July 2014 Central Iowa Monthly Climate Summary


Figure 1: Average Temperature for the month of July 2014 was 69.0°F for the entire state, while the DMX CWA was 68.8°F

Figure 1: Average Temperature for the month of July 2014 was 69.0°F for the entire state, while the DMX CWA was 68.8°F. The DMX CWA is outlined in white.

For the state of Iowa, the average temperature was 69.0°F which was 4.6°F below normal for the month of July (See Figure 1).  July 2014 is now the 5th coolest July on record for the state among the 142 years of records.* The average temperature for the Des Moines County Warning Area (CWA) was 68.8°F with the coldest spots located over the west-central and far southeast portions of the CWA (See Figure 2).

Figure 2: Average Temperature Departure from Mean for the month of July 2014 was 4°F to 5°F below normal for the month.

Figure 2: Average Temperature Departure from Mean for the month of July 2014 was 4°F to 5°F below normal for the month. The DMX CWA is outlined in white.

There were a few individual days that rose above normal, but no extended heat wave occurred during the month.  The hottest days of the month were on the 12th, 21st, 22nd, and 25th when several locations rose into the lower 90s (See Figure 3).  The hottest temperature of the month, within the Des Moines CWA, was 95°F at Osceola on the 22nd. The highest temperature throughout the entire state was 102°F on the 25th at Sidney in far southwest Iowa. Des Moines reached at least 90°F only 3 times during the month and normally averages 10 days with 90°F or greater during the month of July.  In similar fashion, Waterloo only had 1 day that topped 90°F but normally averages around 8 days with maximum temperatures reaching 90°F or higher in July.  Mason City continued its streak of not reaching 90°F in July as the highest temperature at the station was 89°F.  By the end of the month, this streak extended to 325 days since the last time the station reached 90°F on September 9, 2013.

Figure 3: Average Maximum Temperatures for July 21-22, 2014. These were a couple of the hottest days of the month across central Iowa.

Figure 3: Average Maximum Temperatures for July 21-22, 2014. These were a couple of the hottest days of the month across central Iowa.

On the flip side, there were many more colder than normal days in July.  Table 1 illustrates how often central Iowa was below normal during the month. Ottumwa set a couple of new daily temperature records. The first was a daily low maximum temperature of 65°F on the 2nd which broke the previous record of 67°F set back in 1944. The second record was a record low of 49°F set on the 16th breaking the previous record of 51°F in 1930.  Lamoni also set a new record low temperature of 52°F on the 16th, breaking the previous record of 55°F set in 1961. Another record low maximum of 66°F was set at Waterloo on the 2nd breaking the old record of 67°F from 1941.

Table 1: Automated Surface Observation Stations (ASOS) across central Iowa were well below normal for a good portion of the month of July 2014.



Figure 4: the accumulated precipitation percent of mean for the month of July 2014. Much of the state was well below normal, except for the southeast.

Figure 4: the accumulated precipitation percent of mean for the month of July 2014. Much of the state was well below normal, except for the southeast. DMX CWA outlined in white.

Little precipitation fell during the month of July 2014 across central Iowa and there was only one major severe weather event and that occurred on July 6. Other than southeast Iowa, the remainder of the state was well below normal for precipitation (See Figure 4). The majority of the precipitation only fell on a handful of days which includes the 5th, 6th, 12th and the 25th. The Des Moines International Airport recorded 1.99” on the 5th, while several other locations recorded at least an inch of rain on the same day (See Figure 5).  The highest precipitation total during the month, for the state and DMX CWA, was in Montezuma in Poweshiek County with a monthly total of 10.21 inches. The lowest monthly total for the state also occurred in the DMX CWA and that was 0.71 inches at Atlantic in Cass County.

A total of 9 tornadoes occurred on July 6, 2014, 8 dropped down in the DMX CWA. There were two tornadoes rated EF1 and they occurred near the towns of Reinbeck and Dinsdale. The rest were EF0 tornadoes and you can find more information about the event as well as all the tornadoes from 2014 our Tornado Page.

Figure 5: Q3 radar estimated precipitation for July 5, 2014.

Figure 5: Q3 radar estimated precipitation for July 5, 2014. http://nmq.ou.edu/

Reservoir Information

Figure 6: Saylorville Lake Reservoir pool height trend graph during the month of July 2014.

Figure 6: Saylorville Lake Reservoir pool height trend graph during the month of July 2014.

