Flash Flood and Hail in South Dakota, July 8 to 9, 2009

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
900 PM CDT WED JUL 8 2009

.DISCUSSION...
SCT TSTMS MOVG EWD INTO CNTRL SD ATTM...FASTER AND FURTHER SOUTH
THAN ALL MODELS...BUT APPEARS TO HAVE A TEMPORARY WEAKENING TREND
BASED ON RADAR AND LIGHTNING CHART. MAIN DYNAMICS AND TSTMS MOVG
EWD INTO WRN ND ATTM...WHICH SHOULD TURN EITHER SE AS DVLPG MCS WITH
HELP FM CORFIDI VECTORS TNGT...OR STRENGTHEN CURRENT ACTIVITY IN
CNTRL SD...ESP IF TSTMS TAP INTO MOIST UNSTABLE VEERING SLY LLJ.
DID SPEED UP AND RAISE POPS WRN CWA BY MIDN...AND ALG AND NORTH OF
I90 REST OF NGT...AND EVEN DOWN INTO NWRN IA BY THU MRNG. STILL
APPEARS NORTH OF I90 AND ESP HIGHWAY 14 HAS MAIN CHC OF SVR SQUALL
LINE DVLPMT AFT MIDN WITH STG EVAP COOLING POTENTIAL WITH STEEP
LAPSE RATES ALOFT. OTRW PREV FCST IN THE BALLPARK. /RSR

&&

.AVIATION...

MVFR STRATOCUMULUS HAS LIFTED OVER ALL BUT THE KHON TAF SITE THIS
EVENING...THOUGH EXPECT THIS TO CONTINUE TO LIFT THROUGH THE EVENING
HOURS. STRONG STORMS ARE PRESENTLY DEVELOPING OVER WESTERN SOUTH
DAKOTA WITH MID LEVEL WAVE...AND COULD SEE THESE PUSH INTO THE KHON
AREA AROUND 05Z...BEFORE STRONGER STORMS PUSH ACROSS THE REGION
BETWEEN 06Z AND 12Z. WHILE THE BIGGEST IMPACT WILL BE SEEN AT THE
KHON TERMINAL...FURTHER SOUTH DEVELOPMENT MY BRING THEM INTO KFSD
AFTER 09Z. THESE STORMS MAY PRODUCE STRONG GUSTY WINDS AND HAIL AS
THEY PUSH ACROSS THE AREA. /JM

&&

.PREV DISCUSSION...

MID LEVEL CUMULUS DEVELOPING NEAR HETTINGER AND DICKINSON NORTH
DAKOTA AS STRONG UPPER WAVE PUSHES INTO EASTERN MONTANA. EXPECT
CONVECTION TO DEVELOP OVER THE NEXT FEW HOURS AND CONGEAL INTO AN
MCS PROPAGATING EASTWARD INITIALLY...HOWEVER AS THE LOW LEVEL JET
INCREASES AND STORMS MOVE TOWARDS EASTERN EDGE OF THERMAL
RIDGE...EXPECT STORMS TO PROPAGATE SOUTHEAST. EXPECT STORMS TO WORK
INTO NORTHERN PORTIONS OF FORECAST AREA AFTER MIDNIGHT...AND HAVE
INCREASED POPS INTO THE LIKELY CATEGORY ALONG HIGHWAY 14. LOW LEVEL
JET OF 30-40 KNOTS ACROSS FORECAST AREA BETWEEN 2000-4000 FEET AGL
MAY BE BROUGHT DOWN TO THE SURFACE BY DOWNDRAFTS WITH ONLY A MINIMAL
SURFACE INVERSION BELOW 875 MB.  DOWNDRAFT CAPE IN EXCESS OF 1000
J/KG NEAR KHON AROUND 12Z TOMORROW...BUT EXPECT LINE TO BE SLIGHTLY
FURTHER EAST DUE TO ACCELERATION OF COLD POOL.  HAVE ALSO INCLUDED
COLD POOL IN WIND FIELD...AND DROPPED LOWS ACROSS NORTHERN EDGE OF
CWA.  /BT

