June 24-25 Raccoon River Flooding Review

Blog post by Kenny Podrazik – NWS Des Moines
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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.

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 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.

June 2015 crests along central Iowa Rivers.

Figure 4: June 2015 crests along central Iowa Rivers.

Figure 3: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple) on June 26, 2015.

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.

A Recap of 2015 Central Iowa Tornadoes

By Kevin Skow, Meteorologist Intern

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.Radar

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.

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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

EF3MonroeCountyThe 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.AdamsCountyTornado

Additional information on 2015 tornadoes for the state of Iowa can be found here!

A Blizzard that People Remember

The Blizzard of December 8-9, 2009

It’s hard to believe that it has been 5 years since a large blizzard blasted Iowa on December 8-9, 2009. NWS Des Moines Meteorologists still reminisce and discuss this high-impact event as it produced some memories they won’t soon forget. For instance, it took a few forecasters at least 3 times longer to make it to work to cover their shift on that snowy Tuesday into Wednesday and a couple even stayed the night in the office on cots to wait out the storm! A recently hired intern had the task of measuring snowfall in the blizzard and when she went out at midnight Tuesday night on the 8th, she learned the hard way to always bring her security card with her in order to re-enter into the building as she locked herself out. Other forecasters remember being snowed-in and digging out 6 foot drifts from their driveways on Wednesday.

The storm began early Tuesday morning on December 8th when light to moderate snowfall extended across central to southern Iowa. The snow slowly spread over the rest of the state and intensified during the afternoon and evening hours and significantly affected the Des Moines rush hour Tuesday night. Strong northwest winds increased to 25 to 35 mph and gusted to over 50 mph at times by late Tuesday night and continued into early Wednesday morning. There were even a couple of reports of wind gusts to 60 mph overnight Tuesday into Wednesday. The strong winds and heavy snow produced large drifts throughout the state, with several reports over 6 feet! Visibility became reduced significantly as widespread whiteout conditions were observed from the overnight hours Tuesday into Wednesday before improving Wednesday afternoon. Travel became impossible or nearly impossible across much of Iowa due to the large drifts and widespread white-out conditions (See Figure 1).

The powerful storm dropped anywhere from 8 to 17 inches over much of Iowa with the highest swath oriented southwest to northeast across the state (See Figure 2). In fact, the statewide average storm total snowfall was 10.2 inches became the highest storm total since 1971 and the third-highest on record in Iowa. Some of the higher totals around the state included 15.5 inches in Des Moines, 16.0 inches in Atlantic and Belle Plaine, 16.2 inches in Corning and Osceola, and a whopping 17.0 inches in Knoxville. Des Moines recorded its this fourth-heaviest snow storm on record behind the December blizzard of 1888, the New Year’s storm of 1942, and the blizzard of March 2004.

The severity of the storm affected the surrounding states of Iowa as well. Blizzard warnings covered portions of eastern Nebraska, southern Minnesota, northern Missouri, the southern half of Wisconsin, and far northeast Kansas. All 99 counties in Iowa were under a blizzard warning Tuesday night into Wednesday (See Figures 3 and 4). A temperature gradient across Iowa late Tuesday night was about 25 degree difference from southeast to northwest Iowa (See Figure 5). By Wednesday afternoon, temperatures plummeted into the single digits to around 10 above, leading to wind chill values as low as -31F at Mason City on the 9th. The snow finally tapered off by Wednesday afternoon but the howling winds continued into Wednesday night. High pressure built into the region and brought clear skies on Thursday allowing the large swath of snow to be seen on visible satellite (See Figure 6). If residents of Iowa remember, this big blizzard was just the beginning of a very active winter with frequent snowfalls and persistent deep snow pack across the state.

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Figure 1: Road conditions Wednesday morning December 9, 2009. Image courtesy of Iowa State Patrol, IDOT, and IEM.

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Figure 2: Total snowfall amounts across Iowa from the December 8-9, 2009 Blizzard. Snow totals ending at 7 pm on December 9, 2009.

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Figure 3: The blizzard had a huge impact across much of the central United States. Image is courtesy of MRCC.

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Figure 4: All 99 counties in Iowa had a blizzard warning in effect at 5pm on December 8, 2009.

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Figure 5: Temperature difference across Iowa at 2 am Wednesday morning December 9, 2014. Temperatures in the lower 30s were found in southeast Iowa while the northwest part of the state dropped to 5 to 10 above. Image via IEM.

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Figure 6: Visible satellite image from Thursday December 10, 2009 shows nothing but snow across much of the Midwest and Plains except for a few clouds over eastern Iowa.

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Snow removing equipment in Mahaska County. Photo from Mahaska County Emergency Management.

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Sundog on December 9, 2009.

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Snowy office.

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North side of Des Moines. Photo from Matt Kelley.

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Grimes, IA on December 9, 2009.

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Large drift in front of the NWS Des Moines office.

Blog post by Kenny Podrazik – NWS Des Moines

August 23, 2014 – Iowa Heavy Rain Event Summary

Here’s a look at radar estimated precipitation products and rainfall/flash flood reports from heavy rain event across portions of central Iowa on August 23, 2014.

Q3 radar estimated rainfall from midnight to 6 pm CDT on August 23, 2014. Image courtesy Iowa Environmental Mesonet.

Q3 radar estimated rainfall from midnight to 6 pm CDT on August 23, 2014. Image courtesy Iowa Environmental Mesonet.

Q3 radar estimated precipitation.

MRMS (Multi-Radar Multi-Sensor System) Q3 24-hour estimated precipitation across central to southern Iowa. Time ending at 3 PM CDT (20z 8/23/2014). This image is a different color curve and a zoomed in from the image above. You can find more detailed information from the MRMS website on how Q3 works: http://nmq.ou.edu/

Dual pol storm total accumulation from August 23, 2014.

Dual-pol storm total accumulation from August 23, 2014. This dual-pol product provides an estimate of how much rain has fallen since the beginning of a precipitation event. Follow this link here for more details on dual-pol products: http://www.wdtb.noaa.gov/courses/dualpol/
http://www.srh.noaa.gov/jan/?n=dualpolupgrade-products#dsa

Google Map of Storm Reports from 8/23/2014

Google Map (Alternate Version) of Storm Reports from 8/23/2014

Text Summary of Storm Reports

 Blog post by Kenny Podrazik