Weather Watch

Air pressure key to accurate forecasting

To maximize fuel efficiency on a road trip, keeping your tires properly inflated is paramount. Something similar can be said about providing an accurate weather forecast. Simply put, air pressure is key.

The strong invisible force of gravity accelerates trillions of air molecules towards the Earth’s surface and produces weight. The weight of the air is what we feel as pressure.

Imagine a 1-square-inch column of air measured from to the top of the atmosphere down to sea level — it would weigh about 14.7 pounds. All the air surrounding the Earth would weigh about 5,600 trillion tons! Normally we don’t notice it because our bodies maintain an internal pressure that balances the external pressure. But swift changes in elevation can cause us to detect atmospheric pressure changes. One local example can be found driving over the Cuesta Grade, which is 1,522 feet above sea level. Maybe you have felt your ears “pop.” This is caused by your inner ear trying to equalize the pressure from the outside air.

Meteorologists often express air pressure in units called millibars. The standard atmospheric pressure is defined as being equal to 1,013.25 millibars at sea level. Depending on the surrounding atmospheric conditions, levels above this value may be considered areas of high pressure. Along the Central Coast, 1,030 millibars and above is considered strong high pressure. Strong high pressure usually produces clear and dry weather.

The highest pressure ever recorded in the continental United States occurred in December 1983 in Miles City, Mont., which reached 1,064 millibars. The highest recorded sea level pressure in the world occurred in Agata, Siberia, which reached 1,084 millibars in December 1968.

On the flip side, and once again depending on the surrounding atmospheric conditions, levels below standard atmosphere may be considered areas of low pressure. Generally speaking, the lower the pressure, the more unsettled the weather will become. Along the Central Coast, I consider anything less than 990 millibars to be a deep low-pressure system.

On Jan. 21, 2010, an intense low-pressure system moved down the California coastline. The barometer at the Cape San Martin marine buoy, 55 miles west-northwest of Morro Bay, reached 978.3 millibars, while the Diablo Canyon Ocean Lab barometer decreased to 978.7 millibars. These atmospheric pressure readings were some of the lowest that I have seen for our area.

Later that day, according to Joe Sirard, a meteorologist at the National Weather Service in Oxnard, the pressure dropped to 980 millibars at Los Angeles International Airport. This set an all-time record for that location since readings began in 1931.

In August 2005, Hurricane Katrina reached 920 millibars. In October 2005, Hurricane Wilma reached 882 millibars over the open waters of the Caribbean.

The lowest sea-level pressure ever recorded worldwide was 870 millibars when Typhoon Tip passed Guam and rapidly intensified as it moved toward Japan during October 1979. The passage of Tip through the region resulted in eight ships being grounded or sunk. A Chinese freighter broke in half as a result of the typhoon, but its crew of 46 was rescued.

Tragically, 13 U.S. Marines lost their lives at Camo Fuji in Japan due to this storm.

This week’s forecast

A transitory 1031 millibar area of high pressure over the Great Basin of the western United States will produce gusty northeasterly (offshore) winds this morning. These offshore winds will produce dry, clear and mild weather with temperatures reaching the mid-to-high 70s throughout the Central Coast. Absolutely beautiful weather to view the wildflowers of the Central Coast.

The eastern Pacific high will shift southwestward later today and will allow the northwesterly (onshore) winds to increase. This condition will produce cooler temperatures later today, especially along the beaches and in coastal valleys.

This onshore flow will continue through Monday, allowing coastal low clouds and fog and cooler temperatures to develop along our coastline and in a few of our coastal valleys. The interior will remain mostly clear and warmer.

A major change in the weather pattern will develop early this week as the storm track shifts southward.

Increasing clouds, cooler temperatures and few light rain showers will develop late Tuesday as the first in a series of weather systems crosses our area.

A stronger 998 millibar low pressure system and its associated cold front will spread rain and a much cooler air mass southward across the entire Central Coast on Wednesday.

Total rainfall amounts with this system should range between 0.25 and 0.75 inches with heavy snowfall in the Sierra through Wednesday night. A few rain showers will continue into Thursday morning with much lower snow levels in the vicinity of 2,500 feet in the northern Sierra Nevada mountains to 3,500 feet in the south.

Temperatures in a few of the interior valleys could reach near-freezing levels by early Thursday morning.

After a break in the rain on Friday and Saturday, another weather system will migrate southward on Sunday into Monday with increasing southerly winds and moderate rain.

Current longer-range forecast models indicate a dry pattern developing by midweek the following week.

Surf and sea report

A 4- to 6-foot west-northwesterly (285-degree deep-water) swell (with a 16- to 18-second period) will arrive along the Central Coast this afternoon and tonight, increasing to 7 to 9 feet (with a 14- to 16-second period) on Monday. This swell will further build to 11 to 13 feet (with a 13- to 16-second period) on Tuesday.

A 13- to 15-foot northwesterly (290-degree deep-water) swell (with a 15- to 17-second period) will arrive along the Diablo Canyon coastline on Wednesday. Swell heights along the northwesterly-facing beaches (Montaña de Oro State Park) and the offshore buoys could reach over 20 feet by late Wednesday.

This swell should decrease to 11 to 13 feet on Thursday, further lowering to 8 to 10 feet (with an 11- to 13-second period) by Friday. Another increase in northwesterly swell is forecast next weekend.

Seawater temperatures will range between 51 and 53 degrees through today, increasing to 52 to 54 degrees on Monday through Wednesday.

Conservation tip

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John Lindsey is a media relations representative for Pacific Gas and Electric Co. He is also a local weather expert and has lived along the Central Coast for more than 23 years.