EXPECT MATURE ESEWRD TRACKING BOW ECHO TO BE PLOWING THRU NRN/NERN
CWA ON THURSDAY MORNING...FOLLOWING THE VEERING LLJ THRU SWRN MN.
EVEN IF AREAS TO THE SOUTH AND WEST OF WHERE BEST COVERAGE EXPECTED
NORTH OF A BKX TO MWM LINE THRU EARLY MORNING...WL HAVE SOME MINIMAL
THREAT OF CONVECTION WITH NICE ELEVATED MIXED LAYER...AND ADDITIONAL
INFLUENCE OF COLD POOL LIFT AS WELL DEFINED OUTFLOW BNDRY PRESSES
S/SW OUT OF AREA.  THIS WL MERGE TO THE W WITH MAIN CDFNT SLIDING
THRU THE DKTS AS MCS LIKELY TO OUTRUN THIS BNDRY OVERNIGHT AND EARLY
MORNING HOURS. LKLY THAT EXTENSIVE OUTFLOW WILL REALLY IMPACT
CONVECTIVE REDEVELOPMENT ALG THE BNDRY THRU THE AFTN...WITH MOST
LKLY FIRING POINT ALG SRN OUTFLOW EXTENT...WHERE INSTABILITY WL FOCUS
1500-2000 J/KG MLCAPE...WITH SOME CAPPING ESPLY WITHIN OUTFLOW POOL.
FOR NOW...WL LIMIT SEVERE MENTION TO SRN PORTIONS OF NW IA ALG
OUTFLOW AND TRIPLE PT FOR LTR AFTN...AND KEEP CHC TO SLIGHT CHANCE
FARTHER NWRD ALG SYNOPTIC CONVERGENCE. WIND FIELDS ARE CERTAINLY
MORE FAVORABLE FOR ORGANIZATION FARTHER NWRD...BUT BETTER LOW LVL
SHEAR/EXPLOSIVE INSTABILITY ALG SRN OUTFLOW...PROBABLY LEADING TO A
CONDITIONAL SIGNIFICANT SVR THREAT INTO THE EARLY EVENING THRU NW
IA/NERN NEBRASKA/EXTREME SERN SD.

BNDRY SLOWS TO S ON THU NIGHT...AND AS GET INTO THE RIGHT ENTRANCE
REGION OF JET...WL SEE PCPN EXPAND NWRD AGAIN LTR THU NIGHT AND
FRIDAY THRU MAINLY AREAS ALG/S OF I90. FOR NOW...KEPT AT LOWER CHC
LVLS WITH MUCH CONVECTIVE EFFECTS TO YET DETERMINE. WL TEMPER HIGHS
SOMEWHAT WITH CLDS/POTENTIAL PRECIP. INSTABILITY GRADIENT SUGGESTED
TO LINGER INTO FRI EVNG THRU SRN CWA...AND KEPT LOW POPS IN DESPITE
THREAT THAT LOWER LVL BNDRY TO S MAY TAKE OVER BY THIS TIME.

SAT...COOLER HIGH SLIDES INTO NRN CWA AND INTO MN...CLEARING SKIES
AND LOWERING HUMIDITY.

FOR THE EXTENDED RANGE /SAT NIGHT THRU WED/ REMAINS A CHALLENGE WITH
FLOW BETWEEN THE ERN CANADIAN TROUGHING...NRN ROCKIES RIDGING...AND
TIMING ANY STRONGER WAVES CROSSING THRU INTO NRN PLAINS CONFLUENCE.
SOME CONSIDERABLE VARIATION ON THE RELATIVE STRENGTH OF THIS RIDGING
AND ADVANCE OF STRONGER TROUGH IMPULSE FROM PAC NW INTO CONFLUENCE
ZONE.  OPERATIONAL GFS CONTINUES TO BE WELL TO THE AGGRESSIVE SIDE
OF THE ENVELOPE...AND CONSIDERING OTHER SLNS FAVOR A LITTLE SLOWER
IMPULSE...BUT NOT TOO MUCH SLOWER GIVEN THE CONFLUENT NATURE TO FLOW.

