It happens every year like clockwork. October’s shorter days and colder nights trigger weather prophets to come forth with their long-range winter forecasts. It’s traditionally a time to look forward to the upcoming storm season.
In the West, precipitation (rain combined with the liquid equivalent of snow) is measured in water years from Oct. 1 to Sept. 30 as opposed to a calendar year. Early October is historically the time of minimum stream flow and reservoir levels, as well as the traditional start of California’s rainy season.
Is another dry season in the offing as we enter winter 2022-23 with yet another moderate to weak La Niña episode influencing storm patterns over the Golden State?
Highly erratic 2021-22 season
Last winter’s weather was highly erratic, partially influenced by similar La Niña conditions in the equatorial Pacific Ocean — with cooler than normal sea-surface temperatures.
This negative phase of the El Niño Southern Oscillation (ENSO) helped establish extended high-pressure blocking systems in the mid-latitudes of the Northern Hemisphere. These quasi-stationary anticyclones deflect the eastbound jet stream and its associated weather systems. Atmospheric blocking comes in various shapes and forms, but the pattern is not unusual in the eastern Pacific Ocean off the California coast. When the atmospheric ridge is directly over California or just to the west, weather systems are deflected into the Pacific Northwest and Northern Rockies. But when the high-pressure cell retrogrades west or south, it opens the storm door.
Retired National Weather Service forecaster Jan Null recently shared his analysis of 24 La Niña events … It’s clear, statistically at least, La Niña-influenced winter seasons are generally a good signal for close to normal precipitation in our neighborhood.
This scenario seems to have become more common in recent years and has even been dubbed the Ridiculously Resilient Ridge, but blocking is a sporadic weather pattern and highly variable from year to year and even from decade to decade. Some scientists are concerned that climate change may increase these blocking episodes in the Northern Hemisphere, but other experts affirm that so far there are no clear trends indicating that this is the case.
The pattern is often caused by amplified Rossby waves — dramatic north and south meanders in the jet stream — an atmospheric configuration that drives frigid Artic air deep south; warm, moist air far north or brings a period of quiescent weather depending on location.
Early start to last winter
Last winter started off with an epic blast of rain and snow in late October when a powerful low-pressure system tapped into an atmospheric river out of the Pacific Ocean. Precipitation values in the Northern Sierra quickly reached 200 percent of average for that early in the season. On Oct. 25, Blue Canyon on the Sierra west slope was deluged with more than 10 inches in 24 hours, beating the previous one-day record from 1964. Nearly 10 inches of rain drenched Tahoe City, also a new record. About 5 feet of snow fell at the Central Sierra Snow Lab near Donner Pass.
December barrage
The moisture tap turned off in November and for the first half of December, but then a nine-day megastorm roared into the Tahoe Sierra. In an overwhelming display of nature’s raw power, Interstate 80 and Highway 50 were closed to trans-Sierra traffic for days. Travel by train, plane or automobile was shut down, while gasoline supplies ran out in many communities.
Many mountain and foothill communities lost electricity. Nearly 100,000 customers huddled in the cold and dark for days waiting for utility crews to fight through downed trees and deep snow to repair damaged lines.
Lead scientist Dr. Andrew Schwartz at the Central Sierra Snow Lab tallied almost 18 feet of snow during the barrage, the most December snow since at least 1970. (The lab was established in 1945, but snowfall data are not yet digitized prior to 1970.)
Palisades Tahoe topped its previous snowiest December (1970) with 211 inches for the month. Both Homewood Mountain Resort and Diamond Peak were hammered with 10 feet, while Northstar California reached a seasonal total of 23 feet — before New Year’s Eve. The first snow survey at the end of December 2021 revealed the Sierra snowpack was 160 percent of normal.
Ski resorts were elated with the prodigious snowfall and according to the U.S. Drought Monitor, the exceptional drought category had nearly been eliminated from the Golden State. And all this happened within eight weeks since NOAA’s Climate Prediction Center (CPC) issued its drier than normal forecast for California’s winter season.
Months of dry weather
But after the exceptional snowfall in December — crickets. Strong high pressure with an amplified jet stream built in over the eastern Pacific, effectively shutting down the storm track for all of January. Overnight temperatures in the mountains dropped to near zero under the clear skies, but there was no precipitation. It was the third driest January in Tahoe City since at least 1909. Throughout the whole month, the upper mountain at Palisades Tahoe picked up just 6 inches of snow. By the second snow survey on Feb. 1, 2022, snow water equivalent (SWE) in the mountain snowpack had plummeted from 160 percent to 90 percent for the date.
Twenty out of California’s 30 major climate stations reported their Top 5 driest January’s on record. It was an ominous sign. In February, no significant storms materialized but a mid-winter heatwave began to melt the Sierra snowpack in earnest. At South Lake Tahoe, sunny skies with temps in the mid- to upper 50s corresponded to record warm conditions more than 20 degrees above normal in the lower elevations.
