May 9, 1999 Brief Tornado Event

On May 9th, 1999 a series of brief tornadoes struck south-central South Dakota. We will take a short look at conditions that lead to the tornadoes.

Sunday morning, we issued a Severe Weather Outlook outlining the potential for large hail and isolated tornadoes.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE  RAPID CITY SD
1035 AM MDT SUN MAY 9 1999

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
TONIGHT OVER THE SOUTHERN HALF OF SOUTH DAKOTA...SOUTH OF A LINE FROM
HOT SPRINGS TO RAPID CITY AND THEN EAST ALONG INTERSTATE 90. THE
MAJOR THREAT WILL BE LARGE HAIL.

DISCUSSION...
LOW PRESSURE OVER SOUTHEAST WYOMING WILL MOVE INTO NEBRASKA/S
PANHANDLE THIS EVENING. A WARM FRONT EXTENDING EAST FROM THE LOW WILL
MAKE IT TO THE NEBRASKA/SOUTH DAKOTA BORDER. A LOW LEVEL JET UP TO 60
MPH WILL BRING AMPLE MOISTURE NORTH TO THE WARM FRONT. SUNSHINE WILL
DEVELOP IN THIS REGION LEADING TO DESTABILIZATION OF THE ATMOSPHERE.
WIND SHEAR PROFILES SUPPORT SEVERE THUNDERSTORMS NEAR THE WARM FRONT
WITH THE POSSIBILITY OF ROTATING UPDRAFTS...WHICH COULD LEAD TO
ISOLATED TORNADOES. HOWEVER...THE MAIN THREAT WILL BE LARGE HAIL.
CURRENT THINKING IS THAT THUNDERSTORMS WILL DEVELOP BY 500 PM WITH
SEVERE WEATHER POSSIBLE IN THE OUTLOOK AREA. THE GREATEST THREAT OF
SEVERE WEATHER WILL BE EAST OF A LINE FROM MARTIN TO MURDO.
STAY TUNED TO YOUR FAVORITE MEDIA OUTLET FOR WEATHER UPDATES.

Check out the 22z surface analysis for Sunday, May 9th, 1999.  22z is 4:00 PM MDT or 5:00 PM CDT. Low pressure is centered over northeast Colorado with a well-defined warm front into south-central South Dakota. The low is represented by the "L" and the warm front by the red line with half-circles on it. The blue triangles represent a trailing cold front. Take a close look in Nebraska's panhandle. There you will find a dry-line. A dry-line is defined as a narrow zone of strong contrast between moist and dry air masses. It frequently can be a focus for severe thunderstorm development. The green shading represents dew points greater than 60F with a surrounding area of 55F dew points.

Also at 22z, there was strong Surface Moisture Flux Convergence over south-central South Dakota, where you see the 22 "bulls-eye". This indicates moisture is rapidly flowing into that area, which happens to be near the warm front on the surface analysis above. We looks for things like this when forecasting thunderstorms.

On the satellite images listed below, you can see a line of clouds developing along the moisture gradient depicted above. Note, this is a very large file, but it is quite useful.

1km Satellite Images: 2040z, 2045z, 2115z, 2125z, 2145z, 2210z, 2215z, 2225z, 2240z, 2245z, 2310z, 2315z, 2345z, 0015z, 0045z, 0055z.

When we try to determine what kind of thunderstorm we expect during an event, we look at a Skew-T diagram and Hodograph. This lets us look at the vertical profile of the atmosphere. We constructed a representative sounding from KSFD, which is located in Winner, SD. Winner is located near the bulls-eye of moisture depicted above. Check out the sounding.

The CAPE (convective available potential energy) was 3363 J/kg with rather strong shear noted in the 0-6km layer of the atmosphere. Without a cap as indicated in this sounding, thunderstorms were free to develop. When they did develop, they developed rotation in their updrafts which lead to short-lived tornadoes.

Here is a summary of the storm reports received:

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RAPID CITY SD
1101 PM MDT SUN MAY 09 1999

TIME(MDT)  .....CITY LOCATION..... STATE   ...EVENT/REMARKS...
....COUNTY LOCATION....

0600 PM    MARTIN                    SD    1.75 INCH HAIL
05/09/99   BENNETT                         GOLFBALL SIZE HAIL REPORTED BY BENNET COUNTY SHERIFF
0632 PM    12 NW WINNER              SD    TORNADO
05/09/99   TRIPP                           REPORTED BY PUBLIC
0645 PM    HAMILL                    SD    WIND DAMAGE
05/09/99   TRIPP                           SHERIFF REPORTED POWER LINES DOWN AND BARN DESTROYED.  
                                           POSSIBLE TORNADO
0712 PM    5 N PARMELEE              SD    TORNADO
05/09/99   TODD                            REPORTED BY ROSEBUD POLICE
0725 PM    WHITE RIVER               SD    TORNADO
05/09/99   MELLETTE                        TORNADO SOUTH OF WHITE RIVER REPORTED BY SPOTTERS
0725 PM    WHITE RIVER               SD    TORNADO
05/09/99   MELLETTE                        TORNADO NORTH OF WHITE RIVER REPORTED BY SPOTTERS
0742 PM    8 W MISSION               SD    TORNADO
05/09/99   TODD                            REPORTED BY ROSEBUD POLICE
0820 PM    4 W WINNER                SD    1.75 INCH HAIL
05/09/99   TRIPP
0826 PM    MISSION                   SD    TORNADO
05/09/99   TODD                            REPORTED BY POLICE UNITS
0830 PM    NORRIS                    SD    FLOOD
05/09/99   MELLETTE                        1 FOOT OF WATER GOING OVER COUNTY ROAD 
                                           BETWEEN NORRIS AND PARMELEE
0850 PM    MISSION                   SD    1.75 INCH HAIL
05/09/99   TODD                            REPORTED BY ROSEBUD POLICE
0850 PM    NORTH ANTELOPE            SD    FLOOD
05/09/99   TODD                            FLOODING OF SOME ROADS
0930 PM    WALL                      SD    1 INCH HAIL
05/09/99   PENNINGTON                      REPORTED BY SPOTTER
1045 PM    WINNER                    SD    FLOOD
05/09/99   TRIPP                           STATE ROAD 183 UNDER WATER...REPORTED BY
                                           WINNER POLICE

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