How Many Tornadoes Can One Squall Line Produce?

Looking Back on August 31, 2014 from the Air

Residents of west-central Iowa may remember the powerful squall line that tore across the region on the evening of August 31, 2014, leaving a wide swath of wind damage from Crawford to Dallas counties.  More intermittent wind damage was reported from Greene to Howard counties.  In the days following the event, the NWS reviewed the wind damage reports/photos and could not conclusively distinguish any damage that could have originated from a tornado.  Tornadoes produced by squall lines are oftentimes weak, transient, rain-wrapped, and sometimes embedded within broader straight-line winds, making it very difficult to differentiate tornado versus non-tornado damage in post-storm ground surveys.  Mature corn crops can play a vital role in showing the unmistakable convergent path left by a tornado, and two tornadoes from August 31 were located by the author in cropland on a scientific survey on September 10, 2014 (see photo below).

Convergent damage path through corn for one of the ground-surveyed tornadoes on September 10, 2014 just north of Dayton in Webster County.

Convergent damage path through corn for one of the ground-surveyed tornadoes on September 10, 2014 just north of Dayton in Webster County.

However, in the weeks following the event, entire 350-km path of the squall line was imaged at ≈1-m resolution using aerial photography through the USDA National Agriculture Imagery Program.  As stated earlier, the predominantly flat, mature agricultural land cover of central Iowa provided an excellent medium on which to document all scales of wind phenomena.  The imagery (discovered and analyzed during the last 6-8 months) revealed an astounding 111 discrete damage tracks that could have originated from a tornado.  These tracks ranged in length from a mere 130 m to nearly 18 km.  Given the uniqueness of this dataset and the high likelihood that some of these tracks were from surface vortices that did not meet the formal definition of a tornado (a circulation reaching to cloud-base), a probabilistic testing scheme was developed.  This test weighted various track characteristics (length, strength, circulation nature, and damage) and radar data to determine which tracks had the highest chance of being from a tornado.  Using this test, 35 of the 111 tracks (31%) were classified as tornadoes.  Four of the tracks were rated EF-1 using the aerial data and the rest as EF-0.

Imagery collage of nine tracks from August 31, 2014, highlighting the wide variety of damage patterns observed. Track direction and identification numbers are provided with each event. All images rendered to the same scale.

Imagery collage of nine tracks from August 31, 2014, highlighting the wide variety of damage patterns observed. Track direction and identification numbers are provided with each event. All images rendered to the same scale.

The sheer number of damage paths revealed by this dataset is truly unprecedented.  No other study has ever uncovered so many tracks produced by a single squall line.  The aerial data also showed two tornadoes merging into one single entity just northeast of Stratford (see image below), one of only four mergers ever documented and the first with tornadoes from a squall line.  This event unearthed far more questions than answers, not only concerning how these tornadoes formed, but also with how the NWS should handle these types of situations.  Using hypothetical tornado warnings that would have encompassed the area most at risk to tornadoes on August 31, it was calculated that only 0.24% of the warning area would be impacted by a tornado.  Should the NWS be issuing tornado warnings for these low-impact, short-lived tornadoes that would rarely cause damage greater than EF-0/EF-1?  Is it worth false alarming over 99.8% of the warning area to try to capture these fleeting tornadoes that would likely go undocumented?  In addition, how should the NWS document such tornadoes for historical records?  Since aerial data will not be available for all events, a bias would be introduced into Storm Data.  The August 31, 2014 squall line exemplifies this bias to the extreme.  To put this event in context, the 35 tornadoes from August 31, 2014 would rank as the single greatest tornado outbreak in Iowa history, an outbreak that no one has ever heard of!  These questions and more will need to be addressed by the NWS over the coming years not only as the organization moves towards impact-based warnings, but also as the availability and accessibility of aerial/satellite imagery datasets increase.  This case highlights the incredible utility of aerial and satellite datasets for storm surveying, a benefit that the NWS will hopefully capitalize on in the years to come.

(a) Polygon paths for tornadoes 62-T19 (red) and 64-T21 (blue) with the start and end times (in UTC) for each tornado annotated. Local streets are provided as a map background. The black dotted outline denotes the region encompassed by the aerial imagery shown in (b) of both track crossing points and the merger, with the estimated times of the first crossing point and merger noted. Tracks are outlined and in the same colors as (a). (c) Close-up imagery of the merger point with different stages of the merging process highlighted.

