Breaking Down Our Severe Weather Season

With Winter Finally Over, Severe Weather Is Ready to Begin

Via Chad Vis, Brookings 2017

Winter has officially ended for the Sioux Empire and Spring has begun (hopefully there’s no more snow but… we can’t rule it out quite yet)! Now our precipitation makes the transition from a frozen state to liquid as warmer temperatures make their way back into the area. Along with warmer temperatures moving northwards so will moisture along with instability, three of the four key ingredients needed for severe weather. The last ingredient being a source of lift. But when does the severe weather season typically start for the KDLT forecast area, when do we generally see our peak for tornadoes and when does the severe weather season typically end? This blog will break it down for you!

With Spring officially beginning on March 20th, the jet stream is beginning to make its way back north for the Summer. Typically located to our south during the Winter months, it cuts moisture as well as energy off from the Sioux Empire. Keeping most it over the southern states (unless there is a particularly strong low pressure system moving through the United States), these states will sometimes see severe weather during the months of November through February/March. In fact, the latest severe weather outbreak occurred in portions of Alabama, Mississippi, Tennessee and Georgia on March 19th. The two images below show the comparison of CAPE (energy) between the National Weather Service in Aberdeen (left) and the National Weather Service in Jacksonville, MS (right). You can see that the amount of energy returns to the Sioux Empire in late March while it’s always overhead even though the values decrease in the Winter months. Image courtesy of the National Weather Service.

Throughout the next several weeks, during April, the jet stream will continue to settle northwards. As it does, the energy and moisture that was isolated to the South is now free to move northwards too. There may be an occasional severe thunderstorm or tornado warned thunderstorm however, the areas severe weather season doesn’t actually start until May. After collecting a looking at data over 26 years, the southeast usually sees it’s severe weather season begin on May 3rd. For those living farther north and west, into central South Dakota, the severe weather season starts a few days later on May 5th. Now that the severe weather season has started, let’s break down when the area typically sees hail, wind as well as tornado events.

Due to the mechanics needed for hail to form, the southeast typically averages 3 monthly hail events in March with northern and central South Dakota counties skipping out on hail events during the month… again based on collected data. Recall that hail forms in a thunderstorm with a strong updraft. Strong should be noted because all thunderstorms have updrafts but they need to be strong enough to suspend water droplets above the freezing level in the atmosphere. Once the droplets have been suspended for long enough, they freeze and become larger in size as more water freezes onto the developing stone. Once it becomes too heavy, the stone falls to the surface.
As you can imagine, starting in the spring time through the fall, the freezing layer is much higher in the atmosphere due to warmer temperatures. That means the thunderstorm will have to work that much harder to keep the droplets suspended for longer. And even though they will have to work harder during the summer months, the KDLT forecast typically sees the most hail events during June during the late afternoon/early evening hours.

Via National Weather Service Burlington

Next, we’ll break down the tornado climatology across the KDLT forecast area. While there may be hail events occurring earlier in the Spring season across the forecast area, the tornado season typically starts later here in the Sioux Empire. Whether they are part of a larger outbreak or a single cell tornado, a tornado warned cell needs 4 ingredients (and even then, they may not form): moisture, lift, instability and shear. These ingredients typically are overhead starting in April for those across the southeast while we across the north/west won’t see our first tornado until May.
Based on the 26 years of data collected by both National Weather Service offices in Aberdeen and Sioux Falls, the KDLT forecast area typically doesn’t see violent tornadoes. Defined by the National Weather Service, a violent tornado is one that is rated as an EF4 or higher and only make up around 2% of all tornadoes. Across the southeast, the probability of a violent tornado forming is 0.019%! Across the north, it’s even less at 0.009%! Since the KDLT forecast area doesn’t typically see violent tornadoes, what does the data say about how many non-violent tornadoes we see? Again, non-violent tornadoes will be any tornado rated an EF0 to EF3.

Table of Significant Tornadoes from 1980-2006 for the southeast (left) and north (right) via
National Weather Service offices in Sioux Falls and Aberdeen

All Severe Weather from 1980-2006 image via National Weather Service Offices
in Aberdeen and Sioux Falls

Across the southeast, which includes parts of northwestern Iowa, southeastern South Dakota and southwestern Minnesota, June is typically the month with the most amount of tornadoes reported with an average of about 19 tornadoes. While rare, but not unheard of, tornadoes typically do not occur at night; between the hours of 11pm/midnight through the late morning/early afternoon, the southeast has seen between 0 and 7 tornadoes over the 26 years of data collected. The likelihood of a tornado occurring in the late afternoon/early evening hours is much higher; between 100 and 200 tornadoes have occurred across the southeast between the hours of 4pm and 8pm.
Across our northern and western counties within the KDLT forecast area, it’s a similar story. June tends to have the most tornadoes with an average of about 12 tornadoes for the month. And, like the southeast, July averages about 8 tornado events. The table below shows the monthly averages of hail, wind and tornado events for the southeast.

Wind events, like straight line winds as well as derechos (a long lived, straight line wind event that moves quickly across an area), start to pick up as we go into the Summer months. From June to August, the KDLT forecast area sees the number of wind events increase from twenty or so to nearly sixty events with July being the month which averages the most wind events. Similar to tornadoes, wind events typically start in the late afternoon; however, unlike tornadoes, the overnight hours is when the Sioux Empire typically see their peak in wind events. On average, between 9-10pm is when, over the course of 26 years of collecting data, we’ve seen our most wind events.

So what does this information mean to you? Well, knowing when we typically see severe weather can help you, your family or your friends, plan ahead if severe weather is in the forecast. You and your family can develop a plan in case there is severe weather to stay safe/limit property damage. However, keep in mind, severe weather can happen at any time! You can stay up-to-date by downloading the KDLT Weather app for Android as well as Apple users, following KDLT Weather, as well as the individual meteorologists, on Facebook and Twitter and purchasing a NOAA Weather Radio.

Though there was snow falling on the 23rd of March, Spring has begun and so has severe weather season. We here at KDLT and the KDLT Weather Center want to make sure you are staying safe this season! We’ll do all that we can to make sure that happens.

Blaise Keller
KDLT News Morning Meteorologist
Follow Me on Twitter – @blaisekellerr

Categories: News, Weather, Weather Blog