP AND HBR
PROFILER...BUT LACK OF PLAINS PROFILERS ACRS NRN NEBRASKA DOES NOT
MAKE IT ANY EASIER TO DIAGNOSE LLJ.  LOW LVL FRONTAL BNDRY FM SERN
ND TO NWRN SD...WITH SECONDARY WMFNT WELL TO S IN NRN KS. MAIN LO
LVL MOISTURE TRANSPORT BACK THRU CNTRL NEBRASKA WITH RETREATING SFC
RIDGE HOLDING DRIER AIR IN THRU ERN CWA.

NOTING DEVELOPING ACC FIELD OVR WRN NEBRASKA/SRN SD...EXTENDING TO
STRONGER CONVECTION IN NWRN KS/SWRN NEBRASKA...AS EFFECTS OF LEADING
SHORTWAVE MOVE INTO PLAINS AND INTERACT WITH LLJ. EML PUSHING INTO
PLAINS ALSO CONTRIBUTING TO INCREASING POTENTIAL INSTABILITY AXIS
THRU CNTRL NEBRASKA/SD THIS MORNING. OVERALL...LIKE IDEA SHOWN IN
THE HRRR AND IN SOME FORM THE NAM/GFS WITH INCREASING PCPN AREA
EXPECTED TO EVOLVE OVER NEXT 2-3H ACRS WRN/SRN CWA IN WAA WING...
SPREADING AND BECOMING MORE WIDESPREAD TO THE NORTHEAST THRU MID TO
LATE MORNING. MOST SOUNDINGS INDICATE LITTLE RESISTANCE TO THIS
ELEVATED CONVECTION /PRIOR TO INITIATION/ AND THUS CONCERN THAT WL
JUST GO WITHOUT CESSATION...WHICH WOULD NOT ONLY LIMIT SVR THREAT
TO THE N AND W...BUT EVEN MORE TO THE S THRU THE AFTN. HOWEVER...
0-6KM BULK SHEAR INCREASES TO 40 TO 50 KTS BY AFTN ACRS THE AREA...
USING SOME CREATIVE NON CONVECTIVE PROFILES. THIS MORE THAN
SUFFICIENT TO ORGANIZE STORMS WHICH DEVELOP EITHER ON EDGE OF THE
ELEVATED CONVECTIVE AREAS BY EARLY AFTN AS DESTABILIZATION WITH
L80S/L60S T/TD COMBO...OR ON MAIN FRONTAL BNDRY WITH ELEVATED
INSTABILITY. THREAT FOR SVR CERTAINLY FAVORS SERN HALF OF AREA...
AND CHOSE EXEPECT GRADIENT IN SHEAR/INSTABILITY IN DEEPER FRONTAL
CONVERGENCE AREA FOR DEVELOPMENT.  WOULD EXPECT BULK OF THREAT WOULD
BE HAIL TO WIND TRANSITION AS STORMS GROW UPSCALE INTO MCS...BUT MORE
DISCRETE SUPERCELLS LATER AFTN MAY BRING A NOMINAL TOR THREAT WITH
LOWEST ATM SHEAR FROM NON-CONVECTED AREAS JUST ON THE SHY SIDE OF
FAVORABLE.  CONCERN FOR HEAVY RAINFALL IN PLACE...BUT WITH FORWARD
PROPOGATION EXPECTED TO GENERALLY REIGN...EXPECT ANY PROBLEMS WOULD
BE MORE PROPERLY HANDLED ON THE STORM SCALE...HENCE NOT A WIDESPREAD
THREAT FOR FLASH FLOODING.

TEMPERATURES YET ANOTHER CHALLENGE...AND IF PCPN FAILS TO
MATERIALIZE IN TIMELY MANNER DURING THE MORNING...A WARM AIRMASS
COULD EASILY YIELD MUCH WARMER MAXES BEFORE FROPA SINKS SWRD THRU
THE AFTN. SOMEWHERE FROM LOWER 70S IN EARLIER RAIN AREAS THRU ERN SD
TO MID 80S IN SRN PORTIONS OF NW IA AHEAD OF BNDRY/PCPN.

PUSHED OUT PCPN JUST A BIT FASTER TONIGHT AS JET ENTRANCE REGION
AIDS IN BRINGING DRIER NLY FLOW IN AT LOW LVLS...AND DRIVES THE
LOWER LVL FRONTAL BNDRY INTO KS/MO BY EARLY WED. HOWEVER...WL
CONTINUE TO FIND A FAIRLY STRONG WAVE ALFT RIDING ALONG LINGERING
MID LVL BNDRY INTO WED MORNING...SO DIFFICULT TO FULLY REMOVE POPS
THRU SERN 2/3 OR SO OF AREA. YET ANOTHER IMPULSE LOOKS TO COME OUT
OF THE WRN HIGH PLAINS BY LTR WED...BUT AS MAIN BAROCLINICITY ALSO
WORKS SWRD...KEPT ONLY SOME CHC LVL POPS ALG FAR S/SE CWA. CLOUDS
AND COOLER TMPS SHUD KNOCK QUITE A BIT OFF SRN HIGHS WED...WITH ONLY
UPR 60S TO LOWER 70S MOST AREAS.

SFC RIDGE BRINGS PARTIAL CLEARING WED NIGHT INTO THU AS SETTLE SE
ACRS THE AREA. CONTINUE TO FAVOR A BIT SLOWER PROGRESSION PER
ECMWF...AS SEEMS CONVECTION HAVING ITS WAY WITH ESPLY GFS.

NEXT BETTER THREAT IN THE SHORT RANGE FOR PCPN LOOKS TO START UP BY
LATER THU AFTN ALG WRN FLANK...AND SPREADS EWRD THU NIGHT AND
FRIDAY. WAA PCPN EXPECTED TO START UP LTR THU CNTRL SD...AND PUSH
EWRD THU NIGHT BECOMING MORE WIDESPREAD. ENOUGH CONFIDENCE IN
DYNAMIC STRENGTH AND INSTABILITY TO PUSH MENTION TO SCT.

DID VERY LITTLE TO FCST AGAIN BEYOND FRI...AS INCREASING UNCERTAINTY
IN FLOW MAKES FOR POTENTIALLY UNNECESSARY FLIP FLOPS.  DID LOOK LIKE
COULD BE A DECENTLY MIXY DAY BY SATURDAY...FEATURING A BIT MORE WLY
COMPONENT THAN EARLIER FCST...LEADING TO A SMALL RAISING OF TMPS
THRU WRN/SRN CWA MORE LKLY TO BE OUT OF PCPN/CLDS EARLIER. /CHAPMAN

&&

.AVIATION...
SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS WILL BECOME POSSIBLE
EARLY THIS MORNING OVER CENTRAL SD...WITH THIS ACTIVITY SPREADING
TOWARDS I29 AND SW MN BY LATE MORNING. MAINLY VFR CEILINGS AND
VISIBILITIES ARE EXPECTED DURING THIS TIME. BY AFTERNOON...STRONGER
THUNDERSTORMS WILL BECOME POSSIBLE WITH A FEW MVFR CEILINGS AND
VISIBILITIES. SOME OF THESE THUNDERSTORMS MAY ALSO PRODUCE HEAVY
RAIN AND SEVERE WEATHER. THE MORE ORGANIZED THUNDERSTORM THREAT
SHOULD BE AROUND THE FSD AREA FROM ABOUT 19Z THROUGH 0Z WHILE SUX
SEES THE BETTER CHANCE FROM ABOUT 23Z TO 5Z. /08

&&

.FSD WATCHES/WARNINGS/ADVISORIES...

SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.
&&

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