Gusty Winds This Morning From Apparent Heat Bursts

During the morning hours of Tuesday, May 14th, several "heat bursts" or hybrid "heat burst/wake low" induced wind gusts were observed across portions of northeastern South Dakota.  For example, between 0700 AM CDT and 0800 AM CDT observations from the Sisseton Airport (K8D3) indicated:

  • a dramatic increase in temperature (+21F)
  • decrease in dewpoint temperature (-2F)
  • minor surface pressure falls
  • gusty surface winds (a peak wind gust of 37 mph)

Sisseton ASOS:

 

Time
Temperature
Dewpoint
Pressure
Winds
Precip
856AM CDT
72
48
29.54 in
E 15 mph
0.00”
756AM CDT 78 44 29.52 in SW 17 mph 0.00"
741AM CDT
79
43
29.56 in
W 21G37 mph
0.00”
656AM CDT
58
45
29.55 in
SE 7 mph
0.00"
556AM CDT
58
44
29.57 in
ESE 9 mph
0.00"

 

Other strong wind gusts across northeastern South Dakota included:

  

Location Peak Wind Gust Time
Near Frederick, SD (RWIS) 57 mph 0700 AM CDT
Near Lake City, SD (RAWS) 57 mph 0730 AM CDT (est)
Near Victor, SD (RWIS) 57 mph 0750 AM CDT
Near Britton, SD (SuperAWOS) 47 mph 0715 AM CDT

 


How do "heat bursts" form?

So what caused these apparent heat bursts?  During the morning hours weak, high based showers and thunderstorms moved across portions of North Dakota and northeastern South Dakota.  As the precipitation fell out of the high based clouds, it fell through a very dry airmass near the surface (see the Aberdeen sounding in Figure 1).  Thus, much of the rain likely evaporated in the dry air.  Initially, downdrafts caused by evaporating precipitation warm at a moist adiabatic rate (about 6C/km). However, as a majority of the moisture evaporated, the air continued to descend and compress due to momentum effects.  At this time, air parcels likely warmed at the higher, dry adiabatic lapse rate (approximately 10C/km).  If these downdrafts contain enough momemtum, they then "overshoot" the shallow layer of cooler air near the surface.  This results in dramatic warming - along with gusty winds.  Often times, heat bursts also produce a strong pressure gradient at the surface between the warm air (low pressure) associated with the heat burst compared to the cooler (high pressure) ambient airmass.  

Although these events are considered "relatively" rare, they may not occur as infrequently as as previously thought.  Research being conducted by the NWS in Aberdeen and the South Dakota State Climatologist's Office have noted several of these events over South Dakota during the past few years.  The reason for more frequent detection is due in part to a higher density of automatic weather stations in South Dakota (such as South Dakota State University's AWDN Network).   

 


Figure 1:  Upper Air Sounding at Aberdeen, SD at 7am CDT Tuesday (12 UTC on 14 May) - Click to enlarge.

Click image to enlarge

  

 



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