National Drought Mitigation Center

Regional Probability of Exceedance

How to use the tool

There are three dropdown menus to choose from. The first, “Region”, lets you select the northern part of the Great Plains, the southern part of the Great Plains, or the Arizona/New Mexico region of Grass-Cast. These regions were not scientifically defined, but were named to make them easier to find on a map. The second dropdown, “Amount (in)”, allows you to select a minimum precipitation amount. The third dropdown, “Start Date”, lets you select a start date for keeping track of precipitation. The colors on the map will then tell you the percentage chance, based only on past years’ weather data, that you’ll get more than the precipitation amount you selected between your start date and the end of August.

Amount (in.)
Start Date

In Brief

These maps show the chances that, from your selected starting date until the end of August, you’ll see total precipitation amounts higher than a certain number of inches. From the dropdown menus, please select a start date and then click a precipitation amount (in inches) to see the chances of getting more than that in your area through the end of August. The cooler the colors, the better the chance that you’ll get more than the selected amount of precipitation; the warmer the colors, the lesser the chance. To match up your location on the map with a probability value (the percent chance of getting at least the amount of precipitation you’re interested in), please refer to the color bar on the right side of the map labeled “Historical Probabilities (%)”.

Please note that the percentages on the maps are not human-made forecasts. Rather, they are purely based on historical precipitation data collected at weather stations. The more years any location has seen at least a certain amount of precipitation fall during the weeks and months you’re interested in, the higher the probability you’ll see on the map.

Finally, please disregard any data outside of the boundaries of the U.S., as the data feeding the probabilities there is erroneous.

In Depth

For this project, our goal was to calculate the probability of different precipitation thresholds being met from certain points during the growing season through the end of August (as forward-looking windows). These probabilities were derived from historical climate data. Historical precipitation data was considered on a grid, for the years 1950-2022, with data gridded from Applied Climate Information System (ACIS) station records. For more information on the gridded ACIS precipitation data, please see the metadata at Multiple methods were considered for computing precipitation exceedance probabilities. For each grid cell, the sum of daily precipitation for a growing season from the date in question (every two weeks starting from April 1) through the end of August was considered for each year in the record. A basic probability was then derived from the historical record, where the number of years with precipitation from date_0 through August 31 that exceeded the threshold was expressed as a probability.

Mathematically, P = (N_years_exceed_threshold/N)*100, where N is the number of years in the record.

As a proof of concept, this process demonstrated that the probabilities can be calculated efficiently with the daily gridded precipitation dataset. In future work, we may reformulate this, such that a gamma distribution is assumed for the historical precipitation timeline in question. This way, the threshold amount may be added to the end of the time series, and its SPI calculated. From the SPI value of the threshold amount, the probability of that amount being exceeded may then be calculated.