Comparison of Soil Moisture Response in Hurricanes Harvey and Irma

Comparison of Soil Moisture Response in Hurricanes Harvey and Irma

After a record [nearly] 12 years between landfalling major hurricanes [cat 3 or higher], the United States has now experienced two major hurricanes making landfall less than three weeks apart from one another.  Hurricane Harvey brought exceptional record rainfall to southeastern Texas and southwestern Louisiana because it stalled shortly after landfall due to a lack of atmospheric steering currents.  Less than 3 weeks later, Major Hurricane Irma made landfall twice in Florida: once in the Lower Keys and again near Marco Island on the southwestern coast.  A long-lived cat 5 hurricane prior to landfall, Irma had a very large wind field which resulted in far-reaching impacts along the Florida East Coast, up to Charleston, SC, and inland to Atlanta, GA, with millions of households and businesses without electricity and/or water.

Here at the NASA SPoRT Center, we have been closely monitoring these two hurricanes through numerous social media and blog posts of unique satellite products and through SPoRT’s real-time instance of the NASA Land Information System (“SPoRT-LIS”).  This blog post serves to compare the soil moisture responses to hurricanes Irma and Harvey rainfall, as depicted by the real-time SPoRT-LIS output.  The Relative Soil Moisture (RSM) variable is shown throughout this article, since it takes into account the variations in soil composition by scaling the moisture availability between the wilting point (plants cannot uptake moisture) and saturation point (soil cannot hold any more water).  The SPoRT-LIS runs the Noah land surface model, which estimates soil moisture through 4 layers: 0-10, 10-40, 40-100, and 100-200 cm depth.  We first examine the response during Irma in the top 0-10 cm layer, followed by 0-100 cm layer for both storms, and then compare the total column (0-200 cm) values relative to historical values from a climatological database spanning 1981-2013 (33 years).

Figure 1 compares the weekly rainfall accumulation primarily from Hurricane Irma over the Southeastern U.S. to the August monthly rainfall total over Texas/Louisiana, primarily contributed from Hurricane Harvey during the final week of August. Rainfall from Irma was quite substantial in the Florida peninsula up to coastal South Carolina, where numerous locations measured over 10″ of rain in less than 2 days. Rainfall of 3-5″ extended inland to northern Georgia and central South Carolina, with lesser amounts generally below 3″ across eastern and northern Alabama (Fig 1, left panel).  The highest totals were along the southwestern and eastern Florida coasts.  This rainfall still pales in comparison to the widespread 20″+ that fell across a huge part of southeastern Texas and western Louisiana, albeit over a 5-6 day span.  Highest totals exceeded 50″ near Beaumont/Port Arthur, TX!

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Fig 1.  Comparison of weekly rainfall estimate associated with Hurricane Irma (left), and August monthly rainfall estimate associated with Hurricane Harvey (right).

The 0-10 cm RSM animation in Fig 2 for hurricane Irma shows how quickly the top soil layer responds to incoming rainfall within the Noah land surface model in SPoRT-LIS.  The heavy rainfall rates up to 4″ per hour or more led to a quick saturation during 10 September across the Florida peninsula, eventually extending up to coastal Georgia and South Carolina on the 11th.  Similarly, as rainfall ends we can see the 0-10 cm RSM quickly decrease from south to north as the moisture infiltrates into deeper model layers and/or evaporates back to the atmosphere.  We also see that the top soil layer does not completely saturate across interior Georgia and Alabama, likely due to lower rain rates, drier initial soils, and different soil composition compared to the fast-responding sandy soils across Florida.

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Fig 2.  Hourly animation of SPoRT-LIS 0-10 cm relative soil moisture (RSM) and Multi Radar Multi Sensor (MRMS) quantitative precipitation estimates (QPE) from 0000 UTC 10 September through 1200 UTC 12 September 2017, associated with Hurricane Irma.

Meanwhile, the RSM averaged over the top 3 layers (0-100 cm; Fig 3) takes a longer time to moisten up during the heavy rainfall of Irma. We do see values approaching saturation across southwestern, central, and particularly northeastern Florida near the end of the rainfall event as the deeper soils have had an opportunity to recharge.

