Weather Watch

When the jet stream revs up, look for a cold front

At our latitude, the jet stream is typically a tubular ribbon of high- speed winds some 18,000 to 40,000 feet up, flowing in wavelike patterns from the west to the east for thousands of miles.

The jet stream is usually about 300 miles wide at its core. Its speed averages about 100 mph in winter and 50 mph in summer. This may rise to 250 mph or greater during 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 a low-pressure system.

When these low-pressure systems or storms develop in the northern Pacific Ocean, cold fronts are often associated with 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.

These cold fronts usually move in from the northwest, in a southeasterly direction. As the cold front approaches the Central Coast, it often produces increasing southeasterly (prefrontal) winds.

When the cold front passes our area, the winds often shift out of the northwest (post-frontal).

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 post-frontal northwesterly wind speed behind it.

The strongest southerly winds ever recorded at the Diablo Canyon Power Plant meteorological tower occurred Feb. 3, 1998. The southeasterly winds reached 63 mph sustained with gust hitting 79 mph, just shy of 80 mph.

Some of the strongest northwesterly winds I have seen occurred last April 14. The winds reached 50 mph sustained with gust to 64 mph.

As the frontal boundary sweeps by our area, the heavier cold air at the leading edge of the front cuts under the warmer air from the south forcing it upward.

As the air mass rises in the atmosphere, the reduction in surface pressure enables the air to expand, which results in temperatures falling.

As the air cools, it reaches its dew point, producing precipitation. In other words, it’s like wringing out a sponge. This is also referred to as the adioabatic lapse rate.

This condition will often set off a heavy rain in a fairly narrow line. Cold fronts in winter can bring severe cold spells and even snow.

If moisture is not sufficient, cold fronts can pass without producing any precipitation, but these dry cold fronts can still produce gale force northwesterly winds.

This week’s forecast

A ridge of high pressure will move over the Great Basin and will produce gusty northeasterly (offshore) winds during the night and morning hours.

This offshore flow will give clear and cold mornings and mild afternoons through Monday.

Today’s high temperatures will range between the high 60s to the low 70s throughout our area.

Overnight lows will drop below freezing in the interior, while the coastal valleys will reach into the low 40s by early Monday morning.

These lows are likely the coolest minimums this week.

These northeasterly winds are forecast to reach moderate to fresh (13-24 mph) levels with higher gust Monday morning, especially in the eastern part of San Luis Obispo County, near the Santa Lucia mountains and our coastal canyons.

A series of low pressure systems will move through the Gulf of Alaska this upcoming week with rain reaching as far south as the Bay Area on Tuesday.

The main effect along our coastline will be increasing northwesterly winds, partly cloudy weather and night and morning coastal low clouds.

Fair weather is expected through the remainder of the week.

Surf and sea report

Yesterday’s long-period northwesterly swell reached well over 20 feet at many of the marine buoys along the Central Coast.

The swell peaked at Diablo Canyon Power Plant Waverider buoy at 13 feet with a 20-second period, the highest-energy swell so far this season.

This morning’s 9- to 11-foot northwesterly (310-degree deep-water) swell (with a 15- to 17-second period) will gradually decrease to 8 to 10 feet by tonight.

Note: Swell heights will be much higher at the offshore buoys today.

Combined with the increasing northwesterly swell will be 3- to 4-foot northwesterly (320-degree shallow-water) seas today.

This northwesterly swell will continue to decrease to 7- to 9-feet (with a 14- to 16-second period) Monday.

Storm activity in the Gulf of Alaska will produce a 6-to 8-foot northwesterly (310-degree deep-water) sea/swell (with a 5- to 13-second period) along our coastline Tuesday through Saturday. Conservation tip

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To sign up, visit John Lindsey is a media relations and nuclear communications representative for Pacific Gas and Electric Co. He is also a meteorologist who specializes in forecasting for San Luis Obispo County.

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