(a) Polygon paths for tornadoes 62-T19 (red) and 64-T21 (blue) with the start and end times (in UTC) for each tornado annotated. Local streets are provided as a map background. The black dotted outline denotes the region encompassed by the aerial imagery shown in (b) of both track crossing points and the merger, with the estimated times of the first crossing point and merger noted. Tracks are outlined and in the same colors as (a). (c) Close-up imagery of the merger point with different stages of the merging process highlighted.

Blog post by Kevin Skow, Meteorologist Intern, NWS Des Moines

New DSS Events Calendar Tool to Aid NWS Des Moines Partner Support

NWS DMX has been providing support for many partners through our Decision Support Services (DSS) for several years, and this year has a new tool to help with this support, the DSS Events Calendar. This tool is being rolled out across the Central Region NWS offices so each office has a consistent baseline for partner support. The calendar works by NWS partners such as Emergency Managers (EMs) providing input about their events through a web-based form. This information is then organized into a web-based calendar so that our office and meteorologists are aware of each event being held across our county warning area. These events tend to be outdoor events that gather a large attendance which are most vulnerable to the impacts of weather, and more specifically severe weather. 

Many days several events are ongoing across Central Iowa with as many as 15 to 20 on one of the busier days. NWS DMX meteorologists stay aware of these locations while issuing warnings and can place special statements in the severe warnings about the outdoor event locations that may be occurring in a warned area. The event location is included in the input and then is directly loaded into the office computer system used to issue warnings, so that these event locations can be included in a warning if necessary.

County Emergency Managers maintain contact with our office throughout the event as well, by logging into a chat program or via the phone to obtain the latest information about any threatening weather. If requested by an EM, NWS DMX will also provide a localized briefing detailing any hazardous weather that may be expected throughout the duration of the event. One example is a video briefing held with the Jasper County Emergency Management officials for high-attendance races held at the Iowa Speedway, another example is a daily email briefing during RABGRAI provided to public safety officials, County EMs, neighboring NWS office management, and regional NWS management.

This tool has allowed NWS DMX to streamline its operations regarding event support to the local county officials, helped NWS DMX meteorologists to maintain awareness of these large events in case hazardous weather is approaching, and help keep everyone safe so they can enjoy these events to their fullest.

If you have weather hazard concerns for an event, please contact your county emergency coordinator who will work with the National Weather Service before and during impactful weather conditions.

DSSCalendar

Blog post by Mindy Beerends, Senior Forecaster, NWS Des Moines

2016 – Cooperative Observer Awards

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Bev Chaplin of Hubbard, Iowa receives her Benjamin Franklin (55 year Length of Service) Award. A large group of Bev’s family and friends attended the presentation. Picture (Left to Right): Brad Fillbach, Coop Program Manager (CPM), WFO Des Moines, Iowa; Bev Chaplin, Hubbard, Iowa; Ken Harding, Chief Program Officer, Central Region Headquarters; Andy Bailey, Acting Meteorologist-in-Charge, WFO Des Moines, Iowa.

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The staff at Perry Water Works in Perry, Iowa, receive their 25 year Institutional Length of Service award. Presenting the award is Brad Fillbach, CPM, WFO Des Moines, Iowa.

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Ardell McCunn(left) and Kevin McCunn (right) receive their 15 year Length of Service award. The award was presented by Brad Fillbach, CPM, WFO Des Moines, Iowa

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Margaret and Ed Long of Lorimor, Iowa receive their 20 year Length of Service award. The award was presented by Brad Fillbach, CPM, WFO Des Moines, Iowa

