New Weather Satellite Will Improve Forecasting and Warnings

On November 19, 2016, the GOES-R weather satellite was successfully launched into space from Cape Canaveral, FL. After reaching geostationary orbit a little over a week later, GOES-R was renamed GOES-16.  While GOES-16 is still undergoing testing, once fully operationally later this year it will provide your National Weather Service meteorologists along with many other users with faster updates, higher resolution images, and more (spectral) channels than any GOES satellite ever before. This satellite will also provide continuous monitoring of lightning activity. It’s essentially like going from standard definition, black and white television to high definition, color television.

GOES-16 will boast 5 times faster updates. With the current satellites, images are typically received every 15 minutes. With GOES-16, images will be received every 5 minutes and during high-impact weather they can be received every minute or even every 30 seconds! This means that we’ll be able to view thunderstorms in near real-time as they develop and before they are even detected on radar.

Images from GOES-16 will be high resolution. The current GOES satellites provide 1 kilometer visible images with infrared and water vapor images at 4 kilometer. With GOES-16, visible image will be at 0.5 kilometers with infrared and water vapor images at 2 kilometers. Along with faster updates, this means we’ll view finer features important to forecasting and warning operations.

Finally, GOES-16 will have 3 times the number of spectral channels. Basically, a spectral channel is able to ‘see’ different features better than another spectral channel. For example, one of the new satellite channels can differentiate between land and water while another channel will be able to monitor high-level cirrus clouds.

What this all means locally to the public and the partners we serve is increased lead time for thunderstorm and tornado warnings, enhanced general and commercial aviation forecasts, better detection of grass and cropland fires, and improved fog formation and dissipation forecasts.

Check out the high resolution image comparing GOES-13 to GOES-16 imagery:

Blog post by: Andrew Ansorge, Meteorologist

Hazard Services – Improving Hazard Communication

Currently, National Weather Service (NWS) forecasters must use several different software applications to compose and provide warnings, watches and advisories and related information for hazardous weather.  Each tool also has different abilities to support the forecasters in their decision-making.  Since each tool is uniquely designed, forecasters must learn each one and be able to quickly switch between them multiple times while on duty.

Hazard Services–an AWIPS software application presently in development–is an integral part of the NWS’s Weather-Ready Nation vision.  Hazard Services represents a paradigm shift in how the NWS will communicate hazard information and aims to streamline NWS operations by integrating the functionality of the aforementioned tools into a single interface for providing timely, accurate and actionable hazard information.  Within this single interface, Hazard Services will analyze data from various inputs and assist the forecaster in diagnosing and communicating the hazard information.  In addition, Hazard Services will also shift the present focus on legacy text products to multiple pathways of communication such as social media, cell phones, graphics, text and XML.  Thus, Hazard Services will act as a conduit for transforming leading-edge science into timely, accurate and actionable information to the end user in ways that meet their needs.

Hazard Services is also designed to be highly configurable, flexible and extensible.  While work continues on the operational version for NWS local forecast offices, experimental efforts are underway to extend Hazard Services to regional and National offices such as River Forecast Centers and National Centers.  An added advantage of using the same application across various levels of the NWS is the opportunity to share forecast information for consistency and collaboration which will help facilitate a unified NWS message.

Thus far, most of the Hazard Services development and testing work has involved hydrologic hazards such as river floods and flash floods.  Hydrologic hazards were chosen for the initial work because local NWS offices presently use three different software applications to inform its partners and users about them—the most of any hazard.  All other weather-related hazards require only one or two of the three software applications.  In addition, hydrologic hazards have uniquely complicated considerations regarding watches, warnings, advisories and outlooks for them.  Once the complexities of the three different software applications—as well as the inherent complexities of hydrologic hazards can be successfully addressed—then the lessons learned can be used address the other weather-related hazards.

