SIOUX CITY, Iowa (KCAU) – Siouxland was placed under an enhanced and slight risk for severe weather for Father’s Day but as the day progressed severe chances gradually decreased.
Throughout the week leading up to Sunday, June 21, weather patterns were showing signs for a good severe weather day in Siouxland, so good that the Storm Prediction Center (SPC) issued an enhanced risk and slight risk over Siouxland.
During the morning of June 21, Siouxland saw a few severe thunderstorms move into the area between 6 and 8 a.m., with the main threats being high wind gusts and large hail up to the size of a quarter. After those storms cleared the area, things seemed to have calmed down a bit until the early afternoon hours.
Between 2 p.m. and 3 p.m. a storm moved into western Siouxland with severe potential. The National Weather Service (NWS) then issued a new severe thunderstorm warning for parts of Cedar County in Nebraska. The storm was producing wind gusts around 60 mph and hail up to an inch in diameter. That particular storm continued to be severe as it traveled northeast around 50 mph, eventually crossing into South Dakota (Union and Clay Counties) and then into northern Iowa starting in Plymouth, Sioux, and Lyon Counties. The NWS continued to mark that storm as severe into the late afternoon and early evening hours as it moved out of Siouxland. This storm also prompted a severe thunderstorm watch for Siouxland until 10 p.m. June 21.
The weather from that point forward continued to show signs of less favorable severe weather in Siouxland after the first severe storm cleared the area. This meant that severe weather potential was gradually decreasing as the day went on.
Then around 8 p.m. the Storm Prediction Center (SPC) down graded the enhanced risk in Siouxland, and placed Siouxland in a slight risk.
This meant that the chance for severe weather was not as prominent and things started looking much better as far as severe weather, and better meaning less likely to happen throughout the rest of the evening.
There were severe storms expected to pop up in Siouxland during the late evening hours as there was a boundary (weak cold front) stretching across several states, including portions of central Nebraska, southeast North Dakota, and northern Iowa; however, as the evening went on, the severe potential was actually decreasing.
So, why did we not see as much severe weather as originally thought?
There’s a lot that has to be in place for severe weather, such as shear (winds that turn clockwise as you go from the surface into the atmosphere), lift (typically a boundary like a warm or cold front, or even mountains for orographic lift), instability (an air pocket that is warmer than its surrounding environemnt), and moisture in the air (basically humidity/water particles). The easiest way to remember this the acronym SLIM (shear, lift, instability, moisture).
Our atmosphere during the morning and early afternoon on Sunday had favorable conditions for severe weather development, which is why we had a few early morning severe storms and an afternoon severe storm move through Siouxland.
So, what changed between the morning hours and late afternoon hours?
As the day progressed, Siouxland lost a lot of severe weather potential due to severe storms located towards our south in the Kansas area. The storms in that area produced a large amount of cloud coverage that actually stretched north/northeast into the Siouxland area.
Cloud coverage prevents the sun from heating up the surface, which is another important ingredient for storm production. The heat from the sun can make the air unstable, which gives you your lift.
Another reason we didn’t see a lot of severe weather in the area was because of Sunday morning’s (between 6:30 a.m. and 8 a.m.) and afternoon’s (between 2:30 p.m. and 5:30 p.m.) severe storms. These severe storms “sucked up” (used) all the ingredients needed to support severe potential.
The early storms (or convection) don’t always cause problems for later storms/convection, but on Sunday they did because:
- We didn’t have enough heating in our area to support another round of severe potential (sunshine).
- The early morning and afternoon storms “sucked up” (used) all the heat we had, as well as the moisture in the air. We didn’t have enough time to produce more moisture (again, due to the limited amount of sunshine).
- Winds at the surface (ground) were very weak until a storm moved through the area, and winds up higher in the atmosphere were also very weak. (Typically, for severe weather it’s best to have winds increasing with height, meaning, faster speeds as well as turning clockwise as you go higher into the atmosphere. This is important because this supports updraft formation, which is billowing cumulonimbus/thunderheads clouds.)
Early storms don’t always cancel out the chance for more severe thunderstorm production later in the day, but on Sunday, the atmosphere just couldn’t bounce back from the early morning and afternoon thunderstorms.