The Saylorville Reservoir pool height rose a about 7 feet during the first week of July to 877.57 feet on the 6th before gradually falling throughout the rest of the month to 851.50 feet on the 31st (See Figure 6). The maximum pool height corresponded to a maximum storage of 446,670 Acre-feet on the 6th while the lowest storage of the month was 171,070 Acre-feet on the 31st. The Des Moines River downstream of Saylorville Lake maximum stage height was 14.29 feet on the 7th and the lowest was 8.45 feet on the 31st. The maximum outflow at the same location was 19,500 CFS on the 2nd to a minimum of 5,600 CFS on the 31st.

Figure 7: The Lake Red Rock Reservoir pool height trend graph during the month of July 2014.

Figure 7: The Lake Red Rock Reservoir pool height trend graph during the month of July 2014.

Lake Red Rock Reservoir pool height rose from 755 feet on the 1st to 766 feet during the middle of the month before dropping back to around 757 feet by the 31st (See Figure 7).  The pool storage nearly reached 1,000,000 Acre-feet when it maxed out at 910,820 Acre-feet on the 11th.  The lowest storage was 483,840 Acre-feet on the 1st. The stage height of the Des Moines River downstream of Lake Red Rock Reservoir ranged from 90.37 feet on the 2nd to 93.69 feet on the 13th.  It remained above 93 feet from the 1th to the 28th and it was only below 91 feet the first two days of the month.  The outflow from Lake Red Rock increased from 13000 CFS on the 1st to around 22,000 CFS by the 12th where it remained steady until the 29th when it dipped down to 18,400 CFS.

*Statewide averages courtesy of State Climatologist Harry Hillaker: http://www.iowaagriculture.gov/climatology/weatherSummaries/2014/pms201407.pdf

 Blog post by Kenny Podrazik – NWS  Des Moines

New Radar Tools for the Des Moines WSR-88D Radar

It has been an exciting year in the National Weather Service with some new advances in radar operations technology. Some additional tools are now in use by the National Weather Service staff at the Des Moines Weather Office. A recent upgrade to the WSR-88D Doppler Radar included Dual Polarization and now AVSET and SAILS have been added to the list of available tools to the analysis toolkit of the storm interrogation meteorologist.


Figure 1: Radar Volume Coverage Pattern Configuration for VCP 12.

AVSET is a short-hand for Automated Volume Scan Evaluation and Termination. This feature can be turned on or off during the normal operation of radar and is particularly useful for lessening the time of one complete radar volume scan. First, let’s back up a minute and review some terms!  One complete volume scan is the pattern of vertical scanning the radar makes from near the ground to the top of its elevating cycle. The example below is for Volume Coverage Pattern 12 (VCP 12):


Figure 2: Radar Volume Coverage Pattern Configuration in VCP 212 with AVSET running.

In this example the radar begins to scan at 0.5 degree above the horizon and continues scanning through elevating angles up to 19.5 degrees to complete one volume scan. This process takes about 4 minutes and 30 seconds to complete. So, in our standard operations of the WSR-88D radar system, a storm interrogation meteorologist would expect to see new data arriving every 4 minutes, 30 seconds, regardless of how far away or close a storm is located to the radar. With AVSET, the data arrives faster!  AVSET can be best explained by looking at the diagram in Figure 2, which shows how AVSET would work for VCP 212 with a storm far from the radar.

With AVSET running, the radar scans until it no longer detects much return from the target of interest – in this case, a thunderstorm located about 100 nautical miles from the radar site. The radar would scan from 0.5 degrees up to an elevation of 5.3 degrees and then would stop moving upward and not complete any other elevation slices from 6.4 degrees to 19.5 degrees because the radar no longer detects much of a measurable radar return signal. At this point, the radar is complete with the present volume scan and then moves onto the next volume scan by beginning near the ground level of 0.5 degrees above the horizon and starting all over again.

The advantage is pretty significant since we are able to cut the time of one complete volume scan from 4 minutes and 30 seconds to as little as 3 minutes and 30 seconds – a sizable savings in time!  With the standard operations of having AVSET turned off, the storm interrogation meteorologist might see up to 13 scans per hour using a VCP 212 radar configuration. With AVSET on, it is possible to see up to 17 scans per hour. Now this might not seem like that great an improvement over standard operations, but any additional scans that we NWS meteorologists can view during a rapidly changing severe storm environment means that we have a much greater ability to complete our mission of “Protecting Life and Property.”  This has been a welcomed change in the Des Moines NWS Weather Office that enhances not only storm interrogation, but can also result in greater lead times when issuing severe weather warnings which in turn gives you additional time to prepare for dangerous weather events such as damaging thunderstorm winds, large hail, and tornadoes.


Figure 3: SAILS in action:
Top) Radar scans up to 3.1°
Middle) Radar lowers to 0.5° and completes another volume cut
Bottom) Radar resumes scan at 4.0° and completes the remainder of the volume scan.

