Time and tide wait for no man

August 18, 2012 

Like a siren’s song, the Morro Bay sandspit regularly lures shallow-water sailors across the Back Bay. This alluring and dynamic peninsula of sand that separates Morro Bay from the Pacific Ocean is a realm unto itself. Kind of like a scaled-down version of Point Reyes National Seashore.

Problem is, as the Baywood Navy, whose motto is, “We don’t sail in water deeper than we can stand in or when the wind is calm,” will tell you, the relatively deep waters of the bay at high tide can change to silvery mud flats at low tide in just a few hours. If you plan to paddle or sail across the Back Bay, close attention to the tide tables is a must. Late-to-get-back kayakers have become stranded, or rather were stuck in the mud waiting for the flood tide to arrive.

The Central Coast usually experiences two high and low tides per day, but because the tidal cycle is over 24 hours, some days have only one high or low tide.

The maximum tidal range along the Central Coast can reach from more than 2 feet below the mean low water mark, to more than 7 feet above it, for a tidal range of more than 9 feet. The greatest tidal range that I know of is in the Bay of Fundy on the east coast of Canada, which can exceed 52 feet!

Tides are caused by the gravitational forces exerted by the moon and sun. These “tidal forces” are not the total gravitational forces exerted by the sun and moon, but the difference between these gravitational forces over the surface of the planet. The bottom topography — or underwater bathymetry of our coastline — also plays an important part in the changing tides.

You may wonder why the high tide does not occur when the moon is directly overhead or over our meridian, but actually happens hours later. It may seem counterintuitive, but the gravitational forces of the moon and sun tug at the oceans primarily on the horizontal tangent and not the vertical plane to the Earth’s surface. This is partly why the high tide lags between eight and nine hours on the Central Coast — also referred to as the lunitidal interval. This value can be used to program certain tide watches.

This tugging produces a tidal “bulge” or area of higher sea level on the ocean’s surface. As Earth rotates eastward on its axis, the Central Coast moves into this bulge, which produces a flood tide, and eventually a high tide.

As Earth rotates, we move into an area of below-normal sea level or nodes, which produce an ebb tide, eventually reaching low tide. The slack tide is when the tide is not coming in or going out. During high tide, this tends to be a nice time to be kayaking on the Back Bay.

During a full as well as new moon, the sun and moon are aligned, producing strong tidal forces. This is also referred to as spring tide, even though it doesn’t have anything to do with the spring season. These are the highest and lowest tides of the month.

During the moon’s quarter phases, the gravitational forces of the moon and the sun are perpendicular to each other. Nearly canceling each other’s tidal forces out, the result is smaller high and low tides. This is referred to as neap tide.

Earth’s orbit around the sun is really an ellipse, a shape that can be thought of as a “stretched out” circle or an oval.

Tides are enhanced when Earth is at perihelion, when it comes closest to the sun. This occurs during winter in the Northern Hemisphere and summer in the Southern. Aphelion, when Earth is farthest from the sun, occurs during our summer.

Because the Northern Hemisphere leans away from the sun when we’re closest to it, and toward it when we’re farthest from it, our axial tilt tends to moderate our temperature extremes.

The minus low tides during summer usually occur at night, preventing intertidal creatures from receiving too much solar radiation, while during the winter minus low tides usually occur during daytime when the days are much shorter and the sun low in the southern sky.

When we look at the tide tables, you should remember that these are only predicted values utilizing gravitational forces. During El Niño events, actual tides can be up to one foot higher because of thermal expansion of the water column. Storms can also affect sea level. Low atmospheric pressure associated with storms can allow ocean waters to expand, resulting in a temporary increase in sea level. Strong winds can cause additional elevation of sea level due to storm surges.

Not quite the hottest

On Friday, the temperature soared to 106 degrees at the Paso Robles Airport. This was nearly the high temperature for the entire nation.

Extensive monsoon cloudiness and moisture from the south produced cooler high temperatures throughout the Desert Southwest Region on Friday. Twentynine Palms and Needles both reached 103 degrees, while Las Vegas peaked at 100 degrees and Phoenix only reported a high of 96 degrees. In the San Joaquin Valley, Bakersfield peaked at only 98 degrees and Fresno reached 105 degrees. Closer to home, both Creston and the Carrisa Plains Elementary School reported a toasty 105 degrees. However, the hot spot for the country on Friday was 114 degrees at Stovepipe Wells in Death Valley.

Monsoon moisture interacted with a low pressure system and brought widely scattered sprinkles throughout the county Saturday morning, with a few locations reporting trace amounts of precipitation. The monsoon moisture also scoured out the marine layer along the beaches, leaving behind clear skies.

Today’s forecast

A 1,030-millibar Eastern Pacific High about 600 miles to the west of San Luis Obispo will remain nearly stationary, while a thermal trough will persist over the Central Valley.

Today’s high temperatures will once again reach the low 100s in the North County, while the coastal valleys will hit the mid-80s. Fresh to strong (19- to 31-mph) afternoon northwesterly winds along the coast will produce high temperatures in the low 70s along the southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach). The westerly facing beaches will reach the high 60s. The northwesterly facing areas will see high temperatures range between the low to mid-60s.

Cooler weather is on tap early this week. A trough of low pressure will move into the Central Coast on Monday with the return of coastal low clouds and fog. This will deepen the marine layer and allow it to advance further inland.

Another trough of low pressure will form off the coast, providing additional cooling. Highs in the North County will return to normal values for this time of year by Tuesday while coastal locations will be at or slightly below normal temperatures. The coastal stratus will become more extensive during night and morning hours with areas of drizzle developing along the beaches. At this time, no extreme heat is forecast.

Today’s surf report

Today’s 3- to 5- foot northwesterly (315-degree deep-water) sea and swell (with a 7- to 9-second period) will remain at this height and period through Wednesday.

A 3- to 4-foot northwesterly (300-degree deep-water) swell (with a 7- to 11-second period) is forecast along the coast Thursday through next Sunday.

Arriving from the Southern Hemisphere:

Today’s 1- to 2-foot Southern Hemisphere (180-degree deep-water) swell (with a 14- to 16-second period) will gradually decrease Monday.

A 1- to 2-foot Southern Hemisphere (220-degree deep-water) swell (with a 16- to 18-second period) will arrive along the Pecho Coast on Tuesday, and will remain at this height and period through Wednesday.

Seawater temperatures

Seawater temperature will range between 54 and 57 degrees through Wednesday, increasing to 55 and 58 degrees Thursday and will remain at this level through Aug. 26.

• • •

PG&E delivers some of the nation’s cleanest electric power. More than half of the electricity we provide to our customers comes from sources that are renewable and/or emit no greenhouse gases. In fact, PG&E’s electricity creates only one-third as many greenhouse gas emissions per kilowatt-hour compared to the industry average.

John Lindsey is a media relations representative for PG&E. He is a local weather expert and has lived on the Central Coast for more than 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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