Jeff Zogg, Senior Service Hydrologist at NWS Des Moines has been involved in Hazard Services development and testing for a few years.  He also participates in the national Hazard Services Tiger and Test Teams.  These teams help guide software enhancements, fixes and tests.  Jeff has also participated in some of these tests.  During the tests Jeff was part of a group of approximately five people who used Hazard Services as they would during an actual hazardous weather event.  The tests lasted around two to three days and used weather and water data from actual past events.  Testers documented software performance and stability issues—as well as enhancement suggestions—and then submitted them to the Tiger Team for prioritization on the list of items for software developers to address.

To date, 16 formal tests have occurred.  Two additional tests are planned through mid-September.  Within the several months, a readiness review will be conducted to determine if the process can move to the next step which involves assessment tests at four or five different sites around the country.  There, Hazard Services will be put through additional rigorous but longer-term testing for a variety of different hazards.  Once Hazard Services passes another readiness review after that step, it will be tested at local NWS offices.  A final readiness review will then determine if and when Hazard Services will be ready to become operational and thus replace the existing software applications.  Although the exact timetable of these steps is unknown, it is hoped to be completed within the next few years.

Blog post by Jeff Zogg, Senior Service Hydrologist, NWS Des Moines

IRIS Comes to NWS Des Moines

The NOAA/National Weather Service’s goal of building a Weather Ready Nation is all about building relationships with our core partners including emergency managers, first responders, government officials, businesses and the general public and helping them make better decisions to save lives and property and enhance livelihoods.  Communicating with everyone can be a daunting task as the National Weather Service in Des Moines has contact information for thousands of core partners and over 4,700 SKYWARN severe weather spotters.

One of our newest tools to help gather and disseminate information is called IRIS which is an acronym for Integrated Real-time Impacts and Services.  This is a web based application that integrates current weather data and keeps a database of all contacts allowing our staff to call partners and spotters and disseminate Local Storm Reports quickly and easily with just a few clicks of a button. With IRIS, spotters can note their information by name and/or spotter number with their report sent out in just a matter of seconds.  Under previous software, severe weather reports would need to be located with other geocoding software before being sent to the media and the world.  It also has the capability of parsing out subsets of our large database and emailing those folks when necessary. 

IRIS also monitors dozens of local observations alerting NWS staff of critical wind speeds and precipitation accumulations and making that information easy to disseminate with just a few clicks of a mouse.  Future capabilities also include enhancing our ability to provide critical Decision Support Services to our core partners by cataloging key events and impacts important to them. Operational staff will be notified when weather elements reach critical levels.

In order for IRIS to work efficiently and optimize its information our partners and spotters need to have current location and contact information noted. If you are a member of one of these key groups and have had your address, phone number and/or email address change recently please let us know so we can update our information and database.


Blog post by Brad Small, Senior Forecaster, NWS Des Moines

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

15-Year Anniversary of Agency, Iowa Tornado

On April 10-11, 2001, a significant severe weather outbreak produced 28 tornadoes across Iowa which was part of a region wide 2-day tornado outbreak that spawned 79 tornadoes across Nebraska, Iowa, Kansas, Missouri, Oklahoma, and Texas.  The 28 tornadoes in Iowa became an all-time state record for tornadoes in a single day.  The majority of these tornadoes were toward the weaker end of the Fujita scale.  However, an F2 struck Agency, Iowa in Wapello County that caused two deaths and three injuries.  Another tornado, a long-lived tornado, tracked from northern Missouri to Madison County and caused extensive damage in Ringgold County.  In addition to the widespread tornadoes, large hail was also reported ranging up to golf ball size at Parkersburg and there were scattered reports of 70 to 85 mph wind gusts with the worst straight-line wind damage in Black Hawk and Franklin counties.

A very strong low pressure system moved from central Kansas into southeast Nebraska on April 11, 2001.  A warm front extended east along the Missouri/Iowa border during the late morning and surged northward throughout the afternoon (see the 3 hand drawn surface analysis below).  A dryline oriented northwest-southeast moved from southwest Iowa into central to south-central Iowa in the afternoon. Storms erupted along this dryline in the afternoon, but there were also storms that focused along the warm front.  As you can see from Figure 1 below, the radar operator had a very busy day identifying the rotation within each storm that were rapidly moved north-northeast through the afternoon hours. There were 12 tornadoes that occurred within the NWS Des Moines county warning area. 


