Flash Flood Warning 20, NWS Rapid City, SD

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
836 PM MDT MON JUL 4 2011

.DISCUSSION...02Z SURFACE ANALYSIS HAD COLD FRONT FROM SOUTHEAST
ND TO KPIR TO KIML. MLCAPE ALONG FRONT IN THE 1.5-3KJ/KG
RANGE...WITH A DECENT CAP...SEE 00Z KUNR SOUNDING. CAP WEAKEST
ALONG SURFACE BOUNDARY AND IT APPEARS SHORTWAVE ENERGY OVER EASTER
WY STARTING TO INTERACT WITH SURFACE FEATURES BRINGING LARGE SCALE
ASCENT AHEAD OF IT. NOT MUCH CHANGE TO GOING FORECAST...SEVERE
THUNDERSTORM WATCH WILL CONTINUE ALTHOUGH THREAT OF SEVERE STORMS
HAS DIMINISHED SOME OVER SOUTHWEST SD BEHIND BOUNDARY. HEAVY RAIN
THREAT CONTINUES WITH SLOW MOVING STORMS OVER SOUTH CENTRAL SD AND
LOW FLASH FLOOD GUIDANCE VALUES.

&&

.PREV DISCUSSION... /ISSUED 1249 PM MDT MON JUL 4 2011/

DISCUSSION...WATER VAPOR IMAGES REVEALED A MONSOONAL MOISTURE FLOW
TOWARD THE CWA AS A MIDLEVEL SHORT-WAVE TROUGH WAS EXITING THE
EASTERN DAKOTAS. A WEAK SURFACE COLD FRONT HAS PUSHED THROUGH ALL
BUT FAR SOUTHERN SD WITH A MOIST NORTHERLY FLOW IN ITS WAKE. MLCIN
WAS STILL 100-250 J/KG WHICH WAS KEEPING A LID ON CONVECTION. A
SHORT-WAVE TROUGH APPEARED TO BE OVER WESTERN/CENTRAL WY.

FOR THE REST OF THIS AFTERNOON AND TONIGHT...EXPECT CONVECTION TO
INITIATE OVER THE BLKHLS BY 200-300 PM AND THEN GRADUALLY BUILD
ALONG THE SURFACE BOUNDARY BETWEEN 400-600 PM. A FEW CONVECTIVE
COMPLEXES LIKELY WILL ENSUE AS THE APPROACHING SHORT-WAVE TROUGH
INTERACTS WITH THE ABUNDANT LOW-LEVEL MOISTURE...SURFACE BOUNDARY...
AND MLCAPE OF 1000-3500 J/KG. IN ADDITION...DECENT Q-G FORCING IS
PROGGED AS A 70-KT UPPER JET PASSES TO OUR NORTH...PLACING MUCH OF
THE CWA IN UPPER-LEVEL DIVERGENCE. THE 0-6KM SHEAR IS SOMEWHAT
MARGINAL SO DO NOT EXPECT MANY SUPERCELLS. HAIL WILL BE THE MAIN
THREAT GIVEN THE LARGE CAPE...ALONG WITH LOCALLY HEAVY RAINFALL AS A
WEAK LOW-LEVEL JET FOCUSES MOISTURE AND MASS CONVERGENCE ACROSS
SOUTH-CENTRAL SD LATER TONIGHT.

ON TUESDAY THE UPPER TROUGH MOVES EAST BUT CONVECTION WILL LINGER
THROUGH MUCH OF THE MORNING. DRYING WILL WORK IN FROM THE NORTH AS A
SURFACE RIDGE SETTLES ACROSS THE DAKOTAS AND THE MONSOONAL MOISTURE
PLUMES SHIFTS SOUTH. THIS WILL LEAVE A THETA-E RIDGE ACROSS EASTERN
WY...BUT WITH MINIMAL FORCING DO NOT EXPECT MUCH MORE THAN ISOLATED
AFTERNOON STORMS ACROSS SD...WITH SCATTERED COVERAGE AT BEST IN
EASTERN WY. IT WILL BE SEVERAL DEGREES COOLER THAN TODAY DUE TO COLD
AIR ADVECTION IN THE LOW LEVELS.

ON TUESDAY NIGHT THERE WILL BE SOME RIDGING ALOFT WITH NORTHWEST
FLOW. A SUBTLE WAVE MAY MOVE ACROSS WY AND SUPPORT EVENING STORMS
OVER MAINLY EASTERN WY...CLOSER TO THE MONSOONAL MOISTURE PLUME.
OTHERWISE THIS WILL BE A RELATIVE LULL IN CONVECTIVE ACTIVITY.