ADDED IN A SMALL THREAT FOR NOCTURNAL STORMS SAT NIGHT/EARLY SUN AM
WITH .  THESE WOULD HAVE TO BE INDEPENDENT OF ANY LATE DAY SAT
STORMS WHICH DEVELOP THRU THE HIGH PLAINS...WHICH WOULD TEND TO
PROPAGATE ESE TOWARD NEBRASKA RATHER THAN E/NE. BETTER THREAT FOR
STORMS LOOKS TO INITIATE BY LTR SUN AFTN AND INTO SUNDAY NIGHT
WITH CONSENSUS OF JET ENTRANCE REGION NEAR AREA AND DECENT LOW TO
MID LVL FOCUS.  COULD STILL HAVE A LINGERING THREAT THRU FAR SRN CWA
ON MONDAY...BUT EXPECT THE MOST OF THE DAY AND MOST OF THE AREA WL
BE DRY. LEADING WAA AHEAD OF STRONGER SYSTEM STARTS MON NIGHT...
AND AGAIN SEEMS TO BE ON TRACK WITH NOCTURNAL TIMING.
OVERALL...DOWNTREND IN PCPN AFTER FROPA TUE/TUE NIGHT...WITH ONLY
HANGING ON TO A SMALL DIABATICALLY INDUCED THREAT UNDER MN COLD POOL
ON WED. HARD TO PICTURE TMPS GETTING TOO WARM WITH LARGE SCALE FLOW
REMAINING CONFLUENT...WITH BEST CHANCE OF SQUEEZING SOME PREFRONTAL
WARMTH AHEAD OF APPROACHING BNDRY ON TUE. OTHERWISE...GENERALLY AT
OR BLO NORMAL...DEPENDING ON CONVECTIVE DEBRIS.  /CHAPMAN

&&

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

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1220 PM CDT WED JUL 08 2009

SDZ038>040-050-052>071-MNZ071-072-080-081-089-090-097-098-
IAZ001>003-012>014-020>022-031-032-NEZ013-014-091200-
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-
1220 PM CDT WED JUL 08 2009

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

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS...
THUNDERSTORMS WILL DEVELOP OVER WESTERN NORTH DAKOTA LATE THIS
AFTERNOON. THESE STORMS WILL MOVE EAST ACROSS NORTH DAKOTA AND
BUILD SOUTHWARD INTO NORTH CENTRAL SOUTH DAKOTA DURING THE
EVENING HOURS. STORMS WILL CONGEAL INTO A COMPLEX OF
THUNDERSTORMS...WHICH IS EXPECTED TO MOVE ACROSS NORTHEAST AND
EAST CENTRAL SOUTH DAKOTA AFTER MIDNIGHT...AND INTO SOUTHWEST
MINNESOTA THURSDAY MORNING. THIS COMPLEX OF THUNDERSTORMS WILL BE
CAPABLE OF PRODUCING VERY STRONG DAMAGING WINDS IN EXCESS OF 70
MPH AND HAIL TO THE SIZE OF HALF DOLLARS.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

THUNDERSTORMS WILL AGAIN BE POSSIBLE ALONG AND EAST OF INTERSTATE
29 THURSDAY AFTERNOON AND EVENING AS A COLD FRONT PUSHES ACROSS
THE AREA. SOME THUNDERSTORMS MAY BECOME SEVERE...PRODUCING LARGE
HAIL...DAMAGING WINDS...AS WELL AS ISOLATED TORNADOES.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL BE LIKELY ALONG AND NORTH OF HIGHWAY 14
EARLY THURSDAY MORNING.

&&

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