Unlike last winter, we have a major player which could affect our weather pattern through winter. A strong El-Nino is developing and we’re expecting it to play a big part in this winter’s temperatures and snowfall.
When forecasting snow and temperature for the upcoming winter, teleconnection patterns are often used. Teleconnection patterns reflect large scale changes in the jet stream patterns, and influence temperature, rainfall, snow, storm tracks, and location and intensity over certain areas. In particular, the El Nino Southern Oscillation and the Pacific Decadal Oscillation.
El Nino and La Nina phases oscillate every 6-18 months. To determine the phase, the equatorial Pacific waters are looked at, either being above or below the normal temperature of 28 degrees Celsius or 82 degrees Fahrenheit. If the waters are 0.5 degrees Celsius or more above normal, this trend is towards an El Nino. When temperatures are cooler over the equatorial waters, this results in a lower sea level pressure.. The reverse is true for warmer temperatures over the equatorial waters. The change in the sea level pressure changes the location of the jet stream, which is responsible for bringing weather to the u.S. An in result impacts our weather in Mid-Missouri.
To determine the PDO, the waters and sea level pressure of the interior northern pacific and over the Pacific North American coastline are looked at. When the sea surface temperatures are cool in the interior north pacific and warm along the pacific coast, and the sea level pressure is below average over the north pacific, the PDO has a positive value. When the conditions are reversed, and there is above average sea level pressure over the northern pacific, the PDO is negative.
I went back and looked at the past strong El Nino winters, where the Oceanic Nino Index (ONI) was at a value of 1.5 or greater. Since 1957, there have been 6 strong ONI winters. During these times, a range of 4″ to 27″ fell during the winter in Mid-Missouri.
History tells us during the strong El Nino winters of 1957-to 1958, 1965 to 1966, and 1972 to 1973, the PDO was in a cool phase. This brought above average snowfall to Mid-Missouri. The PDO switched however in the late 70s and early 80s to a warm phase. This resulted in below average snowfall from during the strong El Nino years in ’82-’83, ’91-’92 and ’97-’98 with ’87-’88 being an anomaly, and an average of 16″ of snowfall during the season with the avg temperature above normal at 35.
Typically a strong El Nino and warm PDO phase means below average snowfall and warmer temperatures like we’ve experienced this month.