La Niña is main reason for our long dry spell

It would be easy to dismiss Arizona’s current dry spell as just an ordinary occurrence in the desert.

However, a recent tweet from the National Weather Service in Phoenix points out that these conditions are unusual.

From September (roughly the start of the current dry spell) through the end of January, just 0.44 inch of rain was recorded in Phoenix. That’s the second-driest total ever for that peri-

“In the wintertime ... colder air and ... cold fronts are what bring us our storm systems. When we don’t have that colder air, there is not a whole lot to trigger the storm systems we need.” Nancy Selover State climatologist

od. The normal would be 2.11 inches.

During that same span, Flagstaff experienced its driest period on record, with 2.14 inches of precipitation. The normal amount is 9.72 inches. Much of the water we use in the Phoenix area comes from snow in the mountains of northern Arizona.

Neither city saw any precipitation through the first 11 days of February.

The main culprit has been the weather phenomenon known as La Niña, but it is not the only factor.

La Niña is characterized by coolerthan-normal waters in the eastern equatorial Pacific Ocean. It is the opposite of El Niño, which is a result of warmer-than-normal waters in that same portion of the ocean.

Those water conditions influence weather throughout the world. In the southern half of the United States, La Niña winters tend to be warmer and drier than normal. The current season definitely falls in that category.

The current La Niña isn’t a particularly strong one, but it is notable in other ways, said University of Arizona climate scientist Mike Crimmins.

“Interestingly, it is a bit stronger this year than last and I believe it will go in the books as moderate event,” said Crimmins, who also serves as a drought-monitoring expert on the Governor’s Drought Task Force.

When climatologists first began to confirm La Niña conditions last fall, they initially believed it would be a weak La Niña winter. Recent observations have shown it to be stronger than predicted.

It is also somewhat unusual for La Niña conditions to repeat for back-toback winters, which makes the current situation something Crimmins described as a “double-dip La Niña event.” And when back-to-back La Niña events occur, typically the second is weaker than the first. That’s not the case this year.

The other factor in our dry spell is something called the Madden-Julian Oscillation, which can be described as a slow-moving wave of storm activity that moves around the Earth in the tropics. Crimmins said that condition has combined with La Niña to keep this winter’s storms north of Arizona.

“It was working with La Niña in January to kind of supercharge the typical La Niña pattern,” Crimmins said.

State climatologist Nancy Selover said the trend of storms tracking farther north isn’t unique to this year.

“We’ve been seeing a movement over the past few years of the jet stream moving further north,” Selover said. “The jet stream is essentially the dividing line between cold air to the north and warm air to the south in the Northern Hemisphere. When that jet stream moves further north, it’s keeping that colder air further north.”

Colder air dipping through Arizona is important for the formation of winter rain and snow, Selover said. Cooler weather causes water vapor in the atmosphere to condense sufficiently to fall as rain or snow.

“In the wintertime, that colder air and those cold fronts are what bring us our storm systems,” Selover said. “When we don’t have that colder air, there is not a whole lot to trigger the storm systems we need.”

So, this winter is unusually dry. Is that likely to change soon?

There is some hope: This week’s forecast includes significant chances of rain or snow for most of Arizona.

And the latest report from the National Oceanic and Atmospheric Administration says La Niña conditions are expected to weaken, and it’s unlikely that they will carry over into spring. That could clear the way for storm systems to make their way through the state.

Crimmins isn’t very confident that will happen, however.

“La Niña impacts on atmosphere typically linger, which is why I am still a bit pessimistic that we will get much of a turnaround through the spring,” he said. “The damage has largely been done with the (precipitation) deficits and I am concerned that we are headed towards a rough fire season and lots of impacts on water resources.”