It doesn't feel right to let this rainy season fade away without writing about the February storms--especially when they included a rare thunderstorm, and some were bigger than the December storm, which I did write about. So here it is May, the fourth average or wetter-than-average year in a row, and while the soil is already cracking due to the dry spring (although March was average with 6.3" of rain), things look good: the grasses are tall and there is still much green, and the now-disappearing flowers have been wonderful. And a pleasant surprise--the invasive deeply evil (in California, I'm sure it is nice where it is native) star thistle seems to be crowded out by other plants in many areas.
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| 2025 water year total to-date is 47.87" as of the end of May. |
Let's turn the clock back to February.
On January 31st, a light rain all day added up to 0.4 inches by the evening, and there was a half inch in the rain gauge by 5 am the next morning. The rain continued for the next two days, generally staying below a tenth of an inch per hour. Early in the morning of February 3rd, the first atmospheric river (AR) ended, with a total of 4.2 inches over three days. San Geronimo Creek rose from 5 cfs to 200 cfs during that time, peaking at 335 cfs at 10 am on February 2nd.
After a break in the rain during the day on February third, the second AR arrived at 5 pm and peaked early the next morning. By 4 am, 3.4" had fallen, and 0.37 inches per hour for 5.5 hours caused San Geronimo Creek to peak at 1,712 cfs, the highest flow since 1,625 cfs in March 2023. This is also remarkably similar to the 12/23/2012 flood (described here). The rain stopped by noon, and the creek dropped below 800 cfs, but afternoon thunderstorms dropped more rain.
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| The February 4, 2025 AR caused the pond on Larsen Creek near Roy's Redwoods to overflow in seven places--something I hadn't observed before (although I had seen evidence of it previously). |
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| I counted seven overflow spots. |
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| Buried Larsen Creek trying to daylight: Overflow from the pond following the path Larsen Creek naturally would have taken before it was put in a pipe. |
Between 2:10 and 3 pm gusty winds blew down trees and knocked out power on Meadow Way. Rain rates were 0.6 inches per hour from 2:10-2:45 pm, and 1.04 inches per hour from 2:45-3 pm. At 4pm San Geronimo Creek had risen back up to 1,341 cfs. By 6 pm, the sky was clear and the second AR had dropped a total of 6.2 inches of rain.
There was a break in the storms on February 5, and by February 6th the creek had dropped to 60 cfs by the time the third storm arrived. This storm dropped only 1.5" of rain, most of it in 12 hours, but it did have remarkable rain rates of 0.3 inches per hour (2:30-3 pm) and 0.66 inches per hour (8:05-8:15 pm).
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| San Geronimo Creek flow change by rain rate, with longer lengths of lines along the x-axis longer periods of time. A flat slope indicates a steady flow maintained by that rain rate, positive slope is a rising flow and negative slope is a falling flow. 0.09 in/hr maintained a flow of about 270 cfs over 5.5 hours, a quarter inch per hour supports flows of 900-1200 cfs, 1/3 in/hr supports 1200 cfs, 0.37 in/hr supports 1700 cfs, and 0.5 in/hr supports flows over 3000 cfs (some data based on previous years). |
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| The Larsen Creek pond overflowed again on February 13, 2025 |
- 20-25 cfs at Roy's Redwoods
- 5 cfs overflowing pond, 20 cfs in the pipe, for a total 25 cfs below broken culvert
- 30 cfs at the first tributary
- 40 cfs above the confluence at the dog park bridge
- 47 cfs below the dog park bridge
- 50 cfs at the school
- 52 cfs at the Community Center
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