Over southeastern Texas and Louisiana (Fig 4), the 0-100 cm RSM animation shows how the prolonged, training heavy rainfall led to near saturation of the top meter of the Noah model, despite dry antecedent conditions (especially west of the Houston metro, where the RSM transitioned from less than 10% to nearly saturation!).  The much longer rainfall duration with hurricane Harvey led to sustained higher values of soil moisture in the top one meter.

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Fig 3.  Hourly animation of SPoRT-LIS 0-100 cm RSM and MRMS QPE from 10-12 September 2017, associated with Hurricane Irma.

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Fig 4.  Hourly animation of SPoRT-LIS 0-100 cm RSM and MRMS QPE from 25-30 August 2017, associated with Hurricane Harvey.

Finally, the total column 0-200 cm layer can require months or years to respond to rainfall events (or lack thereof), depending on the soil composition.  However, with major rainfall events like hurricanes Harvey and Irma, the total column RSM does respond dramatically and subsequently can depict substantial wet anomalies.  To that end, the SPoRT-LIS has a daily, county-based climatological database of modeled soil moisture from 1981-2013 from which current conditions can be compared to depict anomalies via percentiles relative to the 33-year distribution.  Fig 5 shows these percentiles color-coded to depict dry anomalies (less then 30th percentile) or wet anomalies (greater than 70th percentile) according to the scales beneath the figure.

Following hurricane Irma, we see that portions of southwestern and northeastern Florida have 0-200 cm RSM greater than the 98th percentile, as well as parts of west-central Georgia (Fig 5; left panel).  In general, the extreme wet percentiles are fairly spotty across the domain.  However, following hurricane Harvey (Fig 5; right panel), the 0-200 cm RSM percentiles are “off the charts” high, with dozens of counties experiencing soil moisture exceeding the [33-year] historical 98th percentile.  In fact, the soil moisture was SO anomalously moist following hurricane Harvey that the average daily value across all of Jefferson County, TX (Beaumont/Port Arthur) exceeded all values in the entire 33-year database by the end of August!  This unusual condition is highlighted in Fig 6, which shows a daily animation of historical 0-200 cm RSM histograms for Jefferson County, TX, with the current 2017 county-averaged values in the vertical dashed line.  We see that by the end of hurricane Harvey, the vertical dashed line is well above any values from the 33-year historical distribution, thereby quantifying how exceptionally unusual this rainfall event was in southeastern Texas.

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Fig 5.  SPoRT-LIS 0-200 cm RSM percentile, valid at 1200 UTC on 12 September 2017 (post-Irma; left), and 30 August 2017 (post-Harvey; right).

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Fig 6. Animation of daily distributions of 0-200 cm RSM for all SPoRT-LIS grid points residing in Jefferson County, TX (Beaumont/Port Arthur) during the month of August 2017.  Gray bars are the frequencies of 0-200 cm RSM from the 33-year SPoRT-LIS climatology; colored vertical lines are reference percentiles according to the legend in the upper right; and the bold vertical dashed line is the county-averaged value for the present day in August 2017.

Soil Moisture Conditions over Southeast Texas Prior to Hurricane Harvey

Soil Moisture Conditions over Southeast Texas Prior to Hurricane Harvey

As much-anticipated Hurricane Harvey approaches the southern and eastern coast of Texas today, it is worth examining the pre-existing soil moisture over the region to understand the capacity of the land surface to absorb the upcoming rainfall.  Granted, the amount of rainfall simulated by numerical guidance is off-the-charts high (e.g., today’s 0600 UTC initialized NAM model [Fig. 1] shows 84-hour maximum accumulated rainfall of over 60″ between Corpus Christie and Houston!!).  Thus, extreme flooding is anticipated, regardless of the amount that can be absorbed by the soils.

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Figure 1.  The NCEP/NAM model 84-hour forecast of total accumulated precipitation (inches) over Southeastern Texas, from the simulation initialized at 0600 UTC 25 August 2017 [image courtesy of College of DuPage forecast page].

