Authors: Christopher Redmond, Mary Knapp

On Monday, May 14, a thunderstorm developed outside of Denver metro and moved several hundred miles east/southeast through the day into western Kansas - dropping severe hail nearly the entire way (Figure 1). Hail that is 1 inch in diameter or greater is considered severe and will do damage to roofs, crops, vehicles, and even people. Hail also fell across central/east Kansas on this same day. However, this event was much more sporadic.

Figure 1. Storm Prediction Center (spc.noaa.gov) severe storm reports from Monday, May 14, 2018. Hail is marked by a green “H”, wind - blue “W”, and tornado - red “T”.

Unfortunately for the narrow swath in western Kansas, this severe storm consisted of hailstones near the size of baseballs (2.75 inches). In combination with this large hail, strong winds of 40+ mph occurred with some gusts reaching up to 50 mph (measured at Lane Mesonet station, mesonet.ksu.edu). These winds increase the force of the hailstones impact resulting in increased damages - as was seen in wheat fields in the path of the storm (Figure 2). For wheat, this is poor timing due to the lack of time to recover before a typical June harvest. The hail damage is compounded by the pre-existing drought and freeze impacts many areas have already seen this year.

Figure 2. Wheat damage in Lane County observed by Kansas Wheat (@kansaswheat) on Tuesday, May 15, 2018.

The swath of hail across western Kansas was also visible on satellite images. Infrared satellite band detects the temperature of the earth. With copious amounts of hail residing on the ground behind the storm and clear skies, the cooler temperatures associated with the hail was easily observed from space (Figure 3).

Figure 3. GOES East satellite imagery via National Weather Service in Dodge City (@NWSDodgeCity). Green represents the clouds associated with the persistent storm moving eastward. Arrows point out the swath of cooler temperatures left behind from the hail.

Meteorology behind a hailstorm

It is very rare for a storm to travel this far, especially of this magnitude, for the majority of its life. Such storms are called supercells and are known for a long-lived, persistently rotating updraft - typically responsible for the most severe weather: large hail, damaging winds, and tornadoes. That strong rotating updraft is ideal to suspend hailstones, allowing them to accumulate more layers, becoming larger, until they are either thrown or fall out of the updraft.

Thunderstorms need four main ingredients in order to develop and survive: moisture, instability, lift, and shear. On Monday, shear was the biggest player with Effective Bulk Shear 50+ knots (kt) during the afternoon/evening (Figure 4). An environment 25-40 kt (or more) is needed for supercell development, which was easily achieved during this event.

Figure 4. Effective Bulk Shear as observed with the Storm Prediction Center (spc.noaa.gov) mesoanalysis data.

Lastly, instability in the corridor of the storm’s track ranged from 1,000 - 2,000 joules per kilogram (J/kg). This is measured as the Convective Available Potential Energy (CAPE) as seen in Figure 5 below. While this isn’t incredibly high, it was focused in an area of very cold temperatures aloft. By pushing hailstones into colder air, the super-cooled cloud droplets can accumulate more effectively, thus creating larger stones. On this day, temperatures around 18,000 feet ranged between -12 to -14 degrees F, while at 10,000 feet temperatures ranged from 8-10 degrees F. Therefore, with a strong updraft, upper level temperatures well below freezing were reached, optimizing hail growth.

Figure 5. Convective Available Potential Energy (red lines) of the layers optimized by the thunderstorm (Mixed Layer) via the Storm Prediction Center (spc.noaa.gov).

While these are only a few factors, they played a significant role in developing and sustaining this hailstorm. The storm, like most severe weather in the Plains, was very isolated and didn’t have substantial spatial coverage outside the narrow corridor. Most significant impacts were quite localized, especially from north to south with sharp cut-offs.

From the Kansas Mesonet

Lastly, there was an interesting observation from weather stations across the region that were impacted either directly or indirectly. All measured a significant rapid drop in temperature associated with either hail, cloud cover, or cold outflow winds (Figure 6).

Figure 6. Kansas Mesonet stations observed a substantial drop in temperatures (shaded area) from west to east with storm’s passage at Wallace, Leoti, and Lane stations (mesonet.ksu.edu).

Another interesting perspective is melting hail. Hail accumulates in the rain gauge orifice and must melt before being measured. Temperatures were above-freezing, however, melting is a much longer process due to the dense nature of hailstones. Therefore, light moisture was measured for up to 20 minutes after the event despite rainfall having already ended (Figure 7).

Figure 7. Orange-shaded time period represents actual duration of the storm (15 minutes) while the blue-shaded period is the additional 20 minutes of melting hail. Data from the Lane Mesonet station (mesonet.ksu.edu).

Christopher Redmond, Kansas Mesonet

Mary Knapp, Weather Data Library