April weather can be as gnarly as bark on a pine tree or as soft as talcum powder. This month of April has certainly lived up to its reputation as a transition month between the cool and blustery weather of winter and the warm and dry weather of summer.
The month started roughly when a storm off the California coast butted against a strong Eastern Pacific high and produced 16-foot swells at the Diablo Canyon Waverider buoy April 1. That was the biggest swell event since January 2010. The Cape San Martin NOAA marine buoy, stationed 55 nautical miles northwest of Morro Bay, reached even higher, at 23 feet.
Afterward, a fast-moving front swept across the Central Coast. Cold air filtered in behind it and produced widespread frost throughout San Luis Obispo County. On April 5, a strong Eastern Pacific high developed about 500 miles west of San Luis Obispo and combined with an area of low pressure over Nevada to produce a steep pressure gradient. Consequently, the northwesterly winds reached 37 mph with gusts of 56 mph along the coast. Very windy, indeed.
As these northwesterly winds blew parallel to our coastline, the friction of the wind caused the ocean surface water to move. The Coriolis force turned the surface water to the right, or offshore, causing upwelling along the coast as very cold and nutrient-rich subsurface water ascended to the surface along the shoreline. As any surfer, scuba diver, fisherman or casual beach visitor will remember, seawater temperatures plummeted to a bone-chilling 48 degrees. Not only was the Pacific cold, but air temperatures also remained below normal during the first part of the month.
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True to its nature, this month just couldn’t help itself, and another major change in the weather pattern developed. By mid-April the Eastern Pacific high moved far out to sea and allowed a series of late-season storms to pass over our area. A tightly wound low-pressure system off Cape Mendocino produced a cold front that approached from the west, or just about parallel to the coastline, and slowly moved eastward over the county.
Gentle but steady rain developed along the coastal areas of San Luis Obispo County.
Just a few days later, a vigorous low-pressure system delivered moderate to heavy rain, gale-force southerly winds and a great amount of lightning. These late-season storms produced between 1 and 4 inches of much-needed rain throughout San Luis Obispo County. Cal Poly recorded 2.26 inches of rain, while SLOweather.com in western San Luis Obispo reported 3.48 inches.
According to climate records for San Luis Obispo kept at Cal Poly, the wettest April was in 1967, when 6.90 inches of rain was recorded. The second-wettest April occurred in 2006, which recorded 5.28 inches of the wet stuff. This month, the rain gauge at Diablo Canyon has recorded 2.75 inches.
Last year’s rainfall during April was only 0.16 inches. Completely dry Aprils have occurred in 1973 and 2004, with no measurable rain reported.
Never satisfied, April decided to throw us another change-up. A strong ridge of high pressure moved over the Central Coast on April 20 and produced gusty northeasterly offshore winds and warm temperatures. Warm and balmy Indian summer conditions settled across our area, even along the beaches. On April 21, Paso Robles reached 96 degrees.
On April 22, a low-pressure system that was cut off from the jet steam formed off the California coast and lingered off the shoreline for most of the week. This condition produced an extended period of southerly winds and mostly cloudy conditions. These winds created a persistent northerly onshore flowing current with eddies and gyres spinning away from it. A review of ocean current data along the coast revealed this was one of the more persistent northerly flowing current events during the month of April. This low-pressure system also produced unusually low swell conditions, especially for April and warmer seawater temperatures.
Millions, if not billions, of sea salps — 1- to 3-inch-long, transparent barrel-shaped animals that look and feel much like jellyfish — came ashore last week with these onshore currents. These creatures feed on plankton and follow the deep scattering layer coming to the surface during the night and descending to the depths during the day.
The cutoff low finally moved over our county later in the week with warm and gentle rain that reminded me of Florida. It was followed by a more typical spring type of weather pattern of afternoon gale-force northwesterly winds, plenty of upwelling and mostly fair weather that closed out this eventful month.
Today’s weather report
A 1,033-millibar Eastern Pacific high will remain nearly stationary about 700 miles to the west of San Luis Obispo for an extended period of time.
This area of high pressure will keep the storm track far to the north into British Columbia. At the same time, a 1,006-millibar low will remain nearly stationary over the Four Corners area of the desert Southwest. This condition will produce a steep pressure gradient along the California coastline that will give fresh to strong (19- to 31-mph) afternoon northwesterly winds along the beaches.
This morning’s marine low clouds and areas of fog along the northwesterly facing beaches of San Luis Obispo County (Morro Bay, Los Osos, Montaña De Oro State Park and the Nipomo Mesa) will clear by the afternoon. Otherwise, mostly clear skies will continue through this afternoon. Today’s maximum temperatures will reach into the 80s across the North County, the high 70s in the coastal valleys and the mid-70s along the southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach). Temperatures will reach the high 60s at Pismo Beach and low 60s along the northwesterly facing beaches.
A weak cutoff low will move along the coastline Monday morning and will produce extensive marine low clouds and fog along the beaches and in the coastal valleys and gentle variable winds.
A classic Central Coast spring weather pattern will return Tuesday with strong to gale-force (25- to 38-mph) northwesterly winds during the afternoon hours and night and morning coastal low clouds and fog. This weather pattern will continue through next weekend.
Today’s surf report
Fresh to strong to gale-force (19- to 31-mph) northwesterly winds will generate a 5- to 7-foot (310-degree deep-water) sea and swell (with a 7- to 11-second period) through tonight.
Wave heights will reach 12-plus feet at the offshore buoys.
A 5- to 7-foot northwesterly (290-degree deep-water) swell (with a 9- to 11-second period) is forecast along our coastline Monday morning.
Increasing northwesterly winds will generate a 6-to 8-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) along the coastline Monday afternoon and will remain at this height and period through Wednesday.
A 5- to 7-foot northwesterly (310-degree deep-water) sea and swell (with a 7- to 11-second period) is forecast along the Pecho Coast on Thursday, increasing to 6 to 8 feet Friday through next Sunday.
A 1-foot Southern-Hemisphere (200-degree deep-water) swell (with an 18- to 20-second period) will arrive along the Central Coast this morning. This particular swell train will be overlapped by another Southern-Hemisphere swell and will gradually increase to 1 to 3 feet (with a 15- to 20-second period) this afternoon through Monday. A higher-energy Southern-Hemisphere swell is expected May 12.
Seawater temperatures will range between 49 and 51 degrees through Monday, decreasing to 48 to 50 degrees Tuesday, and will remain at these temperatures through next weekend.
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John Lindsey is a media relations representative for PG&E. He is a local weather expert and has lived along the Central Coast for more than 25 years.
If you would like to subscribe to his daily weather forecast or ask him a question, send him an email at firstname.lastname@example.org.