What Twenty Years of Rain Looks Like in the San Lorenzo Valley
Two years apart, this valley recorded its second-driest complete water year in this station's record and its third-wettest. Water year 2021 brought 21.51 inches to the Ben Lomond gauge. Water year 2023 brought 85.13. Nothing about the way we talk about rain up here prepares you for a four-fold swing inside two winters, and yet that is the actual record.
The National Weather Service and the water district both keep this data, but almost nobody sees it laid out. So here it is: nearly ninety years of rainfall from the valley's longest continuous gauge, what the typical year really looks like, and why the word "average" is close to useless for planning a mountain property.
I've lived every one of these winters. The chart below is the same story my culverts, my generator, and my firewood stack would tell you, just with better data.
Where these numbers come from
The valley's long record belongs to a National Weather Service cooperative station called Ben Lomond No. 4, at 435 feet, which has been logging daily rainfall since 1937. It is about five miles down-canyon from downtown Boulder Creek, and it is the only gauge nearby with a record long enough to say anything about normal.
Rain here is measured by water year, October 1 through September 30, named for the calendar year it ends in. That matters: a water year holds one winter, which is how storms actually arrive. Splitting rainfall by calendar year cuts every wet season in half.
Of the 86 water years in the record, 67 are complete enough to report, meaning five or fewer missing days. The gaps are real gaps, mostly 2011, 2012 and 2024, and they are excluded rather than patched.
The station is NOAA cooperative Ben Lomond No. 4, GHCN identifier USC00040673, read on 2026-07-25 through the NOAA Regional Climate Centers ACIS service. A Boulder Creek volunteer station, US1CASZ0021, is used for comparison further down. Rainfall at your own property will differ, sometimes by a lot across a single ridge.
The typical year is drier than the average year
- Mean water-year total: 49.1 inches
- Median water-year total: 42.7 inches
A handful of enormous years drag the mean upward, so the "average" year is wetter than most years actually are. For describing a typical season, the median is the more representative number. Sizing drainage is a different question: that depends on peak storm intensity, the area draining to the pipe, soil and slope, not on how much rain a year brings in total.
The extremes
| Wettest water years | Inches |
|---|---|
| 1983 | 95.65 |
| 2017 | 94.62 |
| 2023 | 85.13 |
| 1998 | 82.80 |
| 1958 | 80.61 |
| Driest water years | Inches |
|---|---|
| 1977 | 19.97 |
| 2021 | 21.51 |
| 1976 | 21.60 |
| 2014 | 22.83 |
| 1990 | 24.30 |
Three things jump out. Two of the five wettest years on an eighty-six-year record have happened since 2017. So has one of the five driest. And 1976 and 1977 sit back to back, the two-year drought older residents still measure other dry spells against.
The last twenty years, in order
| Water year | Inches | Against the 42.7-inch median |
|---|---|---|
| 2006 | 74.62 | well above |
| 2007 | 29.03 | well below |
| 2008 | 38.79 | below |
| 2009 | 38.55 | below |
| 2010 | 56.20 | above |
| 2013 | 36.85 | below |
| 2014 | 22.83 | 4th driest on record |
| 2015 | 34.44 | below |
| 2016 | 46.62 | near median |
| 2017 | 94.62 | 2nd wettest on record |
| 2018 | 33.77 | below |
| 2019 | 65.26 | well above |
| 2020 | 31.38 | well below |
| 2021 | 21.51 | 2nd driest on record |
| 2022 | 44.58 | near median |
| 2023 | 85.13 | 3rd wettest on record |
Water years 2011, 2012 and 2024 are missing too many days at this station to report. Water year 2025 came in around 37 inches with fifteen days missing, so treat it as approximate and probably a slight undercount. The current water year, 2026, stood at about 45 inches through July, and since August and September are reliably dry here, that is close to its final figure.
Is Boulder Creek different from Ben Lomond?
Less than you would guess. A volunteer gauge three miles northwest of Boulder Creek, up at 875 feet, has overlapped the Ben Lomond record for seven reportable years. Boulder Creek came in an average of six percent lower, and within a few percent in five of those seven years.
Two cautions before anyone reads that as "Boulder Creek is drier." Volunteer gauges miss days, and a missed day during an atmospheric river costs inches, which is the likeliest explanation for the one big outlier, 2017. The fair conclusion is narrower: across the few years where both gauges reported, they generally moved in the same direction. A wet year in Ben Lomond is a wet year in Boulder Creek, so the long Ben Lomond record is a reasonable stand-in for the whole valley, and the exact inches at your own house will differ with elevation, canyon and aspect.
What this means for a house up here
This isn't a test for a long-term climate trend. It describes how wide the year-to-year swings are in one station's record. It says the swings are what you plan for. A 21-inch year and an 85-inch year both appear in the modern record, sometimes only two winters apart.
Practically, that argues for the unglamorous work: clear culverts and ditches before the first atmospheric river, because a 40-inch year and an 85-inch year both come through the same pipe. Know where water goes on your parcel. And treat any single dry winter as a pause rather than a trend, because 1977 was followed by decades that included four of the five wettest years on record.
Our maintenance calendar puts the drainage work in the right months, and the before the first major rain checklist covers the September and October push.
One threshold sits alongside the totals. The California Geological Survey, reviewing research that followed the destructive 1982 storms, notes that once a season has accumulated about ten inches of rain, storm rainfall of only a fifth to two fifths of an inch an hour has been enough to trigger widespread shallow landsliding in these mountains.
Set that against the numbers above and the picture changes. Ten inches is not a big year here, it is an early-season figure in most years. Reading nineteen years of Ben Lomond station data, the same study counted an average of thirteen occasions a year when rain fell hard enough, on ground already wet enough, to meet that combination. A season's total isn't what decides whether a slope fails. Storm intensity and how wet the ground already is matter more. It is the intensity of one storm arriving on ground that is already full.
Frequently asked questions
How much rain does Boulder Creek get in a year?
The valley's long gauge at Ben Lomond averages 49.1 inches per water year across 67 complete years since 1940, but the median is 42.7 inches, and individual years have ranged from about 20 to about 96 inches. There is no single normal.
What was the wettest year on record?
Water year 1983, at 95.65 inches. Water year 2017 came within an inch of it at 94.62, and 2023 was third at 85.13.
What was the driest?
Water year 1977, at 19.97 inches, part of the back-to-back 1976 and 1977 drought. Water year 2021, at 21.51 inches, is the second driest in the record.
Why measure by water year instead of calendar year?
Because a water year, October through September, contains exactly one winter. Calendar years split each wet season across two totals and make comparison meaningless.
For current conditions and forecasts use the National Weather Service, and for road and storm conditions see Mountain Status.
