Last winter was generally warmer-than-normal in the Mid-Atlantic region with below-normal snowfall and 2024 began with a rather strong El Nino event in the tropical Pacific Ocean. However, those warmer-than-normal water temperatures have since flipped to below-normal and this upcoming winter season is quite likely to feature weak La Nina (colder-than-normal) conditions. Typically, La Nina winters feature a more active polar jet stream that helps to transport cold air masses from northwestern Canada into the Northern Plains while, at the same time, much of the southern US often experiences warmer and drier conditions. La Nina winters are somewhat random in the Mid-Atlantic region with respect to temperatures and precipitation with some years featuring more snow than normal and others less.
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Numerous signs point to an active tropical season this year in the Atlantic Basin with more tropical storms, hurricanes, and “major” hurricanes compared to the long-term averages. The average number of named tropical storms in an Atlantic Basin tropical season is 14.4 with 7.2 of those reaching (minimal) hurricane status, and 3.2 becoming “major” (1991-2020 baseline period).
Based on my overall analysis of current and forecasted atmospheric and oceanic conditions, I expect around 16 named storms this season with around 9 reaching hurricane status and of those perhaps as many as 4 to achieve “major” classification level. Another metric to use in the assessment of overall tropical activity is known as the accumulated cyclone energy or ACE which utilizes both strength and longevity of tropical storms in its calculation and I expect this to be 150-160% of normal in the Atlantic Basin this tropical season.
The two most important parameters that have been factored into the “2024 Tropical Outlook” include: (1) the development of La Nina in the equatorial part of the Pacific Ocean and (2) warmer-than-normal sea surface temperatures in much of the breeding grounds region of the tropical Atlantic Ocean and Caribbean Sea…both of which are favorable for the development and intensification of tropical activity. I believe the most vulnerable areas to be directly impacted this upcoming tropical season include those in and around the Caribbean Sea, Gulf of Mexico, and Southeast US.
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Above-normal sea surface temperatures continue this month across most of the equatorial part of the Pacific Ocean, but there are signs that this El Nino episode which began about a year ago will flip to La Nina conditions (colder-than-normal) by the early part of the 2024 summer season in the Northern Hemisphere. A flip from El Nino to La Nino across the equatorial Pacific Ocean can have big implications on the upcoming 2024 Atlantic Basin tropical season. In fact, this expected dramatic change in sea surface temperatures across the Pacific Ocean may be a major contributor to a very active tropical season in the Atlantic Basin as atmospheric conditions are typically more favorable (lower wind shear) during La Nina episodes for the development and intensification of tropical storms. A second favorable factor for a very active tropical season in the Atlantic Basin is the likely continuation there of widespread warmer-than-normal sea surface temperatures. Finally, as oceanic cycles play a critical role in global temperatures, a flip from El Nino to La Nina in the world’s largest ocean could mean a return to closer-to-normal levels following a spike during the past year or so.
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A major change has come to the tropical Pacific Ocean since last winter with respect to sea surface temperatures and this transformation will have a big impact on the upcoming winter. Specifically, above-normal sea surface temperatures (El Nino) have replaced the cooler-than-normal conditions (La Nina) of the previous winter. In fact, the tropical Pacific Ocean uncommonly featured “back-to-back-to-back” La Nina winters with cooler-than-normal water temperatures, but drastic changes began to take shape earlier this year.
The return of El Nino to the tropical Pacific Ocean increases the chance for a “blockbuster” snowstorm (or two) in the eastern US which hasn’t happened in the DC-to-Philly-to-NYC corridor since January 2016 when up to two feet of snow fell near all the big cities along I-95. In an El Nino winter, the strongest part of the jet stream tends to shift southward and extends farther east across the Pacific Ocean. As a result, this southern or Pacific sub-tropical branch of the jet stream can steer storms into the southern and eastern US with enhanced moisture content.
The temperature pattern across the nation in an El Nino winter is largely influenced by the magnitude of the El Nino as well as its ultimate location in the equatorial Pacific (i.e., central, or east based). It appears to me this will be kind of hybrid El Nino in terms of its location with the warmest water “relative-to-normal” in eastern sections near the west coast of South America early on this winter season and then becoming focused over central sections of the tropical Pacific.
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Tropical activity is likely to be nearly normal this season in the Atlantic Basin with competing factors in the Atlantic and Pacific Oceans. In a normal Atlantic Basin tropical season, there are about 14 named storms, 7 hurricanes, and 3 of those actually attain “major” classification status (i.e., category 3 or higher on the Saffir-Simpson hurricane wind scale).