SPoRT manages a real-time simulation of the NASA Land Information System (hereafter, “SPoRT-LIS“), running over the Continental U.S. at ~3-km grid resolution.  The SPoRT-LIS product is a Noah land surface model climatological and real-time simulation over 4 model soil layers (0-10, 10-40, 40-100, and 100-200 cm).  The climatological simulation spans 1981-2013 and forms the basis for daily-updated total-column soil moisture percentiles (forthcoming in Fig. 3), in order to place current soil moisture values into historical context.  For real-time output, the Noah simulation is regularly updated four times per day as an extension of the long-term climatology simulation.  It includes NOAA/NESDIS daily global VIIRS Green Vegetation Fraction data, and the real-time SPoRT-LIS component also incorporates quantitative precipitation estimates (QPE) from the Multi-Radar Multi-Sensor (MRMS) gauge-corrected radar product.  The climatological SPoRT-LIS is based exclusively on atmospheric analysis input from the NOAA/NASA North American Land Data Assimilation System – version 2.

Relative Soil Moisture output from the SPoRT-LIS over the 0-100 cm layer is shown in Fig. 2 over Southeastern Texas and Louisiana at 1200 UTC this morning.  A marked gradient between very dry soils to the west and moist soils to the east occurs in the vicinity of the greater Houston metropolitan area.  The soils in the region bounded by Corpus Christi, San Antonio, Austin, and Houston (areas forecast to have the greatest rainfall from Hurricane Harvey) are extremely dry prior to Harvey’s landfall.  This dryness will help to some extent in absorbing the initial rainfall from Hurricane Harvey.  But with such excessive rainfall being forecast over a prolonged time period (3-5+ days), it won’t be long before the upper portions of the soil column saturates and widespread areal flooding occurs.  In addition, the high forecast rainfall rates could easily result in flash flooding (despite prevailing soil dryness), especially further inland where terrain plays a more important role in runoff and flash flooding.

The total column relative soil moisture percentile from 24 August shows that historically-speaking, the soil moisture is slightly drier than normal, particularly along the coastal plain between Corpus Christi and Houston (Fig. 3).  In this corridor, the soil moisture is generally between the 10th and 30th percentile compared to the 1981-2013 climatological distribution for 24 August.

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Figure 2.  SPoRT-LIS relative soil moisture (RSM) distribution in the 0-1 meter layer across Southeastern Texas and Louisiana, valid 1200 UTC 25 August 2017.  RSM values of 0% represent wilting (vegetation cannot extract moisture from soil) and 100% represents saturation (subsequent rainfall becomes runoff).

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Figure 3.  Total column (0-2 m) relative soil moisture percentile valid 24 Aug 2017, as compared to all 24 August soil moisture values from a 33-year climatological simulation of the SPoRT-LIS.

Finally, an hourly animation of the 1-day changes in 0-10 cm (top model layer) relative soil moisture show that the near-surface soils are quickly moistening between Corpus Christi and Houston, as the initial rainbands of Hurricane Harvey began impacting the coastal plain this morning.  As the soils continue to moisten rapidly from the top-down, subsequent rainfall will quickly lead to runoff and flooding.

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Figure 4.  Hourly animation of 1-day change in top-layer (0-10 cm) relative soil moisture, for the time period spanning 0000-1400 UTC 25 August 2017.  Each hourly image is a simple difference in 0-10 cm relative soil moisture between the current and previous day at the same valid hour.  Line contours depict one-hour QPE from the MRMS product, as input to the real-time SPoRT-LIS.

Detecting tornado tracks using Synthetic Aperture Radar (SAR) imagery

NASA SPoRT has been working to support the NWS’s use of the Damage Assessment Toolkit (DAT) by integrating multiple satellite datasets into the DAT framework to assist in damage surveys.  Imagery from MODIS, VIIRS, and Landsat 8 are available daily within the application while imagery from higher resolution satellites, such as Terra ASTER and other high-resolution commercial imagery are facilitated by our partnership with the USGS’s Hazard Data Distribution System (HDDSexplorer.usgs.gov). One new area being explored is the application of Synthetic Aperture Radar (SAR) imagery to detect tornado damage.

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Zoomed-in section of the SAR change detection RGB generated from Sentinel-1B imagery from May 10 and May 22, 2017.  The damage indicators show preliminary track information as of June 2, 2017 and are not considered final.

