How much rain is considered normal in SLO County? Average annual rainfall is dropping

I have often been asked the average yearly amount of rainfall for a particular location and how it is calculated.

I saw a tweet from Jan Null last week that answered this question. He has been a meteorologist in the San Francisco Bay Area for more than four decades and runs an informative website called Golden Gate Weather Services.

“Every decade, the 30-year normals that are the de facto climatological standard are recalculated,” Null wrote on his site. “Sometime within the next six months, an update from the 1981-2010 normals to the 1991-2020 normals will be published by NOAA and other agencies around the world.

“It is interesting to note that over the past decade, all the California stations, except Eureka, show a drop in their 30-year normals, averaging approximately 5%. This shift was largely the result of the drought years across much of the state from 2011 to 2015 that pushed values downward.”

I performed an analysis of rainfall data from key Central Coast weather stations and found roughly the same magnitude of decline in rainfall normals. I utilized rainfall data from the Southern California Climate Summaries.

From 1981 to 2010, the Paso Robles Municipal Airport reported an average rainfall amount of 15.38 inches. From 1991 to 2020, the normal decreased to 14.99 inches, or, about 3% less.

Cal Poly, the home of climatology for San Luis Obispo, went from 23.62 inches of rain per year to 20.65 inches during the same time period — a 14% reduction.

Santa Maria Public Airport from 1981 to 2010 saw a normal of 14.13 inches of rain but lowered to 13.07 inches from 1991 to 2020, an 8% drop.

Lompoc Airport went from 16.14 inches of rainfall to 14.39 inches, or, 11% less.

These decreasing amounts of rainfall may also be attributed to climate change. The climate models have consistently predicted more prolonged droughts interrupted by higher intensity rainfall events.

Here is why.

Warmer air can contain more water vapor than cooler air before becoming saturated and condensing into clouds or falling in the form of drizzle, rain, sleet, snow or hail.

So why is warmer air capable of containing more water vapor than cooler air?

As the temperature of the air increases, the water vapor molecules contained within are also warmed, giving them more energy. That, in turn, makes it less likely that they will come together and condense or change phases from a gas to a liquid — like a fast dance versus a slow one.

Therefore, with a higher number of higher energized water molecules, warmer air can contain more water vapor than cooler air.

As the atmosphere and oceans continue to warm due to ever-increasing levels of greenhouse gases such as carbon dioxide, the air can absorb more water through evaporation.

If the dew point temperatures are low and air temperatures high, which is so often the case in California during summer and fall, more rain will evaporate before reaching the ground (virga).

Consequently, this reduces soil moisture and stream flows. Not only does this provide less water for trees and other vegetation, but it also allows the air to remove more moisture directly from the plants, creating bone-dry surface fuels.

This condition, combined with other factors, has dramatically increased the severity of wildfires.

In late January, an atmospheric river stalled over Cambria and retrograded northward toward Big Sur, before moving southward into southern San Luis Obispo County and northern Santa Barbara County and produced vast amounts of rain.

Dawn Dunlap lives above Cambria on the Walter Ranch at 1,250 feet of elevation in the Santa Lucia mountains. She told me that this past storm produced more than 17 inches of rain over 54 hours, but the winds were not strong at her home.

Nevertheless, southerly wind gusts estimated at up to 90 mph howled on Jan. 27, around Hearst Castle in San Simeon.

This was due to orographic enhancement, which happens when larger amounts of rain or snow develop on the mountains’ windward side. Like squeezing a wet sponge, moisture from this air mass is released in the form of precipitation.

Doc Miller, who lives a few miles away from Hearst Castle on Pine Mountain, reportedly saw nearly 40 inches of rainfall.

Due to the dry Earth, water levels in many of the local lakes and reservoirs did not increase as much as hoped as a result of the storm. However, Lake Nacimiento saw a rise from 21% of capacity to 41% during this atmospheric river event, which drenched its watershed.

Climate Central is an independent organization of leading scientists and journalists researching and reporting the facts about our changing climate. Climate Central said that the number of days with heavy precipitation has increased in most locations in the United States over the past 70 years.

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