Ready or not, winter will be here soon. Some of us are wishing for lots of snow, while others are hoping it doesn’t get too cold. So what can we expect this season?
Seasonal forecasts are difficult because there are many factors that can influence our weather. To forecast the weather locally out several weeks and months in advance, we look globally to predict locally. While those factors can and will change as the season progresses, one of the big factors that could impact our weather this season is the presence of a La Nina event in the Pacific Ocean.
La Nina is a pattern caused by the upwelling of colder than normal ocean water in the central Pacific Ocean. Even though it is thousands of miles away, it can play a big role in our weather locally by dictating where the polar jet stream sets up. The opposite of La Nina is El Nino, when the water temperatures in the central Pacific are warmer than average.
While not always the case, most La Nina winters feature a strong jet stream that dips into the middle of the country, separating colder air to the north from warmer air to the south. With this setup, our area is usually in the “battleground zone” where those air masses meet. This usually leads to an active weather pattern. Remember the main ingredient for large scale storms that traverse the country is a clash of warm, humid air with cold, dry air. The active jet stream brings those two things together.
Remember too that in every winter in the Northern Hemisphere, the jet stream is always stronger than in the summer because there is a larger contrast in temperature from low to high latitudes. In winter, the temperature along the Gulf Coast, for example could be near 90 degrees while in Chicago it is zero.
The official winter outlook from the National Oceanic and Atmospheric Administration (NOAA) is weighed heavily toward what is historically seen during a La Nina winter, featuring a high likelihood of warmer than average temperatures for the season in the southern U.S., and a chance of above average temperatures in our area.
The outlook follows the typical La Nina setup with a slight nod toward above average precipitation in our area and the Ohio Valley. The question is, what kind of precipitation? That is more difficult to determine because we are in that “battleground zone.”
There have been some La Nina winters with big snowfalls. These are La Nina winters where the jet stream took some significant dips south, allowing enough cold air to pour in and support wintry weather. Other La Nina winters when the jet stream remained a little farther north produced only meager amounts of snowfall. And again, there are many other factors aside from La Nina that can influence snowfall. Some of those factors are still uncertain at this time. The bottom line for snow lovers, an active pattern will be favorable for wintry weather if the jet stream takes a few big dips south.
Unfortunately, there is another side to the coin with La Nina here in the winter — a possible uptick in severe weather. This doesn’t play out every year, but research shows that over the past 25 years, our area has seen a higher frequency of cool-season tornadoes during La Nina. Typically, we see an average of 8.8 tornadoes between November and March in our area each year, but that increases to 11.6 per season in La Nina.
Taking this a step further, of the 63 EF-2 or stronger tornadoes to touchdown in the Paducah National Weather Service office’s warning area since 1995 during the cooler months, 36 of those strong tornadoes have occurred during a La Nina setup. Again, there are multiple factors that have to come together to produce these severe weather setup but the active jet stream that come along with La Nina is an important component of that setup.
We have had many significant tornado outbreaks just in the past 20 or so years locally. Of note, the EF-4 tornado in Harrisburg, Illinois, which was a part of 13 tornadoes that hit the area that night. On Jan. 21, 1999, 15 tornadoes hit the area including a pair of EF-2 twisters in southern Illinois and an EF-1 tornado in both McCracken and Ballard counties in Kentucky.
So the big takeaways for this winter: It will likely be a busy winter with an active pattern developing over the course of the season. Above average precipitation looks likely. Temperatures for the season averaged as a whole will likely run above average, but La Nina can bring some big swings and cold snaps, which could bring both wintry precipitation and big thunderstorms.
One freak record-setting cold snap does not define the winter. When all is said and done, the average of all the days this winter season is anticipated to be above normal — but time will tell.