AVSET is just one great recent addition to our radar toolkit…but wait…there’s more!  There is another new method of storm interrogation introduced this year called SAILS. SAILS stands for Supplemental Adaptive Intra-Volume Low-Level Scans (SAILS). Now you know why it goes by the short-hand term SAILS!  So – what does SAILS do for the radar’s operation and how does it benefit the storm interrogation meteorologist?  First, let’s take a look at how SAILS works. In normal radar operations, the radar scans from near the horizon at 0.5 degrees up to a specified height using elevation cuts to complete one volume scan. This is the same process shown above in Figure 1. With SAILS active, the radar would do the following as illustrated by the diagrams in Figure 3.

As shown in Figure 3, when SAILS is operating, an additional low-level volume cut is added to the list of available products for use by the storm interrogation meteorologist. This benefits the staff at our office because many of the important features that lead to a severe thunderstorm or tornado warning often show up in the lowest volume cut at 0.5 degrees. This would be true of a rapidly rotating meso-cyclone that may be lowering in the process of producing a tornado or perhaps more examination of strong to intense base velocities (strong thunderstorm winds) that are lowering toward the ground in a wind producing storm, such as a derecho. Again, with the addition of another slice at 0.5 degrees during each complete volume scan, there are many more available 0.5 degree slices available to the storm interrogation meteorologist per hour. But hold on, you say!  Wouldn’t it take longer to finish one complete volume scan if we add another 0.5 degree slice in the middle of each volume scan?  The answer is “yes”, it does reduce the total number of complete volume scans per hour, but the trade-off is well worth this slight disadvantage in most severe weather applications. Even though with SAILS active, the number of complete volume scans is decreased by two in VCP 212 compared to the standard operation with SAILS off – we gain 9 additional 0.5 degree slices per hour!  The benefits of both reducing the time between low-level slice updates and the nearly  doubling of 0.5 degree slices per hour allows for more low-level observations of intense thunderstorms during severe weather events. This gives our staff an opportunity to better monitor the trends of the lower portion of the thunderstorm and decide how quickly the storm might be strengthening or weakening. This has major implications for warning operations and should subsequently result in more lead time for warnings and more time for you to take shelter for severe weather.

Wouldn’t it be nice to run AVSET and SAILS together?  Yes!  In fact we can and do run them together. In some cases the net advantage is even better than SAILS alone or AVSET alone. The whole can definitely be greater than the sum of the parts. Take a look at the following table for comparison of the original standard operating mode compared to AVSET and SAILS, and then to SAILS and AVSET operating together:

Figure 4 (page 10) shows that for VCP 212 either AVSET or SAILS working alone provide higher numbers of 0.5 degree slices per hour – AVSET (13-17) and SAILS (22) compared to the Standard Operation (13). Figure 4 also shows the slight disadvantage of complete volume scans for SAILS (11) compared to the Standard Operation (13) and AVSET (11-13) per hour. However, with SAILS and AVSET both operating – the number of 0.5 degree slices per hour increases even more – SAILS and AVSET together (22-28) compared to AVSET alone (13-17) compared to SAILS alone (22). Looking back at the last column of Volumetric Product Updates per Hour shows that the combination of SAILS and AVSET both running together brings the total number of complete volume scans per hour back to 11 to 14 – nearly equal or slightly exceeding the Standard Operation of 13 per hour!  It might seem a bit odd to see a range of 0.5 degree slices and a range of complete volumetric product updates per hour when AVSET is being used. However, this is completely normal because the early termination of one complete volume scan depends both on the height of the storm being viewed and the distance the storm is from the radar. If the storm is captured in only three or four elevation cuts due to being not as tall or far away from the radar, then AVSET will terminate the current complete volume scan earlier and more completed volumetric product updates per hour will be available to the radar meteorologist. This same process carries over to the case when both AVSET and SAILS are working together. One more fact about SAILS is that it is only used when the radar is in severe storm interrogation mode – that is, when the radar is in Volume Coverage Patterns VCP 12 or VCP 212.  These are the coverage patterns used when significant severe weather – that which a warning might be issued – is anticipated or already occurring.

The additional number of low level elevation slices at 0.5 degrees can be critical to more lead time and earlier warnings. By issuing warnings faster with more confidence due to all of the additional weather data observed in those low level 0.5 degree slices, this will ultimately provide better warning services to you and enhance our ability to protect life and property!

Blog post by Roger Vachalek – NWS Des Moines


Jeff Johnson Bids Farewell to NWS Des Moines

JeffJohnsonJeff Johnson, now former Warning Coordination Meteorologist (WCM) at the National Weather Service (NWS), Des Moines, Iowa, has departed to the NWS Office in Topeka as the new Meteorologist in Charge. He begins his new position today, Monday, September 22, 2014.