THERE WILL BE A BETTER CHANCE FOR STORMS WEDNESDAY AFTERNOON AND
ESPECIALLY WEDNESDAY NIGHT AS ANOTHER WAVE MOVES THROUGH THE RIDGE
AND THE MONSOONAL MOISTURE PLUME SHIFTS BACK TO THE NORTH. A 50-KT
JET IS PROGGED TO PASS TO OUR NORTH WITH UPPER-LEVEL DIVERGENCE OVER
THE CWA. IT DOES NOT LOOK TO BE AS ACTIVE AS TONIGHT WILL BE GIVEN
THE WEAKER UPPER-LEVEL JET AND MARGINAL DEEP-LAYER SHEAR...BUT SOME
LOCALLY HEAVY RAINFALL WILL AGAIN BE POSSIBLE AS PRECIPITABLE WATER
IS 140-160 PERCENT OF AVERAGE AND A LOW-LEVEL JET DEVELOPS ACROSS
WESTERN SD.

EXTENDED...AN UNSETTLED WEATHER PATTERN WILL CONTINUE AS A VARIETY
OF WEAK SHORT-WAVE TROUGHS PROGRESS THROUGH A LOW AMPLITUDE RIDGE...
BRINGING DAILY CHANCES FOR THUNDERSTORMS. TEMPERATURES WILL REMAIN
NEAR TO SLIGHTLY ABOVE AVERAGE.

AVIATION...THUNDERSTORMS WILL DEVELOP OVER THE AREA LATE THIS
AFTERNOON INTO THIS EVENING...BRINGING AREAS OF MVFR AND LCL IFR VIS
IN HEAVIER PRECIP. SOME STORMS MAY BECOME SEVERE. SHOWERS AND STORMS
WILL PERSIST ACROSS MUCH OF WESTERN SOUTH DAKOTA OVERNIGHT WITH SOME
MVFR CIGS POSSIBLE LATE TONIGHT THROUGH 15Z TUESDAY MORNING.

HYDROLOGY...FLASH FLOOD GUIDANCE ACROSS SOUTH-CENTRAL SD IS FROM
1-3 INCHES OVER A FEW HOURS. THE POTENTIAL FOR SLOW-MOVING STORMS
EXISTS AS THE 0-6KM MEAN WIND IS ONLY 5-10 KTS. MOISTURE TRANSPORT
VECTORS ARE PROGGED TO CONVERGE IN THIS AREA OVERNIGHT...HELPING
BOOST PRECIPITABLE WATER TO 140-160 PERCENT OF AVERAGE. THE SURFACE
BOUNDARY SHOULD PROVIDE A FOCUS FOR STORM INITIATION AND POSSIBLE
TRAINING OF ECHOES. THE FLASH FLOOD WATCH FOR SOUTH-CENTRAL SD LOOKS
REASONABLE GIVEN THIS SETUP.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...FLASH FLOOD WATCH UNTIL 6 AM MDT /7 AM CDT/ TUESDAY FOR BENNETT-
     HAAKON-JACKSON-MELLETTE-TODD-TRIPP.

WY...NONE.
&&

HELGESON

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RAPID CITY SD
614 PM MDT MON JUL 4 2011

SDZ029-050100-
SOUTHERN BLACK HILLS SD-
614 PM MDT MON JUL 4 2011

...SIGNIFICANT WEATHER ADVISORY FOR WESTERN CUSTER COUNTY UNTIL 700
PM MDT...

AT 610 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING A
STRONG THUNDERSTORM NEAR JEWEL CAVE...OR 14 MILES SOUTHWEST OF
DOWNTOWN CUSTER...MOVING SOUTHEAST AT 15 MPH.

HAIL UP TO ONE-HALF INCH IN DIAMETER IS EXPECTED WITH THIS STORM...
ALONG WITH HEAVY RAIN.

LOCATIONS NEAR THE PATH OF THIS STORM INCLUDE...
AREAS WEST OF PRINGLE AND ARGYLE IN WESTERN CUSTER COUNTY.

THIS STORM MAY INTENSIFY...AND A SEVERE THUNDERSTORM WATCH IS IN
EFFECT FOR THIS AREA. BE CERTAIN TO MONITOR LOCAL RADIO AND TV
STATIONS...AS WELL AS LOCAL CABLE TV OUTLETS...FOR ADDITIONAL
INFORMATION AND POSSIBLE WARNINGS FROM THE NATIONAL WEATHER SERVICE.

TORRENTIAL RAIN...WHICH MAY FLOOD LOW LYING AREAS SUCH AS DITCHES AND
UNDERPASSES...IS ALSO LIKELY. DO NOT DRIVE YOUR VEHICLE ON A FLOODED
ROADWAY.

TRIMARCHI

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RAPID CITY SD
715 PM MDT MON JUL 04 2011

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0655 PM     TSTM WND GST     6 S DOWNTOWN HOT SPRING 43.35N 103.48W
07/04/2011  E60 MPH          FALL RIVER         SD   PUBLIC

&&

MERICKSO

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