Figure 1: Operator identified low-level mesocyclone tracks on 11 April 2001 between 1900-2130 UTC (2:00 PM CDT to 4:30 PM CDT). The dots indicate location of the rotation center every 5 minutes and the dash indicates that a “cell” was still identifiable on radar, but no velocity couplet was found. “FO” or “F1″ to the right of the tracks indicate location of tornadoes and Fujita scale intensity. The “M” to the left of track indicates WSR-88D mesocyclone detection and the “MA” has a base above 5 km. From Figure 1 via Karl A. Jungbluth; “The Tornado Warning Process During a Fast-Moving Low-Topped Event: 11 April 2011 in Iowa.”


1800 UTC (1:00 PM CDT) 11 April 2011

2000 UTC (3:00 PM CDT) 11 April 2011

2100 UTC (4:00 PM CDT) 11 April 2011


F1 damage to a barn three miles northwest of Murray, Iowa along the Union and Clarke county line. Three cows were killed along with multiple large trees down. NWS Des Moines Survey.

F3 damage to an apparently well built home. The destruction included the roof being completely gone and several interior and exterior walls destroyed. Location of the home was along highway 2 one mile east of Mount Ayr, Iowa. NWS Des Moines survey.

F2 damage along J55 three miles north of the Missouri border, or seven miles south of Mounty Ayr, Iowa. A partial underground home severely damaged. Roof completely off and destroyed. Interior walls and main wall facing south remained intact limiting damage to F2. NWS Des Moines survey.

F2 damage along P38 1.5 miles north of the Missouri border. Weak structure, or old house, that was completely destroyed. This was similar to a mobile home being destroyed. NWS Des Moines survey.

agengy dmg photoA damaged home in Agency, Iowa.

Blog post by Kenny Podrazik – NWS Des Moines

Reference: Karl A. Jungbluth; “The Tornado Warning Process During a Fast-Moving Low-Topped Event: 11 April 2011 in Iowa.


Training…So Much Training!

Post by Allan Curtis – Meteorologist Intern

DLOCEver wonder what sort of training meteorologists in the National Weather Service (NWS) go through during their first couple of years? Well, the short answer is a plethora. The amount and type of training can vary a bit depending on the office and area of the country. One piece of training I attended recently, and every meteorologist must go through, is called the Distance Learning Operations Course (DLOC). DLOC is typically attended within the first 12-18 months after being hired by the National Weather Service and is probably the bellwether piece of training provided by the NWS.

The name itself does not give much of an explanation for what it is. So what is DLOC then? It is a crash course in Dual Polarization Radar including its history and predecessors, its nuts and bolts, conceptual models, the algorithms used to create radar products and how to interpret and utilize them during severe weather warning scenarios. And that is just the short explanation. The course covers applications that can be used year-round, including winter weather, but really concentrates heavily on convective weather. One of the primary goals of the course is to prepare meteorologists to issue severe weather warnings, such as severe thunderstorm, tornado, flash flood warnings, and more. In all, the 2014-2015 course included 80 online modules, three instructor led webinars (equaling the “distance” part in the course name), and lastly, a full week at the National Weather Center in Norman, OK for additional classroom sessions and real-time case studies.

All of the online modules and webinars built up a knowledge-base, some of which was new and some was a refresher from college, to be applied, not just during the week in Norman, but throughout one’s career as a meteorologist in the NWS. The week of on-site training was really where headway was made in terms of determining whether or not you knew the material, were able to apply it, and helped highlight areas to be worked on and polished in the future. Certainly no one exited DLOC a bona-fide expert, but it provided a rock solid base and highlighted areas for growth. A typical day during the on-site training included a morning classroom session that highlighted a special topic, such as tornado forecasting or flash flooding, and a related afternoon lab session that allowed direct application of knowledge and simulated weather events from real cases across the country. The lab sessions then consisted of teams of three working together to analyze and issue any and all necessary warnings for a given event. The events were run in real-time in order to simulated the actual severe weather event itself. The simulations were supervised by instructors and they answered questions, provided insight, and pointed out nuances throughout the event. The lab sessions were able to drive home the importance of application of knowledge, time management, team work, and communication, of which would be impossible if strictly done at a distance like the online modules and webinars.

