Watch out for some changes in the weather as El Niño gives way to La Niña

Friday, April 8, 2016

For Ontario, it looks like a winter with a bit of everything but nothing too extreme and a summer with above-normal temperatures with a higher risk of the dangerous type of thunderstorms

by PHIL CHADWICK

Verification is perhaps the most important part of the forecast cycle. How did the prediction work out? We should learn as much from our failures as from our successes. If we have more successes than failures, then we should have more confidence in our forecasts. As a meteorologist, the only time one does not have a forecast failure of some kind is when one is off-duty.

The strong El Niño of 2015 and 2016 certainly played its role in the weather and produced significant global impacts – and even more headline news. There were some big rainfall events in California as well as some extreme snowfalls for the U.S. east coast. For Ontario, the winter of 2015-2016 had a slow start, but the predicted pattern has been in place a few weeks now and will continue through the rest of the winter.

 
The jet stream tells the basic story on the weather. It is simple and reliable. The heart of the Arctic vortex was more over northern Hudson's Bay than James Bay, but the predicted El Niño pattern is still very evident. Freezing rain and snow were big problems for most parts of Ontario. Cooler than normal temperatures can be found anywhere within the cold trough that the jet stream meanders around – but not as cold as the record-breaking year of 2015.

The past is the past, though, so what about the rest of 2016? The weather and climate typically behave like a wave overshooting any equilibrium position and letting momentum carry it into the opposite extreme. All indications are that the strong El Niño will weaken and slowly transition to El Niño-Southern Oscillation (ENSO) – neutral during the late spring or early summer 2016. Furthermore, as depicted in the accompanying graph, the predictions for the fall of 2016 are for La Niña or basically the opposite of La Niño. 

The phrase "La Niña" is Spanish for "the girl" and describes conditions where the equatorial Pacific waters toward the South American coast are anomalously cool. The fishing gets better along that coast of South America, leading the fishermen to give a name to the associated phenomenon. La Niñas  appear approximately every three to five years and typically last one to two years. There are lots of physical and numerical modelling reasons to predict a La Niña event starting in the fall of 2016 but, to be prudent, nothing is certain in life except death and taxes.

The impacts of a La Niña can be best described by using the changes it makes on the jet stream.

Both the west coast ridge of high pressure and the trough over the eastern continent are weaker. The impacts on weather can be summarized as: above average precipitation in British Columbia; colder-than-normal temperatures in the Prairies; and above average precipitation in Ontario and Quebec. The United States can experience drier conditions in both the western Pacific and the southeastern United States. It is also believed that La Niña's cooling of the equatorial Pacific tends to favour hurricane formation in the western Atlantic.

For Ontario, it looks more like a winter with a bit of everything but nothing too extreme. The cold waves will come, but they will also go and not linger. Precipitation will be above-normal, especially in the snow belts around the Great Lakes. From personal research, I have also found that La Niña summers tended to produce more supercell-type thunderstorms than single cell pulse storms. I have very valid scientific reasons for this, but it also came out in the statistics that I produced.  Supercell thunderstorms are responsible for most convective severe events like large hail, damaging winds and tornadoes. This research was not published though and I would be sticking my neck out to assert that this prediction as 100 per cent reliable.

So expect some changes for the rest of 2016. For Ontario, above-normal temperatures for the summer with a higher risk of the dangerous type of thunderstorms. As the La Niña gets established, add in above normal precipitation especially in the snow belts around the Great Lakes. Remember, we are all in this together. BF  

Phil the Forecaster Chadwick has been a professional meteorologist since 1977, specializing in training, severe weather and remote satellite and radar sensing.