On the evening of May 16th, 2017, a supercell tracked across Wisconsin producing a strong tornado. The resulting 83-mile long track tornado produced EF-3 damage.  Shown in the image above is a prototype change detection RGB using data from Sentinel-1(A/B), a European Space Agency (ESA) satellite with a SAR instrument on board. Unlike optical sensors, which observe surface reflectance and temperature, SAR instruments measure backscatter from the surface, allowing the instrument to be used at all times of the day and in any sky conditions. SPoRT has been working with the Alaska Satellite Facility, NASA’s SAR Distributed Active Archive Center (DAAC) to receive these products for evaluation and put them in the DAT to help with the identifying of damage tracks, especially in areas where damage surveys can be more challenging (i.e. forested areas, poor road network).  The RGB takes advantage of the dual polarization from the sensor, assigning the VV and VH corrected polarization from the post-event granule to the red and green channels of the RGB, respectively.  The blue channel is a difference image of the VH polarization (same as what is used in the green channel) from the before and after granules.  The resulting RGB will show any changes between the two granules in a aqua/periwinkle/purple-color.  Although the RGB will show all change between the granules over the ~12-day period (i.e. agricultural growth), tornado tracks tend to be linear, making it a possible to discern/identify the damage track.  Without the hindrance of clouds that constantly plague damage detection in optical imagery, SAR imagery offers another tool to operational forecasters for use during damage surveys.  The team is also working on other change or anomaly detection techniques to facilitate mapping of tornado and severe weather damage.

Stark contrast in Eastern U.S. soil moisture following Hurricane Matthew

Stark contrast in Eastern U.S. soil moisture following Hurricane Matthew

Major Hurricane Matthew left a trail of destruction in its wake from the Caribbean up through the U.S. East Coast.  As Hurricane Matthew tracked northward along a large portion of the U.S. Southeast Coast from Florida to North Carolina, the rainfall impacts worsened.  Figure 1 shows the weekly rainfall spanning 4-11 October, ranging from ~2-8 inches along the Florida East Coast to 10-20 inches in the eastern Carolinas.  Since antecedent soil moisture was highest in the eastern Carolinas (Fig. 2), the extreme rainfall led to the most serious flooding in this area.

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Fig. 1.  Weekly rainfall totals from 4 – 11 October 2016.

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Fig. 2.  Total Column (0-2 m) relative soil moisture prior to Hurricane Matthew’s impact on North and South Carolina, valid at 0000 UTC 7 October 2016.

Referring back to the precipitation totals in Fig. 1, we can see that there was a sharp rainfall gradient on the northwestern edge in the Middle Atlantic region.  Interestingly, this gradient in Hurricane Matthew’s rainfall coincided with a pre-existing transition zone between wet conditions near the Atlantic coast and drought conditions further inland from the Appalachians through New England.  The net result was to accentuate the wet-dry contrast already in place.  The animation in Fig. 3 highlights this contrast nicely by presenting the SPoRT-LIS daily total-column relative soil moisture percentiles from 1-12 October.  The percentiles are based off a 1981-2013 daily soil moisture climatology that SPoRT produced from its ~3-km resolution SPoRT-LIS simulation.  By 9 October, notice the incredible transition from excessively wet soil moisture exceeding the 98th percentile (Carolinas through the southern half of Delaware) to extremely dry soil moisture less than the 5th percentile across Pennsylvania into the Northeast (as well as much of the inland Southeastern U.S.).  In fact, total column soil moisture values are less than the 2nd percentile over a large part of Ohio, Pennsylvania, New York, and the New England states, indicative of the ongoing severe drought there.

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Fig. 3. Daily animation of SPoRT-LIS total column relative soil moisture percentile from 1 to 12 October 2016.

Louisiana Flooding Captured by SPoRT-LIS

About a week ago, southern Louisiana began to experience heavy rainfall from a storm system that remained relatively stationary over the Gulf Coast.  The SPoRT-LIS, a real-time, high-resolution implementation of the the NASA Land Information System, captured trends in soil moisture that provide some insight into the evolution of this flooding, including hints at precursor conditions that may have led to the extreme nature of this event.

