Drought is a costly weather situation in the United States that is of high interest to TV viewers. Droughts impact crop growth, water supply, and natural vegetation. Soil and vegetation conditions during drought create risks for wildfires and flash floods. Depending on time frame and situation, the relevance of the drought may differ, in that short-term rain deficits affect soil conditions and agriculture, while long-term droughts impact water reserves and can cause dust storms. Through several drought related products, broadcasters can convey an accurate representation of the drought risk to their viewers on a regular basis.

Palmer Drought Severity Index and Palmer Z-Index

Long-term droughts are quantified by The Palmer Drought Severity Index (PDSI), calculated by NOAA from a combination of precipitation, temperature, and soil moisture data. It represents the accumulation or deficit of water over a long-term period, about 9 months. Soil moisture data is calibrated to a homogeneous climate zone. PDSI has been widely used to recognize long-term agricultural drought and hydrological drought, and to identify the abnormality of a particular drought in a region.

Short-term droughts are calculated by the Palmer Z-Index (or short-term Palmer Index). The Z-Index corresponds to monthly drought conditions with no memory to previous monthly deficits or surpluses. This is beneficial because it is possible to have a short-term moist period in the midst of a long-term drought or vice versa. The Z-index corresponds to meteorological drought and short-term agricultural droughts.

Because the Z-index is not affected by moisture conditions in the previous month, its values can vary dramatically from month to month. On the other hand, the PDSI varies more slowly because antecedent conditions account for two-thirds of its value, although heavy rain events can dramatically change PDSI values.

Current Palmer Drought Index for the US
Current Palmer Z-Index for the US indicating short term drought

Percent of Normal Rainfall

Another drought aid being produced incorporates products from NOAA/NWS’s Advanced Hydrologic Prediction Service (AHPS). From this service, monthly rainfall deficit/surplus and percentage of normal, updated daily, will be provided to the TV market. The analysis incorporates radar-derived accumulated rainfall, which is bias corrected through validity testing against rain gauge data, as well as the rain gauge data itself, combined into a multisensor field of total rainfall. The data is then compared to a climatological dataset of monthly normal rainfall encompassing observations from 1971-2000. While this product does not truly depict drought conditions, it can be used to communicate the shorter-term trends in rainfall deficit or surplus. The monthly percent of normal rainfall accumulation can be useful to dispel the notion that any single rainfall event has ended a drought. This Percent Normal Rainfall can also explain the great difference in soil moisture in a single region when areas of rainfall surplus border areas of drought. The percent of normal rainfall can be expressed for any time frame, not just monthly, as seen below in the 90, 180, and 365 day percent of normal precipitation maps.

Drought Recovery – Rainfall Deficit in Inches

WorldWinds has developed the “Rainfall Deficit (inches)” product as a tool for drought recovery discussions. This product uses radar-estimated rainfall totals, that are bias corrected by rain gauge observations, and compares it to the climatological norm (30 year average, 1971-2000) for that time frame. This will allow the broadcaster to convey local variations in rainfall deviations (in inches) from climatology. Displaying a 30-day deficit could help signal the beginnings of a drought in an area, or signal the relief of a drought in an area, as well as convey the current effects on an agricultural season. While it does not take into account climate and soil properties, the deficit can be used as a rough estimate for the rainfall needed to end an area’s drought. Rainfall deficit is updated daily for 30-day, 90-day, 180-day, and 365-day time frames.