Severe thunderstorms developed across central Iowa during the afternoon of 19 July 2018, producing multiple tornados. Fortunately, 1-minute imagery from GOES-16 was available over the region to aid forecasters during the event.

GOES-16 derived motion winds in the area of tornadic development indicated enhanced low-level and deep layer shear just prior to initiation. With surface winds of 12 knots from 160 degrees and GOES-16 DMWs indicating ~1 km winds of 16-20 knots from around 220 degrees, 0-1 km shear was around 16 knots, which is favorable for supercell tornados (Fig 1). DMWs around 6 km were 40-50 knots from 275 degrees, which yields a 0-6 km shear vector of around 49 knots, favorable for supercell development. Figure 2 combines the mesoscale and CONUS winds to show the abundance of winds vertically and horizontally that can be available from GOES-16. Numbers from the nearby DMX VAD wind profiles are similar.

GOES-16 CAPE showed convection develop on the western edge of a CAPE max. As has been found in previous experiments, although CAPE derived from GOES may often be lower than other sources, the gradients in the moisture/instability fields are captured well.

NUCAPS profiles from Suomi-NPP were available ahead of the line of the line of storms around the time of initiation (Figure 4). Considering central Iowa does not have a balloon launch, temperature and moisture profiles from polar orbiting satellites have exceptional value. The surface conditions of the profiles need to be modified to match nearby obs. After doing so, the profiles near Des Moines indicated around 2,000 j/kg of MLCAPE and up to 4,000 j/kg of SBCAPE, supportive of strong updrafts (Figure 5). Further, the mid/upper levels of the profiles were dry above moist low levels, indicating favorable convective instability.

GOES-16 1-min imagery revealed progressively enhanced cumulus development and clumping northwest of Des Moines between 1800 UTC and 1900 UTC. Initiation took place shortly thereafter, confirmed by the first GLM total lightning detections at 1926 UTC (Figure 6). The first tornado was reported around 1950 UTC, just as GLM FED values began to rapidly increase. A max of 3 flashes/5-min was detected at 1940 UTC, and a max of 23 flashes/5-min was detected at 1950 UTC. At 2000 UTC, a max of 100 flashes/5-min was detected with this storm as it continued to produce a tornado. The updraft weakened briefly thereafter, before strengthening again from 64 flashes/5-min at 2037 UTC to 142 flashes/5-min at 2112 UTC when 1.5″ hail was reported.

Further south, severe thunderstorms produced widespread damaging winds, including a reportedly fatal storm that capsized boats on Table Rock Lake. GOES-16 1-min visible imagery showed rapid updraft growth with this storm through old anvil clouds up to the tropopause, quickly producing overshooting tops and an above anvil cirrus plume, the latter of which is a sign of a particularity strong storm.

Bill Line, NWS

GOES-16 GLM data was utilized to help monitor storm activity near the site of the El Paso County Fair during the week of July 14 – July 21. The fair takes place in Calhan, which is in northeast El Paso County. During the late afternoon of July 17, a cluster of thunderstorms approached the fair grounds from the west. Given the potential for rain, breezy winds, and lightning (confirmed in GLM with CG’s confirmed by NLDN), EL Paso County Sheriff’s office was notified at around 2345 UTC. Figure 1 shows a 1-hour long loop updating every 1-minute, while Figure 2 is the same animation but with the break points discussed in the rest of this post.

By 0000 UTC (the middle of the loop), 0.5 degree radar reflectivity showed the eastern edge of a cell moving into Calhan, with NLDN confirming cloud to ground lightning strikes. However, GLM FED indicated lightning activity still well to the west of town. This discrepancy is due to parallax effect inherent in the GLM data. The GLM gridded data in AWIPS is formatted to the 2 km ABI fixed grid, so is influenced by the same parallax effect. In Colorado from GOES-East, lightning objects will appear slightly to the north and west of where it actually is occurring over the surface. Users will need to get used to the degree of the effect in their CWA, and use radar, NLDN, etc to confirm where that lightning is actually occurring.

By 0029 UTC (the end of the loop), radar and NLDN data would indicate Calhan in a gap of thunderstorm activity. However, GLM shows mainly long flashes continuing to extend over and around Calhan, implying a continued lightning threat away from the updraft cores and over the fair grounds.

The Total Optical Energy Product was not found to provide any additional value over FED and AFA in this case.

Bill Line, NWS