Jeff started his tenure at the NWS Des Moines office in 1992 and then became the WCM in 1994. While at the NWS Des Moines, Jeff experienced the 1993 and 2008 major floods, numerous tornado events including the Parkersburg EF5 tornado and too many winter storms to mention.

Over the years, Mr. Johnson developed many professional relationships with emergency managers, members of the media and many others in the weather and public safety sectors. He will miss working with them and he appreciates all of the support through the years.

“I bid all of my work colleagues and partners a fond farewell and all the best in the future” Jeff said when reflecting on his departure. If you wish to contact Jeff, please do so via e-mail at jeff.johnson@noaa.gov.


June 2014 Central Iowa Monthly Climate Summary

Temperature Overview

The statewide monthly average temperature for Iowa in June 2014 was 70.3°F which was 0.6°F above normal. June 2014 ranks 55th warmest June among 141 years of records in the state of Iowa. The average temperature within the NWS Des Moines County Warning Area (DMX CWA) was 70.0°F for June 2014 and the average maximum temperature was 79.8°F and the average minimum temperature was 60.1°F. The DMX CWA is outlined in white in Figures 1-4.  Roughly 3 out the 4 weeks in June remained above normal with a cold stretch occurring from June 7 to 13 (See Figure 1).

The coldest temperature across the DMX CWA was 39°F in Emmetsburg on June 9th while the lowest in the state was 38°F at Battle Creek on the 13th. However, much of the departure from normal only ranged about 1 to 5 degrees above normal during the month. In fact, only one day topped 10 degrees above normal, within the DMX CWA, and that occurred on the 1st (See Figure 2). Since no long duration heat wave developed during June, the majority of stations only reach 90°F a couple of times. Des Moines reached exactly 90°F on the 16th, 18th, and 20th while Waterloo’s hottest temperature was only 88°F on the 1st. Typically, Des Moines averages 4.5 days and Waterloo averages 4.1 days with 90°F or greater maximum temperatures during the month of June. The two stations combined for 4. Other ASOS (Automated Surface Observing System) stations such as Lamoni, Ottumwa, Marshalltown, and Mason City never topped 90°F during the month. By the end of the month, Mason City still had not reached 90°F since September 9, 2013 and this streak continued into July.

June7-13-AVG TempDFM

Figure 1: Average Temperature Departure from Mean from June 7 to June 13, 2014. The map shows much of the state well below normal.

June1-AVG TempDFM

Figure 2: Average Temperature Departure from Mean on June 1, 2014.

Precipitation Overview

The statewide monthly average precipitation for June 2014 was 9.94 inches which was a whopping 4.92 inches above normal (See Figures 3 and 4).  June 2014 became the 3rd wettest June among 141 years of records and the 4th overall wettest month for Iowa (See Table 1).

June 2014 Monthly (Top 4) Precipitation Records for the entire state of Iowa. 2014 comes in at number 4th overall.

Table 1: The Top 4 Monthly Precipitation Records for the entire state of Iowa. June 2014 comes in at number 4th overall for all months.

Daily precipitation records were set at Mason City, Lamoni, and Waterloo during the month (See Table 2). Waterloo, Iowa had its 3rd highest June precipitation with a total of 9.63” for the month, which was just ahead of the 8.79” total in 2008 (See Figure 5).  Most sites across central Iowa reported precipitation for at least two weeks (not consecutively) out of the month of June. For instance, Ottumwa recorded at least a trace of precipitation 19 out of the 30 days in June and 6 days had at least a half inch or more. Des Moines, Waterloo, Lamoni, Estherville, and Marshalltown all had 4 days with 1” or more of precipitation. Ames came in with 5 days with daily totals of 1” or more. All this precipitation led to significant flooding and flash flooding across the DMX CWA during June 2014. The upper Des Moines River and the Cedar River were affected the most as 13 river stations topped flood stage.

A very active severe weather pattern occurred in Iowa during June 2014 with June 3, June 16, and June 30 receiving the most significant and widespread severe weather. On June 16, 9 tornadoes occurred within the DMX CWA, while altogether there were 12 tornadoes in the state. June 29-30, separate systems, but the two-day total for tornadoes was 13 for the state, in which 11 occurred in the DMX CWA. For the entire month, there were 31 tornadoes across the state of Iowa resulting in a total path length of over 107 miles long. The highest rating was EF2 for two tornadoes. The first one occurred on June 3 in Pottawattamie County southeast of Bentley and the second dropped across Tama County northwest of Traer. For more detailed Iowa tornado information, visit our tornado page: http://www.crh.noaa.gov/dmx/?n=iators2014

Tornadoes were not the only severe weather hazards that affected Iowa in June as the aforementioned dates, along with a few other events, produced significant damage from very large hail and damaging winds. On June 3, southwest Iowa was hardest hit with wind-driven hail as it produced significant damage to crops and buildings, where siding was completely shredded off several homes (See Figure 6). June 30 produced similar wind-driven hail that caused major damage across portions of central Iowa and more detailed information the event can be found here: http://www.crh.noaa.gov/dmx/?n=june30event


Figure 3: Statewide Accumulated Precipitation: Percent of Mean for the month of June 2014

Figure 3: Statewide Accumulated Precipitation: Percent of Mean for the month of June 2014

Figure 4: Accumulated precipitation for June 2014.

