NASA

This image shows La Niña conditions from last month, collected by NASA's Jason-2 satellite.

This has been a historically dry winter, dry enough that it’s likely to land a spot as one of the top ten driest since the Gold Rush. And even though La Niña is waning, that probably won’t make much of a difference, because there’s a delay between when ocean surface temperatures change, and when that change actually has an effect on our weather.

“March 20 is just around the corner, and that’s the first day of spring. Our winter — our snowpack and our rain — is essentially over,” NASA Jet Propulsion Laboratory climatologist Bill Patzert told me. Though Patzert’s observation comes as three Pacific storms are poised to potentially bring a week of rain to Northern California, he said, “a weakening La Niña won’t necessarily give us a March miracle in terms of snowpack and rainfall.”

La Niña is caused by colder-than-average ocean surface temperatures in the equatorial Pacific. It typically makes for warm, dry winters in California. But not always. Last year was also affected by La Niña, and it was historically wet.

“I find that as a great teaching moment,” adds Jan Null, from Golden Gate Weather Services. “We look at 20 La Niña years since 1950, and you come up with the average conditions. Well, those averages are made up of a wide range and we’ve seen both ends of that range in the past two years.”

Or, as Jim Mathews, lead forecaster at the National Weather Service Forecast Office in Sacramento put it with a laugh, “Normal here in California is the average of two extremes. It seems to be wet and dry and then we take an average and call that normal.”

Some atmospheric scientists think that could change soon.

By Andrew Freedman

While some may be cheering the lack of snow as welcome relief, the widespread lack of it spells trouble for the ski industry, which pumps billions into the wintertime economy in states from California to Maine, and requires cooperation from Mother Nature to stay in business.

Craig Miller/KQED

Snow from last year's big winter storms could still be seen on the mountains near Lake Tahoe on August 30th. This winter has been one of the driest on record.

Ski area operators across the country are already reporting drops in lift ticket sales, and are hoping for a major change in the weather pattern to bring colder, snowier weather. So far, die-hard skiers have been forced to either ski on man-made snow or travel to one of the few far-flung areas that have benefited from the unusual weather, such as the mountains of New Mexico or Alaska (where one town has had 18 feet of snow).

Compared to last winter, this wimpy winter weather is coming as quite a shock.

Snow was so widespread last winter that at one point in January, every state except Florida had some snow on the ground. But this year, the U.S. had the 11th least extensive December snow cover in the 46-year satellite record, said David Robinson, the director of the Global Snow Lab at Rutgers University.

“Is it fair to call it a snow drought? We’re getting there,” Robinson said. “It’s certainly an early season snow drought.”

Michael Berry, president of the National Ski Areas Association, said this winter is noteworthy for how many ski areas are seeing below average snowfall. “Typically, we have one region or another in the country that might be off to a slow start. But the thing about this year that’s somewhat unique is that it’s kind of an across-the-country problem, at least to date.”

Nowhere is the contrast from last winter so evident as in California’s Sierra Nevada Mountains, home to Lake Tahoe’s ski resorts.

National Oceanic and Atmospheric Administration (NOAA)

Observed snow depth on December 29, 2010 (top) and December 29, 2011 (bottom).

Last year, skiers hit the slopes all the way through July 4th, and several mountains set all-time seasonal snowfall records, with totals surpassing 65 feet. Squaw Valley, for example, received a staggering 810 inches of snow.

This December, at nearby Alpine Meadows, just 1.5 inches of snow fell, well below the average of 72.3 inches, and far below last year’s 134-inch December total.

The lack of snow has allowed Yosemite’s Tioga Pass, a road that would normally be buried in snow by now, to remain open later than at any time since record-keeping began in 1933.

In the Northeast, an October blizzard and other early season snows were followed by warm temperatures that melted the natural snow cover, and prevented ski areas from making snow. With a recent shot of cold air, New England’s ski resorts have finally cranked up their snow making operations, but attendance is down from last year, in part due to the bare ground in major media markets, such as New York and Boston. With just a trace of snow in December, Boston had its second-least snowiest December on record, for example.

