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July-August 2010

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Weather Queries

I have enclosed a firsthand narrative concerning a freak hailstorm that struck our community on Sunday, August 9, 2009, at approximately 10:30 a.m., and some photos taken about 45 minutes after the storm. I was unaware of any NOAA warning. We were declared a disaster area. One thousand four hundred out of 1,900 homes were severely damaged. Eleven people were injured. The local farmers lost all their crops. The roar of the storm was heard six miles away. Our local county anemometer broke after registering a wind speed of 107 mph, all of this apparently with no warning from NOAA. I have just one question. What happened?

Stephen W. Throssel
  Eldora, Iowa

It was a cloudy morning on Sunday, August 9, 2009. Cornelia and I were enjoying our morning coffee. There was an archery shoot with vendors at the Wildlife Club. It looked like rain was approaching, so I left home to purchase a handmade bow from one of the vendors.

While purchasing my bow, several men nearby were talking and looking at the approaching storm on the viewing screen of a Blackberry minicomputer. I overheard the words, “It's purple. That's the worst there is; that means hail.” I finished my transaction and raced home as fast as I could. The clouds in the west looked ominous.

I told Cornelia about my concerns and told her it was a bad storm. By this time, it was as dark as night outside, darker than I can ever remember before. The storm was almost upon us when I lifted the cellar hatch. By now you could hear an approaching roar. I was sure it was a tornado, but my ears always pop when a tornado is close by, and my ears weren't popping.

Cornelia raced upstairs to get our son Christopher. They both came running down in bare feet. I went outside to close the garage door that was stuck open. The first wind gust was a mighty one, and a few hailstones the size of marbles fell. I was about to cross the 20 feet from the garage to the main house when hailstones the size of baseballs began to fall. The wind was howling at 60–70 mph, and I couldn't cross without being hit. I grabbed a life vest and put it on for protection, when the wind suddenly doubled in speed, blowing out the window on the north side of the garage. I couldn't believe what I was seeing. The house was no longer visible. All I could see was green leaves and branches intermixed with huge white hailstones, all moving well over 100 mph. Before I lay down on the garage floor, I looked up into the sky. There was no sky. Instead, I saw a swirling blur of tree limbs, leaves, trash, and debris, all punctuated with huge, white hailstones that slammed into anything and everything. They bounced off the driveway and went higher than I was standing and didn't come back down. The leaves and branches broke off and went up into the swirling maelstrom, not down. The wind got even stronger.

The garage groaned, and I got down on the floor and covered my head, waiting for what I thought would be the tornado behind the hailstorm. The tornado never came. The wind howled and the hailstones fell for a long time, 13 minutes before it quit.

When I got up off the garage floor, I looked out the south-facing door and saw six to eight inches of leaves and broken sticks covering everything. Figure 1 shows the litter in a nearby street, swept away near the curbs by torrential rains. My wife and son emerged unharmed, as did I. We were all in stunned disbelief and shock. Our house was severely damaged, with a large hole in the west slope of our roof. The hailstones punched through two layers of shingles and wooden underlay that, before the storm, was sturdy enough to support a man. Our house was damaged to about half its total value. Several homes, probably closer to a dozen, were completely destroyed. Figure 2 shows damage to the clubhouse at Pine Lake Country Club. As we picked up the debris in the yard and on the driveway, a city snowplow came by to open the street.

Karl Jungbluth, Science and Operations Officer at the National Weather Service (NWS) office in Johnston, Iowa (near Des Moines), answered this question:

The “Eldora” hailstorm was part of a very long-lived and isolated thunderstorm system that tracked all the way from Nebraska into Michigan on August 9, 2009. Across Iowa, there was a single, very intense thunderstorm core that led to the production of extremely large hail in combination with damaging winds in excess of 70 mph. In the case of Eldora, the NWS received a wind report of 102 mph. The storm began breaking windows and destroying crops as far west as Wall Lake, Iowa, around 8:15 a.m., more than 100 miles west of Eldora. The damage continued through Otho, Iowa, in Webster County, where vehicles were totaled by the large hail. As the storm hit Eldora, the hail swath and hail size became even larger, and the winds even more extreme. Figure 3 is a MODIS image of several counties in central Iowa captured by a NASA satellite three days after the storm. The long brown smudge across central Hardin County (and the town of Eldora) marks vegetation shredded and killed by the storm.

Figure 1. Leaf and twig litter on an Eldorado, Iowa, street about 45 minutes after the hailstorm.

Figure 1. Leaf and twig litter on an Eldorado, Iowa, street about 45 minutes after the hailstorm.

Figure 2. Siding shredded and stripped from the windward side of the clubhouse by a severe wind and hail storm at the Pine Hills Country Club, Eldora Iowa. Broken windows had been repaired by the time this photo was taken.

Figure 2. Siding shredded and stripped from the windward side of the clubhouse by a severe wind and hail storm at the Pine Hills Country Club, Eldora Iowa. Broken windows had been repaired by the time this photo was taken.

NOAA's NWS in Des Moines provided long advanced warning ahead of this thunderstorm all the way across Iowa, with warning lead times often exceeding 30 minutes. This type of intense, long-lived thunderstorm produces a clearly defined radar signature that allows forecasters to provide warnings well in advance. A Severe Thunderstorm Watch was also in effect for north central Iowa during the morning hours of August 9, and the Storm Prediction Center had placed counties east of Interstate 35 in a moderate risk for severe weather.

