11:10 AM | *Some of the factors being closely monitored in the preparation of this year’s upcoming “Winter Outlook”*
Paul Dorian
Sea surface temperature anomalies around the world are an important factor in any seasonal outlook. La Nina conditions (colder-than-normal; shown in blue) currently dominate the scene in the equatorial part of the Pacific Ocean (boxed in region) and will likely continue to do so right through the upcoming winter season. Map courtesy Canadian Met Centre, NOAA
Overview
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.
Water temperatures in the Atlantic Ocean are largely warmer-than-normal as we head into the month of October and this overall pattern has contributed to an active tropical season in the Atlantic Basin. There are three separate systems being monitored in the Atlantic Basin as we end the month of September. Map courtesy NOAA/NHC
Discussion
La Nina conditions persisted last winter season in the equatorial part of the Pacific Ocean with colder-than-normal sea surface temperatures and it appears this type of pattern will continue right through the upcoming winter season. In general, La Nina winters are characterized by warmer and drier weather across the southern and western states and colder-than-normal conditions from the Pacific Northwest to the Northern Plains. In the Mid-Atlantic region, La Nina winters have been somewhat mixed with no overriding signals one way or the other in terms of temperature and precipitation anomalies. In fact, in terms of final snowfall amounts, last winter which was a La Nina type of season, snowfall amounts were mixed in the Mid-Atlantic region and largely dependent on specific location relative to the PA/MD border. There above-normal snowfall amounts in Philly (23.9” at PHL Airport) and NYC (38.6” at Central Park), and way below-normal levels in DC (5.4” at DCA Airport).
The Atlantic Ocean sea surface temperatures certainly can play an important role in a given winter season; especially, in the Northeast US and Mid-Atlantic region. The Atlantic Ocean is currently featuring warmer-than-normal sea surface temperatures in most areas and this, in turn, has contributed to an above-normal and active tropical season. There is some empirical evidence that an active tropical season in the Atlantic Basin is often correlated with an early start to the winter season in the Mid-Atlantic (i.e., a colder-than-normal month of December) and I do believe that this is indeed a likely scenario for this reason and others.
Solar cycles are crucial when it comes to the long-term with respect to weather and climate and activity levels can even play a role on a shorter time scale. Solar cycle 25 is now underway and low solar activity is pretty likely through the upcoming winter season as we are not too far removed from the last solar minimum. Plot courtesy “climate4you.com”, NOAA
In addition to oceanic conditions, solar activity can be important in a given winter season as there is empirical evidence that low solar activity years are often correlated with more frequent “high-latitude blocking” events. “High-latitude blocking” refers to higher heights and pressure than normal in places like Greenland, Iceland and northeastern Canada which can dictate whether cold air masses can penetrate into the central and eastern US. The more “high-latitude blocking” events occur in a given winter season, the more likely is colder-than-normal weather across the Northeast US and Mid-Atlantic region which, in turn, increases the chance for snow. I do expect this winter season to generally feature low solar activity as we are at the beginning stages of solar cycle 25 and not too far removed from the last solar minimum.
Snow has begun to build up across the northern hemisphere as we end the month of September and it can play an important role in winter weather conditions across the US in subsequent months. Map courtesy NOAA
One other factor of interest this time of year – and yet to be determined – is the amount of snow that builds up across the northern hemisphere as we head through the fall season. A deeper snowpack in the fall season across places like Canada and Asia on the other side of the North Pole would tend to increase the chances for colder-than-normal and more extensive air masses to develop in these important cold air source regions for US. There is some empirical evidence that a deeper and higher-than-normal snowpack in Siberia this time of year correlates pretty well with a higher number of cold air outbreaks into the central and eastern US in subsequent winter months.
Another consideration for the upcoming “Winter Outlook” will be an analysis of what I call “analog” years in which overall environmental conditions are somewhat similar to today (and similar to what is expected this winter). Of course, no winter season is exactly like any prior one, but “analog” years can provide some clues as to what to expect in an upcoming winter season. The “Winter Outlook” should be ready within a few weeks or so and global sea surface temperatures, solar activity, and northern hemisphere snowpack will be just some of the factors being monitored closely in coming weeks to help provide some clues on the winter season of 2021-2022.
A final note, I am quite confident the month of October will be warmer-than-normal for much of the eastern half of the nation including the Mid-Atlantic region and colder-than-normal for the western states. Also, it is likely that the Atlantic Basin tropical season will remain pretty active as we head through October given the large area of above-normal sea surface temperatures.
Meteorologist Paul Dorian
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