Rare Heat Burst Saturday in Yankton: What is it and how does it happen?
SIOUX CITY, Iowa (KCAU) — This past Saturday, some parts of Siouxland saw isolated showers/storms. However, not everyone experienced the rare phenomenon that those in Yankton, South Dakota did.
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As those showers and storms in the area began to weaken and wind down and move through, the National Weather Service out of Sioux Falls says temperatures in Yankton warmed from 70° to 90° in around 10 minutes. All thanks to what is called a Heat Burst.
But what exactly is that and just how rare is it?
To explain, let's start with a refresher on basic thunderstorm development.
At the start of storm development, warm, moist air is rising and forms those classic cumulus and eventually cumulonimbus clouds
As the storm matures, that warm, moist updraft strengthens and we see a downdraft of cooler air as rain falls from the now mature storm. Eventually we see the cooler downdraft and drier air undercut or overtake the storm which weakens and dissipates it as it loses the warm, moist updraft.
This is when we see precipitation wind down. Now, typically, at this stage, we get a push of cooler air out of the dying storm and maybe some light rain or drizzle along with it.
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Think of it like breathing. The inflow is your inhale, and the outflow is your exhale. When you exhale, that breath spreads in all directions, out and away from your nose/mouth. The storm is inhaling the warm, moist air and exhaling the rain-cooled air at maturity. And when the storm starts to dissipate and die out, it's essentially like the storm is giving a big exhale and all that air is pushed outward from the storm.
With a heat burst though, the air aloft, instead of being moist and producing rain aloft and at the surface, is dry and produces little to no rain aloft and in turn at the surface.
As a result, as the storm dissipates and we start to get the downdraft dominating and flowing out from the very highest part of the decaying storm, any moisture/rain within that downdraft is rapidly evaporated as it descends to the surface.
Now, a few things to remember, air pressure and temperature DECREASE with altitude. So as this outflow of air descends, pressure rapidly increases, as does the temperature. These rapid increases in pressure compress the air in the downdraft as it also warms. And since the air is so dry (meaning it's able to warm much faster than a moist air parcel), it warms rapidly and quite substantially as it is forced down and out of the dissipated storm.
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By the time this downdraft reaches the surface, it is much warmer than the surface temperature, sometimes as much as 15-20° warmer, as it reaches the ground and produces a burst of hot, dry wind.
In Yankton, on the morning of Saturday, September 21st, the temperature at 4:56 AM was 68° with a dewpoint of 57° and relative humidity of 68%. The winds were recorded at 6 mph out of the SE.
One hour later, at the next update at 5:56 AM recorded a temperature of 89.1°, dewpoint of 44.1° and relative humidity 21%. In addition a southerly wind gust of 41 mph and sustained wind of 30 mph was recorded.
By the 6:56 AM update, the temperature was back down to 66.9 degrees with a dewpoint 57.9° and relative humidity back up to 73%. Winds were out of the west at just 3 mph with no gusts reported.
NWS Sioux Falls data shows that between 4:56 and 6:56 AM, the temperature reached a maximum of 91°, which would go on to be the official high temperature for the day Saturday.
