True to its nature, April is a transition month regarding weather conditions: not quite summer and not quite winter. You may have stormy weather one day followed by record-breaking heat the next, and this week’s weather should live up to this reputation.
Post frontal gale-force northwesterly (onshore) winds returned along the Central Coast on Saturday afternoon and will continue into Sunday.
At this time of the year, the winds blow out of the northwest around 80 percent of the time at the Diablo Canyon Power Plant’s meteorological tower. In old weather reports from the early 1900s in San Luis Obispo, locals often called these winds the “Los Osos winds” as they blew in from the Los Osos Valley off the Pacific Ocean.
So why is spring so windy?
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At this time of the year, the Eastern Pacific High off the California coast strengthens and gradually shifts northward. This condition frequently weakens cold fronts as they head down the California coastline, which diminishes the strength of the pre-frontal (southerly) winds but tends to increase the post-frontal (northwesterly) winds. Due to the lack of southerly winds and the orographic enhancement they yield, rainfall totals tend to be more uniform across the entire region. That’s what occurred Saturday: Cambria recorded over a half an inch of rain, while Rocky Butte — typically one of the wettest locations in the county — only received about a quarter of an inch.
The northwesterly winds of spring are further enhanced by the higher amount of sunlight as the days grow longer. Let me explain: As the days grow longer in April, the air in the inland valleys naturally warms. As the valley’s surface air heats, it expands and rises like a hot air balloon. This in turn produces lower pressure at the earth’s surface. Meteorologists refer to this as a thermal low. Nature never likes anything out of balance, and consequently, the higher air pressure out over the ocean forces air inland to fill the void left by the thermal low.
As these northwesterly winds blow along the coastline, they cause upwelling along the shoreline.
Upwelling brings cold and nutrient-rich subsurface water from the ocean’s deep to the surface along the beaches. The cold surface water cools the air that travels over it and acts like a gigantic air conditioner.
Nature’s air conditioner will kick in Sunday afternoon as gale-force northwesterly winds develop along the coastline.
However, due this month’s nature, a transitory ridge of high pressure will produce strong-to-gale-force Santa Lucia (northeasterly) winds Monday. These offshore winds will create dry and warm conditions with the beaches, coastal and inland valleys reaching the mid-80s.
A trough of low pressure will approach the Central Coast by midweek and promote widespread cooling with a chance of scattered rain showers, mountain snow and moderate gale-force to fresh gale-force (32 to 46 mph) northwesterly winds along the coastline Wednesday into Thursday, producing much cooler temperatures. Additional wet and unsettled weather may arrive next weekend.
These fluctuating atmospheric conditions could lead to a pleasant surprise. The “mini-miracle of March” combined with this April’s rain showers, heat and cold have created the conditions needed to germinate many of the annual wildflowers. In fact, the intensity of the annual bloom seems to be increasing throughout the Central Coast.
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Join PG&E employees April 21 to celebrate Earth Day at Montaña de Oro State Park. The event is one of a number of service projects sponsored by PG&E and the California State Parks Foundation. Be sure to dress for outdoor work with long pants, long-sleeved shirt, sturdy shoes, hat, gloves and sunscreen.
Snacks and a light lunch will be provided. Bring your own refillable water bottle. Rangers will provide tools and supervision. Please register at the California State Parks website: www.calparks.org/help/earth-day .