Blog post by Brad Fillbach, Coop Program Manager, NWS Des Moines

Fall and Winter Outlooks for Iowa

Introduction

The talk all winter and spring was El Niño and how it would affect large portions of the United States.  As spring ended and summer took over, El Niño conversations were quickly replaced with La Niña thoughts as conditions began to transition.  Taking a step back, for those that are unfamiliar with the terms El Niño or La Niña, they are phenomena that exist along the equatorial Pacific Ocean.  An El Niño event is characterized by anomalously warm sea surface temperatures that propagate eastward towards the South American coast.  Conversely, a La Niña event is characterized by anomalously cool sea surface temperatures off the coast of South America that propagate into the central Pacific Ocean.  The oscillation between the two is called the El Niño-Southern Oscillation (ENSO).  For an event to be categorized as an El Niño or La Niña, the Oceanic Niño Index (ONI) is used.  The ONI is a measure of above and below normal sea surface temperatures within a specific region of the equatorial Pacific.  Anomalous values of 0.5 C or greater for 5 consecutive over-lapping seasons would be an El Niño.  Conversely, values of -0.5 C or less for 5 consecutive over-lapping seasons would be a La Niña.  In between 0.5 C and -0.5 C or for periods that do not meet the 5 consecutive over-lapping seasons, would be categorized as neutral.

At this point, you may be wondering what Pacific Ocean sea surface temperatures have to do with weather in the United States, let alone Iowa.  The atmosphere is constantly in flux, responding to various inputs such as El Niño or La Niña conditions in the Equatorial Pacific, in an attempt to reach equilibrium.  Phenomena that last long periods of time and occur regularly can affect atmospheric conditions in ways that lead to tendencies in other areas, known as teleconnections.  A number of phenomena across the globe have been researched and found to correlate to weather patterns in other locations across the globe, and El Niño and La Niña are probably the most researched and well known teleconnections.  We’ll proceed to take a look at the tendencies in Iowa during neutral and La Niña conditions and how they compare to the current seasonal outlooks from the Climate Prediction Center.

For more detailed information about El Niño and La Niña, atmospheric conditions, thresholds, U.S. impacts, global impacts, and more, check out: https://www.climate.gov/enso

For more detailed information about the Oceanic Niño Index, check out: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

Fall

As hinted at above, El Niño conditions existed through the winter and spring months, but weakened into summer and are forecast to continue into at least neutral conditions and possibly transition into La Niña conditions through the fall months (Image 1)

(Image 1: Official probabilistic ENSO Forecast from the Climate Prediction Center and International Research Institute for Climate and Society)

Image 1: Official probabilistic ENSO Forecast from the Climate Prediction Center and International Research Institute for Climate and Society.

First, focusing on neutral conditions, when looking at previous historical neutral events that occurred during the fall months, temperatures have a tendency to be cooler throughout the state (Image 2).  On the other hand, precipitation does not have a strong tendency state wide, and instead is a mixed bag, indicating a weaker correlation (Image 3).

Should La Niña conditions set in, once again the strongest tendency is seen in temperatures, however this time it is for warmer temperatures throughout the state (Image 4).  Then similarly to neutral, precipitation does not show a strong tendency, but does lean toward slightly drier conditions (Image 5).

While all of that is well and good, how does all of it compare with the Climate Prediction Center’s (CPC) official fall outlook?  For their most recent fall outlook, temperatures nationwide show an increased chance for above normal temperatures (Image 6).  The precipitation outlook shows no tendency for above or below normal precipitation for much of the United States, including Iowa (Image 7).  When comparing to the neutral and La Niña tendencies above, the outlooks do not closely resemble either, possibly indicating low confidence or another teleconnection playing a significant role.

Winter

Shifting towards winter, forecasts show a likely probability of La Niña conditions setting in and that’s what we will focus on here.  La Niña has a pretty strong signal across the United States, however Iowa is caught in the transition zone of above normal temperatures to the south and below normal temperatures to the north (Image 8).  Precipitation, also has a decent signal across the United States, but once again Iowa does not fall within an area of anomalies one way or another (Image 9).  Instead, the best signals are found through the Ohio River valley, Pacific Northwest, the Southeast, and California.

When compared to the CPC outlook for winter, striking resemblances are immediately seen. The strongest resemblance is seen within temperatures, where the CPC highlights the Southeast and South for above normal temperatures and the North for below normal temperatures (Image 10).  Strong resemblances are also seen in the CPC outlook for precipitation, where the Ohio River Valley and Pacific Northwest are highlighted for above normal precipitation, and the Southeast, South, and California for below normal precipitation (Image 11).  Unfortunately for Iowa, with the seeming strong tendency for the CPC outlooks to lean towards La Niña conditions this winter, the CPC does not depict strong feelings one way or the other for precipitation or temperatures. 