Figure 4: Accumulated precipitation for June 2014.

Figure 3: Waterloo Accumulated Precipitation for June 2014 as well as June 1947, 1993, and 2008.

Figure 5: Waterloo Accumulated Precipitation for June 2014 as well as June 1947, 1993, and 2008.

Figure 4: Significant wind-driven hail event occurred on June 3, 2014. This image from a resident in Treynor, IA had major damage to the siding and roof.

Figure 6: Significant wind-driven hail event occurred on June 3, 2014. This image from a resident in Treynor, IA had major damage to the siding and roof. Image courtesy of KWWL.

Reservoir Information

The Saylorville Reservoir pool height began at 837.84 feet on the 1st and rose over 30 feet to a height of 868.23 feet by the 30th (See Figure 7). The normal pool height is 836 feet. The pool storage monthly maximum was 333,960 Acre-feet on the 30th while the minimum pool storage was 67,068 Acre-feet on the 13th. The Des Moines River downstream of Saylorville Lake fluctuated throughout the month before cresting at 15.25 feet on the 27th with a flow of 19,500 cubic feet per second or CFS (See Figure 8).

Lake Red Rock Reservoir pool height increased from 743.77 feet on the 1st to 752.30 feet by the 30th. The pool storage doubled in volume within two weeks as it rose from a minimum of 189,420 Acre-feet on the 15th to a monthly maximum of 404,370 Acre-feet on the 30th (See Figure 9). The stage height of the Des Moines River downstream of Lake Red Rock Reservoir increased from a low of 86.73 feet on the 1st to a maximum height of 92.50 feet on the 29th.  The outflow maxed out at 19,000 CFS on the 28th and 29th while the lowest flow was 3,950 CFS on the 16th.


Figure 7: Stage height trend graph for Saylorville Lake in June 2014.


Figure 8: Output, measured in cubic feet per second, along the Des Moines River downstream of Saylorville Lake Reservoir.

RedRock Pool Storage-June 2014

Figure 9: The storage trend for June 2014 at Lake Red Rock Reservoir.



Blog post by Kenny Podrazik – NWS Des Moines

Tornado Survey – “The Old School Way”

Take yourself back 75 years to August 10, 1939 when World War II was less than a month from getting underway, the U.S. was slowly climbing out of the Great Depression and on the brink of war, the Studebaker Champion was introduced and cost about $660 (or $11,312 in 2014), and the Cubs actually had a winning record! Sorry Cubs’ fans. The country was just learning about ALS or Lou Gehrig’s disease as he was just diagnosed and had to retire from baseball that summer. The price of gas was $0.10, a pound of hamburger was $0.14, and a loaf of bread was $0.08. The average cost of a new house in 1939 was $3800 but now, the average cost of a new home in 2014 is $339,100 (per U.S. Census).

Okay, so you’re back in August 1939 in Iowa when no weather radar coverage or tornado warnings were available to meteorologists. Folks literally could say “It struck without warning” and be honest about it! They still did damage surveys, but the Fujita scale wouldn’t be introduced until 1971.  So when C.D. Reed and/or S.E. Decker, from the Iowa Department of Agriculture or IDA (See Figures 1a and 1b) had the daunting task of surveying three destructive tornadoes that occurred on August 10, 1939 in central Iowa, they did an amazing job (See Figure 2).  Back then, surveying included talking with eyewitnesses hit by the tornado and whatever sort of geodetic survey equipment they had available. They had limited resources, but the detail of what buildings were hit, livestock killed, or persons injured was phenomenal. Granted there were less people and fewer buildings to destroy, but traveling and communication was more cumbersome in 1939 than 2014; especially since the survey covered several counties.

Figure 1a: C.D. Reed Author of the Iowa monthly climate review for August 1939.

Figure 1a: C.D. Reed Author of the Iowa monthly climate review for August 1939.

Figure 1b: S.E. Decker was the author of the damage survey or storm section in the Climatological Data: Iowa Section.

Figure 1b: S.E. Decker was the author of the damage survey or storm section in the Climatological Data: Iowa Section.

