What will winter bring?


Last updated 9/26/2023 at 9:43am

Photo by Lisa May

The Sisters got their first dusting of snow last weekend. Does it portend a snowy winter? Or will El Niño exacerbate drought?

There is an increasing probability that the upcoming winter weather will be orchestrated by a moderate to strong El Niño. What that means in our part of world is that warmer and drier than normal conditions will likely prevail. But please read on, for as it's oft said, the devil is in the details.

After influencing global weather patterns for three straight years, La Niña is now gone. Her departure is making way for the El Niño Southern Oscillation's (ENSO) warm phase, namely El Niño himself.

Atmospheric scientists explain that the easterly trade winds in the tropical Pacific Ocean have weakened, or even reversed direction. This is allowing warmer water in the western Pacific to be transported eastward. When sea surface temperatures (SST) in the central and eastern Pacific become warm enough, an El Niño is officially born.

An often-used indicator for monitoring the ocean component of the seasonal climate pattern is the Oceanic Niño Index (ONI). When the value of this index becomes greater than 0.5 degrees C, an El Niño is said to exist. The most recent observations show the ONI to be 1.6 degrees C, already putting the developing El Niño in the strong category.

The Climate Prediction Center has stated that there is a 95 percent chance El Niño will continue through the winter, with a greater than 70 percent chance it will become a strong event at its peak.

A typical El Niño weather pattern across the U.S. features a quasi-stationary high-pressure ridge situated over Oregon and Washington, extending into British Columbia. This often causes the polar jet stream to split just offshore, weakening and dividing incoming Pacific storms. Part of this energy is diverted northward into British Columbia and southeastern Alaska, while some of the moisture finds its way to California, leaving the Pacific Northwest high and dry.

If the blocking ridge has enough amplitude, the jet stream will occasionally draw arctic air southward from higher latitudes into the Great Lakes area, overspreading the northeastern part of the country.

Meanwhile, El Niño sometimes displaces the subtropical jet stream farther to the north than normal, causing wetter weather to overspread southern California and the southern tier of states.

But there is a caveat here: If the El Niño is sufficiently strong, it can push the subtropical jet far enough north to impact our region to produce warmer and wetter weather.

The last time El Niño paid a visit was the winter of 2015-16, and it was a strong one too. So, is there any evidence to suggest that winter was warmer and wetter than normal here in Central Oregon?

Available data from the Cooperative Observer Network collected at Redmond and Sisters during the 2015-16 El Niño shows that both stations recorded precipitation amounts far above normal in December, while temperatures were in the normal range. February was warm and exceedingly dry at both stations.

November, January, and March were about average with regard to temperature and precipitation. Hence, there appears to be no clear signal that the subtropical jet was active in our region.

For the three-month period November through January 2023-24, the Climate Prediction Center is calling for a 50 to 60 percent chance of above-normal temperatures in the Pacific Northwest, northern New England, and northern Alaska. In no part of the country are below-normal temperatures expected.

Less than normal precipitation amounts are expected in the Northwest eastward across the northern plains into the Great Lakes region. Greater than normal levels of moisture are called for from New Mexico eastward through the southern states, extending up the eastern seaboard to New Jersey, and in northern Alaska. This pattern is much the same for December through February and January through March, except that Southern California gets wetter as winter progresses.

If the actual weather we get this winter is anywhere close to what is projected, the result will negatively impact the skiing industry, with less snow in the mountains, and at higher elevations, and further exacerbate our ongoing drought.

But it's important to remember that these prognostications describe general trends over a period of a few months.

According to Dr. Nicholas Siler, associate Oregon state climatologist, "seasonal forecasts have never been particularly accurate, in part because most of the year-to-year variability in our weather is a result of chaos in the atmosphere, which is inherently unpredictable."

Asked whether he thought global climate change is making forecasting more difficult, Siler said, "It's hard to say...we're seeing sea-surface temperature patterns around the globe that have no historical analog."

Pete Parsons, lead meteorologist for the Oregon Department of Forestry, believes winter-like conditions should arrive early this year. He points out that in years with similar ENSO signals to what we're seeing now, volatile weather has occurred in December, such as windstorms and arctic intrusions.


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