California is approximately halfway through what may be the most closely watched wet season in state history. A rainy October and snowy December brought some relief from the extensive, multiyear drought, but a vanishingly dry January portends continuing water challenges.
Parts of central California have seen a record lack of precipitation so far this month.
What happens in the weeks ahead will have huge implications for the summer dry season. Almost all of the precipitation that nourishes soil and fills reservoirs in the western United States falls from November through March. The amount that it rains and snows in these five crucial months has a substantial influence on a region home to tens of millions of people and billions of dollars of agricultural production.
Years with insufficient precipitation have seen huge wildfires, widespread farming woes, domestic water rations and extreme heat. For much of the West, including California, another dry winter could turn a worrying situation dire.
The water year began with the immense bang of a record-setting October storm system, which dragged a Category 5 intensity atmospheric river into California. The deluge set Sacramento’s all-time single day rainfall record, ignited flooding and all but ended a disastrous wildfire season.
Though California had seen a lot of rain in that October storm, mountain snowpack – essential for easing drought conditions – remained largely absent. It was not until the middle of December, when a series of significant storms made landfall in the state, that this changed. Nearly continuous precipitation fell in hefty snowstorms that blanketed California’s higher elevations, bringing adverse short-term effects such as power outages and road closures. But the barrage was overwhelmingly good news, and brought Sierra snowpack well above normal.
By Jan. 1, total snowfall in the Sierras was over 50% higher than normal for the date.
The weather pattern responsible for the heavy snow allowed so-called atmospheric rivers to slam the coast repetitively. These “rivers” are plumes of moisture sucked from the tropical Pacific by long tongues of northerly wind. Weather patterns in which the high-altitude jet stream bends into a u-shape over the eastern Pacific Ocean steer such storminess toward the coastline. December saw the jet stream stuck in this offshore u-shape.
But an abrupt pattern shift in early January saw this persistent u-shape flip to an n-shape. Suddenly, the moisture-rich southwesterly wind stopped flowing, and the tap turned off.
Since then, only scant showers in January’s first week have managed to blow through Southern and far Northern California. The rest of the state, including much of the Sierra Nevada range, has remained bone-dry through what is typically among the wettest times of year.
This will probably end up as the driest January on record for much of central California. Sacramento has only received 0.05 inches, tied for fifth least on record. Just a trace of precipitation has fallen in Stockton.
In Nevada, Reno has seen no measurable precipitation so far this month.
Conditions have been so dry that an unusual midwinter wildfire erupted amid gusty offshore winds Jan. 21 and 22 just south of Monterey, prompting evacuations in Big Sur and closing a portion of Highway 1.
The weather pattern that enveloped much of the West in January also promoted unusual warmth, with temperatures three to seven degrees above normal. The important Sierra snowpack, built by an ideal December pattern, has actually decreased somewhat at many sites due to melting through the warm, dry weeks.
But just as a month of monumental rain and snow is insufficient to end California’s massive drought, a month of dry warmth is not enough for a complete backslide.
Precipitation for the season to date is generally at or just above average, with Sierra snowfall around 10 percent above normal. This means that a very wet season like 2016-17, and a very dry season like 2020-21, are both largely out of the question for the state. But whether California’s drought situation remains dire into 2022, or improves significantly, depends largely on what the next month brings to the state.
A shift from the weather pattern that all but blocked precipitation for much of the West Coast during January appears likely. However, that does not assure a wet February. In fact, dry weather may continue on balance.
While the n-shaped jet stream will likely transition quickly into a u-shape to end January, potentially bring more storminess toward the coast, it may well shift inland, shutting down any storminess thereafter.
The American modeling system, which averages dozens of computer simulations, can clue forecasters in on atmospheric patterns weeks in advance. Here, it shows the jet stream rapidly switching structure over the West to begin February from more of an n-shape to a u-shape.
While a period of storminess will be possible as this shift occurs, the precipitation potential may quickly wane deeper into February.
There are signals that the pattern could once again switch to a wet one into March, but it is still too early to project such a development with much confidence.
The official drought outlook from the National Oceanic and Atmospheric Administration calls for drought to persist in California into the spring.