Normally, April is a transition month between the wet season and the dry.
Average rainfall for the month of April tapers off to about 1.7 inches in San Luis Obispo and 0.9 in Paso Robles, but there have been exceptions. According to general climate summaries for San Luis Obispo at Cal Poly — home of record for climatology for San Luis Obispo — the wettest April occurred in 1879, when 8.8 inches of rain were recorded.
The wettest April on record in Paso Robles occurred in 1958, with 4.9 inches. However, many Aprils have been bone dry, such as the Aprils in 2013 and 2002, with no measurable rain.
So far this April, the only rain recorded was Tuesday’s cold front that produced 0.7 of an inch of rain at Cal Poly, 0.4 of an inch in Paso Robles and 0.6 in Santa Maria.
What April lacks in rainfall, it makes up by the intensity of its storms. As the days grow longer, the temperature gradients tend to increase not only from mid- to northern latitudes at the Earth’s surface, but also as you ascend into the sky, creating a greater amount of atmospheric instability.
Cal Poly physics professor Bob Echols said, “It’s amazing how much the vertical wind influences the rain in frontal boundaries.”
Cold fronts such as Tuesday’s storm contain colder and denser air that push the lighter and warmer air upward — somewhat like snow moving up the shoveled surface of a fast-moving snow plow. The warm air cools as it rises and condenses into clouds. As the water vapor transforms into clouds or liquid/frozen water, it releases tremendous amounts of latent heat — the energy that intensifies storms.
For example, last year’s April tempest produced 1-inch-diameter hail, lightning strikes and gusty winds in the Cayucos area toward San Luis Obispo. Updraft winds of between 37 and 56 mph are needed to produce hail of that size.
Long-time Cayucos resident Pat Molnar commented about the hail strikes: “What a way to wake up in the morning; you could not believe the racket it made.”
Cayucos resident Mike Smith said, “It sounded like a freight train coming through the house.”
An extensive review of weather records for San Luis Obispo County indicates that these were probably the largest hailstones ever documented along the Central Coast.
Tragically, April storms can be deadly, as seen April 7, 1926, when a Pacific storm came in from the west and produced lightning in San Luis Obispo County. The lightning struck large oil tanks along Tank Farm Road. More than 5 million gallons of oil burned over five days.
Burning oil reportedly made it all the way to Avila Beach by way of San Luis Obispo Creek. The intense heat from those fires produced hundreds of fire whirls, many of which showed characteristics of true tornadoes. One of the fire tornados traveled 1,000 yards, picked up a house and carried it 150 feet, killing the two people inside.
Of course, the month is also known for its strong northwesterly (onshore) winds and cold seawater temperatures. On one day in April 2008, the Diablo Canyon Power Plant’s primary meteorological tower recorded northwesterly winds at 50 mph sustained and gusts up to 64 mph.
After this winter’s near-record-breaking warm seawater temperatures, this April’s northwesterly winds and the upwelling they bring has brought seawater temperatures along the Central Coast back to seasonal norms. However, the blob of abnormally warm seawater temperatures continues along the Pacific Northwest and Southern California coastlines. The Scripps Waverider buoy, near La Jolla, is reporting 65-degree ocean temperatures.
So what will the rest of April bring? At this time, it’s looking dry for at least another week if not longer as a moderately strong ridge of high pressure builds over California with periods of breezy night and morning Santa Lucia (northeasterly) winds. Consequently, temperatures will warm with the North County reaching the upper 80s by the week’s end.
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