Weather Watch

Radar: f­rom busting speedsters to saving lives

You may feel a surge of anxiety when an officer points a radar gun at your vehicle, especially, if you’re late for an appointment. But I think of radar as a life-saver.

While serving in the U.S. Navy as a systems operator in a Kaman H-2 Seasprite helicopter, I used its radar to help vector our helo around severe storms and to find our way back to our pocket-sized ship in the vast expanse of the oceans.

On rare instances, our helicopter crew would be temporarily stationed on aircraft carriers. Various aircraft stationed on these massive ships had amazingly powerful radars, capable of radiating thousands of watts of power.

As a new member of the aircrew, the maintenance chief gave me some steel wool and asked me to carry it across the flight deck to a maintainer working on another aircraft.

About halfway across the flight deck, the steel wool exploded in sparks, like metal in a microwave oven, as a powerful beam of electromagnetic energy swept across me from a nearby aircraft. I looked back at our chief and he was laughing hysterically.

As the years have passed since its first use in World War II, radar — actually an acronym for radio detection and ranging — has become an essential tool for weather forecasting.

Today’s radar units send out short pulses of radio waves traveling at the speed of light, while receiving the reflected energy coming back from water droplets suspended in clouds or falling to the ground.

Like a dentist using X-rays to look for cavities, radar allows a meteorologist to examine areas of the atmosphere previously inaccessible.

The weather radar at Vandenberg Air Force Base is part of a vast network of 159 National 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.

The weather radars send out a beam of 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.

Rainfall is shown as blue and green; heavier rainfall is indicated by yellow with orange and red indicating the heaviest precipitation.

Doppler radar uses the principle called Doppler shift, which is a change of frequency caused by movement.

A classic example is a fast-moving truck passing your location. This sound from the truck is shifted to a higher pitch (higher frequency) as it moves toward you. As the truck moves away from you, its sound is shifted to a lower pitch or frequency.

This Doppler shift allows weather forecasters to peer into thunderstorms that can produce tornadoes. It also allows easier discrimination between snow and rain.

The Central Coast mountains block some of the radar energy in the lower levels of the atmosphere. The antenna is tilted up to get longer ranges. Like a baseball batter getting under the ball and hitting it out of the park, the Doppler radar can see ranges out to nearly 150 miles away!

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

The final cold front in a long series of weather systems passed the Central Coast Saturday with light rain and gentle to moderate (8 to 18 mph) southerly winds.

Rainfall totals were sparse with many areas only reporting the few hundreds of an inch to a quarter of an inch.

However, our southwesterly facing slopes are reporting significant rain due to orographic enhancement. Further up the coast, Cambria at Santa Rosa Creek at Main Street reported 0.88 inches and Rocky Butte near San Simeon reported more than 2 inches of rain as of 4 p.m. Saturday.

As much as 6 inches of snow accumulated as low as 2,500 feet. Another 1 and 2 feet of snow was recorded at the higher elevations of the Central Sierra.

A major change in the weather pattern will take place today as the Eastern Pacific high slides eastward toward the Central Coast, shifting the storm track into the Pacific Northwest and closing the storm door.

Today will be partly cloudy but dry with high temperatures mostly in the low 60s.

High pressure will build over our area Monday and will produce springlike weather with strong to gale (25- to 31-mph) northwesterly “springtime” winds along the coastline and temperatures climbing into the 70s Tuesday and Wednesday under mostly clear and sunny skies.

At this time, the models and surface charts are indicating a northeasterly (offshore) wind event developing Thursday and Friday morning. If this condition develops, temperatures should reach the low 80s throughout San Luis Obispo County during this period.

Surf and sea report

The swell at the Diablo Canyon waverider buoy peaked at 13.9 feet with a 17-second period Friday afternoon. The Cape San Martin marine buoy, about 55 nautical miles west-northwest of Morro Bay, reported the peak of the swell at 22 feet with a 16-second period Friday morning.

Today’s 6- to 8-foot northwesterly (310-degree deep-water) swell (with a 12- to 14-second period) will remain at this height and period through tonight.

Strong to gale-force (25- to 38-mph) northwesterly winds will produce a 6- to 8-foot northwesterly (300-degree deep-water) sea and swell (with a 7- to 11-second period) Monday and will remain at this height and period through Wednesday.

A 5- to 7-foot northwesterly (310-degree deep-water) swell (with a 7- to 11-second period) is forecast along our coastline Thursday.

A 959-millibar storm is forecast to develop near the International Date Line on Monday. If the storm develops as advertised, a 7- to 9-foot northwesterly (290-degree deep-water) swell (with an 18- to 22-second period) will arrive along the Central Coast on Friday, building to 9 to 11 feet (with a 15- to 17-second period) Saturday.

Seawater temperatures will range between 52 and 54 degrees through today, decreasing to 51 and 53 degrees Monday. The seawater temperatures will further decrease to 49 and 51 degrees Tuesday and will remain at this level through Saturday.

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John Lindsey, media relations representative for PG&E and local weather expert, has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, email him at pgeweather@ pge.com.

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