Rare Yankton, South Dakota Heat Burst Creates Weird Dawn One-Hour Hot Spell.

 I have a big affinity for Yankton, South Dakota, since that's where my husband was born and raised, and I have several relatives and friends there. 

The Meridian Highway Bridge, now a
pedestrian walkway, in Yankton,
South Dakota. Anyone out on that
bridge around 6 a.m. Saturday
will have experienced a rare
"heat burst" that sent temperatures
up by 20 degrees in 15 minutes.

So I was kind delighted in a weather nerd sort of way when Yankton, a  very nice southeastern South Dakota city about the size of St. Albans City and Town combined, experienced a weird and rare weather event early Saturday morning. 

Bonus: The event wasn't dangerous, but just really strange.

At around 5 a.m. Saturday, the weather was pretty routine in Yankton, if a bit warm for the season. The temperature was 68 degrees, it was vaguely humid for the season. The wind was from the southeast at just 6 mph. 

Then, an hour later a 6 a.m., things took a bizarre turn. The temperature abruptly soared to 91 degrees, rather rare for an afternoon high this time of year  never mind 6 a.m.  During this event, the temperature rose 20 degrees from 70 to 90 degrees in Yankton in just 20 minutes.

Other weird stuff happened in the moment.   The humidity crashed to near desert dry levels, and the wind abruptly started gusting to 40 mph. 

Within an hour, it was over. By 7 a.m., the temperature was down to where it was at 5 a.m., the humidity came back up to where it was, and the wind was light again, but this time from the west. 

So what the hell was all that about?

What happened in Yankton is a wild effect from a dying thunderstorm. A mature thunderstorm ingests hot air and rises. As the storm weakens, the hot air stops rising, and the rain from the thunderstorm starts to fall into dry air.  That leads to the thunderstorm collapsing. Initially, the descending air from the collapse is cool as the evaporating water tends to chill the air. 

Once the raindrops are gone, compressional warming quickly heats up the air due to compression. That air reaches the ground as a big burst of wind that has become hot. 

This kind of thing is rare, and needs certain conditions to happen. The air that starts to descend must begin from really high up.  That gives any rain a head start to evaporate on the way down. There must also be a pretty thick layer of dry air so that the rain evaporates quickly well before reaching the ground.  

For this to work, the downdraft must have a high velocity.  Warmer air tends to rise. So the velocity of the sinking air with a heat burst must offset the fact the warming air is becoming less dense that under normal circumstances would want to rise.

That's mostly the reason it got so windy in Yankton when this heat burst was happening. 

Heat bursts almost always happen late at night or in the pre-dawn hours. 

Heat bursts are more common out west than the are in the East, where we are. They're similar to microbursts, which we do have quite often. The difference is in a microburst, a lot of the precipitation does not evaporate. So you end up with a huge gush of damaging wind and rain. 

Even though heat bursts are rare, this is the second time this year South Dakota has experienced one. In the pre-dawn hours of July 31, the temperature at Mobridge, South Dakota went from 78 to 91 in 15 minutes as winds gusted to 50 mph. 

Some heat bursts can contain damaging winds, so Yankton was lucky in that the wind wasn't too terrible. A heat burst in Oklahoma back in 1996 produced wind gusts as high as 105 mph the cause $15 million in damage.