The 0-200 cm integrated relative soil moisture (RSM) fields have been used in the past to identify flood precursor conditions.  These fields give an indication of the total amount of water in the soil moisture column and provide information about how much additional precipitation can be accepted by the soil before all becomes runoff into nearby streams and rivers.  About 2 weeks ago (August 3 00Z; Fig. 1), Southern Louisiana showed soil moisture values in the 50% range, which are higher than other parts of the country, but likely about normal given the swampy nature of that region.  However, following a couple of precipitation events in that area on August 3, 7, 9, and 10), these integrated RSM fields bump up the 60-65% range (Fig. 2), which has become somewhat of an unofficial threshold for antecedent saturated soils that could lead to areal flooding events.  Based on various reports, it appears that the official start of the flooding event began on August 11.

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Fig. 1: SPoRT-LIS valid at 00Z on 03 August 2016 showing 0-200 cm integrated relative soil moisture values around 50% over Southeastern Louisiana.

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Fig. 2: SPoRT-LIS 0-200 cm integrated relative soil moisture values valid at 00Z on 10 August 2016 showing impact of multiple precipitation events since the 03 August figure above. Soil moisture values are elevated in southeastern Louisiana to values around 60-65%.

 

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Fig. 3: SPoRT-LIS 0-200 cm integrated relative soil moisture values valid at 00Z on 14 August 2016 following the heaviest precipitation. Soil moisture values are above 90% in most areas, indicating major ongoing flooding across much of southern Louisiana.

Starting on 11 August, the 0-200cm integrated RSM begins to exhibit signs of flooding (starting to get into 70-80%; not shown).  By Aug. 12, most of SE LA is above 80% Integrated RSM with pockets above 90% (not shown).  By Aug. 14 (Fig. 3), nearly all of southern Louisiana is covered with soil moisture values above 85-90%, which indicates major ongoing flooding in this area.

These products are provided to select National Weather Service partner offices to aid in these flooding forecast challenges.  For more details on this product and to view additional days or hours, please visit the real-time SPoRT-LIS page.

April 27, 2011–Five Years Later: A Satellite Imagery Perspective

On April 27, 2011, a severe weather outbreak occurred across the southeastern United States, resulting in 199 tornadoes across the region and over 300 fatalities (NWS 2011 Service Assessment).  Alabama was among the states hardest hit, with 68 tornadoes surveyed by the National Weather Service (NWS) Weather Forecast Offices (WFOs) in Huntsville, Birmingham, and Mobile, Alabama, and over 250 reported fatalities in the state. Huntsville, home to NASA’s Marshall Space Flight Center and the Short-term Prediction Research and Transition (SPoRT) Center, lost power along with most of Madison County after tornadoes severed major utility lines.  The power outage lasted well over a week in some areas. Once power was restored, SPoRT team members were able to provide satellite imagery to our partners in the National Weather Service to help clarify some of the high-intensity tornado damage tracks that occurred throughout the state. SPoRT provided pre- and post-event difference imagery at 250 m spatial resolution from the Moderate Resolution Imaging Spectroradiometer (MODIS) and 15 m false color composites from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). These surveys helped our NWS partners confirm their ground surveys, but also helped to correct the characteristics of several tracks (Molthan et al. 2011). Many of these products remain available through the SPoRT web page (link) and also through the USGS Earth Explorer portal (link).

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The MODIS Band 1 difference image above shows some of the scars left behind by the April 27, 2011 tornado outbreak. Radar snapshots were taken from various times to identify the supercell thunderstorms associated with each track.  Reproduced from Molthan et al. 2011.

Follow-on studies examined the capability of various NASA sensors for detecting and measuring the length and width of scars visible when using the Normalized Difference Vegetation Index, or NDVI, a measurement of vegetation greenness and health commonly derived from multiple satellite imaging platforms.  SPoRT examined NDVI products from MODIS (250 m), Landsat-7 Enhanced Thematic Mapper Plus (ETM+, 30m) and ASTER (15 m) collected in May and June 2011. Possible tornado tracks were identified, mapped, and were then measured to compare against the official NWS damage surveys.  In general, many of the major tornadoes (defined here with maximum intensity EF-3 and greater) were at least partially visible at resolutions of 15-250 m, though weaker tornadoes or those that occurred in complex terrain were more difficult to detect using NDVI and a single snapshot in time. As tornadoes initiated and increased in intensity, or dissipated and decreased in intensity, some of their characteristics became more difficult to detect.  However, some weaker tornadoes were also apparent in Landsat-7 imagery (30 m) in well-vegetated areas.  A summary of the study is available as a publication in the National Weather Association’s Journal of Operational Meteorology. In 2013, SPoRT received support from NASA’s Applied Sciences: Disasters program to partner with the NWS and facilitate the delivery of satellite imagery to their Damage Assessment Toolkit (DAT).  The DAT is used by the NWS to obtain storm survey information while in the field. Satellite imagery from NASA, NOAA, and commercial sensors (acquired in collaboration with USGS and the Hazards Data Distribution System) helps to supplement the survey process by providing an additional perspective of suspected damage areas.