“Psychologically, we do better when people see snowfall in their backyards,” said Bonnie MacPherson of Okemo Mountain in Vermont, where skier visits were down about 30 percent compared to last year over the same Christmas holiday week. “We’re fortunate in that we’ve invested pretty heavily in the snow-making system that we have,” she said.

With each passing day of below average snowfall, it becomes more difficult to make up for lost time, especially when spring is arriving earlier on average due in part to global warming. This forces ski area operators to squeeze the same amount of earnings into a shorter time period.

Rutgers’ Robinson said ski areas would have to rely more on the President’s Day school vacation week in February to compensate for lost revenue during the Christmas holiday period. “Many of the ski resorts are going to have to really count on it this year because they were hurt by the holiday week between Christmas and New Year’s,“ he said. A poor turnout during that week could mean ski areas are “going to be running in the red.”

Forecasters are quick to caution, though, that the rest of the winter may not play out the same way that the season has started. Much depends on two key factors that affect winter weather in North America: La Niña and the Arctic Oscillation.

Two Very Different La Niña Winters

La Niña, which is a natural climate cycle characterized by cooler than average water temperatures in the tropical Pacific Ocean, tends to nudge the wintertime storm track to favor heavier snows in the Pacific Northwest, northern Rockies, Ohio Valley, and northern New England.

But this winter has not been behaving like a typical La Niña winter. Instead, storms have moved across southern California, into New Mexico and Colorado, and then on up into the Midwest.

Klaus Wolter, a researcher at NOAA’s Earth System Research Lab in Boulder, CO, said there are indications this will change later this month. “I’m pretty confident that it will probably revert back to more typical La Niña winter weather,” he said.

Part of the reason why this winter has departed so dramatically from La Niña’s script has to do with the Arctic Oscillation, which is a pattern of atmospheric pressure that helps steer the jet stream in the Northern Hemisphere. When it’s in a “positive phase” as it has been so far this winter, cold air tends to remain bottled up in the Arctic. So far in 2012, 130 daily high-temperature records have been broken in California alone.

Wolter and many other forecasters think the Arctic Oscillation may shift during the next few weeks, making it easier for a more typical La Niña weather pattern to emerge. This would begin to favor ski areas in the Pacific Northwest and northern Rockies, such as Jackson Hole, Wyoming and Big Sky, Montana.

A version of this post also appears at Climate Central, a content partner of Climate Watch.

Precipitation outlook for winter 2011-12, showing the likelihood of below average precipitation in Texas and other drought-stricken states.

Does this mean Texas is toast?

By Michael D. Lemonick

As most Californians know, El Niño is a periodic unusual warming of the surface water in the eastern and central tropical Pacific Ocean. Actually, that’s pretty much a lie. Most people don’t know the definition of El Niño or its mirror image, La Niña, and truthfully, most people don’t much care.

What you do care about if you’re a Texan suffering through the worst one-year drought on record, or a New Yorker who had to dig out from massive snowstorms last winter (tied in part to La Niña), or a Californian who has ever had to deal with the torrential rains that trigger catastrophic mudslides (linked to El Niño), is that these natural climate cycles can elevate the odds of natural disasters where you live.

At the moment, we’re now entering the second year of the La Niña part of the cycle. La Niña is one key reason why the Southwest was so dry last winter and through the spring and summer, and since La Niña is projected to continue through the coming winter, Texas and nearby states aren’t likely to get much relief.

But Niñas and Niños (the broader cycle, for you weather/climate geeks, is known as the “El Niño-Southern Oscillation,” or “ENSO”) don’t just operate in isolation. They’re part of the broader climate system, which means that climate change could theoretically change how they operate — make them develop more frequently, for example, or less frequently, or be more or less pronounced. Climate change could also intensify the effects of El Niño and La Niña events.

Climate scientists have been wrestling with the first question for a while now, and they still don’t really have a definitive answer. Some climate models have suggested that global warming has already begun to cause subtle changes in ENSO cycles, and that the changes will become more pronounced later this century. But a new study, published in the Journal of Climate, doesn’t find much evidence for that.

But on the second question, the new study is a lot more definitive. “Due to a warmer and moister atmosphere,” said co-author Baylor Fox-Kemper, of the University of Colorado in a press release, “the impacts of El Niño are changing even though El Niño itself doesn’t change.”

