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North Carolina has an active community of drought monitors through the Drought Management Advisory Council (DMAC), which meets weekly to evaluate conditions and make recommendations on the depiction of drought severity as part of the US Drought Monitor. Current drought conditions are synthesized from a range of drought impacts and indicators. These include precipitation over various intervals; streamflow, reservoir, and groundwater levels; drought indices, such as the Standardized Precipitation Index or Palmer Drought Indices; as well as reports of impacts to agriculture, forest health, and utilities. Information about North Carolina's specific conditions can be found under the "Current Conditions" section of the DMAC's webpage while more national information can be found on the US Drought Monitor's website. Descriptions of some commonly used drought indices, which are standardized measures of drought based on varying initial conditions, follow.

The Standardized Precipitation Index (SPI) is a precipitation-based drought index that relates an observed precipitation amount to its historical probability. The primary advantage of SPI is that it can be calculated for any time scale, giving it the ability to monitor simultaneously occurring events, such as a short-term wet spell embedded within a much longer-term drought. Additionally, since SPI is normalized to the historical precipitation time series for a given location, SPI values from different locations - which may have different climates - can be compared without modification. Values represent the number of standard deviations away from the mean, which is centered at zero, and typically range within ±2. Negative numbers indicate drier than normal while positive numbers indicate wetter than normal conditions. A variety of agencies calculate the SPI over various spatial and temporal scales. The NC SCO calculates a daily-updated SPI by incorporating a variety of surface gauge-based and gridded datasets which can be found here. Additionally, the SPI is computed by NCDC at a climate division level for several time scales, ranging from one month to 24 months.

The Palmer Drought Severity Index (PDSI) attempts to measure the duration and intensity of drought through a simplified climatic balance that takes into account precipitation, temperature, and the available water content of the soil. It was designed primarily for use as an agricultural index but has an inherent time scale that ranges from 9 to 12 months. PDSI values are centered at zero and typically stay within ±6, with negative values indicating drier than normal conditions and positive values indicating wetter than normal conditions.

The Palmer Hydrological Drought Index (PHDI) is a long-term drought index that tries to reflect hydrological impacts, which accumulate and diminish more slowly. Consequently, the PHDI responds more slowly to changing conditions than the PDSI, but its values are also centered at zero and can range from -6 to +6.

The Palmer "Z" Index is a short-term drought index that attempts to reflect how the moisture conditions depart from normal. Its shorter time scale makes it more suitable for agricultural drought monitoring. Like the PDSI and PHDI, Z-index values can range from -6 to +6 and are centered at zero.

The Modified Palmer Drought Severity Index (PMDI) was instituted by the National Weather Service Analysis Center, which modified the PDSI for operational meteorological purposes. The selection for the modified PDSI value for a given month is made based on probabilities, in that the modification incorporates a weighted average of the wet and dry index terms using probability as the weighting factor. In general, PMDI and PDSI values will be the same during an established wet or dry spell, but will differ during periods of transition.

Additional Drought Indicators

Other useful information for drought monitoring includes heating and cooling degree days. These are calculated by finding the difference between the temperature on a specific day and a base temperature to which a home would be heated or cooled. Both values are calculated through comparison with a base temperature of 65°F. These values are provided as a reference along with temperature and precipitation, and are typically used to estimate energy consumption. Climate division heating and cooling degree day data for North Carolina from the National Climatic Data Center is available in the Historical Data section of this page.

While it is not currently available on our website, the SPEI (Standardized Precipitation Evapotranspiration Index) is another drought index which attempts to calculate a more accurate water balance for multiple time scales with the inclusion of the effects of temperature on evapotranspiration. As the index becomes available the High Resolution Drought Trigger Tool page will be changed to include the new data.

More Information

More information on these drought indices as well as others not discussed here can be found on the National Drought Mitigation Center's website.

Recent data for SPI and other drought monitoring data sets are available as part of our Experimental Drought Trigger Tool. This tool provides high-resolution drought index information for the contiguous United States and will continue to be updated with more drought indices and drought monitoring information.

Percent of normal precipitation is offered as part of the Experimental High Resolution Drought Trigger Tool, although it is not available through the historical data page. It is calculated by simply dividing observed precipitation by normal precipitation and converting to a percentage. This method assumes that precipitation is normally distributed, which is rarely the case. SPI corrects this by using a normalized distribution. Accumulated Multi-Sensor Precipitation Estimates, which use many data sources such as radar, satellite, and rain gauge measurements to provide a better estimate of local precipitation, are also available from this experimental tool.

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