The key in this case was that the hailstorm struck on a Sunday morning, an unusual time of day for life-threatening severe weather to occur. This catastrophic event was preceded by cloudy and foggy weather, so Iowans might not have been checking for warnings or other critical weather information. Warnings were in effect, but TVs and radios might have been switched off, which probably allowed the storm to sneak up on some people. In addition, the intensity of the morning hailstorm shocked all who experienced it.

For Eldora specifically, a Severe Thunderstorm Warning was issued at 10:07 a.m. for the storm “CAPABLE OF PRODUCING DESTRUCTIVE GOLF-BALL SIZE HAIL…AND DAMAGING WINDS IN EXCESS OF 70 MPH.” Figure 4 depicts the area covered by the warning along with a display of radar reflectivity at the time the warning was issued. Amateur radio and law enforcement spotters reported that the storm struck Eldora at 10:38 a.m., and by Mr. Throssel's timing, this warning provided at least 22 minutes of warning lead time for the people of Eldora. In addition, the warning stated “THIS STORM HAS A HISTORY OF PRODUCING DESTRUCTIVE WINDS AND LARGE HAIL AND IS CAPABLE OF PRODUCING A UNIQUE COMBINATION OF WIND AND HAIL DAMAGE TOGETHER. LARGE WIND-BLOWN HAIL…AND SEVERE STRAIGHT LINE WINDS CAN CAUSE EXTENSIVE DAMAGE JUST LIKE A TORNADO.” Recommended preparedness actions were similar to those issued for a tornado, but since no tornado was expected and no tornado occurred, a tornado warning was not issued.

A NOAA All-Hazards Weather Radio would have been very useful on the morning of August 9. The Severe Thunderstorm Warning triggered the Weather Radio's alarm at 10:07 a.m. for Hardin County and Eldora, alerting people with a Weather Radio that this damaging storm was headed their way.

Figure 3. MODIS image of several counties in central Iowa from a NASA satellite three days after a severe hailstorm left a swath of destruction across Hamilton, Hardin, and Grundy counties. The destroyed crops show as an indistinct, brown smudge, stretching west to east across the center of the image and particularly evident in Hardin County. A few scattered clouds are also visible on the image. For scale, Hardin County is about 22 miles wide.

Figure 3. MODIS image of several counties in central Iowa from a NASA satellite three days after a severe hailstorm left a swath of destruction across Hamilton, Hardin, and Grundy counties. The destroyed crops show as an indistinct, brown smudge, stretching west to east across the center of the image and particularly evident in Hardin County. A few scattered clouds are also visible on the image. For scale, Hardin County is about 22 miles wide.

Why was this particular storm so severe? Questions like this are often difficult to answer. Although atmospheric instability and wind patterns were certainly favorable for severe thunderstorms, the storm's ability to isolate itself was a big factor in producing the combination of extreme winds and hail. By being the only show in town, the storm probably adjusted its own inflow environment, adding to the intensity of its updraft and increasing the ingestion of warm, moist, unstable air. This is exhibited by the radar reflectivity at low levels, where the storm displayed a persistent inflow notch, characteristic of strong updraft organization. It also had a huge updraft tower of very high radar reflectivity, plus an extremely well defined weak echo region (WER), both key indicators of thunderstorm severity. The height of the 65–70 dBZ echo, a strong indicator for very large hail, exceeded 30,000 feet as the storm approached Eldora (Figure 5), a full 15,000 feet above the freezing level in the surrounding atmosphere.

Although the storm exhibited mid-level rotation in association with its updraft, one would have expected a stronger rotational signature, e.g., a clearly recognizable supercell thunderstorm. By my experience, it is unusual to have such a severe hail and windstorm without the thunderstorm also being a strong supercell. Mysteries such as these, and the impacts of this hailstorm on the people of Iowa, will keep us working on severe thunderstorm research for years to come.

Figure 4. Depiction of Severe Thunderstorm Warning #172 issued at 1007 AM on 9 August 2009. Courtesy of the Iowa Environmental Mesonet. The red polygon depicts the areal coverage of this storm-based warning, including the town of Eldora. The counties under the severe hailstorm were already in a warning, but their storm based warning polygons are not shown on this map.

Figure 4. Depiction of Severe Thunderstorm Warning #172 issued at 1007 AM on 9 August 2009. Courtesy of the Iowa Environmental Mesonet. The red polygon depicts the areal coverage of this storm-based warning, including the town of Eldora. The counties under the severe hailstorm were already in a warning, but their storm based warning polygons are not shown on this map.

Figure 5. Three-dimensional depiction of NWS Des Moines WSR-88D radar data as the hailstorm struck Eldora, Iowa. Magenta contours show echo intensity greater than 65 dBZ towering to a height over 30,000 feet, indicators of an extremely severe thunderstorm. Image produced using Gibson Ridge Software. Viewing angle is toward the east-northeast.

Figure 5. Three-dimensional depiction of NWS Des Moines WSR-88D radar data as the hailstorm struck Eldora, Iowa. Magenta contours show echo intensity greater than 65 dBZ towering to a height over 30,000 feet, indicators of an extremely severe thunderstorm. Image produced using Gibson Ridge Software. Viewing angle is toward the east-northeast.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.

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