That’s because global warming has begun to change the playing field on which El Niño and La Niña operate, just as it’s changing the background conditions that give rise to our everyday weather. The Texas drought is a prime example. It’s most likely cause is reduced rainfall from La Niña-related weather patterns. But however dry Texas and Oklahoma might have been otherwise, the killer heat wave that plagued the region this past summer — the sort of heat wave global warming is already making more commonplace — baked much of the remaining moisture out of both the soil and vegetation. No wonder large parts of the Lone Star State have gone up in smoke.

A map of sea surface temperature anomalies, showing a swath of cooler than average waters in the central and eastern tropical Pacific Ocean - a telltale sign La Niña conditions.

When the next El Niño occurs in a year or two, it will probably bring heavy rains to places like Southern California, whose unstable hillsides tend to slide when soggy. Except now, thanks to global warming, the typical El Niño-related storms that roll in off the Pacific may well be turbocharged, since a warmer atmosphere can hold more water. This is the reason, say many climate scientists, that downpours have become heavier in recent decades across broad geographical areas.

La Niña, plus the added moisture in the air from global warming, have also been partially implicated in the massive snowstorms that struck the Northeast and Mid-Atlantic states during the last two winters. Those could get worse as well, suggests the new analysis. “What we see,” says Fox-Kemper, “is that certain atmospheric patterns, such as the blocking high pressure south of Alaska typical of La Niña winters, strengthen…so, the cooling of North America expected in a La Niña winter would be stronger in future climates.” So to pre-answer the question that will inevitably be asked next winter: no, more snow does NOT contradict the idea that the planet is warming. Quite the contrary.

Finally, for those who really do want to know what El Niño and La Niña actually are, as opposed to what they do, you can go to NOAA’s El Niño page. But be warned: there will be a quiz, and the word “thermocline” will appear.

A version of this post also appears at Climate Central, a content partner of Climate Watch.

Frank Gehrke conducting last year's first snow survey of the season. (Photo: Gretchen Weber)

All the wet weather that’s been drenching much of the state has left the Sierra Nevada with an extra-thick blanket of snow, which has water officials optimistic about the state’s water supply for 2011.

Using a combination of manual and electronic measurements, the state’s Department of Water Resources conducted its first snow survey of the season on Tuesday, and found the water content of the state’s snowpack at 198% of normal for this time of year.   Last year at this time, the statewide average was just 85% of normal.

Surveyor Frank Gehrke said all this precipitation has California off to an unusual start this winter season, especially because it’s a La Niña year, which usually means a drier winter throughout much of California. “You generally don’t expect to see these really significant accumulations at all,” said Gehrke. “So, we’re a little bit scratching our heads going ‘Hmm, this is a little outside of the pattern.”

Particularly unusual, he said, is the situation in the southern Sierra, where the water content of the snowpack is currently 261% of normal for this time of year. Water content is a better indicator of spring runoff than snow depth alone.

But just because we are starting out wet doesn’t mean we’ll stay that way.  Gehrke said he predicts a lull in the storms sometime soon, and just how long that lull is could determine what the water allocations will be in the spring.

“A common feature of a Niña is a pretty decent start to the year with a lapse in the January/February timeframe,” said Gehrke.  “And that could put us in a less than optimal circumstance come April 1.”

That didn’t dampen the optimism of Gerhke’s boss at DWR. “This boosts our hopes that we will have an adequate water supply for our cities and farms as we continue to shake off the effects of the 2007-2009 drought,” said director Mark Cowin in a press release Tuesday.

Earlier this month the agency said it would likely deliver 50 percent of the water requested for the coming year, which is what it delivered in 2010, but the agency said Tuesday that cities and farms are “all-but-certain” to get more water from the state in 2011 than they have in years.

Most of California’s reservoirs are at or above normal storage levels for this time of year.  You can track the state’s water supply with our interactive map, which now includes key information from the snow survey: California Reservoir Watch.

View KQED: California Reservoir Watch in a larger map

Scientists like to joke that “climate is what you expect, weather is what you get.” The relatively soggy winter so far is a classic example of that.