The major factors involved with this year’s tropical outlook include the development of El Nino in the equatorial Pacific Ocean which generally acts to suppress activity in the Atlantic Basin due to increased wind shear. The development of El Nino in the Pacific Ocean with warmer-than-normal sea surface temperatures comes after three years of La Nina conditions. Meanwhile, the Atlantic Ocean features plenty of warmer-than-normal water which is generally favorable for the development and/or intensification of tropical activity in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Indeed, the warmer-than-normal sea surface temperature pattern in the western Atlantic Ocean makes the east coast somewhat more vulnerable than normal to what I like to call “home-grown” tropical hits during this upcoming tropical season.
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The past two winters have featured La Nina conditions in the equatorial part of the Pacific Ocean and all signs point to a third such event for the winter of 2022-2023. While “back-to-back-to-back” La Nina episodes are not unprecedented, they are rather uncommon and analog years suggest that the third winter in this extended oceanic event is typically colder-than-normal across much of the eastern half of the nation with the coldest core of the air centered over the Upper Midwest. In addition to La Nina, another factor considered in the 2022-2023 “Winter Outlook” is the stratospheric cooling that developed earlier this year in the southern hemisphere partly as a result of unusually high levels of water vapor that were injected aloft by the powerful Hunga Tonga volcanic eruption back in January. The connection between southern hemisphere stratospheric cooling and high-latitude blocking (negative NAO) patterns in the subsequent northern hemisphere winter season is investigated.
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The winter season of 2020-2021 featured La Nina conditions in the equatorial part of the Pacific Ocean which then weakened to near neutral status during the spring and summer of this year. New cooling has formed in the same region and it appears that a second year La Nina will take place for the upcoming winter season. Sea surface temperature anomalies in the central Pacific Ocean were but one of several factors considered in the 2021-2022 “Winter Outlook”.
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October begins on Friday and there are several factors that I am closely monitoring which may give some clues as to what kind of winter weather we can expect in the Mid-Atlantic region. To begin, whenever dealing with longer-range forecasting such as with seasonal outlooks, I like to examine the current and projected sea surface temperatures around the world which can play an important role in an upcoming winter season across the US. The Pacific Ocean is most crucial as it is by far the largest body of water, but the water temperatures in the Atlantic Ocean can be critical to a winter season; especially, in the Northeast US and Mid-Atlantic region. In addition, though the sun is clearly crucial in the long-term when it comes to all weather and climate, it can actually play an important role in the shorter-term – such as during an upcoming winter season - depending on its overall activity levels. Finally, the building snowpack across the northern hemisphere this time of year is always of interest as it can play a role in the formation of cold air masses that can impact the US in subsequent winter months.
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The number of named tropical storms this year is likely to be above-normal in the Atlantic Basin, but not as high as the very active 2020 tropical season. While the number of named tropical storms may be above-normal, the number that actually attain hurricane classification and “major” status (i.e., category 3 or higher) is likely to be right near normal. The average number of named storms in an Atlantic Basin tropical season is 12.1 with 6.4 of those reaching minimal hurricane status and 2.7 becoming “major”. Based on the overall analysis of current and forecasted conditions, I expect around 15 named storms in the Atlantic Basin this season with around 6 or 7 reaching hurricane status and of those perhaps 3 to achieve “major” classification level.
Two parameters factored into this year’s tropical outlook include the expected slight weakening of La Nina in the tropical Pacific Ocean and the warmer-than-normal sea surface temperatures in parts of the tropical Atlantic Ocean. The lack of an El Nino (i.e., warmer-than-normal water) in the tropical Pacific and the warmer-than-normal water in the “breeding ground” region of the tropical Atlantic tend to favor a more active tropical season in the Atlantic Basin compared to normal, but there will be some limiting factors compared to the extremely active 2020 season.
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The winter season of 2019-2020 actually got off to an impressively cold start in the Mid-Atlantic region during the month of November, but it transitioned in December to one with warmer-than-normal conditions and those positive temperature anomalies continued for the vast majority of the time in the months of January, February and March. Perhaps not surprisingly, snowfall turned out to be well below-normal last winter to go along with the overall relatively warm weather conditions. In fact, some spots in the Mid-Atlantic region featured their lowest snow totals for a winter season since 1972-1973.
Numerous factors have been examined for this year’s “Winter Outlook” including the strong prospects for a moderately strong La Nina event in the tropical Pacific Ocean, somewhat favorable signs for high-latitude blocking events, and an analysis of “analog” winters that featured similar oceanic conditions to today’s environment. Many signs point to colder-than-normal conditions this winter season across the northern US including in the Mid-Atlantic region and the virtually snowless winter of last year is quite likely not going to repeat itself.
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