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California's fire season is expanding, while the wet season is compressing

by Matthew Cappucci, Washington Post |

In California, raging wildfires seem to materialize like clockwork every summer and fall, damaging property and claiming lives. What was once traditionally the “wet season” is no longer immune from bouts of dangerous fire weather. California’s wildfire season is expanding, and human-induced climate change is a leading cause.

The greatest risk for wildfires comes between August and November, when parched vegetation left in the wake of summer’s warmth provides ample kindling at a time of year when seasonably strong winds fan any flames. But in recent years, that dry season has been swelling, the threat of fires lingering when winter rains are delayed.

We looked at the data to reveal just how conditions have evolved in the past 60 years as climate change and other factors alter California’s weather.

>> What we studied

We examined more than half a dozen cities throughout California, beginning at Mount Shasta in Northern California and extending as far south as San Diego. All stations we reviewed had uninterrupted or nearly continuous data from as far back as 1960, save for Mount Shasta, where the earliest records dated to 1979. That provided us 60 years’ worth of data.

Modern research suggests that California’s wet season isn’t necessarily drying out, but it is becoming more compressed. Atmospheric scientists point to a tendency for winter rains to arrive later and later, rather than ramping up around mid-fall, with fewer heavy rain events during the fall fringe season. The biggest events are most concentrated in December and January, precipitation more compactly squeezing into the heart of the wet season.

To evaluate changes in the wet season, we probed the average first and last dates of heavy rainfall, or calendar days with a half-inch or more of precipitation bookending the wet season. We also looked at the number of days with a half-inch or more of rainfall during the cool/wet season, as well as the amount of rain to come down each fall.

>> What we found

We first discovered that the dry season is getting longer — especially in Northern California. Northern California traditionally has a more pronounced wet vs. dry season because it is in a better position to intercept winter storms that dive southeast surfing the jet stream. We defined the wet season as the window between a winter’s first and last rainstorm with a half-inch or more in a calendar day.

Our findings showed that at Mount Shasta in Northern California, the dry season has grown 22 days just since 1979. In Redding, the dry season is a bit shorter but sticks around later — during the peak of wildfire season. San Francisco saw its dry season expand by 14 days, while Sacramento has to wait more than three weeks longer for the wet season to arrive.

Farther south, such places as Bakersfield saw a growth of its dry season, but it was a toss-up in San Diego and Fresno. That’s unsurprising, since the transitions between dry and wet seasons are more blurred in Southern California. What matters is that the autumn wet season is arriving later in San Diego and Fresno, heightening wildfire risk.

>>> Mount Shasta (Dunsmuir Treatment Plant): Dry season 22 days longer

* Since 1979, the average first date of a half-inch rainfall has jumped from Sept. 27 to Oct. 7.
* In the same time frame, the average date of a winter’s last half-inch storm leaped dramatically, from June 11 to May 27.

>>> Redding (Shasta Dam): Dry season eight days shorter but arrives later

* Since 1960, the average first date of a half-inch rainfall has jumped from Sept. 27 to Oct. 9.
* In the same time frame, the average date of a winter’s last half-inch storm trended later, extending from May 19 to June 8.

>>> Sacramento: No change in duration but much later arrival of wet season

* Since 1960, the average first date of a half-inch rainfall has jumped from Oct. 19 to Nov. 15.
* In the same time frame, the average date of a winter’s last half-inch storm crept from March 28 to April 22.

>>> San Francisco: Dry season 14 days longer

* Since 1960, the average first date of a half-inch rainfall has jumped from Oct. 30 to Nov. 13.
* In the same time frame, the average date of a winter’s last half-inch storm remained consistently around April 2.

>>> Fresno: Slightly longer wet season but later onset

* Since 1960, the average first date of a half-inch rainfall has jumped from Nov. 10 to Dec. 6.
* In the same time frame, the average date of a winter’s last half-inch storm shifted from March 18 to April 10.

>>> Bakersfield: Dry season 19 days longer

* Since 1960, the average first date of a half-inch rainfall has changed from Dec. 23 to Dec. 9.
* In the same time frame, the average date of a winter’s last half-inch storm shifted from March 8 to March 3.

>>> San Diego: Dry season five days longer; wet season arrives later

* Since 1960, the average first date of a half-inch rainfall has crawled from Nov. 21 to Dec. 8.
* In the same time frame, the average date of a winter’s last half-inch storm shifted from March 7 to March 17.

So far, that paints the picture of the dry season arriving later and, in many places, lasting longer. It also matters which end of the wet season is shifting faster.

We found that the most dramatic changes came in the fall, with the wet season being delayed later in the year. That means the dry season is extending longer into September, October and November, at the same time when Santa Ana winds, or systems that bring strong gusts that blow offshore and increase fire danger, are most common.

To be sure that this trend was legitimate, we also investigated the frequency of heavy rain events during September, October and November and found that there has indeed been a dramatic reduction in average fall rain since 1960. In some areas, the shortcomings were dramatic:

* Mount Shasta (since 1979): -5.85 inches, or 40 percent decrease.

* Redding (Shasta Dam): -6.24 inches, or 41 percent decrease.

* Sacramento: -0.29 inches, or 8 percent decrease.

* San Francisco: -2.57 inches, or 57 percent decrease.

* Fresno: -1.28 inches, or 52 percent decrease.

* San Diego: -0.65 inches, or 32 percent decrease.

The wet season as a whole didn’t really exhibit any significant trends in rainfall, attesting to the fact that the wet season isn’t dissipating but is rather being squeezed into a narrower window. We looked at the number of days each winter with a half-inch or more of rain and found little change:

* Mount Shasta (since 1979): No significant change.

* Redding (Shasta Dam): No significant change.

* Sacramento: Marginal increase.

* Fresno: Marginal decrease.

* San Diego: Marginal increase.

>> The bottom line

Putting it all together, the data points to a shrinking — and delayed — wet season. Winters in California are, for the most part, just as wet as they used to be — but the winter rains are arriving progressively later. That’s allowing autumn to become drier and the fire season to lengthen.

Our findings were consistent with peer-reviewed results, including those detailing a later onset of California’s rainy season published in Geophysical Research Letters in January. Among the most important shared findings is that the fire season is disproportionately expanding later in the season — which is already the most dangerous time of year.

Daniel Swain, a climate scientist at the Institute of the Environment and Sustainability in Los Angeles, penned a commentary in the same journal noting that the extension of the dry season increases the “overlap between extremely dry vegetation conditions and fire‐promoting downslope winds in late autumn.” Those winds ramp up at the start of fall, boosting wildfire danger dramatically.

With the dry season growing by as much as three weeks and eating further into fall, the stage is more frequently set for a dangerous overlap between hot, dry weather and the seasonal winds of autumn in the years ahead.