Weather
This post first appeared on climate.gov and was written by Tom Di Liberto
Spring across the United States is prime weather-whiplash season. One day it could be sunny and 75° F. The next, it might be snowing. But on a monthly timescale, what is favored for April 2021? The April 2021 outlook from NOAA’s Climate Prediction Center predicts a tilt in the odds towards warmer-than-average temperatures across much of the contiguous United States and drier-than-average conditions across the southern tier.
A reminder: the climate outlook maps are not a forecast for the absolute temperature or precipitation amounts in April. Instead, they are the probability (percent chance) that April temperatures or precipitation will be in the upper, middle, or lower third of the climatological record (1981-2010) for April. (Note: the climatological record will shift to 1991-2020 later this year). We refer to these categories as “well above” and “well below” average. The colors (red or blue for temperatures, brown or teal for precipitation) indicate which outcome is the most likely. Darker colors reflect higher chances of a given outcome; not more extreme conditions. Head to the end of this post for more on the math behind the outlooks.
Lots of warmth except in the Pacific Northwest
Like the outlook last month, the Climate Prediction Center’s April forecast favors temperatures to lean on the warmer-than-average side for the vast majority of the contiguous United States. The highest likelihood for a warmer-than-average April is over the western Great Lakes (60-70%), though the entire Plains has over a 50% chance (50-60%) of observing above-average monthly temperatures.
The only area of the Lower 48 forecast to see odds tilted to below-average temperatures are across the Pacific Northwest (though the southern half of Alaska also has the same colder-than-average forecast).
Normally when forecasters are discussing the influences on the monthly outlook, phrases like ENSO, Arctic Oscillation, and the MJO are thrown around. But one influence you might not think about is how much moisture is in the soil. In places with dry soil (a soil moisture deficit), temperatures could be even hotter as less energy can be “diverted” from heating to evaporating water from the soil. The opposite holds true, too. Wet soil can tamp down temperatures. In April, the large soil moisture deficits over the West, especially the Southwest, are part of the reason the forecast favors above-average monthly temperatures.
Drier-than-average conditions favored parts of the southern United States
For the Gulf Coast, the signal in April is similar to what’s common when a La Niña is occurring in the Pacific. And that signal is a dry one. The tilt in the odds towards a drier-than-average month extend from the Southeast towards the western Plains and Rocky Mountains. This means that once again the forecast favors less precipitation than usual across a region of the country—the Southwest and West—locked in an extended and deep drought.
The one area in the contiguous United States where April precipitation tilts towards wetter than average is across North Dakota, Minnesota, and Wisconsin. Western Alaska also is favored to observe a wetter-than-average April.
While these climate forecasts are monthly, what happens with the weather during the first couple of weeks can have a large impact on the monthly averages. For instance, in northern California, southern Oregon, western Nevada, and western Idaho, the forecast for the first ten days of April is for near-average precipitation. After that the precipitation signal gets muddled. But because of that expected precipitation during the beginning of the month, the likelihood for a below-average precipitation month was decreased compared to the April outlook made in the middle of the March.
Dry, dry, dry, dry, dry, dry, dry
Nearly 44% of the country is currently in some form of drought with nearly 9% in the worst drought category, D4-exceptional drought. The epicenter of that drought is located across the Southwest stretching from the Mexican border north through Utah and Colorado. But drought is pretty much present across the entire West and northern Plains.
And the next month’s forecast provides little relief. In fact, the drought outlook for April has drought expanding in Texas, Colorado, and Kansas, as well as drought developing across Florida and the Northeast. However, the wet forecast for Minnesota and North Dakota is expected to help improve drought conditions there.
To read the entire discussion of the monthly climate outlooks from the Climate Prediction Center, check out their website. And head back to Climate.gov later this month for a United States and global climate recap of March 2021.