Blog post by Allan Curtis, Meteorologist Intern, NWS Des Moines

 

Iowa Monthly Climate Summary – July 2016

Temperatures

The statewide average temperature for July 2016 was 73.0°F which was 0.6°F below normal (See Figure 1). July 2016 ranks as the 50th coolest July on record out of 144 years of statewide climatic records.  The monthly average temperature at Des Moines was 76.8°F or 0.5°F above normal with the highest temperature coming in at 97°F on the 21st and lowest temperature at 58°F on the 1st.  Waterloo’s monthly average temperature was 72.8°F or 0.8°F below normal.  The highest temperature in Waterloo for the month of July was 93°F on the 21st and 52°F on the 3rd.

Through the 4th of July weekend, cooler than normal temperatures prevailed across the state (See Figure 2).  In fact, a few locations recorded low temperatures in the 40s on the morning of the 3rd (See Figure 3).  Estherville, Elkader, and Cresco all had low temperatures at 47°F on July 3rd.  Then the hot and humid conditions developed and dominated throughout much of the remainder of the month.  There were a few days peppered into the last 3 weeks of the month when cloud cover and thunderstorms kept temperatures below normal.  Regardless, the hottest stretch occurred from the 17th to the 27th. The hottest temperature of the month throughout the entire state was 97°F on the 20th at Sioux City and at Des Moines and Lamoni on the 21st.  Much of the state had heat index readings well over 100°F between the 20th and 23rd (See Figures 4 & 5) due to the high humidity and dew points.

Precipitation

The monthly statewide precipitation total for Iowa during the month of July 2016 was 6.13 inches (See Figure 6).  This was 1.63 inches above normal, resulting in the 16th wettest July on record among 144 years of statewide climatic records.  Des Moines totaled 6.98 inches for the month, with the bulk of that total occurring on the 19th when 3.53 inches of rain fell. This daily total nearly doubled its monthly total up to that point. In fact, only 0.02 inches of additional rainfall accumulated the remainder of the month, but Des Moines was still 2.51 inches above normal for the entire month.  Waterloo had a different outcome with 4.01 inches or 0.90 inches below normal for July.

After a dry first five days of the month, an active weather pattern developed and brought a multitude of precipitation through the 20th.  The statewide average from July 6th to the 20th was 4.86 inches or 2.63 inches above normal during that stretch (See Figure 7 & 8). The heaviest rainfall occurred from the southwest to north central to northeast portions of the state. Enough rain fell over central and southern Iowa to relieve some of the drought conditions except for a small area in far south central Iowa (See Figure 9).

Iowa Monthly Climate Summary – May 2016

Temperatures

The statewide average temperature in May 2016 was 59.5°F which was 0.6°F below normal (See Figure 1). This ranks as the 65th coolest May on record out of 144 years of statewide climate statistics.  Des Moines’s average temperature was 63.0°F or 0.7°F above normal while Waterloo’s monthly average temperature 59.9°F or 0.6°F below normal.

A cold snap occurred during the first five days of May as a few spots reported light frost on the mornings of the 2nd, 3rd, and 5th (See Figure 2).  However, by the second week of May, temperatures were above normal with a few locations topping 90°F for the first time in 2016 in Iowa. Marion, Guttenberg, and Hawarden reached 90°F on the 6th. More cool temperatures filter into the state from the 12th to the 21st.  In fact, freezing temperatures were recorded on the 14th, 15th, and 18th with the coldest day and most widespread frost occurring on the 15th (See Figure 3). Cresco, Decorah, and Elkader all recorded 27°F low temperatures on the morning of the 15th.  The remainder of the month, temperatures finished above normal with a string of 10 consecutive warmer than normal days. The hottest temperature was 89°F in Little Sioux and Lamoni on the 30th.

Precipitation

The statewide average precipitation for May 2016 was 4.64 inches or 0.08 inches above normal. This ranks as the 52nd wettest May among 144 years of statewide climate records. Des Moines was 0.62 inches below normal for the location’s monthly precipitation as it totaled 4.12 inches. Waterloo totaled 3.24 inches of rainfall for the month of May, which was 1.29 inches below normal.