Satellite image from last weekend, showing storm systems marching across the Pacific toward California. (Image: NASA)

A closely-watched oscillation in the Pacific is in the La Niña phase this winter, creating colder-than-normal surface temperatures and distorting weather patterns. Usually a La Niña means drier-than-normal conditions for Southern California in particular and often for northern parts of the state as well. Not this year–at least not so far. The rain set multiple records over the weekend. Los Angeles has had a third of its average annual rainfall in a week. So what’s going on?

Kevin Trenberth, who heads the Climate Analysis Section at the Nat’l Center for Atmospheric Research, says lately there’s a monkey wrench in the works, in the form of a “blocking anticyclone.”

“In La Niña conditions, which is what we have now, the main storms that come into North America come barreling into Washington, Oregon and British Columbia more,” Trenberth told me in a phone interview.

But lately a persistent region of high pressure in the north Pacific is diverting storms south, into California. Trenberth says: “There’s a crapshoot or a random component to it, if you like, in the more northern latitudes, that’s adding some extra flavor to what’s going on, I think.

But that doesn’t mean it’ll keep raining. The tap could be shut off at any time and Trenberth, for one, still thinks it’ll happen. He says this is considered a “strong” La Niña and is still likely to wield influence over the winter as a whole. One clue is ocean temperatures in the central-to-eastern Pacific, which are running 2 degrees C (3.5 F) below normal. “That only occurs—probably less than 10% of the time, so it’s a relatively rare event and certainly stronger than anything we’ve seen in recent years,” said Trenberth.

Earlier in the season, Jet Propulsion Laboratory climatologist Bill Patzert told the Los Angeles Times that 82% of La Niñas since 1949 have produced below-average rainfall.

That’s more the case for the Southland. As you move north, La Niña’s influence on precipitation reverses itself and the Pacific Northwest gets doused. Trenberth says the transition line is right about 40 degrees north latitude. Around that line, La Niña’s effects become murky and things can go either way. San Francisco is at 37 degrees north and Reno is at 39.

With a global average surface temperature of 79 degrees Fahrenheit, 2008 was the coolest year since 2000, according to climatologists at the NASA Goddard Institute for Space Studies (GISS). But it’s also the ninth-warmest year since 1880, so it’s probably not time to invest in a ski resort just yet.

Including the 2008 dip, the 10 warmest years on record (since 1880) have all occurred between 1997 and 2008, according to NASA.

The NASA scientists attribute 2008′s relatively lower temperature to a cooler Pacific Ocean, due to a strong La Nina pattern in the first half of 2008. La Nina and El Nino are opposite phases of a natural oscillation of  upwelling and subsequent temperatures in the equatorial Pacific Ocean.

2008 temperatures in the United States were cooler than any other year this decade, but, as illustrated on the map below, other parts of the world such as Eurasia and the Arctic were exceptionally warm.

Director of GISS James Hansen predicts that because a shift to El Nino is expected to start this year or next, it “still seems likely” that we’ll see a new record high for the average global surface air temperature in that time frame.

The short version: It’s bleak.

As I noted last week, Lake Oroville, a key reservoir on the Feather River, stood at 31% of capacity as of midnight on September 30. Readings from the same hour showed the state’s largest reservoir, Shasta Lake, at 30%; Folsom Lake (American River, east of Sacramento) at 28%; and San Luis Reservoir, east of Silicon Valley at 12–yes, twelve percent of capacity.

Capacity figures by themselves can be misleading. We expect reservoirs to be low this time of year, right at the end of the dry season. But as DWR was taking these readings, Oroville, to use one example, was at 49%–less than half–of “normal” for this time of year.

So, much depends on the coming winter. Even with all the advanced tools that forecasters have at their disposal in this first decade of the 21st Century, it’s hard to say how much water we’ll wring out of the skies this winter. Lynn says we’re in a “La Nada” pattern, meaning the Pacific Ocean isn’t giving a strong signal for either El Nino or its opposite, La Nina. The two conditions describe the degree–or lack–of cold water upwelling from the ocean depths, which has a strong influence on California’s precipitation patterns.

But Lynn says the consensus among forecasters is “leaning toward a dry-to-average” winter and average won’t get us there. We’ll need several soggy months to make up for lost water and avoid more severe water restrictions throughout the state next summer.