It is early August and an El Niño is likely developing for Fall 2018 and Winter 2018/2019. The ENSO (El Niño Southern Oscillation) phases – neutral, cold (La Niña) and warm (El Niño) – have a significant impact on weather over large areas of the world. Let’s discuss where we are at right now in ENSO observations, where ENSO forecasts take us into Winter 2018/2019 and what the United States and Canada can expect based on the forecast.
Current ENSO Observations
Since last winter, water temperatures in the tropical Pacific have shown a steady climb. Figure 1 below is a time series of water temperature in NINO 3.4. Last winter, we were in La Niña – November 2017 through April 2018. Water temperatures have been in a steady climb since March 2018 and are now above the long term average. This is the area on the right of the time series that is orange.
Figure 1 Sea Temperature Departure from Normal Niño 3.4
Figure 2 shows Niño 3.4 with a temperature 0.3°C above the long term average. Niño region 3.4 is the benchmark for forecasting ENSO phases.
Figure 2 Niño Regions Sea Temperature Departures from Normal
When the temperature is 0.5°C above average three consecutive months for five consecutive overlapping three month periods, El Niño conditions are met. For example, conditions are met October, November and December (OND) through February, March and April (FMA). Last winter Niño 3.4, using the same thresholds for water temperature departure and time, met La Niña conditions. So, sea surface temperatures, as of June 2018, are above the long term average in Niño 3.4 and are close to reaching the temperature threshold for El Niño.
Computer modeling is used to take these observations and create an outlook or forecast into the future. Figure 3 is a probabilistic forecast. There are three colored bars representing the three phases of ENSO. The red bar represents El Niño. This forecast was issued a few weeks ago in early July. A greater than 50% probability of El Niño is forecast in the three months of August, September and October. We are just entering that time period. By the end of this year and early 2019, probabilities of El Niño rise to 70%.
Figure 3 July 2018 CPC ENSO Probabilistic Forecast
Figure 4 is a plume or ensemble forecast. This forecast is more recent – updated August 3, 2018, but backdates to early July. All members of this forecast favor above average sea temperatures in Niño 3.4 through early 2019. The forecast mean favors sea temperatures in Niño 3.4 from August through April 2019.
Figure 4 August 3 2018 CFSv2 Niño 3.4 Plume Forecast
Forecasts favor El Niño developing soon and continuing through early 2019. Anticipating this, The National Weather Service issued an El Niño Watch for this fall and winter.
El Niño Winter Impact on United States and Canada
Historically El Niño during the winter months has a significant impact on temperature and precipitation distribution. In some areas of North America, these impacts are quite consistent. And in other areas, the impact is less certain.
Figure 5 North America El Niño Winter Correlations
Figure 5 maps temperature and precipitation anomalies from January through March over North America during El Niño conditions. The southern United States experiences consistently below average temperatures and above average precipitation. The western Canada and northwestern United States can expect above average temperatures.
Figure 6 shows snowfall anomalies for the United States during El Niño winters. The large brown and orange areas in the Midwest and Great Lakes represent a strong signal for below average snowfall. The Midwest and Great Lakes consistently experience below average snowfall. Along the east coast, there is an above average anomaly for snowfall. However, East Coast snowfall varies widely during El Niño winters – there are winters with above average snowfall and below average snowfall.
Figure 6 United States Snowfall Anomalies during El Niño Winter
Now these are seasonal forecasts. A big snowstorm, or two, could hit an area, like the Midwest, that can expect below average snowfall. Understand that a seasonal forecast looks at the long game and not individual storms. These El Niño anomalies can verify and you still have snow or cold. That is why forecasts are so important. They focus on the time period that impacts us most – now and the next few days. Make sure you pay attention to your local forecast and winter updates. In the meantime, I have to go out and cut my grass before thunderstorms move in.