TRI STATE AREA HEAT BURST
MARCH 26, 1998
PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE SIOUX FALLS SD 740 AM CST THU MAR 26 1998 ...TRI-STATE AREA EXPERIENCES HEAT BURST OVERNIGHT... AS THUNDERSTORMS WERE PROGRESSING ACROSS THE TRI-STATE AREA DURING THE NIGHT...A HEAT BURST DEVELOPED BEHIND THE STORMS. A HEAT BURST IS DESCRIBED AS A RARE WEATHER PHENOMENA CAUSED BY DESCENDING AIR FROM WEAKENING THUNDERSTORMS THAT WARMS RAPIDLY...WHICH THEN CAUSES THE SURFACE TEMPERATURE TO RISE SHARPLY. THIS WAS EXPERIENCED BY MUCH OF THE TRI-STATE AREA OVERNIGHT. DURING A TWO HOUR TIME PERIOD...TEMPERATURES INCREASED ANYWHERE FROM 10 TO 20 DEGREES. MARSHALL MN INCREASED 20 DEGREES SIOUX FALLS INCREASED 18 DEGREES BROOKINGS INCREASED 13 DEGREES MONTROSE INCREASED 13 DEGREES THE LAST TIME THIS OCCURRED (OF THIS MAGNITUDE) IN EASTERN SOUTH DAKOTA WAS IN PIERRE. LAST NIGHTS EVENT EQUALED THE TEMPERATURE INCREASE ON JUNE 20 1989. HEITKAMP/HOLMES
During the early morning hours of March 26 at 08:30 UTC (2:30 am) weak, high-based showers and thunderstorms were moving across the Sioux Falls county warning area. The showers, in combination with very dry air in the low layers, were the cause of the rare heat burst we experienced that morning. A heat burst occurs when air descends and warms rapidly due to a natural effect called compressional warming. Since air is more dense at the surface than aloft it becomes more compressed and warms as it descends through the atmosphere. This is analogous to when a bicycle pump needle gets warmer as you pump air into a tire. When the air is drier in the lower levels of the atmosphere it warms at a faster rate than if there was moisture in the low layers.
The satellite derived sounding for Sioux Falls at 0900 UTC (3:00 am) shows there was a deep layer of dry air from 600 mb down to just above the surface with high based moisture above 600 mb. Any weak precipitation falling out of the high based showers likely evaporated in the dry air. Initially the weak downdrafts caused by evaporating precipitation warm at a moist adiabatic rate of 6C/km. However, as all the moisture evaporates the air continues to descend, partly due to momentum effects, and warms at the higher, dry adiabatic lapse rate of 10C/km. Therefore a deeper layer of dry air at low levels allows a greater increase in temperature because the air continues to warm at the higher lapse rate for a longer period of time.
The water vapor satellite loop during this time also showed a notch forming to the rear of the clouds over the Sioux Falls area indicating that drying and evaporation was occurring. The notch becomes more pronounced as the cloud mass moves into southwest Minnesota. The radar imagery also showed that precipitation activity had diminished significantly by 0930 UTC (3:30 am) as this dry notch moved over the area.
Automated weather sensors over the tri state area recorded the effect of the heat burst. The Sioux Falls and Montrose sensors showed the most dramatic increase in temperature during a short period with a peak at around 0900 UTC (3:00 am). Farther to the north and east at Brookings and Marshall, MN the peak occurred later at around 1100 UTC (5:00 am) and the temperature increase was not as strong.
