Now that some seasonal weather has arrived, people are interested in the Winter Outlook. Below are the Climate Prediction Center’s outlooks for temperature and precipitation during the winter season -- December through February.
The temperature outlook is neutral, with a slight tendency towards warmer-than-normal temperatures in southwest Kansas. That tendency increases as you move farther south and west in the Plains. It is important to remember that this is the 3-month average. There could be significant cold periods and still have an overall warmer-than-normal winter. One difficulty with that pattern is that neither crops nor livestock develop strong winter hardiness and can be more severely affected by the occasional extreme cold snap.
For precipitation, the outlook is also neutral. It is equally likely to have above- or below-average precipitation over the period. Winter is normally the driest time of the year for most of the Plains. Southeast Kansas is an exception, with a more even distribution across the year. It is also worth noting that neither the temperature nor the precipitation outlook predicts the degree to which conditions will vary. A tenth of a degree warmer than normal would validate the outlook as much as 10 degrees warmer. A hundredth of an inch greater than normal would have a similar result in the precipitation outlook. Significantly wetter conditions, similar to last winter, would be needed to improve the drought conditions in the western Plains.
The major force driving the current outlook is the ENSO signal. At this time it has a weak La Niña pattern – that is, cooler-than-normal waters in the Pacific along the Equator. During a La Niña winter there is typically a more zonal flow across the continental U.S., with the polar jet steering systems across the Northern Plains. Given the weak nature of the La Niña, other factors may have a stronger influence. Two patterns that deserve attention are the North Atlantic Oscillation (NAO) and the Eastern Pacific Oscillation (EPO). Both are comparisons of high and low pressure in the respective basins.
When the NAO is negative (with a weak gradient between high pressure in the subtropics and low pressure over Iceland), the east coast of the U.S. tends to have stronger cold outbreaks, with more snow. Some of that can clip the eastern Plains region. When the EPO is in the negative phase (with strong pressure to the north and low to the south), as it is currently, there tends to be increased cooling in the Central and Eastern US.
Unfortunately, both the NAO and EPO conditions can change rapidly, and forecasts for these patterns are not as well developed as for the ENSO. That makes it difficult to gauge their impacts on an extended basis.
Mary Knapp, Weather Data Library