Global effects of El NiñoThough effects on the atmosphere over North America, and the rest of the world, vary with each El Niño, there are some typical atmospheric responses. The effects are most visible during the winter months, since that is when the waters of the Pacific tend to be warmest during El Niño years, and include:
- Increased precipitation across the southern tier of the U.S, including drought-stricken California, due to a more active Pacific jet stream
- Decreased precipitation across the Pacific Northwest and Ohio Valley
- Warmer than average conditions across the northern half of the U.S. and southern Canada, as the polar jet stream is displaced further north
- More coastal storms affecting the eastern U.S., and
- In summer, suppressed activity in the tropical Atlantic and increased activity in the eastern Pacific.
Regional effects of El NiñoMid-South weather can be fairly fickle during El Niño winters, as we are typically positioned between the persistent Pacific jet stream that brings wet and cool conditions to the southern U.S. and a pronounced dry area over the Ohio River Valley, which can extend as far south of the Tennessee River Valley. If either of those areas shifts slightly due to other variables, Mid-South weather could end up either wetter or drier than average, particularly for short durations over the course of the winter.
Mid-South temperatures over the course of an El Niño winter are generally driven by shorter-term regional variations that occur. El Niño itself generally places us in a "near average" temperature regime, between cooler than average weather to the south (nearer the Pacific jet stream) and warmer than normal weather to the north.
|Typical climate pattern over North America during El Niño winters. Graphic courtesy NOAA/CPC.|
Other factors: TeleconnectionsWhile El Niño will be the primary factor on our winter weather pattern, there are other climate influences (or "teleconnections") that may last only for a week or two at a time that frequently determine local weather conditions. These are much less predictable more than a couple of weeks in advance. These include the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), Pacific-North American Pattern (PNA), and Madden-Julian Oscillation (MJO), among others. Here is a brief description of a few that can affect our weather:
North Atlantic Oscillation (NAO) - Outside of El Niño, one of the primary influencers of winter weather in eastern North America and Europe is the NAO. A positive NAO occurs when atmospheric pressure over the high latitudes of the North Atlantic (i.e., Greenland) is below average and areas in the central North Atlantic have above average pressure. A strong jet stream across the eastern U.S. into the north Atlantic keeps the coldest winter air bottled up to the north, resulting in above average temperatures for the eastern U.S. A negative phase features above average pressure over the high latitudes and usually results in below normal temperatures and a snowy pattern for the eastern U.S. as cold air is allowed to dip into the region due to a weaker jet stream. The NAO can shift from positive to negative multiple times within a season or may vary in strength but remain in the same phase for several months at a time. At least through the early stages of the winter season, a positive NAO is expected.
Arctic Oscillation (AO) - A positive AO occurs when the ring of winds circulating around the polar region (commonly referred to as the "Polar Vortex") is strong, keeping cold weather confined to the highest latitudes around the North Pole. Higher pressure results in the mid-latitudes, along with less frequent intrusions of Arctic air. In a negative AO phase, this wind circulation weakens, allowing the cold polar air to penetrate south into the middle latitudes and increasing storminess in these areas as the polar jet stream dips south. This is what has been referred to in the media last January as the "Polar Vortex" invading the U.S., when in actuality, it is always there, just not always as far south. The AO is difficult to forecast more than a couple of weeks into the future.
Pacific-North American Oscillation (PNA) - The PNA describes a pattern of mid-level pressure readings in distinct areas across the Pacific and North America. Usually, these pressure readings (or "heights") are similarly anomalous in the Aleutian Islands of Alaska and the southeast U.S. In the positive phase, heights are above average around Hawaii and in western North America and lower in the North Pacific and southeastern U.S. The positive phase tends to result in cooler and drier weather for the eastern portion of the country in the winter and also tends to occur during El Niño conditions, but not always.
|Average temperatures during a strongly positive PNA regime in January 1981, courtesy of the State Climate Office of North Carolina. The country was virtually split in half with very cold air in the east and warmer than normal air in the west.|
As you can see, the state of each of these teleconnections can alter or even reverse the impacts of a "typical" El Niño season for periods during the winter, which then affects the overall averages. It's important to remember that "climate is what we expect, weather is what we get." Thus there can be a difference between long-term, or seasonal, averages and day-to-day weather that makes up that average.