The hardest hit counties were Adair, Clark, and Warren from the tornadoes while Polk County endured significant damage due to heavy rainfall. Well what do you know  – heavy rain in August in Iowa? There’s a shocker.  Another county, Montgomery, was hit hard with large hail as noted on the tornado track from Figure 2 and suffered $10,000 worth of crop damage.

August 10-1939 TornadoPaths-resize

Figure 2: hand drawn maps of the tornado paths and hail swaths on August 10, 1939.

The first and second tornadoes occurred in Shelby and Adair Counties respectively. The twister in Shelby County damaged buildings on five farms resulting in a loss of $12,000 (see inflation rate table below) and injured one person, Mrs. Pete Anderson, according to the report.

The Adair County tornado started about 3:30 p.m. near the Summerset Township and traveled northeast through Summerset and Prussia to just east of Fontanelle, Iowa. Miss Mildred Bakerink was fortunate to photo the tornado when it was about 3 miles northeast of her location near Prussia (See Figure 3). Reports suggested the early life of the tornado that “the storm of pendent cloud was shaped more like a cone with a wide V-top.”  The survey determined the tornado path was about “12 miles long and 80 rods wide.” A rod is equivalent to 5 ½ yards or 16 ½ feet in length.  Hence, the tornado width was roughly 440 yards (1320 feet) wide or a quarter of a mile. That’s a pretty significant tornado. To compare it to a recent tornado that occurred in Iowa, the “Belmond” Tornado that passed through the north side of Belmond on June 12, 2013 was 200 yards wide with path length of 6.2 miles. This tornado was rated an EF-3 tornado with a 155 mph peak wind speed. You can draw your own conclusions on where to rate the Adair County Tornado from August 10, 1939. To help you out, damage was estimated to buildings on six farms ranged from $5,000 to $10,000 while the damage to crops, stock, implements “amounted to several thousand dollars”, according to the IDA report. Luckily, there was only one injury and no deaths.


The third and most destructive tornado was on the ground for roughly 35 miles and it originated southwest of Osceola, in Clarke County, and finally dissipated near Milo in Warren County.  The damage surveyor, likely C.D. Reed, visited Liberty Center where several eyewitnesses said they could see five funnel clouds visible at one time southwest of town. There were also several reports from Osceola that suggested seeing the five funnel clouds at the same time.  In fact, a writer from the Osceola Tribune depicted the funnel cloud as “bounding around like a rubber ball, alternately lifting and lowering.” Here’s how the eyewitnesses from Liberty Center described the funnels:

“…as being close together and joined to a common dark cloud mass. They were said to be suspended in the air without touching the ground as long as they remained separated, but that upon joining or merging the remaining funnel grew in length and extended down to the ground.”

This sounds a lot like a what modern day meteorologists call a multi-vortex tornado. It certainly did some damage to Clarke and Warren Counties. The IDA report said “buildings were demolished on at least ten farms” in Warren County. There were 18 of 22 buildings, on one livestock farmstead, “wrecked or seriously damaged.” There was a stretch of corn, roughly a mile wide and 15 miles long, which was completely flattened. Several trees were snapped or uprooted, power and telephone lines blown down, and most fences blown away in the tornado path. The description of the damage near Liberty Center gets even more detailed (See Figure 4). The total damage from the storm, including heavy rain and straight-line winds, in Warren County was estimated to be at least $102,000 as the IDA report stated “several thousand dollars more” to furniture, telephone lines, crops, etc.  There were several injuries but no related deaths. The table below shows the damage adjusted for inflation from 1939 to 2014.

August 10-1939 DamageDescription

Figure 4: very detailed description of the the tornado damage near Liberty, Iowa.

The surveyor calculated the speed of the storm itself at around 40 mph by determining when and where it originated and when and where it dissipated. Something we still do today but with the aid of radar data, satellite imagery, and aerial photos.  The surveyor estimated the “rotary winds indicated at least hurricane velocity of about 75 miles per hour.” From the description of the damage, there’s little doubt the peak wind speeds where likely higher.

As far as the meteorological setup, a surface analysis on the morning of the storms (See Figures 5-7) suggested a warm front draped across Arkansas into eastern Oklahoma. However, further surface analysis along with the description from the IDA report, the warm front extended north-northwest and connected to the cold front near the Grand Island area.  The storms developed along the warm front by the late afternoon as its surged north throughout the day. The cold front, according to the report, pushed through the Des Moines area around 6:30 p.m. time frame.

A fine job done by the folks at the Iowa Department of Agriculture, which was likely down by either C.D. Reed or S.E. Decker or both, on the storm survey from August 10, 1939.


