Why do SLO County rainfall totals differ so much between coast and inland areas?
On Nov. 9, a weak cold front with southwesterly winds moved through the Central Coast and produced some much-needed rain in the coastal regions, but only sprinkles and light rain in the inland areas.
The Santa Maria and Lompoc airports recorded around one-tenth of an inch of rain, while Santa Ynez Airport only saw a few hundredths of an inch.
On the southwesterly side of the Santa Lucia Mountains, Dawn Dunlap at the Walter Ranch above Cambria and the rain gauge at Rocky Butte above San Simeon reported about one inch of rain, while Paso Robles Municipal Airport only saw one one-hundredth of an inch.
So, what caused such a difference in rainfall totals between the coastal regions and the inland valleys of San Luis Obispo County?
The Santa Lucia mountains to the north, and the Santa Ynez mountains, part of the east-west trending Transverse Range to the south, created a rain shadow on the lee side. That’s the side sheltered from the prevailing wind, the opposite from the windward side.
Traditionally, according to wind data recorded at Diablo Canyon Power Plant’s meteorological tower, the winds blow about 60% of the time out of the northwest (onshore) quadrant along the Pecho Coast.
The winds blow 12% of the time out of the northeast (Santa Lucia/offshore) quadrant and out of the southeast (prefrontal) quadrant about 23% of the time.
The other 5% of the time, the winds are spread evenly across the rest of the cardinal headings, such as from the southwest.
Santa Lucia (northeasterly) winds are more common during dry years, while wet years will see more prefrontal (southeasterly) winds.
Regardless of if it is a wet or dry year, southwesterly winds are rare.
The Santa Lucia mountains have a northwest-southeast orientation.
On Nov. 9, southwesterly winds hit the mountain range in a near-perfect perpendicular direction and caused the so-called “orographic lift” that helped to enhance the amount of rain that fell on Rocky Butte and the Walter Ranch.
The southwesterly winds blew off the ocean and carried moist air up and over the Santa Lucia mountains.
The increasing elevation causes the air to cool due to less atmospheric pressure, and the water vapor in the air condensed as it reached its dewpoint temperature, creating clouds and rain. It is like wringing out a sponge.
As a result, the ridgeline of the Santa Lucia mountains received around 50 times more precipitation than the Paso Robles airport that day.
Orographic lift rain or snow can be further enhanced by a mechanism known as the seeder-feeder process.
According to the American Meteorological Society, “Precipitation from a precipitating upper-level cloud (seeder) falls through a lower-level orographic stratus cloud (feeder) capping a hill or small mountain. Precipitation droplets or ice particles fall from the higher seeder cloud and collect cloud water as they pass through the lower feeder cloud by collision and coalescence or accretion, thus producing greater precipitation on the hill under the cap cloud than on the nearby flat land.”
On the lee side of the mountain range, the winds flow downslope and are called “katabatic winds,” from the Greek word katabatikos, which means “going downhill.”
As the air mass descended on the lee side of the mountain range, it warmed at a rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent.
Meteorologists refer to this rate of warming as the “dry adiabatic lapse rate.”
This, in turn, decreased the relative humidity of the air and increased the dew point temperature spread, resulting in less rain, if any at all, in the inland valleys.
On a higher level, that is why the western Sierra Nevada mountain range receives so much more rain and snow than the Nevada side, which is much drier.
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