Thursday, September 4, 2014

Heat Bursts Over the High Plains

On early Thursday morning a somewhat rare and unusual phenomenon occurred in the High Plains. A series of heat bursts were observed in western Nebraska and central and southern South Dakota.

A heat burst is a downdraft of very warm and dry air, associated with decaying showers or thunderstorms that usually occurs in the evening or overnight. Heat bursts at the surface are characterized by dramatic rises is temperature, decreases in dew point, and strong gusty winds.

A heat burst develops when rain falls through a very dry layer in the middle levels of the atmosphere. As the rain falls through the layer of very dry air it evaporates. As the water evaporates, the air cools and accelerates toward the surface. Once all of the precipitation has evaporated the air can no longer be cooled by the evaporation process. This dense air continues to accelerate toward the surface but now rapidly warms as it compresses at a rate of 5.5°F per 1000 feet (called the dry adiabatic lapse rate). This downdraft hits the ground and spreads out in a similar fashion to a downburst from a severe thunderstorm.

This is the radar image from Aberdeen, SD shows the decaying showers and thunderstorms over southern South Dakota generally south of Huron (HON).

Last night's heat bursts were captured by a number of automated weather stations in the South Dakota Mesonet and NOAA's Climate Reference Network (CRN).

Temperature, humidity, and wind plots from White Lake, SD showing the heat burst. The temperature at White Lake rose from 73°F to 89°F and the humidity dropped from 92% to 24% all in the span of 25 minutes,
and the peak wind gust was 47 mph..

Temperature, humidity and wind plots for Oacoma, SD showing heat burst.

CRN temperature and humidity data showing heat burst near Harrison, NE.

I experienced the effects of a heat burst in July 1995 while in Scottsdale, AZ attending a meeting. It was in July and the high temperature for the day was 121°F. That night a number of us where sitting around the hotel pool around 11:00 p.m. when the winds picked up and the temperature went from about 102°F to 111°F in a matter of minutes. That heat burst came from a decaying thunderstorm over Phoenix. The effects were a little more dramatic closer to Phoenix and there was some wind damage as well. It was a very odd thing to experience, especially at those temperatures.

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