Dry season outlook for South Florida is wet, say forecasters

It’s going to be a relatively wet winter in South Florida, according to the Miami office of the National Weather Service, which released its dry season outlook for the region on Friday.

The forecast focused on the potential impacts from what is expected to be a “strong” El Niño event.

“We feel that this El Niño is trending to be one of the stronger El Niños,” said head meteorologist Robert Molleda, “especially compared to those we’ve observed since 1950.”

El Niño is a natural climate pattern causing trade winds to weaken across the Pacific Ocean, thus piling warm water up along the Pacific coast of South America. The phenomenon generally causes warmer weather globally, but can bring wet winters to South Florida.

That’s because El Niño causes the subtropical jet stream, which travels west to east over the Pacific, to shift to the south, picking up moisture over the Gulf of Mexico and steering storms across the southern U.S. to us.

The NWS’s technical definition of a “strong” El Niño is when the sea surface temperatures in the Pacific off South American average 1.5 degrees C above normal. NWS models show that’s very likely this winter.

Cooler? Maybe not, but almost certainly wetter

Will this winter be cooler than normal? Molleda said that even though strong El Niños since the 1950s typically prompt cooler winters for South Florida, this one could be different. That’s because his team incorporated the winter warming trend of the last 10 years. As an example, the strong 2016 El Niño had warmer-than-normal temperatures.

For the temperature outlook, “the recent trends muddy the picture,” Molleda said. “It actually leans to warmer-than-normal winter, but with a low level of confidence.”

Some side effects of more cloudy wet days is that the cloud cover can bring daytime temps down, but the clouds also trap heat at night, reducing the chances of overnight freezes.

That strong subtropical jet stream shooting across the South also tends to buffer us from the polar jet stream sending arctic blasts our way, but it can happen. The brutal 2010 cold spell occurred during an El Niño.

The rain outlook is a bit more definitive. They have “moderate to high confidence” in predicting above-normal precipitation, with an average of five to 10 more days above the norm with measurable precipitation.

Additionally, Molleda said, the last 15 years of winters have been wetter than normal, so coupling that trend with El Niño reinforces the confidence of the forecast.

Three of the top five wettest winters have been in El Niño years, and there are more days with 3 inches or more of rain in South Florida during strong El Niño years.

Storm potential

A strong El Niño doesn’t just add wetness to the winter, it also means more storminess.

Molleda said that with the southern shift in the jet stream, everything gets ramped up.

There’s more moisture, fronts are stronger, and there’s strong wind shear from the high-altitude winds of the jet stream. “This tends to lead to more occasions of severe weather,” Molleda said.

Strong El Niño winters have a higher frequency of tornadoes in South Florida.

The numbers of strong tornadoes are nearly double the long-term average, with February and March being the most likely months.

Molleda said that this El Niño is also likely to be long-lived, possibly lasting until June.