Figure 5: North America Synoptic Weather Map on the morning of August 10, 1939 at 1230 UTC or 6:30 am.


Figure 6: Zoomed in morning surface analysis on the Corn Belt and Central Plains.


Figure 7: CONUS surface analysis on August 10, 1939 at 6:30 am. The map shows the area of low pressure over northern Kansas and a boundary extending from southwest to northeast Iowa. It also depicts thunderstorms over South Dakota, western Nebraska, and far northwest Iowa.

Tornado Damage Adjusted for Inflation

Township County Type of Damage 1939 Cost 2014 Cost
Polk Shelby Buildings $12,000 $205,700
Polk Shelby Crops $1,000 $17,140
Polk Shelby Livestock $100 $1,710
Red Oak Montgomery Crops $10,000 $171,400
Summerset/Prussia Adair Buildings $5,000-10,000 $85,700-171,400
Near Osceola Clarke Buildings $10,000-15,000 $171,400-257,100
Near Osceola Clarke Crops $5,000 $85,700
Near Osceola Clarke Livestock $500 $8570
Near Liberty Center Warren Buildings $75,000 $1.3 million
Near Liberty Center Warren Livestock $2,000 $34,280
Near Liberty Center Warren Crops $25,000 $428,500

Inflation rates rounded and based off cumulative rate of 1614.0%



Blog post by Kenny Podrazik – NWS Des Moines

Tornadic Debris Signatures in Iowa

Between 2011 and 2013, the National Weather Service WSR-88D Doppler radar network underwent a major upgrade to dual-polarization (dual-pol). Now, instead of sending out just one radio wave oriented in the horizontal, the radar simultaneously sends out a horizontal and vertically polarized wave. This enables the radar to take a cross-section of whatever particles it samples and assists meteorologists in determining their size, shape, and concentration. It also helps delineate which scatterers are meteorological (rain, hail, snow, etc.) or biological (birds, dust, and insects).

The dual-pol upgrade introduced three new products on top of the legacy reflectivity, velocity, and spectrum width data. The first, differential reflectivity (ZDR), simply calculates the difference between the horizontal and vertical channel reflectivity values. Positive numbers indicate objects oriented in the horizontal, negative values denote vertically oriented objects, and values near 0 signify spherical objects. The radar samples millions of particles multiple times within a single range bin, and correlation coefficient (CC) measures the similarity of these objects to one another. A value of 1 indicates uniformly shaped particles, while the closer one gets to 0, the more random the shape and size of the scatterers. Usually anything below 0.8 is non-meteorological in nature (the exception being large hail). Finally, differential phase shift (KDP) calculates the attenuation difference between the horizontal and vertical channels. Since rain drops become flattened as they fall and thus will attenuate the horizontal channel more than the corresponding vertical channel, KDP does an excellent job of locating regions of heavy rainfall.

One special phenomenon that has been observed on dual-pol radars with some tornadoes is the tornadic debris signature, or TDS. As the name implies, the radar is actually sampling the debris being lofted thousands of feet into the air by a tornado. Debris identification was possible before the implementation of dual-pol, but involved correlating a small but intense area of higher reflectivity values with a tight velocity couplet. Known then as a “debris ball”, it was difficult to determine in real-time and sampled on only a select few tornadoes. Now, the CC and ZDR products make locating a debris signature much easier. Debris will present a very low CC signal owing to their plethora of shapes and sizes. The tumbling nature of the debris will also result in a near 0 ZDR value since the objects “appear” circular to the radar beam. The colocation of the high reflectivity values, a tight velocity couplet, and low CC/ZDR values together form the text-book TDS. The stronger and closer a tornado is to a radar site, the more likely it is that the radar will display a TDS.

The Des Moines WSR-88D radar was modernized with dual-pol capabilities in September 2012.  A review of radar data for the 49 tornadoes that have been recorded in the NWS Des Moines service area (central third of Iowa) in the last two years turned up six definitive TDSs and four likely candidates whose radar characteristics did not quite fit the traditional TDS model and are still being investigated. Thus, TDSs were only found for 12% of the total number of tornadoes sampled by the radar (20% if the probable TDSs are included). All but one of these signatures were noted during the 2014 tornado season, which was significantly more active than 2013. Each TDS, like the tornadoes that produced them, was unique in its size, shape, and duration. However, many of the signatures behaved like a plume, originating from the tornado and spreading out over time.

There was little correlation between the strength/duration of the tornado and whether it produced a TDS. The Lake Panorama tornadoes of May 11, 2014 and the Zearing to Union tornado of June 30 were long-tracked tornadoes relatively close to the radar that caused substantial damage, yet failed to produce a TDS. Meanwhile, brief and weak tornadoes that hit didn’t strike any major objects produced TDSs. Four TDSs alone were sampled with just one storm system on June 30, 2014 in Adair, Madison, and Warren counties. The strongest and most persistent TDS was sampled on July 6, 2014 with a strong EF1 tornado over northern Tama County near Traer.