The first day of May was cool and wet before generally dry weather prevailed from the 2nd to 8th.  From the 9th to the 13th, a wet period settled into the state with several weak systems that brought light to moderate rainfall. The highest precipitation accumulated over southwest Iowa (See Figure 4).  A couple of brief tornadoes were spotted in Davis and Guthrie Counties County on the 9th, mainly damaging crops and trees. Another relatively dry period dominated the middle of the month from the 14th to the 22nd before a fairly active weather pattern developed to finish off the month. Severe weather plagued the state every day during the final week of the month with the most active night occurring on the 25th. Widespread damaging winds along with some large hail were reported in 19 counties across the northern two-thirds of Iowa. The dries weather was found over central to southeast Iowa, while the bulk of the monthly precipitation accumulated over northwest to southwest Iowa (See Figure 5).  The highest monthly precipitation was 10.75 inches at Atlantic while the lowest monthly total was 2.19 inches in Albia. In fact, Atlantic’s total was its 2nd highest total out of 130 years of records. The highest total is 12.37 inches set in 1903.

Blog post by Kenny Podrazik, Meteorologist, NWS Des Moines

Iowa Monthly Climate Summary – June 2016

Temperatures

The statewide monthly average temperature was 73.2°F or 3.5°F above normal (See Figure 1). June 2016 ranks as the 14th warmest June among 144 years of statewide climate records. A warmer June was last recorded in 1991. The average temperature at Des Moines and Waterloo was 77.3°F and 71.9°F respectively.  Des Moines was 5.5°F above normal while Waterloo was only 1.9°F above its monthly average. On the 15th, the hottest temperature of 96°F was recorded for the month at Des Moines while the coldest temperature was 56°F on the 2nd. The hottest temperature at Waterloo was 95°F on the 10th while just two days earlier the low temperature dropped to 46°F.

Above normal temperatures prevailed for the majority of month of June across the state. A long hot spell occurred from the 9th to the 20th when the hottest temperature of 100°F was recorded at Little Sioux on the 11th (See Figure 2). This 100°F ended up being the hottest temperature for the entire state during June 2016 and was Iowa’s first triple-digit temperature since September 10, 2013. The state’s second longest streak without a 100°F reading ended on the 11th with 1,004 consecutive days. The longest streak is 1,438 consecutive days from August 3, 1991 to July 9, 1995.  The maximum temperature at Des Moines from the 9th to the 27th was 85°F or higher for 19 consecutive days in June.  This became the longest streak of its kind during the month of June since 1878 at Des Moines. The previous longest stretch was 17 consecutive days set in June 1956.

Precipitation

The statewide average precipitation was 3.69 inches which was 1.33 inches below the state’s normal for June (See Figures 3 & 4). This ranks as the 45th driest June out of 144 years of statewide climate records. The monthly totals at Des Moines and Waterloo were 1.47” or 3.47” below normal and 8.97” or 4.98” above normal. Des Moines recorded its 10th driest June since 1878 and Waterloo recorded its 4th wettest June since 1895. This was common throughout the state as the precipitation was highly variable (See Figure 5). For example, Cedar Falls had a whopping 11.22” while Salem only recorded 0.25” for the entire month. By the end of the month, much of south central to southeast Iowa were placed into Moderate Drought by the U.S. Drought Monitor with much of the southwest to western part of the state classified as Abnormally Dry (See Figure 6). Severe weather was limited throughout the month of June, even though June is Iowa’s peak severe weather month. In fact, the first half of 2016 was fairly “quiet” with respect to severe weather (See Figures 7a/7b).  The most active day came on June 14th when several central to northern Iowa counties reported wind damage. Heavy rain and damaging winds plagued a few counties during the evening of the 25th into the early morning on 26th (See Figure 8). Flooding was also limited during the month with only 1 river reaching flood stage and just a handful of Flash Flood Warnings issued by National Weather Service Office in Des Moines.

Blog post by Kenny Podrazik, Meteorologist, NWS Des Moines

Summer 2016 Central Iowa Fire Weather News

It has been a slow fire weather season.  The spring was cool and damp and warm season grasses were a little slower to green up.  As we transitioned into June we did warm up and were dry but grass fires, or at least grass fires that were known by the NWS, were minimal.  As we went into July, we continued to have hot spells though any period of heat through mid July was both preceded and followed by a cooler spell which again helped keep grass fires in check.

