Official Blog of the NWS Des Moines Serving Central Iowa
Figure 1
Figure 2
Two periods of storms affected central Iowa during a 24-hour period from June 24 to June 25, 2015. The first round of storms occurred during the morning hours on June 24 that affected portions of Iowa along and south of Highway 20. This was the “primer” for significant flash flooding and river flooding that would result from the second round of storms to affect the same area later that evening. The second round of thunderstorms began to develop over west-central Iowa late in the afternoon and quickly became severe by the early evening. The first Severe Thunderstorm Warning (Figure 1) issued by the National Weather Service in Des Moines was at 6:20 p.m. CDT on June 24 for Guthrie and Dallas Counties. The final Severe Thunderstorm Warning (Figure 2) expired at 1:50 a.m. CDT on June 25 for Marion and Jasper Counties. There were multiple reports of large hail that ranged from the size of a quarter (one inch) to hen egg (two inches) throughout the night. Winds to 60 mph caused numerous trees to become damaged and there was even a report of an 80 mph wind gust in Guthrie County early in the evening. Extremely heavy rain brought widespread flash flooding from west-central Iowa to southeast Iowa, including the Des Moines Metro, late in the evening into the overnight hours Thursday morning. The video below shows the entire second round of storms unfold on radar from the first development of a thunderstorm around 6 p.m. CDT on June 24 to the final drop ending around 2 a.m. CDT on June 25.
Widespread rainfall amounts of three to seven inches were reported after the event, with the highest report of 7.25” near Dawson, Iowa in northwest Dallas County. From Bagley to Jamaica, Iowa, radar estimated near nine inches of rain fell within this area of the upper reaches of the Raccoon River Basin. The majority of the heaviest rain fell within the Raccoon River Basin (Figure 3). As a result, major to near record flooding occurred along the Raccoon and Des Moines River Basins, including several tributaries (Figure 4). In fact, the Walnut Creek recorded its highest crest ever at two river gauge locations. The Clive I-80/35 (CLVI4) and Des Moines 63rd Street (DOSI4) gauges crested at 13.41 feet and 18.82 feet respectively, both breaking the previous records set back on August 9, 2010. Near record flooding occurred on portions of the Raccoon River, with Van Meter (VNMI4), Des Moines Highway 28 (DMWI4) (Figure 5), and Des Moine Fleur Drive (DEMI4) all cresting at their third highest crest in history. All the flooding on the Des Moines and Raccoon subsided by June 29, 2015.
Figure 3: Storm total radar estimated precipitation ending 7 am June 25, 2015. The yellow oval received 6-9 inches of rainfall in the headwaters of the Raccoon River Basin.
Figure 4: June 2015 crests along central Iowa Rivers.
Figure 5: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple horizontal line) on June 26, 2015.
Temperatures:
Temperatures across Iowa were generally favorable this summer with limited bouts of heat and, in most instances, readings at or below normal. Departures were generally one to two degrees below normal with the largest departures along the Highway 30 corridor. The warmest reading in Iowa during the summer was 99 degrees in Sioux City on June 9. This year was the second year in a row that no summertime 100 degree readings were recorded in the state. Otherwise, temperatures were at or below normal for long stretches, especially in early July and again in mid to late August.
Summer temperature departure from across Iowa – generally slightly cooler than normal.
Precipitation:
Moisture was more than sufficient for much of the state during the summer. The exception was across northeast Iowa where deficiencies were common for much of the growing season. Abnormally dry conditions did creep into the northeast by the second half of the summer but the area still received enough rain to keep widespread drought conditions from developing. Farther south and west, thunderstorms were more common with localized heavy rain from June into August. A couple of events produced flash flooding in and around the Des Moines metro area as well as far south Iowa near the Missouri border. Rainfall frequency did decrease into August, but remained common enough to support crop maturation.
Summer rainfall departures – Drier in the far northeast, quite wet south half.