Winter outlooks from other sourcesAs I state every year, I am not an expert climatologist. At MWN, the focus is, nearly exclusively, on the short to mid-term forecasts - those out to a week to 10 days. So, as usual, in putting together our Mid-South winter outlook, I consulted multiple sources and researched data from winters past that I felt were comparable to what we might experience this year from a general pattern perspective (called analogs, listed at the end of the post).
Significant weight was given to previous winters that had strong El Niño conditions but consideration was also given to winters with weaker El Niños which had similar sea surface temperatures outside the Pacific ENSO region, namely the eastern north Pacific and western north Atlantic, since they would be most likely to have an effect on U.S. weather. In particular, I feel the warm waters near the Pacific coast and those in the north Atlantic will play more than a passing role in weather patterns over the U.S. and influencing the Mid-South.
The winter outlooks that I gave the most credence to were those from NOAA and WeatherBell Analytics, as their reasoning is sound and I generally agree with their premises of their respective outlooks, even though they differ slightly. You'll find those outlooks shown below.
NOAA Winter Outlook
WeatherBell Winter Outlook
The MWN Winter Outlook
All of that said, there is fairly strong consensus that Mid-South winter will start off warmer than it will end. In other words, the temperature anomalies will be positive (above normal) through December, then descend into negative (below normal) territory by February and March. In fact, the average temperature in December for the ten El Niño winters examined (analogs listed below) was 0.9° above normal while the average February temperature was 3.4° below normal. Thus I expect temperatures will be above normal, on average, through the end of the year before beginning to trend downward. Similar to last year, February and early March have the potential to be a fair amount below normal in the temperature arena.
Precipitation-wise, a warmer early season tends to support more atmospheric moisture than a cold season and our analog winters support that as well. By the time the latter half of the winter arrives, precipitation in the analogs falls below normal. In a classic El Niño setup, precipitation is usually near average in the Mid-South, but drier anomalies in the Ohio Valley can sometimes creep south into west and middle TN as well. Therefore, we are of the opinion that the winter will start off with above normal precipitation and the latter half of the season will be slightly below normal. We also believe that severe weather activity will continue to be below average, which is fairly common in El Niño years.
The most anticipated part of the forecast, snowfall tends to be near the long-term average for the season, though it can be fairly variable. This is because one snow storm (or the lack thereof) can result in a season that is well above (or below) normal, since not much typically falls. (Memphis International Airport averages 3.4" of snow each year.) Last year, the December-February snow total was 2.3", or about an inch below normal. However, the first light accumulation actually occurred very early (0.1" in mid-November) and the biggest ice/snow event of the season actually occurred in the first week of March, both outside the typical "winter" season. So for 2015-'16, we are predicting slightly above normal snowfall for the season (4-5") with the majority of that likely to occur later in the winter, not dissimilar to last year. With a forecast of cooler air in place in February to early March, we believe there is an above average probability of a late season snowfall in the Mid-South once again.
So there you have it - the predictions for 2015-'16 winter! If you're interested in how I did last year, take a look at this blog post from last March. In a nutshell, the winter of 2014-'15 was cold, dry and snowy. I had forecast temps below to slightly below average (correct), precipitation near average (incorrect), snowfall near to above average (correct), and large temperature swings with periods of severe weather (partially correct).
The analog years we examined for this year's outlook were 1919-20, 1957-58, 1972-73, 1982-83, 1991-92, 1994-95, 1997-98, 2002-03, 2009-10, and 2014-15. These included the 3 strongest El Niños on record (to this point): 1997-98, 1972-73, and 1982-83. Click here to view the raw data for each of these winters.
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