Are precipitation and snow levels returning to ‘normal’?
Storm after storm has pummeled central and northern California over the past couple of months, denting if not erasing a multiyear drought that was only partially mitigated over the last year or two.
One rainy season doesn’t mean the state’s drought problems are over, however, despite a big gulp of wet weather. Only California Governor Gavin Newsom can officially proclaim that the drought is over.
Despite major storms battering our region as well as up and down the state, California is still currently experiencing the longest duration of drought, which as of Feb. 26, 2019, has lasted 375 weeks beginning Dec. 27, 2011, according to the website www.drought.gov/drought/states/california.
Local weather spotter for the National Weather Service and Feather Publishing contributor Dale Knutsen provided two charts depicting the weather trends associated with snow and precipitation levels over the past 18 years.
The word “normal” gets used a lot when discussing weather, said Knutsen, as in “normal” temperatures; “less than normal” rainfall (leading to drought); a “new normal” climate; etc.
Making clear how the concept of normal is really a misnomer, Knutsen pointed out that, “There really is no such thing as “normal” when it comes to precipitation, nor in any other weather matter; there never has been.”
Instead of normal weather, forecasters generally use long-term “averages” as a measure for comparison, he said, to take into account the ups and downs of weather conditions from year to year.
So what’s going on with local precipitation? “Buckle up for a bumpy ride!” Knutsen remarked.
“Let’s start with how much snowfall we’ve had in Chester,” noting that, “This season we have experienced a lot of it.” Starting off slowly, “we saw only 2 inches in November and 13 inches in December. But January came on strong with 4 feet of snow, and then February brought us a bit more than 10 additional feet.”
The season accumulation at the end of February was 189 percent, he said for that point in the season. “In fact, it exceeded our average annual snowfall!”
So, where do these long-term averages come from? Local weather data started being recorded early in the 20th century, Knutsen said.
Eventually that data ended up in the archives of the Western Regional Climate Center in Reno, where current records are added to the file. “Those records are used to calculate long-term averages of a variety of weather measurements, which means we now have something over a century of records to establish long-term local trends.”
Back to the current year, “Most local residents would say that we have more snow on the ground now than in recent memory. And that is mostly true, largely because most of our current snowpack arrived in a concentrated period (January and February) rather than being spread out over a longer timeframe.”
A look at our recent snowfall history puts things in greater perspective, he said.
Knutsen submitted the chart titled “Snowfall History,” showing the amount of seasonal accumulated snowfall over the most recent 18 years.
“Just to make things interesting, it shows how much had been received in Chester by the end of February in the years designated with cross-hatched bars, and then how much the annual total was in previous years in white bars.”
Now we’re starting to see why the word normal has very little meaning in matters of weather,” he said. “Those snowfall amounts vary dramatically, from hardly any in 2014 and 2015 all the way to very deep in 2011. It’s hard to distinguish any real predictive trends in the pattern of ups and downs,” he said.
It’s also difficult to confidently forecast what this season’s final snowfall amount might be, he added.
“The February 28, 2019, snowfall amount (the far right hand bar) is pretty similar to the Feb. 28 amounts in 2008 and 2011.
But in 2008, hardly any snow arrived after February, while in 2011 “we got a lot of additional snow,” adding that the 2019 total snowfall is “anybody’s guess at this point.”
Not included on the chart is another deep snow season that many residents will recall in the years 1992-1993.
“It was a case where there was very heavy snow in December and January, with moderately heavy snow in February.”
For comparison, the February 28, 1993, seasonal snowfall accumulation came to 291 inches, and the final (June 30) annual seasonal accumulation was 295 inches. “So, no, we’re not in that extreme category this year, even though it may feel like it.”
Water content of snow (the amount of water in melted snow) is an important factor for water managers and for our local vegetation. Weather records keep track of rainfall plus the water content of snow in what is called “total precipitation.”
“Our recent history of total precipitation is illustrated in the chart titled, “Total Precipitation History; Rain Plus Water Content of Snow,” where again, the white bars indicate the season total (as of June 30) and the cross-hatched bar indicates the amount accumulated as of Feb. 28 of each year indicated.
Two observations are immediately obvious: (1) there is again a lot of variation from year to year, and (2) total precipitation doesn’t exactly mirror snowfall.
“That’s because first, there’s a substantial difference in the water content of “dry” snow versus “wet” snow, and second, there is never a consistent annual pattern of rain versus snow (sometimes we get little snow, but a lot of rain).
Going back to 1993, “We find that the total precipitation accumulation at the end of February of that year was 37 inches,” said Knutsen, “which is not much more than the 2019 amount.” So in that regard, the 1992-1993 snowfall was apparently drier overall than what we have received this season, he construed.
As for the bottom line: How is local precipitation doing with respect to the long-term average?
Clearly, 2019 is already above average, but what about our recent history, while the state has been in drought status?
Looking at the full annual seasons illustrated in the two charts “we can calculate the recent average snowfall and total precipitation amounts and compare them with the long-term numbers.”
For annual snowfall, our recent (17 season) average is 148 inches, or 116 percent of the long-term average.
For annual total precipitation, our recent average is 33.8 inches, or 106 percent of the long-term figure.
“When you look at more than a single year, we’re doing pretty well for the 2018-2019 time period.”