The anomalous amplification of the jet stream with dry and balmy weather continued into March.
Arriving late to the party, a strong storm in mid-April finally broke down the blocking high pressure. Palisades Tahoe was stoked with 7 feet of snow at 8,000 feet, but by then many resorts had shut down for the season.
Dry spells & heat waves
The dearth of winter storms was sobering. San Francisco had 44 consecutive rain-free days in January and February, its fourth longest mid-winter dry spell since 1849. Reno, Nev. suffered through January with no measurable precipitation for the first time in nearly 130 years of record keeping. Although winter-season dry slots are a normal part of Northern California’s climate, they are more common and longer during weak ENSO events.
In the heart of the 2022 wet season, Sacramento went 10 weeks without a drop of rain. In 2021, the capital city had set a record streak with 212 consecutive dry days, until the October “bomb cyclone” flooded the town with unprecedented rainfall. Weather extremes to be sure.
The dry winter of 2022 segued into a warmer-than-normal summer before peaking in a blistering heat wave in September that set new all-time high-temperature records across Northern California. Sacramento residents sweltered in a new record of 116 degrees on Sept. 7, exceeding 114 degrees set on July 17, 1925.
During the same event, Reno boiled under a temperature of 106 on Sept. 6, the hottest temperature ever recorded in September and the fourth highest in the city’s history. The Biggest Little City in the World also endured nine consecutive days with triple-digit heat, its third longest streak, just one day short of the 10-day records set in July 2005 and July 2021. September’s average temperature in Reno was 71.3 degrees, 4.3 above average, making it the warmest ever. South Lake Tahoe set new daily highs in the low 90s.
September rain
Smoke transport from The Mosquito Fire near Foresthill was an issue this past summer, but conditions were not quite as bad for the Lake Tahoe Basin as the Caldor Fire was in August/September 2021. Fortunately, in mid-September 2022, as wildfire season approached its zenith, an exceptionally vigorous low-pressure system formed in the eastern Pacific Ocean and slid south along the California coastline, tapping subtropical moisture and drenching the mountains with several inches of rain. For the second year in a row, early season precipitation materially reduced the risk of forest fire across much of California.
As of Nov. 11, Lake Tahoe’s water level is 6,222.82 feet above sea level, dropping below its natural rim at 6,223 feet. It is not an uncommon occurrence because Big Blue has fallen below this point about 15 percent of the time since the dam was completed in 1913.
Since the development of massive irrigation projects in the arid American West, it has become clear that early 20th-Century officials overestimated the amount of water available to allocate. The poster child for this hydrological exuberance is the Colorado River system, which has been desiccated by a mega-drought for two decades. A word of advice: If they tell you that you’re in a 20- or 30-year dry spell, it’s not a drought. That’s your climate.
Virtually all of California was drier than normal during the winter of 2021-22 except for some locations in the Central Sierra and a few rural areas north of Truckee. Unique among California’s 10 hydrologic basins, the North Lahontan Hydrologic Region (NLHR) was the only one that achieved average annual precipitation. The NLHR, whose drainage basins are generally located in the rain shadow caused by the Sierra range, encompass much of the Tahoe Sierra topography.
The Lahontan Region includes a dozen major watersheds; among them the Truckee, Carson and Walker River drainages. Unfortunately, due to the timing and type of this year’s weather events, streamflow runoff was well below average. Precipitation values on the west slope of the Sierra ranged from 81 percent on the Northern Sierra Precipitation 8-Station Index to 63 percent in the Central Sierra, falling to 59 percent on the Southern Index.
What’s in store for 2023-24?
For the past two years NOAA’s CPC long-range forecast has envisioned below average precipitation based on similar La Niña conditions and this year is no different.
On Oct. 13, the CPC confirmed that the current ENSO-negative conditions should prevail and weaken through the 2022-23 winter with a 54 percent chance of a transition to an ENSO-neutral phase during February to April.
There have only been three three-year La Niña episodes in 73 years of record keeping. The CPC’s extended, multi-month outlooks for precipitation and temperature have utility when considered in the broadest sense of large-scale trends but fail to indicate higher-resolution weather events like impactful atmospheric rivers or juiced-up low-pressure systems from the Gulf of Alaska.
For skiers, boarders and outdoor enthusiasts, CPC outlooks are simply the agency’s educated guess, not a pinpoint forecast.
Retired National Weather Service forecaster Jan Null recently shared his analysis of 24 La Niña events and their impacts on the North Lahontan sector. Since 1950, out of two dozen negative ENSO oscillations, our hydrologic region averaged above normal precipitation for weak and moderate events and 97 percent during strong events. It’s clear, statistically at least, La Niña-influenced winter seasons are generally a good signal for close to normal precipitation in our neighborhood.
