Weather Watch

Cold front is the boundary of two air masses

Weather can switch from warm to cold, or rain to dry, in the course of minutes, as if there is some invisible dividing line. Turns out, there is such a line.

Out over the Pacific Ocean, the sun’s light evaporates enormous amounts of seawater into water vapor in the atmosphere.

The sun, along with the Earth’s rotation, is also the driving force behind the jet stream, the upper-atmosphere river of air that drives all this water vapor in an easterly direction toward California.

The polar jet stream moves in a wavy or even looping pattern around the northern hemisphere in a west-to-east pattern at elevations between 6 and 9 miles in the tropopause.

It often moves faster than 100 knots but can exceed 200 knots in the winter.

As the wave amplitude of the jet stream increases, the air flow may transform from a wavelike structure to that of a counter-clockwise circulation, producing low-pressure systems.

Generally speaking, the lower the air pressure, the more unsettled the weather will become. When these low-pressure systems or storms develop in the northern Pacific Ocean, cold fronts accompany them.

Think of a cold front as a wave of energy extending away from the core of the storm — like sweeping your hand from the center of a draining kitchen sink to the edge.

The actual front is the boundary or transition zone between two air masses of different densities, usually caused by temperature or humidity differences.

Fronts are nearly always accompanied by clouds and, if strong enough, by rain, snow, thunderstorms and severe weather.

As cold fronts move into the Central Coast, the heavier, cool air pushes the lighter and warmer air upward.

The warm air cools as it rises, and it condenses into clouds. These cold fronts usually move in from the northwest, in a southeasterly direction.

When the cold front passes, the winds often shift out of the northwest and the atmospheric pressure reaches its lowest point.

In fact, the shift from southerly to northerly winds is used by meteorologist as the indication that a cold front has passed a particular location.

Some of these fronts can be quite narrow and intense, thick with heavy rain and lightning. I’ve seen the wind shift 180 degrees during frontal passage in less than a few minutes. The air behind a cold front is often noticeably colder and drier than the air ahead of it.

Interesting rule of thumb that I’ve noticed over the years: Often, the speed at which the cold front moves down the coastline is about 75 percent of the northwesterly wind speed behind it.

I have seen cold fronts fall apart and seemingly vanish between Ragged Point and Point Sal, or actually intensify and stall over a particular section of this coastal region, producing copious amounts of rain over one part while leaving other areas relatively dry.

On surface charts, a cold front is represented by a solid line with triangles along the front pointing in the direction of movement.

This week’s forecast

A 1,033-millibar high over the Great Basin combined with a stationary 1,039-millibar Eastern Pacific High about 400 miles west-northwest of San Luis Obispo will produce gusty northeasterly (offshore) winds, especially in the coastal canyons and passes this morning.

This condition will also produce Santa Ana winds in Southern California this morning.

These downslope winds will give mild to warm temperatures later this morning into this afternoon with the coastal valleys (San Luis Obispo) reaching the mid- to high 70s.

Variable mid- to high-level clouds will continue to stream over the Central Coast, producing a spectacular sunrise and sunset today.

Fair skies and mild temperatures will continue into Monday morning.

Fresh to strong (19 to 31 mph) northwesterly winds will develop Monday afternoon into Tuesday.

These onshore winds will produce cooler temperatures and will allow the marine layer to redevelop during the night and morning hours Monday night into Tuesday.

Another round of northeasterly (offshore) winds will give clear skies and mild temperatures Wednesday through Friday.

The strong high pressure ridge responsible for this mild weather will gradually weaken and migrate westward late this week.

At this time it looks like our dry weather pattern could change by next weekend, with rain developing over the Central Coast as early as next Sunday night into next Monday.

Surf and sea report

Today’s 5- to 7-foot west-northwesterly (290-degree deep-water) swell (with an 8- to 12-second period) will decrease to 4 to 6 feet (with an 11- to 13-second period) Monday morning, becoming a 4- to 6-foot northwesterly (290-degree deep-water) sea and swell (with a 5- to 17-second period) Monday afternoon and night.

Gale force northwesterly winds off Cape Mendocino will generate a 7- to 9-foot northwesterly (300-degree deep-water) swell (with an 8- to 17-second period) along the Central Coast on Tuesday through Wednesday.

A 3- to 5-foot west-northwesterly (285-degree deep-water) swell (with an 8- to 15-second period) is forecast along our coast Thursday and will remain at this height and period through Saturday.

Seawater temperatures will range between 54- and 56-degrees through Monday, decreasing to 53 and 55 degrees Tuesday and remaining at this level through Saturday.

SmartMeter workshop

The League of California Cities-Channel Counties Division, in conjunction with Pacific Gas and Electric Co., is hosting a workshop on the utility’s SmartMeter program from 3 to 5 p.m. Tuesday at the South County Regional Center in Arroyo Grande.

The purpose of the meeting is to provide PG&E customers with an overview of the SmartMeter program and to explain how it affects the future of energy.

Experts will be on hand to answer questions on SmartMeter installation and operation.

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 24 years. If you have a question, send him an e-mail at