As we roll into fall we will be preparing for the fall fire weather season which will begin on September 1st.  Beginning September 1st we will be issuing the Fire Weather Planning Forecast twice a day…at 6 am and by 4 pm daily.  We will also be alert to the potential for red flag warnings as we head into harvest season then late fall.  In addition, we will use crop curing as a guide to fire danger during the months of September and October and transition to prairie grass curing in later October through mid November.   We have found that using these two different fuels in the fall better assesses the fire threat and will alert people to the proper threats.  The threats will be displayed in the form of a state map on the fire weather web page of our website.  Please check out http://www.weather.gov/dmx/fire for the latest fire weather forecasts.

Assessment of prairie grass curing is provided by local County Conservation Board employees.  They provide the National Weather Service with curing values of prairie grass on a weekly basis through the entire dry down of the grass and I would like to take this time to recognize their efforts in helping us to provide the most accurate forecast and warning service possible.

As we head into the fall season I would like to remind everyone to heed forecasts and warnings of dry conditions.  Harvesting in extreme dry and windy conditions is the number one cause of crop fires in the fall and crop fires by far exceed any other cause of fall fires.

Blog Post by Frank Boksa, Meteorologist, NWS Des Moines

Iowa Summer Weather Review

Temperatures:

Temperatures have remained warmer than normal for much of the spring into the first half of the summer.  The warm temperatures have been interrupted from time to time by cooler periods, especially in May when monthly temperatures averaged just below normal, but for the most part readings have remained warmer than normal.  The warmest readings have been across western Iowa with a couple periods of intense heat during June and July with heat index values going above 110 degrees during the third week of July.

 

Spring and early Summer temperature departure in Iowa – warmer than normal.

Spring and early Summer temperature departure in Iowa – warmer than normal.

Precipitation:

Since the beginning of March, precipitation has been most robust and reliable along the Minnesota border into east central Iowa as well as the far southwest corner of the state.  Much of the remainder of Iowa has seen at or below normal rainfall, especially in central into south central and southeast Iowa.  The month of June was particularly dry in these locations with some areas remaining below one inch of precipitation or only about a quarter of normal.  Recent storms across much of Iowa in July have helped to alleviate the incipient drought conditions in the state, although much of the rain was quickly used by maturing crops.  The map shows the percentage of normal precipitation from the March through July time frame.

Summer rainfall departures – Wet on the edges, dry central and southeast.

Summer rainfall departures – Wet on the edges, dry central and southeast.

Spring 2016 Climate Table
Blog post by Craig Cogil, Senior Forecaster, NWS Des Moines

On This Day in Iowa Weather History – May 21, 2004 Tornado Outbreak

On May 21, 2004, a significant severe weather outbreak began that morning and continued across the state throughout the day/evening and lasted well into the following morning on the 22nd. There were 16 tornadoes that occurred across Iowa including one that injured 15 people in and around the small town of Bradgate, Iowa. The Bradgate tornado was rated an F2 (before the EF scale came into effect).  Another significant tornado just west of Bradgate was a F1 in Rolfe. One thing to note, the Bradgate tornado began just south of the town of Rolfe and in this location was rated F2 (See Survey Map). Below are several damage photos from the NWS survey team along with a few aerial shots courtesy of KCCI and John McLaughlin.  There is also a quick loop of the storm relative velocity data for the Bradgate tornado. In addition to the multitude of tornadoes, large hail was reported across the state including softball sized hail in Spencer.  In fact, there were two separate incidents where softball hail fell, first with a storm around 10 am and then again with another storm around 4 pm. Some of the storms also produced very heavy rainfall and flooding with accumulations of 4.75 inches at Emmetsburg, 5.14 inches at Decorah, and 6.10 inches at Mason City.

NWS Damage Survey from the May 21, 2004 Bradgate/Rolfe F2/F1 tornadoes respectively.

NWS Damage Survey from the May 21, 2004 Bradgate/Rolfe F2/F1 tornadoes respectively.

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Blog post by Kenny Podrazik – NWS Des Moines