2024-2025 Winter Outlook

Winter Seasonal Forecast by Kyle Elliott

* La Niña And Strongly-Negative PDO Lead to Mild Winter *

Monday, November 11, 2024:

After a mild and relatively benign 2023-2024 winter season, snow-lovers may face frustration and disappointment yet again this year. One of the main factors that influence both the weather and climate in the winter are ocean sea-surface temperature (SST) anomalies, or how much above- or below-normal the surface water temperature is, in the Pacific Ocean. When SST anomalies in the equatorial Pacific are below normal, we are in a state of La Niña. When they are above normal, an El Niño is present. Despite last winter's El Niño and more activity in the Jet Stream's southern branch, seasonal snowfall only tallied 18" at Millersville University. Air masses were rarely cold enough for all-snow events, and the "big storms" occurred outside of winter's coldest period in mid-to-late January. El Niño conditions ended in June, and the climate has been in an ENSO-neutral state ever since. However, a weak La Niña is expected to emerge this month and then persist through the upcoming winter, according to various climate models. At first glance, this may not seem like the "worst-case scenario" for snow-lovers, but other factors could make this winter eerily similar to the abysmally mild and virtually snowless winter of 2022-2023. In many first-year La Niña winters, the atmosphere does not fully respond to SST changes in the equatorial Pacific. In other words, changes in large-scale, atmospheric patterns often lag behind the SST changes by several months or more. However, the atmosphere never had a chance to fully respond to last year's El Niño, and La Niña's impacts are already evident in global weather patterns. Thus, the typical "lag effect" won't apply this winter season. Another key factor to consider is the strength of the Stratospheric Polar Vortex (SPV). The SPV can play a major role in preventing or unleashing Arctic outbreaks across the central and eastern United States. When the SPV is strong and centered near the North Pole (as was the case in 2022-2023), cold air tends to remain bottled up in Arctic regions. When the SPV weakens or splits into multiple pieces, bitter cold may plunge into the eastern U.S. and/or Europe just a few weeks later. One such split occurred in January 2021 and was at least partially responsible for the crippling ice storm and record cold that gripped the southern Plains (particularly Texas) that February. This winter, odds favor a strong SPV and one whose strength could rival the winter of 2019-2020.. another "dud" in terms of cold and snow. 

The phase of the Pacific Decadal Oscillation (PDO) is yet another primary winter climate influencer. When SSTs are above normal in the interior North Pacific and below normal along the North American coast, the PDO has a negative value. The opposite is true for a positive PDO. Since 2020, the PDO has been in a highly negative, or "cool," phase and is currently approaching record-low values. The PDO will undoubtedly remain strongly negative through the winter months (see below). In negative PDO winters, the coldest weather (relative to normal) often occurs in the Pacific Northwest, northern Rockies, and northern Plains. By contrast, the warmest conditions are usually found in the Deep South, Southeast, and mid-Atlantic States. To nobody's surprise, I expect this winter to be no exception to that rule.

The last factor that I'll highlight is the Madden-Julian Oscillation (MJO). The MJO, which can be thought of as an eastward-moving area of convection between the Indian and central Pacific Oceans, can be active in 8 different phases. Each phase corresponds to a specific location of this tropical convection. Which phase the MJO is active in at any given time can alter weather patterns across North America on a week-to-week basis. Based on the location of highest SST anomalies between the Indian Ocean and west-central Pacific, a meteorologist can make a reasonable guess as to which phases the MJO should be most active in during a particular winter season. This year, SSTs are highest from the eastern Indian Ocean to areas north/west of Australia (i.e. Indonesia). Meteorologists refer to this area as the "Maritime Continent," and high SST anomalies in these regions promote MJO activity in phases 3-5. These are generally considered "warm phases" for the eastern U.S. during a La Niña winter, but momentum from the eastward-moving convection may carry it into phases 6 and 7 at times. These two phases can produce the opposite effect (i.e. colder periods) across the eastern U.S., particularly in December and January. Thus, the progression and intensity of MJO activity will likely play a significant role in our weather during this year's holiday season.

For those of you wondering how I create a winter forecast, the recipe is actually quite simple. I look at the distribution of temperature and precipitation patterns across the U.S. in past winters with similar conditions to those I expect in the upcoming winter. These are called "analog years," and they're a vital part of seasonal forecasting. In first-year La Niña winters with a highly-negative PDO and strong Polar Vortex (what I'm expecting this year), there tends to be an active, Pacific storm track and lack of big, East Coast snowstorms, or Nor'easters. Most storm systems should follow, or "come out of," the Jet Stream's northern branch this winter, so they'll often track to our north and west and be unable to tap into rich, Gulf of Mexico moisture. As a result, I expect frequent light precipitation events this winter with "moisture-starved systems" being the norm. We will likely experience many "all-rain events" and a few "changeover storms" where precipitation starts as snow but quickly turns over to ice or rain. Any bigger storms will tend to cut into the Great Lakes region or up the Appalachians, placing the Lower Susquehanna Valley on the "warm side" of these systems. The Polar Vortex may wobble, or weaken, at times during the second half of December or early January, creating the potential for a brief but pointed cold shot around the holidays. A similar thing happened in 2022-2023.. one of my analog years for this winter.. during the Christmas holiday. Even if this doesn't happen, any brief periods of high-latitude blocking could force colder air into the eastern U.S. However, the extent, intensity, and duration of any blocking will be highly dependent on the location and magnitude of MJO activity. A strong MJO wave in phases 6, 7, or 8 could cause some blocking to develop around or just after the holidays, but MJO activity in other phases would hinder it. Along these lines, I can't rule out a significant ice storm this winter, especially between Dec. 20 and Jan. 10. However, there should be a predominance of mild air across a large portion of the nation this winter, and any cold/active period around the holidays should be the exception rather than the rule. Thus, temperatures in both January and February should average "well above normal".. perhaps by as much as 5-6°F. Compared to average, December may be slightly colder than January and February but still "mild," as a whole. In aggregate, I am expecting 10-20" of snow this season in Lancaster County and most of the Lower Susquehanna Valley with a below-normal chance of a major, double-digit snowstorm. Total precipitation (rain, sleet, ice, and snow) will likely be near- to slightly below-normal with temperatures at least 2-4°F above normal from December-February.. "par for the course" in recent years. The "snow drought" should continue.. -- Elliott