Sept. 24, 2015

The Climate Prediction Center (CPC) is the federal agency responsible for releasing the official 30- and 90-day climate outlooks for the United States. These outlooks are commonly released as temperature and precipitation maps showing areas forecasted to be above normal, normal, or below normal during a given period. Although the graphics are pretty, they lack sufficient information for agricultural interests to make informed decisions on the amount of moisture or expected temperatures an area is expected to receive during the forecast period.

Customized precipitation and temperature forecasts from the Climate Prediction Center can be a valuable tool in fine-tuning your cropping and management plans for the coming season. In this tutorial State Climatologist Al Dutcher guides you through how to customize the reports for your area.

Most users of CPC climate outlook graphics are probably not aware that "Probability of Exceedance" maps are available to the public. These POE maps quantify the amount of moisture expected for a given area during the forecast period.  These resources can provide information on the past 15 years for the region, as well as confidence intervals, mean precipitation expected, and how much the current forecast shifts from normal.

Step. 1.  Locate your area and associated precipitation forecast from CPC.

The CPC "Probability of Exceedance" forecasts based upon the most recent seasonal outlooks are available at http://www.cpc.ncep.noaa.gov/products/predictions/long_range/poe_index.php?lead=1&var=t

Users visiting this site will notice that the default setting is for "Probability of Exceedance" temperature maps. To get precipitation information, click on the "Precipitation POE" link in the blue left column under "Related Forecasts." You can then select a three-month period from those listed in the column or leave the default setting to get forecast information for the most current three-month forecast available.

This outlook includes two maps in the white area to the right of the "Links" section in blue.  The first map is a visual map with the areas of above normal, normal, or below normal precipitation, along with their associated probability levels.  The second map contains more detailed information on what you might expect to see for your area. You can either click on the map or click on the labeled area below the map.

If you click on area 36 (Southern Nebraska), a precipitation probability map will appear on the screen that contains a host of information including a point forecast, the point forecast anomaly, the 50% and 90% percent confidence limits, and the most recent 15 years of precipitation. Figure 1 (below) is the September-November 2015 precipitation "Probability of Exceedance" plot for southern Nebraska.

Looking at Figure 1, there are several items of interest for agricultural producers.  The most important item is the point forecast and anomaly located in blue at the top left hand corner of the graphic.  Because normal precipitation decreases the further one heads northwestward across Nebraska, users will need to convert the point forecast for southern Nebraska to a county forecast.

Step 2. Find your county's normal precipitation for the time period in question.

County level precipitation is available through the High Plains Climate Center at http://hprcc.unl.edu/datasets.php?set=CountyData

You will need to select your state and the variable you wish to examine.  Once you select your variable, the data chart on the right side of the screen will automatically update.  You can the type in your desired county in the search box and it will appear in the chart below the input boxes.

For this exercise, we are interested in Richardson County precipitation data. This indicates that normal precipitation for Richardson County is 2.57 inches for October, 1.91 inches for November, and 1.07 inches for December.  This gives a three-month value of 5.55 inches. You will notice that the value of 5.55 inches is greater than the 4.41 inches indicated as the point forecast in the upper left corner of Figure 1.

Step 3. Convert your county's normal precipitation into a precipitation forecast.

We need to convert the Richardson County normal data into a point forecast for the October-December 2015 period.  First it is necessary to convert the "Point Forecast" data into percent of normal using the following methodology:

Percent of Normal = (Point Forecast + Forecasted Anomaly) / Point Forecast

In this scenario, the Percent of Normal value becomes 1.04 by inserting 4.41 into the denominator (bottom) position and 4.41 + 0.18 into the numerator (top) part of the equation.  Your equation would look like the following:

Percent of Normal = (4.41 + 0.18) / 4.41 = 1.04

If you take the 1.04 value (104% of normal) for Percent of Normal and multiply it by Richardson County's normal September-November precipitation value of 5.55, the point forecast for southern Nebraska converted to Richardson county data becomes 5.77 inches. This converts to a point forecast of 0.22 inches above normal (5.77 – 5.55) for Richardson County.

The other useful piece of information in Figure 1 of this tutorial is the trend since 2000. At the bottom of the graphics just above the X-axis in red are the values for the forecast period with a two-digit year number (00 = 2001, … , 14 = 2014). The red asterisk indicates the average of the data since 2000. If the value is greater than 50%, the average since 2000 has been wetter than the 30-year normal (1981-2010) and if it is below than it has been drier than normal.

Integrating this Precipitation Forecast into Your Management

The remaining information contained within the CPC Probability of Exceedance maps refers to probability functions and confidence limits, which may be too complicated for the average user. However, irrigators may find these forecast charts useful in determining how much of a shortfall or surplus is projected for an area prior to the onset of irrigation season. Dryland producers will be able to determine whether significant surplus or deficit moisture is forecasted for an area, which would allow producers the opportunity to adjust populations prior to planting.

You do not need to convert average temperature projections into a county forecast because, unlike with precipitation forecasts, normal temperatures do not exhibit large variances over short distances. Normal average temperatures may only vary a couple of degrees from the southeast to the northwest corner of the state for monthly and annual data. Precipitation data can vary over 15 inches on an annual basis for this same transect. Monthly normal precipitation values can vary by 1.5 inches during our wettest months of the year.

Al Dutcher
Nebraska State Climatologist