Droughts – Intensity Notifications


The U.S. Drought Monitor is unique, blending numeric measures of drought and experts’ best judgment into a single map every week. It started in 1999 as a federal, state, and academic partnership, growing out of a Western Governors’ Association initiative to provide timely and understandable scientific information on water supply and drought for policymakers.

The U.S. Drought Monitor’s intensity scale is based upon drought impact guidelines, indicators, and index and model thresholds taken into consideration every week by rotating drought monitor authors. Examples of basic input indicators and indices include the Standardized Precipitation Index, or SPI, Palmer Drought Severity Index or PDSI, precipitation, temperature, streamflow, soil moisture, reservoir levels, groundwater levels and snow pack. In addition, duration and both regional and seasonal influences are taken into account, as well as whether a given location is improving or getting worse in terms of drought condition.[1]

The U.S. Drought Monitor is produced in partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture (USDA) – Drought Response, and the National Oceanic and Atmospheric Administration (NOAA) – Climate Prediction Center.

Example US Drought Monitor Map[2]

The following are Drought Intensity Categories issued by the U.S. Drought Monitor service in the order of severity:[3]

Drought Category D0 corresponds to 'Abnormally Dry' conditions. At this level, an area experiences short-term dryness that is typical with the onset of drought. Such dryness can slow crop growth and elevate fire risk to above average. This level also refers to an area coming out of drought, with lingering water deficits and pastures or crops not fully recovered. D0 is noted when a convergence of indicators fall into the 30th percentile. In other terms, this equates to roughly a 1 in 3 year dryness.

Note: that for the D0 level, the area in question is only considered to be dry, not necessarily in drought conditions.

An intensity level of D1 corresponds to 'Moderate Drought' conditions. At this level, some damage to crops and pastures can be expected. Fire risk is high, while stream, reservoir, or well levels are low. Some water shortages are developing or imminent and voluntary water use restrictions could be requested. In terms of percentile rankings, D1 is the first "drought" class and falls into the 20th percentile, or a 1 in 5 year type event.

An intensity level of D2 corresponds to 'Severe Drought' conditions. At this level, crop or pasture losses are likely, fire risk is very high, water shortages are common, and water restrictions are typically voluntary or mandated. This category is the 10th percentile, which roughly equates to a 1 in 10 year drought.

An intensity level of D3 corresponds to 'Extreme Drought' conditions. At this level, major crop and pasture losses are common, fire risk is extreme, and widespread water shortages can be expected requiring restrictions. Duration and impacts are critical in looking at D3 droughts, which fall into the 5th percentile, or a 1 in 20 year type of drought.

The most severe intensity level, D4, corresponds to 'Exceptional Drought' conditions. At this level, there are exceptional and widespread crop and pasture losses, fire risk, shortages of water in reservoirs, streams, and wells that yield water emergencies. Worst on the scale, D4 can loosely be likened to a "once-in-a-generation" type of drought noted by the 2nd percentile, or a 1 in 50 year drought.


Drought Prediction and Outlook

The most prominent predictive product focused on drought for the general public is the U.S. Seasonal Drought Outlook. This product is issued on a bi-weekly schedule by NOAA’s Climate Prediction Center. For the period extending from the release date to the end of the third month following the release date, the Outlook characterizes medium-term trends in drought persistence, enhancement, and reduction. The Seasonal Drought Outlook tracks areas experiencing moderate drought or worse, corresponding to the intensity levels of D1 or higher as defined by the U.S. Drought Monitor, and is verified using the change in drought intensity over the forecasted period.

Example US Seasonal Drought Outlook Map[5]


The following are Drought Change Categories issued by NOAA’s Climate Prediction Center (CPC) in the order of severity:[4]

Drought to Persist or Intensify means that drought intensity categories as measured by such composite indicators as the U.S. Drought Monitor either do not change or grow worse.

Drought Ongoing with Some Improvement indicates more limited improvement than designated by the other improvement category, usually because surface moisture conditions are expected to improve while hydrologic drought persists, or the improvement is expected early or late in the forecast p

Drought Likely to Improve with impacts easing implies at least a 1-category improvement in the Drought Monitor intensity levels. This results in drought impacts easing, meaning the impact on crops, water supplies, etc. is less serious.

Drought Development Likely means that drought has a better than 50 percent chance of developing sometime in the forecast period.



  1. Understanding Drought (Printable Version): http://www.drought.gov/drought/content/understanding-drought-printable-version#p4_4
  2. Image Source: http://droughtmonitor.unl.edu/ – [Accessed August 16,2013]
  3. U.S. Drought Monitor: http://droughtmonitor.unl.edu/ ; Understanding Drought (Printable Version) – http://www.drought.gov/drought/content/understanding-drought-printable-version#p4_4
  4. U.S. Seasonal Drought Outlook: http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html
  5. Image Source: http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html – [Accessed August 16,2013]