Understanding NOAA’s monthly climate outlooks
Outlook schedule
On the third Thursday of every month, NOAA’s Climate Prediction Center issues an outlook for temperature and precipitation for the United States for the coming month. These outlooks are updated on the last day of each month. Those updated outlooks are the basis for the posts that appear on Climate.gov each month.
Outlooks are not the same as weather forecasts
Most people understand a forecast to be a prediction of the high (or low) temperature or precipitation amount likely to occur on a given day. NOAA’s monthly and seasonal outlooks do not predict specific monthly (or seasonal) temperatures or precipitation amounts. Instead, they try to predict which of three possible outcomes is most likely to occur: above-average, near-average, or below-average monthly mean temperature or total precipitation.
To qualify as above or below average, the Climate Prediction Center requires the monthly temperature or precipitation to be in the upper or lower one-third of values observed from 1991-2020 for a given month. To distinguish this definition from the more literal definition, we often describe the categories as “well above (or below) average,” or sometimes “much wetter (or drier) than average”. If forecasters think all three possible temperature or precipitation outcomes (above-, near-, and below-average) are equally likely, or have no basis for selecting one category over the others, they issue an “equal chances” forecast.
There's more to a monthly climate outlook than meets the eye. A full monthly temperature outlook includes the probability for all three possible monthly average temperature outcomes: well above average, near average, and well below average. The colors forecasters choose for the map show (1) whether one possibility is more likely than either of the other two and (2) how strong the chances of that outcome are. The climate is never 100 percent predictable, which means that even when one temperature outcome is favored, the others remain possible. NOAA Climate.gov map, based on data from the Climate Prediction Center.
About the maps
White areas do not mean average
White areas mean "equal chances". In those areas, forecasters think that all three possible monthly temperature or precipitation categories are equally likely. In other words, the chances of a warmer-than-average month are the same as the chances of a near-average or a cooler-than-average month.
Blue areas
Blue areas mean a much cooler than average month is more likely than a near-average or a much warmer than average month. Darker blues mean higher chances of a relatively cold (lowest one-third) month—not increasingly extreme cold.
Orange/red areas
Orange to red areas mean a much warmer than average month is more likely than a near-average or a much cooler than average month. Darker reds mean higher chances of a relatively (highest one-third) warm month—not increasingly extreme heat.
U.S. map showing the favored precipitation outcome for May 2023 for each part of the contiguous United States (view Alaska). Green colors indicate much wetter than average conditions are favored, while brown colors mean much drier than average conditions are favored. White areas mean equal chances for above- near-, or below average precipitation. NOAA Climate.gov map, based on data from NOAA Climate Prediction Center.
Green areas
Green areas mean a much wetter than average month is more likely than a near-average or a much drier than average month. Darker greens mean higher chances of a relatively (highest one-third) wet month—not increasingly wetter conditions.
Brown areas
Brown areas mean a much drier than average month is more likely than a near-average or a much wetter than average month. Darker greens mean higher chances of a relatively (lowest one-third) dry month—not increasingly drier conditions.
Outlook math
In contrast to the “equal chances” forecast, sometimes things are happening in the climate system that tilt the odds toward or away from the default climatological frequency (an equal 33.3 percent chance for each outcome). When that happens, forecasters subtract the most likely probability (for example, a 60 percent chance of above-average temperatures) from 100 percent and divide the probability of the other two categories over the remainder (40 percent).
The remaining probabilities (40 percent in our example) are not just split evenly between the two remaining categories, however (near- and below-average temperatures). Physically speaking, it makes sense that if the odds have tilted toward one extreme climate outcome by a given percent, they’ve tilted away from the opposite extreme outcome by an equal amount. So by convention, forecasters divvy up the remaining probability by holding the chances for near-normal conditions at the “default” 33.3 percent, and then subtracting that from the total remainder (40-33.3). What’s left (6.7 percent) is the probability of the less-favored category.
When the odds of one climate outcome are so high (more than 63.3 percent) that the remaining fraction is smaller than 36.7 percent, forecasters set a minimum 3.3 percent chance for the opposite outcome, with the remainder being the probability of a near-normal outcome. (The least favored category never decreases below 3.3%, as there’s always some degree of uncertainty in any climate forecast and always some small chance of a surprise outcome).
A full monthly temperature outlook is made up of three probabilities: the chances for a much warmer than average month, the chances of a near-average month, and the chances of a much cooler than average month. As the chances of one outcome increase, the chances of the opposite outcome decrease—but never all the way to zero. NOAA Climate.gov graphic, adapted from Climate Prediction Center histogram.
Seeing what static maps don’t show
In the real world, the probability for a single category will never be 100 percent. We will never be able to say a month in advance that there is a 100 percent chance of a wetter-than-average May, for instance. The climate is not that predictable. The full monthly climate outlook, then, will always include three separate probabilities for each location.
But only one color can be shown on a static map, so forecasters show the color of the most likely category—red and blue for above- or below-normal temperature, green or brown for above- or below-normal precipitation. It’s easy to mistake that single color for a specific forecast, and to not realize that that the other outcomes remain possible.
The Climate Prediction Center’s interactive map view allows users to explore the complete three-part outlook. The map on the right displays the outcome that is most likely: a 40-50 percent chance of a much warmer than average June. The pie chart at top left shows the full outlook: 46 percent chance of a much warmer than average June, 33 percent chance of a near-average June, and 21 percent chance of a much colder than average June. Explore map.
If you’d like a more complete picture of the monthly climate outlook, the Climate Prediction Center offers an interactive map that allows users to see the full suite of probabilities for a given location.
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