Winter Preview: El Niño contributes to a tale of two seasons
Tuesday, September 1, 2015, 2:00 -
EDITOR'S NOTE: This article was previously published as a winter preview immediately following our Fall Forecast release in September. Since then we have published our Official 2016 Winter Forecast, which can be found here.
We are in the midst of a rapidly strengthening El Niño event which will likely peak later this fall as one of the strongest El Niño events on record. So, what does this mean for the upcoming winter season?
El Niño has a reputation for bringing mild winters to much of the country, especially across the northern States. The two strongest two El Niño events on record prior to this year (1982-83 and 1997-98) were quite mild from the Pacific Northwest to the Northeast. Only the Southwestern States saw below average temperatures during those winters.
However, a review of other El Niño winters shows that a strong El Niño does not guarantee a mild winter. At this end of article I will discuss a few of the key variables that we will be evaluating as we develop our final forecast for the months of December, January and February which is scheduled for release on November 30th.
The Weather Network releases it's 2015 US Fall Forecast: See what the next three months have in store!
We do think that this winter will start off rather mild across most of the country. The map below with our forecast for December (and likely into early January) looks a lot like the El Niño winters of 1997-98 and 1982-83. However, we do not expect that this mild pattern will persist through the entire winter.
Later in the season, especially during February, we expect several weeks of classic winter weather over the eastern third of the country. This period will likely feature several winter storms from the south central Plains to the Northeast, with a higher than typical threat for significant snow and mixed precipitation deep into the South.
As we look at the winter as a whole (December through February), we expect that the final numbers will show below average tempratures across much of the South, northward to the Mid-Atlantic States. In contrast, a rather mild winter is expected from the Pacific Northwest to the Northern Plains. Across the Great Lakes and Northeast, the mild start to winter will be offset by the colder conclusion to winter and result in temperatures that are close to normal.
Nearly all El Niño winters in the past have featured an active storm track across the southern United States, with a turn up the East Coast to New England. It looks like that pattern will dominate our winter this year as well. Therefore, we expect above-average precipitation from Southern California to Florida and up to the East Coast to Maine. This will likely also include a higher threat for severe weather near the Gulf Coast, including Florida.
While some temporary relief is expected from the drought conditions in Southern California, unfortunately the wetter pattern is not expected to extend north of Central California and a dry winter is expected for the Pacific Northwest.
Coastal areas Washington and Oregon will likely see a below-average number of rainy days, but the final precipitation totals could approach normal due to a few periods of rather wet weather in which storm systems will tap into subtropical moisture. However, these periods of active weather will typically be associated with very mild temperatures and high snow levels, which will mean rain rather than snow for many ski areas. Well below average snowfall is expected from interior areas of the Northwest to the Great Lakes.
After a few lake-effect events during the fall, there will be rather limited amounts of lake effect snow during December. When colder weather does return for the end of winter, however, the lakes should still be mostly wide open and able to contribute to some significant mid to late season lake-effect snow events.
Forecast Considerations: Strong El Niño with a twist
El Niño is associated with a warming of sea surface temperatures in the tropical regions of the Pacific Ocean, primarily to the west of South America (region highlighted by the oval on the map below). When ocean water temperatures in this region are warmer than normal, it impacts weather patterns around the world, especially during the late fall and winter. As we head into winter, El Niño will likely peak as one of the top two strongest events since 1950.
When the warmest water relative to normal (the darkest shades of red) is found close to the coast of South America, we usually see mild winters across much of the country, much like what we experienced in 1982-83 and 1997-98.
However, when the warmest water is shifted west towards the Central Pacific, the winters over the eastern half of the US typically turn significantly colder compared to what we expect with a classic El Niño. As we progress through the fall we will track changes in the ocean-water temperature pattern, but there are signs that warmest water will be found closer to the Central Pacific and not just off the coast of South America.
Another key to upcoming winter is whether El Niño peaks later this fall and then starts to weaken as we progress through the winter or whether it continues to strengthen during the winter. A strengthening El Niño rather than a weakening El Niño during the heart of winter would likely mean a milder winter for the Northeastern States.
Forecast Considerations: “The Blob”
A unique feature of the upcoming winter compared to other strong El Niño winters of the past is the expected persistence of the warmer-than-normal ocean-water temperatures south of Alaska, shown in the top circle in the map above.
Some have referred to this feature as “the Blob” and it has been a key contributor to the dominant weather pattern across North America for the past two years. This pattern has been associated with extended periods of warm and dry weather in the West and two of the coldest winters in recent memory further to the east, especially in the Great Lakes and Northeast. If “the Blob” does indeed persist through the upcoming winter, then the threat for a cold conclusion to winter in the East will increase.