This week is Severe Weather Awareness Week in South Dakota and Minnesota. Across the eastern Plains, thunderstorms can occur anytime of the day. With thunderstorms come the threat of cloud-to-ground lightning which can cause serious injury or even death. Being aware when thunderstorms are most likely to occur can help you prepare to take action to protect yourself and others.
While thunderstorms can occur at any time of day, they are least likely in the early afternoon across the area. This is typically the time when overnight thunderstorms have weakened and moved to the east and afternoon thunderstorms have yet to develop. The frequency of thunderstorms begins to increase after 3 pm as the late afternoon convection develops. Huron reaches its peak freqency around sunset, 9 to 10 pm LT, while Sioux City and Sioux Falls also reach their first peak around the same time. This is typically the time that thunderstorms that develop from the heat of the day have reached their peak coverage and begin to weaken with sunset. These are the storms that are typically associated with tornadoes across the region.
The frequency of thunderstorms decreases through the remainder of the night in Huron. However, in Sioux Falls and Sioux City, the most common period for thunderstorms is from midnight to 5 am LT. This occurs because after sunset, strongly southerly winds can develop above the surface. In the right conditions, thunderstorms will typically develop after 10 pm - and as late as 4 or 5 am. While these storms rarely produce tornadoes, they can bring large hail, damaging winds and flash flooding. In these cases when you may be sleeping, having access to NOAA Weather Radio can alert you to severe weather warnings for your area. If you have a NOAA Weather Radio which can be programmed for alarming only for your county, see our web page at http://www.crh.noaa.gov/fsd/?n=nwrindex.
Below are graphics of the number of times thunderstorms with heavy rainfall were recorded at each hour for Huron, Sioux Falls, and Sioux City between 1970 and 2008. To qualify as an event, over 0.5 in. of rain had to occur within 2 hours at the observations site.