Many of the damage scars apparent from the April 27, 2011 outbreak exhibited signs of recovery and change in the years following the outbreak.  Other tornado events also brought additional vegetation damage and scarring to the region. With five years passing since the 27 April 2011 tornado outbreak, annual views of cloud-free imagery have been obtained from the Landsat missions, operated and managed as a collaboration between the USGS and NASA.  In the viewer linked below, SPoRT has collaborated with the USGS Earth Resources Observation Systems (EROS) Data Center to acquire 30 m true color and vegetation index information from Landsat 5, Landsat 7, and Landsat 8 during the late spring and summer months when local vegetation is at its greenest, allowing the greatest contrast between damaged and undamaged areas. Users can take a look at these images in a web viewer that allows toggling between different products and years, view some of the tornado tracks surveyed by the NWS following the April 27, 2011 event, and zoom into areas of interest to examine how some of the affected areas have evolved over time:

Tuscaloosa, AL

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The above animation shows the year before and years after the EF-4 tornado impacted the Tuscaloosa area. The tornado track has seen a significant recovery, but a scar still remains in 2015. In addition to seeing how the landscape as recovered from tornado, development in and around Tuscaloosa is also apparent.  Missing pixels in 2012 are due to an issue with the Landsat-7 imager.

Hackleburg-Phil Campbell

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Similar to the Tuscaloosa animation, this animation shows the recovery of the EF-5 tornado that moved through Hackleburg and Phil Campbell, before tracking northeast across the Tennessee River.  Missing pixels in 2012 are due to an issue with the Landsat-7 imager.

SPoRT Disasters Team Provides Post-Storm Imagery to WFO Des Moines, IA Office (DMX)

Following the 10 May 2015 tornado (EF1) near Lake City, IA, SPoRT Disasters team members began to acquire commercial satellite imagery through our partnership with the USGS. Once a request for imagery was made by folks in NWS Central Region, images hosted on USGS’s Hazards Data Distribution System (HDDS) web portal were obtained for processing and distribution to the Des Moines, IA Weather Forecast Office (WFO). Concurrently, teams from the DMX WFO were deployed to assess and survey damaged areas. As survey teams collected information from the field, damage points and polygons were uploaded to the NWS Damage Assessment Toolkit’s (DAT) beta damage editor as early as 12 May 2015.

By 14 May 2015, the Disasters team was able to process and disseminate commercial satellite imagery (in this case SPOT-6 panchromatic) to the DAT application for post-event damage assessment and verification by the office. Ground scarring and swirling (see red inset in the image below) from the tornado were evident as a linear feature moving SW to NE through the image. Once the imagery was delivered, members of the DMX office were able to corroborate their original damage polygon with ground scarring in the imagery. It was in this corroboration that surveyors noted divergence in the two paths, as highlighted by the figure below, and subsequently revised their outputs.

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SPOT-6 Panchromatic image (1.5 m resolution) of an EF1 tornado track near Lake City, IA. Image acquired on 12 May 2015.

Kevin Skow, one of the surveyor’s from the DMX office who worked the event, provided the Disasters team positive feedback on the usefulness of this imagery with regards to their work, “…the modifications you are seeing in the DAT are a direct result of what we found in the satellite data. A second storm produced wind damage within a few miles of the actual tornado path that same night. The ground survey team thought that this damage might be from the tornado. However, the satellite data showed that the path was further to the NW. The satellite data also helped us fine tune the path north of Lake City. The satellite data has proven once again to be a great asset for our storm surveying operations.”

Even with some latency, this event demonstrates the importance of providing satellite imagery to forecast offices during (and following) severe weather. While satellite imagery cannot replace the work performed during ground surveys, this type of data has the ability to provide yet another source of information for surveyors to utilize during their response efforts.