The latest climate forecast models continue to forecast a 60 to 75 percent probability of weak El Nino conditions developing later this fall and continuing through the upcoming winter.
El Nino conditions develop when the trade winds weaken causing warmer than average sea surface temperatures to develop across the Equatorial Pacific. When this occurs, the southern branch of the jet stream becomes more active directly impacting the weather across the United States during the winter months. For the Washington Area, the amount of snow or rain and winter temperatures depend on the strength of the El Nino (weak, moderate or strong).
There are other factors of course that impact our winter weather, one of the most prominent being the phase (positive or negative) of the North Atlantic Oscillation (blocking over eastern Canada & Greenland). Simply put, the more blocking you have in the upper levels of the atmosphere over the North Atlantic the higher the probability of low pressure systems slowing down along the Mid-Atlantic coastline (dumping snow). A weak El Nino and chronic negative NAO during the winter of 2009-2010 produced the snowiest winter since records have been kept here in the National Capital Region.
Let’s take a look at how temperatures vary (cooler or warmer than average during the winter months) during strong, moderate and weak El Nino Winters:
Let’s now take a look at how precipitation varies (wetter or drier than average during the winter months) depending up on the strength of the El Nino:
The stronger the El Nino (warmer the sea surface temperatures are off the Equatorial Pacific) the more noticeable the effects are on the continental United States. A strong El Nino can produce rainy, mild winters across the Mid-Atlantic states while a weak El Nino has a less noticeable impact.
If you like snow, weak to moderate El Nino winters are certainly more conducive in producing southern tracking storms while a strong El Nino (Winter of 1972-73 & 1997-98) produced two of the least snowy (mild winters) in Washington’s history when under a quarter of an inch of snow fell at National Airport. Those winters produced lots of rain as opposed to snow because the temperatures were too warm.
Latest Forecast Model Predictions:
The latest forecast from mid-September shows a slight uptick in probability that a weak El Nino may develop late this fall continuing into the Winter months.
I am a huge fan on the European Forecast Model. It is interesting to note that the European forecast model (ECMWF) is one of the more bullish models in predicting moderate El Nino conditions during the upcoming winter months.
The vast majority of forecast models currently do not suggest a strong El Nino developing. At this time, a weak El Nino seems likely, however if the European Forecast model is on to something and sea surface temperatures continue to warm… we may have a moderate strength El Nino on our hands.
I will be issuing my Washington Winter Forecast in mid-October as by then we should have more confidence in the strength of the potential upcoming El Nino. Stay Tuned.