Weather Watch

Cut-off lows are highly unpredictable

Woe to the weatherman who deals with a cut-off low. Cut-off low-pressure systems are notorious for being unpredictable, and they can produce some very unsettled weather.

Occasionally, an upper-level low-pressure system will break away from the jet stream, becoming a cut-off low. In other words, the jet stream shifts to a higher latitude and leaves a circulating low-pressure system behind.

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 the main mechanism that drives storms from the Pacific Ocean east toward the Central Coast. When a low-pressure system breaks away from the main flow, it becomes difficult to almost impossible to predict the speed and direction of these weather systems. I’ve seen these lows remain nearly stationary for days, or even move westward back out to sea.

Often when a cut-off low develops, the many different numerical weather prediction models that meteorologists consult for guidance — Global Forecast System (GFS), North American Mesoscale (NAM), Short Range Ensemble Forecast (SREF) and my personal favorite, the Navy Operational Global Atmospheric Prediction System (NOGAPS) — can diverge wildly, much like the cut-off lows themselves.

A cut-off low is usually slow moving and often moves in a wobbly fashion. Last Wednesday a cut-off low parked itself over the Central Valley. Combined with a strong eastern Pacific high, it produced very windy and unseasonably cool and wet weather along the Central Coast.

A cut-off low generally won’t exit an area until the jet stream finally shifts southward and pushes it along, or it eventually weakens and dissipates as it slowly spins in a counterclockwise fashion and unravels.

For most of last year, the storm track was over the Pacific Northwest, which was one of the main factors that contributed to below-normal rainfall for the Central Coast. It was also the year of the cut-off lows, more than ever seen before. Even though it was a difficult year to forecast weather, at least some of these systems gave us much-needed rain.

This week’s forecast

A 1,029-millibar area of high pressure over the Great Basin combined with a 1,026-millibar high about 200 miles off the Central California coastline will produce gentle morning northeasterly (offshore) winds and sunny and warmer weather later today. Today and Monday will be the warmest days for a while.

Temperatures will range from the high 70s to the low 80s in the interior, the low to mid 70s in the coastal valleys and the mid to high 60s along the coast.

A weather pattern that resembles the pattern that occurred this past Tuesday and Wednesday is forecast to develop again this week.

Another late-season cold front will approach the Central Coast Monday night into Tuesday morning with increasing southerly winds and mid- to high-level clouds. The cold front will pass over our area on Tuesday afternoon with rain. Total rainfall amounts with this system should range between 0.25 and 0.50 inches.

Once again, the main effect of this system will be to produce a cut-off low, which is expected to park itself over the great Central Valley of California.

This system will produce cold and windy weather along with a few scattered rain showers on Wednesday through Friday.

The strongest northwesterly winds should occur on Friday with sustained winds reaching moderate gale to fresh gale force (32 to 46 mph) with gusts hitting 55 mph along the coastline. The winds will decrease on Saturday with fair and warmer weather returning by next Sunday.

Surf and sea report

Today’s 6- to 8-foot northwesterly (295-degree deep-water) swell (with an 8- to 15-second period) from the Gulf of Alaska will remain at this height and period through tonight.

Combined with this northwesterly swell will be 2- to 3-foot northwesterly (320-degree shallow-water) seas this afternoon.

This northwesterly swell will decrease 5 to 7 feet on Monday, further lowering to 3 to 5 feet (with an 11- to 14-second period) by Tuesday morning.

Increasing southerly (200-degree shallow-water) seas will develop later on Tuesday, followed by a 7- to 9-foot northwesterly (300-degree deep-water) sea and swell (with a 7- to 11-second period) Tuesday night.

Increasing northwesterly winds will generate an 8- to 10-foot northwesterly (300-degree deep-water) sea and swell (with a 7- to 12-second period) on Wednesday. This northwesterly sea and swell will remain at this height and period through Thursday, increasing to 11 to 13 feet on Friday and Saturday.

Arriving from the Southern Hemisphere:

A 1- to 2-foot Southern Hemisphere (220-degree deep-water) swell (with an 18- to 20-second period) is forecast to arrive along our coastline on Wednesday through Thursday.

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

Conservation tip

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John Lindsey is a communications representative for Pacific Gas and Electric Company. 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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