Weather pattern could bring wetter snows, but less Arctic air blasts

So it’s the El Nino that might just eat west Los Angeles, or as one NOAA forecaster put it, the “Godzilla El Nino.”
“Every since he let that slip, it’s given everyone a chance to talk about it on the front page,” noted Colorado State Climatologist Nolan Doesken. “My take is that I know that El Nino is something that interests people every year — anything that you can try to hang your hat on to project more than two months in the future is something that’s of great interest to a bunch of forecasters.”

Doesken said he’s a little hesitant to make an overall state forecast in terms of snowfall, but there are a number of things that all of us could take under consideration, beginning with the overall strength of this El Nino event.

How strong is this El Nino?
The main measure of any El Nino, or the opposite La Nina event, is the Southern Oscillation, the difference in barometric pressure between Darwin, Australia and Tahiti. When the oscillation is positive, higher in Darwin, it means strong Pacific trade winds and a La Nina event, but a negative number means weak winds, allowing warmer waters to pool near the Americas.

That’s the easiest way to think about an El Nino event. Warmer ocean surface temperatures near the Americas.
Normally, the difference between La Nina and El Nino events, is about 3 degrees Celsius, or five to six degrees Farenheit. This year it promises to be about 4 degrees Celsius.

While there is no direct correlation, that 25 percent increase in temperature during this event is very, very significant, both in water vapor available for storms, as well as the convective energy from the ascending water vapor.

“That’s a lot of potential — tons of water vapor and energy a very big big area. It’s a big deal,” Doesken said.

So what does that mean for forecasters?
Well, obviously there’s a lot more water available for this winter’s storms, meaning that a lot of rain or snow could fall. But where it falls pretty much depends on how the storms track, which will follow the atmospheric jet streams. The weak west-to-east winds usually mean, in North America, a split jet stream.

This is why forecasters love a strong El Nino event, it makes their job just a little easier with that split jet stream.

“In in the case of a really strong El Nino event, or the reverse, that’s usually the only time that it gives you more than the flip-of-the-coin change,” Doesken said.

“In a split flow, the subtropical (lower latitude) flow cuts across the southern border of the United States, where the main channel jet stream tends to go well north of the U.S. border. That tends to shunt the cold rather to the north, and toward the eastern United States.”

The split stream, therefore, means that wet storms are more likely in the southern states, while mild temperatures could hold sway over much of the Rockies and MidWest. Colder temperatures will stay above the border falling down on northeastern states, where they will undoubtedly deflate footballs late in the NFL season.

In southern Colorado, which saw a bit of relief from drought this year, precipitation increases could be significant.

So what does that mean for this winter in northern Colorado?
“I am bullish on thinking we won’t have really cold temperatures, at least in the western and probably the northern parts of the state,” Doesken said. “I used to be more bullish, but the older I get the less likely I am to go out on a limb.”

The split jet stream has particular significance in the northeastern Colorado. Far fewer arctic blasts should occur, where temperatures can easily fall to less than zero for days, and the Chinook winds of winter and early spring are expected to be far less severe.

“It’s hard to say why there were so few strong Chinook events last year,” Doesken said. “But we did have three very significant periods of arctic blasts.”

On the other hand, there is one aspect of El Nino events that seems to be harder to predict — early winter storms. The 1983 El Nino is a good example of what this can mean — almost 3 feet of snow fell in early September.

Given the strength of this year’s El Nino, Doesken (perhaps inadvertently) put himself out on a bit of a limb.
“The wild card is the statistic that says there are improved chances for a fall or early winter snowfall in the Front Range or eastern Colorado,” he said. “Look out for an early snowstorm before Thanksgiving.”

What about next spring?
“Now the question is, was El Nino just ramping up last spring (which saw heavy rains) and will the El Nino be ramping down? I think the prevailing opinion is we should still expect a wet spring,” Doesken said.

El Nino springs can bring significant precipitation to northeastern Colorado, in fact the 1983 event did just that. If the prevailing conditions hold until spring, there is a very good chance of spring Four Corners lows being created, which create very wet upslope conditions in northeastern Colorado — multi-day events that are very favorable to winter wheat crops.

“The prevailing idea would be don’t expect cold waves or stronger winds this year,” Doesken said.