A nearly stationary upper-level low-pressure system off the northern Baja California peninsula directed a significant amount of monsoon moisture toward the Central Coast on Saturday and Sunday, producing substantial mid- to high level clouds and beautiful sunrises and sunsets.
If you have been watching the National Weather Service’s weather surveillance radar at Vandenberg Air Force Base on your computer or local television station, you’ve probably noticed large green, yellow and even orange bans of heavy rain marching across San Luis Obispo County in a sweeping counterclockwise direction, but only a few locations reported measurable precipitation.
If the atmosphere near the surface is bone dry, rain can evaporate in midair, somewhere between the base of the clouds and the ground on its downward journey.
This is the reason why we haven’t received much measurable rain over the past week. However, as the water vapor increased in the lower atmosphere, sprinkles and a few rain showers developed.
This phenomenon is called Virga, and is frequently seen trailing in streaks from the bottom of puffy altocumulus or altostratus clouds — almost like fine tentacles hanging from the bottom of a Portuguese man-of-war jellyfish that are being gently swept horizontally by the ocean currents.
You may be wondering if the radar is a reliable source of information or just another way to confuse dedicated weather watchers.
One of my mentors throughout my weather forecasting career was meteorologist Rea Strange. Rea — whose name was pronounced Ray — was one of the most respected weather forecasters on the California coast for more than 50 years.
He reached a point where he no longer looked at the radar. “It only confuses the forecast!” he often said.
The weather radar at Vandenberg Air Force Base is part of a vast network of 159 Weather Service high-resolution Doppler weather radars that cover most of the United States.
They are invaluable in predicting and tracking tornadoes and have probably saved many lives in the United States.
The weather radars send out a beam of electromagnetic radiation or radio waves from their antenna as they rotate 360 degrees.
If the radio waves strike rain, hail, snow or even insects, a small portion of reflective beam is directed back toward the radar antenna.
Computers analyze the strength and frequency shift of the returned radar waves and produce a display of the location and intensity of precipitation that we see.
The Vandenberg weather radar mostly displays precipitation in the middle levels of the atmosphere but can get as low as a few thousand feet off our coastline.
This week’s forecast
A nearly stationary upper-level low-pressure system off the northern Baja California peninsula will continue to direct subtropical moisture over San Luis Obispo County, producing mid- to high level clouds and widely scattered sprinkles and/or light rain showers.
This subtropical flow has also scrubbed out the marine layer from along the coastline today.
Variable mid- to high level subtropical clouds will once again stream across our area later today through this evening and could trigger isolated thunderstorms or light rain showers.
However, most of the thundershower activity will be confined to the southern Sierra and across the Tehachapis through today.
Today’s temperatures will range from the mid- to high 60s along the beaches. The coastal valleys (San Luis Obispo) will reach the mid-70s while the North County will hit the low 80s.
Coastal low clouds will redevelop along the coast tonight and will strain into the coastal valleys and even into the North County by Monday morning.
The cooling trend will continue, then temperatures should cool more substantially Monday and Tuesday as a weak low-pressure trough sets up a steep pressure gradient along our coast.
This pressure gradient will produce strong to gale force (25 to 38 mph) northwesterly winds, which will mix out the marine layer and give plenty of sunshine during the afternoon hours.
At this time, current medium-range forecast models indicate night and morning coastal low clouds and slightly below normal seasonal temperatures.
Surf and sea report
Strong to gale force (25 to 38 mph) northwesterly winds along the entire length of the Northern and Central California coastline will generate a 6- to 8-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 14-second period) this afternoon, increasing to 7 to 9 feet (with a 6- to 13-second period) Monday.
This northwesterly (290-degree deep-water) sea and swell will diminish to 6 to 8 feet Tuesday.
A 5- to 7-foot northwesterly (290-degree deep-water) swell (with an 8- to 11-second period) is forecast along our coastline for Wednesday, decreasing to 3 to 5 feet Thursday through next Friday.
Arriving from the Southern Hemisphere:
A low-height, long-period Southern Hemisphere (220-degree deep-water) swell will arrive along our coastline today at 1 foot (with a 20-second period), increasing to 1 to 2 feet (with an 18- to 20-second period) later today.
This swell will remain at a 1- to 2-foot level but with a gradually shorter period through Tuesday.
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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask a question, e-mail him at firstname.lastname@example.org.