Overall, the 2015 Iowa tornado season has been relatively quiet up through the end of August. A preliminary total of 30 tornadoes have been recorded for the year throughout the state, which is below the average of 46 tornadoes typically seen in a given year. Iowa’s tornado season historically peaks in the months of May and June, though late season outbreaks can occur well into November. With the bulk of the season behind us, here is a recap of some notable tornadoes to strike central Iowa this year.
May 10: Carroll and Calhoun Counties
The first significant tornado of the season to hit central Iowa occurred on the late afternoon of May 10. A small and compact supercell overrode a warm front and, aided by high low-level instability and shear, produced when has been up to this point the longest-tracked tornado of the season for Iowa. This very visible EF1 tornado touched-down at 7:10pm northwest of Lidderdale in northern Carroll County and tracked to the NNE. The broad but thankfully weak tornado made a direct hit on the town of Lake City at 7:32pm, damaging the roofs of a number of homes and businesses, including the school. The tornado continued churning northeastward across rural Calhoun County, hitting several farmsteads along its way, but damage was relatively light. The dying tornado passed just to the west of Rockwell City shortly after 7:50pm and finally dissipated northwest of town at 8:00pm. It travelled for 23.5 miles during the 50 minutes it was on the ground.
The tornado quickly lofted dust and other debris high enough to be sampled by the Doppler radar in Des Moines, 70 miles away and at a radar beam height of 5,000 ft. This shows up as the yellow and green shaded region on the correlation coefficient radar image at right, also provided with a velocity image (left) to show the location of the tornado. This tornadic debris signature reached up to 16,000 ft into the storm! Videos shot of the tornado, as well as satellite imagery of the track in the days afterwards, showed that this tornado was multi-vortex in nature, with the worst damage concentrated in small streaks.
Mid-May to Mid-June
The middle part of May to the middle of June, typically the peak of Iowa’s tornado season, was only characterized by a few weak, but somewhat rare, early morning tornado episodes. The first event early on the morning of May 17 was also responsible for producing a downburst that derailed an 80-car train near Osceola. The five short-lived tornadoes that occurred afterwards were weak but did damage a few farmsteads in Madison and Dallas counties.
Another early morning tornado event took place between 1 and 2 am on June 7 across Webster, Boone, and Story counties. Four very brief tornadoes touched down along the leading edge of a squall line, one of which moved through far southern Ames and caused tree (photo on right) and light building damage. A squall line was also responsible for strong EF1 tornado on the afternoon of June 20 near Eddyville.
June 22: Marion, Lucas, and Monroe Counties
The streak of weak tornadoes came to an end on June 22. A powerful supercell spawned a tornado over far southern Marion County shortly after 5:00pm, which then clipped northeastern Lucas County before strengthening and tracking into northwest Monroe County. While it thankfully stayed out over rural areas of the county during its half hour long life, this now 500 to 600 yard wide EF3 tornado did make a direct hit on one farmstead, completely destroying the house and several outbuildings (photo). Thousands of trees were destroyed by this rain-wrapped tornado as it traversed the hilly and wooded regions of Monroe County on its 11 mile track. The tornado lifted eight miles northwest of Albia, but a second EF2 tornado developed just to the west of town and hopscotched through the southwest part of the city, heavily damaging several businesses and homes.
August 2: Adair and Adams Counties
Following a quiet July, a cold front swept through Iowa on the afternoon of August 2. Thunderstorms erupted along the leading edge of the cold front, aided by over 6000 J/Kg of surface based CAPE. Despite the lack of low level or deep wind shear, the cold front generated enough localized spin along its forward flank to produce a very picturesque tornado just after 6:20pm when this broad circulation was stretched by a developing thunderstorm updraft in southern Adair County. With little in the way of steering winds near the surface, the tornado first drifted southwestward for about a mile, then interacted with another storm outflow boundary and turned southeast and strengthened. Widening to 300 yards, the tornado struck the hamlet of Williamson and inflicted EF1 damage to several buildings. Scouring its way southward, the tornado paused for several minutes two miles south of Williamson before turning to the northeast and dissipating. See the satellite image below that shows this unusual path of this tornado. In the 36 minutes the tornado was on the ground, it traveled only 7.5 miles.