Methods for Developing NRDC's "Extreme Weather Map 2011"

Resource
December 05, 2011

Updated February 2012

The project goal was to map "record-breaking" weather events that occurred in 2011 within the 50 United States and explore the implications of these types of events with a changing climate. We created an animated map that tracks extreme weather events over the course of the year.

A. Criteria for Events' Inclusion in the Map: Record-Breaking

"Record-breaking" was defined as exceeding the monthly maximum for each event type over the past 30 years. We included two different types of weather event information to build the "Extreme Weather Map 2011": (1) specific record-breaking weather events linked to a meteorological station or monitoring location (i.e., point events with latitude and longitude); and (2) record-breaking events that covered larger, multi-state areas and that were notable for their large geographic extent, unusual intensity, or that generated significant damage costs that have already been estimated at over $1 billion.

B. Link to Climate Change

Furthermore, we were interested in mapping some of the types of extreme weather events that have occurred in 2011 and whose occurrence is linked to the influence of climate change. With the November 18, 2011 Intergovernmental Panel on Climate Change (IPCC)'s SREX report—"SREX" being the acronym for The Summary for Policymakers of the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation—some of the linkages between climate change and extreme events have been drawn even more sharply than ever before. For example, the SREX summary finds at least a 66 percent chance that extreme temperatures and coastal extreme high water (which contributes to flooding) have worsened as a result of human activities. And looking to the future, SREX projects that if carbon emissions continue unabated, it is likely that the frequency of hot days will increase by a factor of ten in most regions of the world; that heavy precipitation will occur more often; and that the wind speeds of storms will increase (see the IPCC SREX Press Release [PDF]). It's likely, too, that climate change will intensify drought in the future and that, coupled with extreme heat, wildfire risks will increase.

On the other hand, there are other types of extreme events for which the net influence of climate change is not yet understood fully. These include extreme events like tornadoes, which occurred in 2011 and inflicted significant damages and tragic effects in US communities. Because additional studies are needed to determine the potential influence of climate change in affecting tornadoes' occurrence and severity, we chose to not include these types of events.

C. Event Data Sources

We used the following data sources:

(1) Temperature, rainfall and snowfall records

We mapped record temperature, rainfall, and snowfall using the National Oceanic and Atmospheric-National Climatic Data Center (NOAA-NCDC) data available online for each state. We looked at the monthly records broken at meteorological stations in 2011.

  • Record Temperatures: Monthly Highest Maximum Temperature records and Monthly Highest Minimum Temperature records (i.e., daily records that were higher than recorded temperatures previously set for that month in the period of record for that temperature station) were compiled for 2011. Records, by state from January through November, were downloaded by month and compiled as of January 4, 2012 from NOAA-NCDC. The NOAA-NCDC dataset is based on the historical daily observations archived in NCDC's Cooperative Summary of the Day dataset, and on preliminary reports from Cooperative Observers and First Order National Weather Service stations, and as such is subject to change. (Data was downloaded from these sites:
    http://ncdc.noaa.gov/extremes/records/monthly/maxt/2011/08/00?sts[]=US and http://ncdc.noaa.gov/extremes/records/monthly/himn/2011/07/00?sts[]=US.)

    Values that only tied with prior monthly temperature records were not counted as broken, for mapping purposes, and were removed from the dataset. The Period of Record (POR) represents the number of years with a minimum of 50 percent data completeness. All data was from stations with a POR of at least 30 years.

    The Record Temperature icon means that the monthly highest maximum temperature, the monthly highest minimum temperature, or both exceeded the previous records set at meteorological stations located within the designated county in a given month.
  • Record Rainfall: The record-breaking events we used are daily records that are higher than recorded rainfall for any day in that month in the period of record for that station. Records broken in the US in 2011 were downloaded (in inches) from the NOAA-NCDC site. Values that only tied with prior monthly rainfall records were not counted as broken, for mapping purposes, and were removed from the dataset. NOAA-NCDC records were based on the historical daily observations archived in NCDC's Cooperative Summary of the Day data set and on preliminary reports from Cooperative Observers and First Order National Weather Service stations, and as such are subject to change. The Period of Record (POR) represents the number of years with a minimum of 50 percent data completeness. All stations have a Period of Record of at least 30 years.

    The Record Rainfall icon means that at least one day of record breaking rainfall occurred at a meteorological station(s) located within the designated county in a given month.
  • Record Snowfall: The record-breaking events we used are daily records that are higher than recorded snowfall for any day in that month in the period of record for that station. Records broken in 2011 were downloaded (in inches) from the NOAA-NCDC site. Values that only tied with prior monthly snowfall records were not counted as broken, for mapping purposes, and were removed from the dataset. These records are based on the historical daily observations archived in NCDC's Cooperative Summary of the Day data set and on preliminary reports from Cooperative Observers and First Order National Weather Service stations, and as such are subject to change. The Period of Record (POR) represents the number of years with a minimum of 50 percent data completeness. All stations have a Period of Record of at least 30 years.

    The Record Snowfall icon means that at least one day of record breaking snowfall occurred at a meteorological station(s) located within the designated county in a given month.