A prominent TDS (black circle) with a tornado north of Traer on July 6, 2014

A prominent TDS (black circle) with a tornado north of Traer on July 6, 2014

Blog post by Kevin Skow

Snow in September?

Believe it or not, there have been at least nine years in which snow has been recorded in Iowa in the month of September, most recently in 1995, as detailed in Table 1 at the end of this document. The most remarkable of these events is the very early snowfall of September 16, 1881, which was amazing not only for its earliness in the season but also for the extent and amount of snowfall. The track of the surface low pressure center associated with this storm system is illustrated in Figure 1, a reprint of the original “War Department Weather Map” from September of 1881. At the time weather observations and reports were filed by the U.S. Army Signal Service, the progenitor of the modern National Weather Service. In the report of the Chief Signal Officer for that month, the development of the low pressure center is detailed as follows:

“This storm, which pursued a very anomalous track, was first evident in Texas, where on the 14th it moved in a track nearly due east. At the midnight observation, while the storm center was near New Orleans, a barometric depression extended from the Gulf of Mexico to the Lake Superior region. At the same time [an area of high pressure] prevailed with fair weather in New England. These conditions were unfavorable to an eastern progress of the storm, and on the 15th the depression moved in a northerly course to Lake Michigan. On the 16th, with diminishing energy, the storm center moved into Iowa and Minnesota, and on the 17th into Manitoba. The track on the 16th and 17th is very remarkable, and probably for a storm of such energy will have no parallel in the history of the Signal Service.”

Figure 1: Highlighted track of the low pressure center from September 14-17, 1881.

Figure 1: Highlighted track of the low pressure center from September 14-17, 1881.

On September 14, as the low moved across the Gulf of Mexico, fair weather prevailed across most of Iowa until a cold front moved through late in the day. At Des Moines the high temperature was 80 degrees but then the official observer wrote that, “Low stratus clouds moved rapidly from north and northwest during the afternoon and evening.” On the 15th, as the low pressure center moved northward toward Chicago, it pulled down unseasonably cool air behind the front into Iowa and spread a cold rain across much of the upper Midwest. At Des Moines the temperature fell through the day, with a high of 58 degrees measured early in the morning. The observer noted, “Cloudy and threatening weather prevailed during the day, low stratus clouds moved from the north.”

By the morning of September 16, the low pressure center was moving slowly westward into southern Minnesota and northern Iowa, pushing cold air even further southward into the central U.S. Frost was noted as far south as Arkansas and Texas and at Fort Gibson, Oklahoma ice formed on standing water. Across eastern Nebraska, southern Minnesota, and about the northwestern two thirds of Iowa the colder air allowed rain to mix with or change entirely over to snow at times, mostly in the morning. At Des Moines the high temperature for the day was only 46 degrees and the observer recorded that, “Few flakes of snow was observed.” Further north and west the snow was heavier, in some areas melting as it fell but in others managing to accumulate for a short time. At Algona an estimated 4 inches of snow fell in the morning, breaking some tree branches, but all melted by noon. The snow was observed as “quite heavy” at Creston, while “several inches” were noted between Des Moines and Atlantic and 4-6 inches were estimated on the Rock Island Railroad between Stuart and Avoca.

This stands as one of only two occasions on which fairly widespread, measurable snow has fallen in Iowa in the month of September, the other being on September 25, 1942. In that storm most of the state received snow with amounts ranging up to 4 inches at Allison, Forest City, Mason City, and Millerton and scattered tree and utility line damage noted across the state.

Table 1: Years in which snow has been recorded in Iowa in September.
1881 – widespread measurable snowfall on the 16th (see above)
1895 – “first snowflakes” noted at Madrid on the 28th
1912 – “few flakes” observed at Storm Lake and Marshalltown on the 17th and 18th
1938 – flurries reported at Orleans and Maquoketa on the 18th and 19th
1939 – traces of light snow and sleet across northern IA, 0.1” at Sheldon and 0.2” at Sibley, on the 29th and 30th
1942 – widespread measurable snowfall on the 25th (see above)
1961 – light snow across northwestern half of IA on the 30th, a few measurable amounts ranging up to 3.0” at Swea City
1985 – a few flakes at Des Moines on the 24th, widespread wintry mix with 0.5” at Audubon and Storm Lake on the 29th and 30th
1995 – a few flakes and ice pellets mixed with rain across northern IA on the 22nd

Blog post by Jim Lee