El Paso, TX -
The Climate Prediction Center says that there is a greater than 70% chance of an El Niño this winter, and that can have a big impact on our weather. El Niño occurs when equatorial trade winds weaken, thus allowing surface waters in the central and eastern Pacific to warm up more than normal.
These warmer than average sea surface temperatures are a key feature of El Niño. El Niño is, of course, the warm phase of the El Niño - Southern Oscillation (ENSO). The cold phase is called La Niña. In layman's terms, ENSO is the periodic variation in the trade winds and seas surface temperature of the Equatorial Pacific.
A phase is considered neutral (neither El Niño or La Niña) if the sea surface temperature is not more than 0.5 degrees Celsius above or below normal. Weak is defined as 0.5 to 1 degree Celsius from normal, moderate as 1 to 1.5 degrees Celsius from normal, strong as 1.5 to 2 degrees Celsius from normal, and very strong as more than 2 degrees Celsius from normal.
Warmer than normal Equatorial Pacific waters will often influence the jet stream in the northern hemisphere during our winter. During the winter, we typically see two different branches of the jet stream: the northern branch (polar jet stream) and the southern branch (subtropical jet stream).
The jet stream is a narrow band of strong winds (oftentimes over 100 knots) in the upper atmosphere that separates cold air to the north and warm air to the south. The term jet stream comes from the fact that it occurs at the height that jet airliners fly.
Storm systems are often picked up and directed by the jet stream, so that tends to be the track of many storms. During a typical El Niño year, the subtropical jet stream is often right over the Borderland, which usually means relatively frequent bouts of precipitation.
However, since the jet stream is orientated flat and fast in this situation, storm systems tend to move along fairly quickly. During a strong El Niño, snowfall tends to be well above average for El Paso, but we will most likely see a weak El Niño this winter.
Weak El Niños have historically left us with unimpressive snowfall totals, compared to moderate or stronger ones, as shown below.
Our two snowiest water years (October - September) occurred during a “strong” or “very strong” El Niño. The snowiest being 1982-1983 at 35.0”, during a “very strong” El Niño. The winter of 1987-1988 comes in at number two with 32.5” of snow during a “strong” El Niño.
It’s also important to note that the Polar jet stream is typically situated well to our north, in Canada, during an El Niño year. For us to get snow in El Paso, we usually need that polar jet stream to dig all the way down to the Mexican Border.
Not only do we need a shot of very cold air that would bring our temperatures more than 25 degrees below our normal winter high temperature, but this also has to be timed with enough moisture. Don’t forget, we live in a desert so that in itself can be tough.
Bottom line, the weaker the El Niño is, the less of an influence it has on our weather. Therefore, the harder it is to make an accurate, long term winter forecast based on just El Niño. When this occurs, sea surface temperature anomalies (departure from average) in the north Pacific, and even north Atlantic could contribute as much, or more, to our winter weather patterns than a weak El Niño.