Generally, any degree bearing meteorologist can identify a severe thunderstorm or tornado worthy of a warning for a picturesque thunderstorm on the plains of Texas through Nebraska, but what about lines of storms or clusters of storms? Such as in the Washington D.C. area? Or the Columbia River Valley in Washington state? Or even in the desert areas of Nevada and Arizona? The instructors were cognizant of the text book situations often found on the plains, and were instead eager to provide cases that were not as clear cut and required a more thorough analysis and solid knowledge base. At the end of the week, the result was a worn out group of attendees that gained a better appreciation for what goes into making a warning decision and an understanding that most situations are not clear cut. Another result of the on-site training that is often overlooked and under-appreciated was the networking that began at the course that will ultimately result in colleagues and friends to fall back on for knowledge and support throughout a career.



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


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

A Look Inside the DMX WSR-88D Radar

Here are some photos taken from inside the Des Moines (DMX) WSR-88D Doppler Radar.

Radome-07292014 (1)

Radome-07292014 (2)

View looking down the steps from inside the dome of the radar. To the base of the radar is 100 feet and the dome itself if 24 feet tall.

Radome-07292014 (3)

The back of the radar dish and the inside of the large “soccer ball” you see from the road. The “soccer ball” is to protect the radar the weather elements.

Radome-07292014 (4)

On the backside of the radar dish, weights are needed for balancing. Also pictured here is the dual-pol AME (antenna mounted equipment) attached to the side of one of the arms holding the dish.


The transmitter part of dual-pol.




New Upgrades to the Des Moines Radar

A new software upgrade has been installed at the Des Moines WSR-88D radar that will enable the radar to obtain low level scans more frequently in severe weather events. Previously, the WSR-88D scanned the atmosphere at progressively higher angles to create a 3D profile of a storm. These scans would begin at 0.5 degrees above the horizon (the lowest angle possible) and end at a maximum angle of 19.5 degrees. These scans form what is known as a Volume Coverage Pattern, or VCP. This maximum angle can vary depending on the distance the storms are from the radar. Last year, a feature called AVSET (Automatic Volume Scan Elevation Termination) was installed that allows the radar to automatically restart a VCP if the radar beam travels above the storms.

With this latest upgrade, a new feature called SAILS (Supplemental Adaptive Intra-Volume Low‐Level Scan) will enable the radar to insert an additional 0.5 degree scan in the middle of a VCP. See the illustrations below for more details.

Why is this Important?

When it comes to severe weather, frequent low-level radar scans are crucial to observe the development of tornadoes, which can form in a matter of seconds. Thus, with SAILS the NWS to be able to observe rapidly changing weather phenomenon with a greater degree of precision and issue more timely severe weather warnings. Currently, the WSR-88D radar completes its lowest scan in 3 to 4.5 minutes (during severe weather), depending on the range of the storms from the radar (AVSET). With SAILS, the radar will now perform this low-level scan every 1.8 to 2.5 minutes, giving us low-level data almost twice as fast as before.

Other New Features

This upgrade will also bring several new enhancements besides SAILS. One new addition will be a radial noise filter, which will greatly reduce the “spikes” seen on the radar image at sunrise and sunset (see example from the Minneapolis radar below). Another new feature will enable the radar to automatically determine the best settings for viewing velocity data for the strongest storms in the radar’s coverage area.

A “sun spike” removed from the Minneapolis radar while radars to the north and south (which had not been upgraded yet) still contain these spikes.

(Courtesy of the Minneapolis NWS)

Click here for technical details on SAILS