Splitting Severe Thunderstorms and the Destructive Bow Echo at Ethan, SD
June 19-20, 1997
By Ron Holmes
The first big severe weather event to strike our CWA occurred late in the day on June 19, 1997 producing a nearly continuous 12 hour stretch of severe weather from the early evening hours on the 19th until the morning of the 20th. Some of the highlights of this event included strong straight line winds which produced widespread damage across southern Davison and Hanson counties. Numerous buildings and mobile homes were destroyed and up to 8 people were injured. Other severe weather reports to reach our office were 1.75" hail in Turner county and 2.75" hail in Brule county. Our Warning Coordination Meteorologist, Todd Heitkamp, conducted an extensive storm damage survey in Ethan, South Dakota and determined that this damage was a result of severe, straight-line winds based upon the orientation and type of damage. This survey report also contains many pictures of the storm damage in Ethan and the accompanying severe thunderstorm warning that was issued 27 minutes prior to the severe weather. Click here to see a map of all severe weather reports received at our office for this event.
SYNOPTIC OVERVIEW
The synoptic situation which lead up to this event was quite weak dynamically but there were several features in the thermodynamic fields which helped outline the geographic area in which the storms formed. The 1200 UTC sounding from North Platte (LBF) was analyzed and modified using SHARP which yielded an afternoon CAPE of 3713 J/jg with a lifted index of -7. The wet bulb zero height was low (9,292') which indicated a good potential for hail producing storms. One of the quantities that was weak was the environmental wind shear which yielded a helicity of only 58 m2/s2. This value was based on an estimated storm movement of 320 at 18 knots which was very different from the observed motion of 260 at about 30 knots.
The 850 mb chart for 0000 UTC 6/20 (700 pm 6/19) showed a very tight thermal gradient (red lines) oriented north to south across South Dakota. One can infer (although the lack of upper air observations over our CWA precluded direct measurement of) strong warm advection into south central and southeastern South Dakota associated with a surface frontal boundary (at 0600 UTC or 100 am 6/20) over the southern border of South Dakota. There was also ample low level moisture as 850 mb dewpoints were in excess of 12 C (shaded green) in an east to west direction parallel to the isotherms. This orientation caused the storms to form along the surface frontal boundary within strong low level warm advection. Once they formed, upper level westerly winds caused the storms to track from west to east parallel to the frontal boundary and along the low level moist axis. This helped to form waves of thunderstorms over a preferred region over western South Dakota and track repeatedly through the same areas.
At 500 mb a minor short wave was tracking across North Dakota (red dashed line) with weak height falls (shaded brown) to the east and into northern South Dakota. There was also a tight gradient of temperature at this level across the Dakotas (blue lines) with weak cold advection at least over the northern half of South Dakota. This feature, along with strong low level warm advection, helped to steepen lapse rates and generate unstable conditions for thunderstorms. Further aloft at 300 mb a 110 knot straight-line jet streak was located over eastern Montana at 0000 UTC with much of South Dakota in the right exit region. It is inferred that during the night this jet streak moved east so that South Dakota gradually became under the influence of the right entrance region and favorable ageostrophic lift associated with this quadrant of the jet.
RADAR FEATURES AND STORM SPLIT
During the event the storms evolved from a supercellular structure to a multicell structure which, in one case, became a long lived and destructive bow echo that moved from Aurora to Hanson county. The first supercell severe thunderstorm moved into the western portion of our CWA around 0400 UTC (1100 pm 6/19). Prior to that the VWP product from the Sioux Falls Doppler radar showed a slight veering in the wind field but the speed shear was weak. Plotting this information on a hodograph revealed not a clockwise curved hodograph (which can result from veering winds and cause primarily right moving supercells) but a straight line from the surface up through 3 km. This type of wind shear environment is conducive to storm splitting with both a left moving and right moving severe thunderstorm after the split. Also, since winds favored a southwesterly direction in the 0-3 km layer, in this case, the hodograph suggested that the left mover would move faster than the mean wind and the right mover would move slower. The storm tracking algorithm from the Doppler radar later showed that this in fact was observed after the storm split.
Around 03:47 to 03:57 a single thunderstorm split into a left and right moving supercell over Lyman county. This is shown in a 4-panel time lapse of reflectivity from the Sioux Falls Doppler radar (time increases from the top left to bottom right panel and each panel is labeled 1 to 4 in the upper right corner). Panel 1 (upper left) shows the initial thunderstorm at 03:32 UTC (10:32 pm) and by 04:47 UTC (11:47 pm ) (panel 4) the storm has split into two convective cells with the northern storm moving to the left of the mean wind and the southern storm moving to the right of the mean wind.