(2) Wildfires

In 2011, there were wildfires which broke individual state records; and the total acreage burned in Arizona, New Mexico and Texas was record-breaking. We used the NCDC State of the Climate Report monthly summaries to identify those wildfires which broke state records; and identified all large wildfires, as defined by the National Interagency Fire Center (greater than 100 acres), in Arizona, New Mexico and Texas as contributors to the record-breaking burned acreage. The Rocky Mountain Geographic Science Center online database provided location information for these wildfires. These locations are marked with a wildfire icon and are displayed in the monthly animation according to the date, or span of dates, included in the database.

(3) Extreme Drought

The Palmer Hydrological Drought Index (PHDI), a measure of cumulative soil moisture conditions and hence drought integrated over prior months, was used as a source of data on statewide drought and in which months it achieved "record-breaking" lows. Reports are updated monthly by NCDC/NOAA online (see the October 2011 "State of the Climate-Drought" report). Four southern states experienced record-breaking drought in various months of 2011: Texas (July, August, September); New Mexico (July, August; also had its lowest-ever monthly precipitation in June); Louisiana (August, October); and Florida (September).

To represent the geographical area impacted by extreme drought in 2011, we mapped those areas identified as experiencing "Exceptional Drought" (category D4) by the National Drought Mitigation Center's Drought Monitor.

Drought icons represent locations within areas designated as experiencing exceptional drought.

(4) Extreme Flooding

In 2011, there were record-breaking flood events along the lower Mississippi, in the upper Midwest, and in mid-Atlantic and New England states as a result of Hurricane Irene and Tropical Storm Lee. For the Mississippi and Midwest flooding, we were able to digitize NCDC maps describing the high flood risk area to depict the flood-impacted area. Because these maps were unavailable for the flooding caused by Hurricane Irene and Tropical Storm Lee, we mapped those stream gauge stations where record-breaking peak flows were recorded in the relevant time period by the USGS WaterWatch program. Record breaking peak flows were defined as where the peak flow was ranked #1 (highest) and there were at least 30 years of records. It is important to note that this excluded some newer stream gauge stations which recorded record breaking flows but did not have 30 years of data. This was particularly true in Vermont.

Lower Mississippi flood: Flooded area is depicted as a polygon based on the map found at http ://www1.ncdc.noaa.gov/pub/data/cmb/special-reports/2011-spring-climate-extremes/ustatus_miss-II.jpg and extreme flood icons represent approximate locations within this area.

Upper Midwest flood: Flooded area is depicted as a polygon based on the map found athttp ://www.noaa.gov/extreme2011/midwest_flood.html and extreme flood icons represent approximate locations within this area.

Hurricane Irene flooding: Extreme flood icons represent stream gauge stations with record breaking peak flows from August 20 - August 29, 2011 as identified using the WaterWatch flood tool. (http://waterwatch.usgs.gov/new/?id=wwdp2_2)

Tropical Storm Lee flooding: Extreme flood icons represent stream gauge stations with record breaking peak flows from September 3 - September 9, 2011 as identified using the WaterWatch flood tool. (http://waterwatch.usgs.gov/new/?id=wwdp2_2)

Multi-state Events

For multi-state area events, news stories provided a secondary source of information, along with NOAA-NCDC maps and reports on the record-breaking number of events whose damage costs each have topped $1 billion this year. NOAA-NCDC's Billion-Dollar US Weather-Climate Disasters siteDr. Jeff Masters' blog from Weather Underground, and other news outlets have reported on these events. In all cases, when we could locate specific data on a large-scale weather event that was "record-breaking," it was eligible for inclusion on the map.

The boundaries defining the extent of large-scale storm events aim to capture the general geographic area of the extreme weather being described. Not every location or county within the geographic area shown experienced the particular storm event being described. The 10 area events included on our map (and their dates, in parentheses) follow. Area boundaries were estimated visually from satellite or weather map imagery from NOAA-NCDC information available online for each event unless otherwise described below:

  • Extreme Snow Storms (January)
  • Extreme Storm: Groundhog Day Blizzard (1/31-2/3)
  • Extreme Snowstorm (4/14-4/16)
  • Extreme Floods: (April-May) - See extreme flood description above for description of the data source for the polygon
  • Heat, Drought & Wildfires (April-July)
  • Extreme Floods (May-July) - See extreme flood description above for description of the data source for the polygon
  • Extreme Drought (July-September) - See extreme drought description above for a description of the data source for the polygon.
  • Hurricane Irene (8/20-29)
  • Tropical Storm Lee (9/3-8)
  • Extreme Storm: Early Snow (10/29)

Updates/changes from preliminary (December 2011) map

The following additional data was available in January 2012 to revise and update the preliminary map:

(1) Temperature, rainfall and snowfall records

We added record breaking events from November and December 2011, completing the whole year's worth of data.

(2) Wildfires

We located the database with specific locations for wildfires in 2011, allowing us to be more precise in placing our wildfire icons, and we updated the area impacted to reflect the record that was set based on the full year. Although the first three quarters of 2011 set a record for total acreage burned on a national level, this record did not hold for the entire year. We therefore restricted our depiction of the record-breaking wildfire impact to those states where the record was broken (AZ, NM, TX) and removed the multi-state event called "extreme wildfires" which was based on the record set for only a limited portion of the year.

(3) Extreme Drought

We located geographically more precise information on the areas experiencing extreme drought in 2011 and were able to update the shape of the multi-state event polygon.

(4) Extreme Floods

We located geographically more precise information on the areas experiencing extreme floods in 2011 and were able to update the shape of the multi-state event polygon and icons.