Weather Watch

Will fireworks be fogged in? It’s tricky to tell

Over many years of forecasting weather along the Central Coast, one of the more challenging aspects of the job is predicting the marine layer, especially during the summer months.

If you forecast, “Marine low clouds and fog during the night and morning, becoming mostly clear during the afternoon,” you would probably be correct most of the time. But there are nuances in forecasting fog along our shoreline. All it takes is a small change in the surface air temperature, the temperature inversion layer or the direction or speed in the wind to make the difference between a beautiful Fourth of July fireworks display versus a light show in the coastal stratus.

The type of fog we see along our rugged coastline is advection fog. Advection simply means transport. The persistent northwesterly (onshore) winds produce upwelling along the coast, which brings cold, subsurface water to the surface along the immediate shoreline. As you head further out to sea, the surface water becomes warmer. The northwesterly (onshore) winds transport the relatively warm air from further out to sea across the much colder water along our immediate coastline. The overlying air then becomes chilled and drops to its dewpoint temperature, producing coastal low clouds and fog. If the wind direction is out of the northeast (offshore), clear skies will develop.

Another important aspect of fog development is San Luis Obispo County’s coastal topography. The coastal mountains force the northwesterly moisture-laden winds blowing horizontally from the Pacific Ocean to turn vertical, or upward. As the air mass is lifted up over our coastal mountains (upwind), it cools and eventually reaches its dewpoint temperature. You can often see this at Montaña de Oro State Park.

On the leeward (downwind) side of the mountain, the air mass is forced downward by gravity and warmed by pressure (compressional heating). The sinking air is drier after losing some of its moisture on the upwind side of the mountain range. This is why our southwesterly facing beaches like Cayucos and Avila Beach often see more sun. If the winds shift out of the south, the opposite can occur. The southwesterly facing beaches will struggle with the coastal stratus while the northwesterly facing beaches (Los Osos and Morro Bay) will be mostly clear.

Strong to gale force northwesterly winds can often mix out the temperature inversion layer, producing mostly clear skies, while light to gentle winds can often produce a stubbornly persistent marine layer that can last throughout the day along the entire coastline.

Even though the marine layer may come rolling in tonight, the base of its clouds may be high enough to still allow for a nice viewing of the firework displays. Fireworks are made to burst at different heights, depending on the size of the shell and how the fireworks are packed. Most firework displays along our coastline reach an altitude between a few hundred to 1,000 feet.

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I received a few phone calls and e-mails about the seasonal rainfall total in Atascadero. Rain totals in Atascadero vary greatly depending on where the rain gauge is located. Going from west to east on a straight line, the rainfall totals drop sharply. For this past season, the San Luis Obispo County Water Resources rain gauge off Highway 41 on Upper Toro Road recorded 31 inches of rainfall.

Heading eastward, the Atascadero Mutual Water Co. rain gauge near the intersection of Highway 41 and 101 recorded nearly 27 inches of rain. Heading further east, the gauge near Heilmann Regional Park recorded 19 inches.

Today’s weather

A persistent trough of low pressure will remain over the Central Coast through most of this week, while a strong 1,035-millibar Eastern Pacific high creeps toward the Pacific Northwest.

This condition has shifted the wind fields northward. Without the strong northwesterly winds to mix out the marine layer, low stratus clouds will stubbornly persist along the immediate coastline for most of the day with little sun. Let’s hope that my forecast is wrong and there will be clear skies for the fireworks.

However, gentle to moderate (8 and 18 mph) southerly winds may produce clearing along the northwesterly (Los Osos and Morro Bay) facing beaches. Usually, the northwesterly facing beaches are socked in by fog and the southwesterly (Avila Beach and Cayucos) facing beaches are clear.

Night and morning patchy fog will develop in the coastal valleys, becoming clear during afternoon hours. The North County will continue to see clear skies.

Today’s temperatures will range from the high 80s to low 90s in the interior to the mid-70s in the coastal valleys.

Temperatures along the immediate coastline will range from the high 50s to the mid-60s.

Little change in this forecast is expected through Wednesday.

Thereafter, expect typical July weather with early clouds retreating to the coast by mid-morning and temperatures running about average.

Surf and sea report

The wind fields have shifted northward and will strengthen off Cape Mendocino. In fact, the northwesterly winds are forecast to reach 60 mph off Cape Mendocino today.

These winds off the Northern California coastline will produce a 5- to 7-foot northwesterly (300-degree deep-water) swell (with an 8- to 11-second period) along our coastline today through Monday.

This northwesterly (300-degree deep-water) swell will decrease to 4 to 6 feet (with an 8- to 11-second period) Tuesday further lowering to 3 to 5 feet Wednesday.

A 2- to 4-foot northwesterly (305-degree deep-water) sea and swell (with a 5- to 9-second period) is forecast along our coastline Thursday through next Saturday.

Arriving from the Southern Hemisphere:

A 1-foot Southern Hemisphere (200-degree deep-water) swell with a 22-second period) will arrive along our coastline tonight, increasing to 3 to 5 feet (with an 18-to 20-second period) Sunday. This swell will remain at this height, but its period will gradually decrease Monday, Tuesday and Wednesday.

Note: The NOAA marine buoy station 51004 — 185 NM southeast of Hilo, Hawaii — stopped broadcasting in October 2009 and there’s no way to verify this swell at this time.

Seawater temperatures will range between 51 and 53 degrees through Sunday.

Intake seawater temperatures will increase to 52 and 54 degrees Monday, further rising to 53 degrees to 55 degrees Tuesday through Thursday.

Conservation tip

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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. If you have a question, send him an e-mail at pgeweather@pge.com.

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