Weather Watch

Recent storms show how dramatically topography can affect rainfall

A view of the Pt. Bouchon Trail and Diablo Canyon nuclear power plant in the distance.
A view of the Pt. Bouchon Trail and Diablo Canyon nuclear power plant in the distance.

On a clear day on the Point Buchon Trail at Windy Point just north of Diablo Canyon Power Plant, you can easily see Morro Rock to the northwest and to the southeast, Point San Luis.

To the northeast, towers the Irish Hills. This part of San Luis Obispo County is often referred to as the Pecho Coast. Along this magnificent shoreline, about 14 miles of rugged and diverse topography, I’ve seen cold fronts fall apart and seemingly vanish between Montaña De Oro State Park and Port San Luis.

At other times, fronts will actually intensify and stall over a particular section of this coastal region, producing copious amounts of rain over one part while leaving other areas relatively dry.

These quirky occurrences can be discouraging for any soul attempting to write an accurate forecast.

The topography along the Pecho Coast plays an important role in the various rainfall anomalies. The Irish Hills force moisture-laden winds blowing horizontally from the Pacific Ocean to turn vertical or upward. As the air mass is lifted up over these mountains (upwind), it cools and eventually reaches its dew point temperature.

When this occurs, either clouds, drizzle or rain will develop on the windward side of the mountain. Like squeezing a wet sponge, moisture from this air mass is released in the form of precipitation. This is also called orographic enhancement or uplift.

Areas near or just past the summit will often receive the greatest amount of rain. On the leeward (downwind) side of the Irish Hills, the air mass is forced downward by gravity and warmed by pressure. The sinking air is drier after losing much of its moisture on the upwind side of the mountain range. Locations on the leeward side of the coastal range receive noticeably less rain.

Last week’s two storms illustrated this phenomenon perfectly. Thursday’s cold front produced persistent southerly winds and rain throughout San Luis Obispo County. However, upwind areas of Irish Hills received well over an inch of rain, such as Avila Beach, while communities on the downward side, such as Los Osos, recorded less than quarter of an inch of precipitation. The rain gauge at the Diablo Canyon meteorological tower, at the half way point along the Pecho Coast, reported 0.86 inches.

Last Sunday’s storm, a robust 1,007 millibar surface low pressure system strengthened by upper-level winds, produced convective cells of moderate rain showers and northwesterly winds during overnight hours. These winds enhanced the amount of rainfall on the Morro Bay/Los Osos side of the Irish Hills. Dawn’s weather station in the North Cloisters of Morro Bay reported nearly 2 inches of rain, while on the other side of these mountains, rainfall amounts were generally less than three quarters of an inch. Again, the rain gauge at Diablo Canyon recorded the average of the two sides at 0.83 inches.

Another example of this can be seen in the Sierra Nevada. The lush and fertile Sacramento Valley is fed by the western slopes of the Sierra Nevada, while its eastern slopes descend to the vast Nevada desert. Of course, due to orographic enhancement, wind direction will also affect rainfall totals.

Land stewardship is important to PG&E. The company manages 12,820 acres that surround Diablo Canyon Power Plant. This has allowed for coastal hiking trails open to public use, including the Pecho Coast Trail that leads to the restored Point San Luis Lighthouse and the Point Buchon Trial which leads to Windy Point. To learn more about these preserved lands, visit