The winter temperature and precipitation outlooks were recently released for December through February by the National Weather Service Climate Prediction Center.
Justin Glisan, state climatologist with the Iowa Department of Agriculture, and Dennis Todey, director of the Midwest Climate Hub at Iowa State University, offer their interpretation.
Looking at the short term as winter begins, they don’t think December will be too unusual — maybe a little colder than normal over the northwestern part of the Midwest but close to normal in the southeastern part. As for precipitation, it will probably be near normal in Iowa and most places in the Midwest.
In winter, the phase of the El Niño Southern Oscillation climate signal typically influences temperature and precipitation behavior. The neutral phase of ENSO is now in place. So, in the absence of an El Niño or La Niña, long-term climatology and recent trends become the dominant seasonal predictor.
Winter weather near normal
Other climate patterns are also considered, such as the phase of the Madden-Julian Oscillation and Arctic Oscillation. A negative phase of the AO produces shifts in the storm track into the Midwest along with colder air masses. A positive AO shifts the storm track farther north and generally keeps colder air out of the region. These patterns unfortunately are not forecastable more than a few weeks ahead of time.
Temperature-wise, much of the Upper Midwest, including Iowa, is classified as having an “equal chance” (EC) of above-, below- or near-average climatological behavior. Near-average temperatures are given slightly higher probability of occurring (34%) when EC is present with above- and below-average probabilities equally split at 33%.
“We have a better handle on the expectations for precipitation this winter,” Todey says. “We are seeing a distinct signal for precipitation for the southern part of Iowa to having a slightly elevated chance of wetter-than-normal conditions, with probabilities increasing into northern Iowa.”
The NWS Climate Prediction Center does not produce a snowfall outlook, as winter season systems are extremely variable and are often not predictable more than a week out, Glisan says. The longer-range outlooks continue the above-average precipitation chances into early spring.
Weather impacts on agriculture
Moving into winter, weather forecasters are always mindful of agricultural impacts with a particular focus on moisture variables, including precipitation and subsoil conditions. “As we closed out October 2019, southwestern Iowa has had anywhere from 2 to 5 inches of above-average rainfall,” Todey says. “Much of Iowa has had rainfall amounts 4 to 8 inches above normal over the last 60 days.”
With recent wetness, concerns about abnormally dry and drought conditions have vanished. Modeled subsoil moisture is near capacity. If you think of a sponge, current conditions indicate the sponge is almost soaked.
“We will need to monitor snowfall in the upper Missouri basin as stream flows are near to above average; flooding could continue to be an issue for southwest Iowa moving into spring 2020,” Glisan says. “Early snowfall across much of the state of Iowa during the last few days of October added additional soil moisture and further slowed some harvest operations.”
The good news is, while November is likely to be colder, precipitation chances are weak, and no major storm systems are showing up yet. For a look at the weather prediction maps, visit Winter Weather Outlook for 2019-2020.