After 0400 UTC these storms moved into the western portion of our CWA with the left mover moving towards Beadle county and the right mover moving into northern Brule county. Severe thunderstorm warnings were issued for portions of both counties. At 04:32 UTC (panel 2) and 04:52 UTC (panel 4) a flare echo (near the arrow) was observed extending out from the main precipitation core to the northwest of the left moving thunderstorm. This signature has been shown by research meteorologists to be associated with large hail (>1.00 inch diameter).
The Storm Relative Velocity product for these storms also shows the rotation in each of the storms at 15,600 feet at 04:43 UTC (11:43 pm). The left moving storm had a much more defined area of anticyclonic rotation than the right mover had cyclonic rotation but the calculated Vr shear was 31 knots for both circulations which at this distance (108 nm) is classified as a moderate circulation. You can also see the flare echo extension as a region of outbound velocity directly away (downradial) from the anticyclonic circulation. The cyclonic/anticyclonic couplet rotation is a direct observational measurement of the rotation in a pair of thunderstorms that theory and numerical modeling experiments indicate should happen in a unidirectionally sheared environment. Armed with this knowledge of theory and applying it to observations should lead the forecaster to conclude continued upward forcing on the north and south sides of the left moving and right moving storm, respectively, and continued propagation away from each other.
COMPOSITE REFLECTIVITY, CELL TRENDS, AND SEVERE HAIL
The composite reflectivity product from our Doppler radar indicated the left and right movement of these storms. Note also the forecast movement of storm W4 (left mover) and R5 (right mover) located in the table at the top. The left mover was forecast to move from 259 at 38 knots and the right mover was forecast to move from 285 at 29 knots. The past storm track clearly shows the diverging paths of these two storms and the forecast track extrapolates these cells into southern Beadle and northern Brule counties.
The cell trends product for the left moving storm shows that as the reflectivity in the storm increased the cell based VIL and probability of severe (>3/4") hail increased. The cell trends product for the right mover shows that it was much weaker than the left mover with lower reflectivity, cell based VIL, and probability of severe hail. Severe thunderstorm warnings were issued for both left moving and right moving storms from the Aberdeen and Sioux Falls National Weather Service Offices during the life of these storms. At 03:48 UTC (10:48 pm) 3/4" hail fell at Iron Nation in Lyman county just as the storm was splitting. After the storm split the left moving storm produced golf ball size (1.75") hail at Fort Thompson in northwest Buffalo county at 04:25 UTC (11:25 pm) and 3/4" hail 2 miles southeast of Stephan in southern Hyde county at 04:44 UTC (11:44 pm). We did not receive reports of hail from the right moving storm which could be attributed to the time of day and the track across sparsely populated areas. Based on the hail reports and cell trends it at least appears that in this case the left mover was indeed more severe than the right moving thunderstorm.
DESTRUCTIVE BOW ECHO
As the night progressed additional waves of thunderstorms moved into our CWA from the west. One complex of thunderstorms became organized into a north to south oriented line and evolved into a fast moving bow echo that caused a path of destruction from Aroura county eastward into Mc Cook and Hutchinson counties. A time lapse of reflectivity in 4-panel format illustrates the movement of the bow echo during this destructive phase.
In the first panel (upper left) the line was beginning to organize into a north-south line at 09:03 UTC (4:03 am). Severe winds (70 mph) occurred at White Lake at 3:55 am and 3/4" hail and gusts to 60 mph were being reported at White Lake at the time of this picture. Note this is behind the leading edge of reflectivity. While there may be strong winds ahead of the bow echo along the gust front often the strongest winds occur just behind the leading edge of the bow echo at the apex of the bow. The fact that there is a reflectivity weak area behind the leading edge suggests there is also strong subsidence occurring as well which is the result of evaporational cooling and acceleration of strong downdrafts. By 09:13 UTC (4:13 am) (upper right panel) the echo has moved into Davison county but severe winds (60 mph) were occurring at Plankinton in Aroura county, again, behind the leading edge of the bow echo in the reflectivity free area.
Between 09:13 and 09:43 UTC (4:13 to 4:43 am) the bow echo really increased in strength and became more "bowed out". This period marks one of widespread destruction across southern Davison and southern Hanson counties. In the lower left panel at 4:13 am the leading edge was in central Hanson county but the reflectivity weak area was from west central to southern Hanson county. During this time (4:40 am) widespread wind damage was occurring at Ethan with 5 injuries and a mobile home destroyed. Buildings and trees were also damaged. Based on a storm damage survey by our Warning Coordination Meteorologist, Todd Heitkamp, wind speeds were estimated to be in excess of 80 mph. A short time later at 10:00 UTC (5:00 am) (lower right panel) the bow echo caused more destruction at a Hutterite Colony located about 10 miles east of Ethan in southern Hanson county. Three people were injured at the colony and there was widespread building and tree damage with estimated winds in excess of 80 mph.
