Predicting the weather isn't easy to do

This January, the South African government passed a law that would imprison weather forecasters for up to 10 years — or a fine up to about $100,000 — if they issue incorrect severe weather warnings without official permission.

Thankfully in this country, if you get the weather forecast wrong, you don’t have to worry about going to prison.

Predicting the weather can be extremely difficult because of the massive scale of Earth’s atmosphere and the interactions it has with the sun, ocean and land.

If you wanted to produce a near-perfect forecast, you would need to know the temperature, relative humidity and pressure of every cubic inch of the Earth’s atmosphere. About every second around the clock, all that of information would have to be updated and run on some sort of futuristic computer capable of handling the unfathomable amount of data.

On top of that, the atmosphere behaves chaotically, making it very difficult to model accurately. We probably couldn’t produce a perfect forecast even given the exact initial conditions.

Despite of all these hurdles, the five-day weather forecast today is as reliable as a two-day forecast was 20 or 30 years ago, according to the World Meteorological Organization. This is because of advances in computer modeling that utilize a progressively greater amount of atmospheric and oceanographic data.

Even with these advances, though, weather forecasters are still proved wrong on short-term forecasts.

Tuesday’s storm is a perfect example. The day before, I had predicted that a low-pressure system and associated cold front would produce strong to gale-force (25- to 38-mph) southeasterly winds and between 0.5 to 1 inch of rain. By the end of the day, most locations reported only about a tenth of an inch.

The storm system certainly produced strong winds. However, they were shifted more out of the east than the normal southeasterly incoming direction.

This unusual incoming direction meant that the surface winds were blowing more from the land than from the sea. Consequently, relative humidity levels near the ground were dry.

Both the Paso Robles and San Luis Obispo airports reported relative humidity levels in the low 40 percent range — very dry indeed. Without any real instability, the storm lacked dynamics that could have helped to mix out this layer of dry air.

The Doppler radar from Vandenberg Air Force Base showed a great deal of precipitation moving over the Central Coast at the higher levels of the atmosphere. However, near Earth’s surface, it indicated very little. Most of the rain simply evaporated as virga.

Virga is any form of precipitation that evaporates as it falls into a layer of dry air near Earth’s surface. This dry layer of surface air drastically reduced Tuesday’s rainfall totals.

These sorts of things happen in weather forecasting.

There are times when the different weather models are poles apart, and you just have to make an educated guess. If someone has been forecasting over a number of years in the same geographic area, he or she will base predictions on previously observed weather patterns. Tuesday’s system will certainly add to my experience.

Today’s weather report

Except for some dense morning fog in the North County, today will start mostly clear and then clouds will increase from the north as a cold front drops out of the Gulf of Alaska toward the Central Coast. Rain is expected tonight into Monday morning. Rainfall amounts across the Central Coast should be light, less than a quarter of an inch.

Snow will spread to the central Sierra by early Monday morning and will further spread southward into the southern Sierra late Monday morning. Snow levels will start out near 5,000 feet and fall to 3,500 feet by Monday. Snow totals should generally range between 2 and 6 inches.

A strong 1,030-millibar high will build behind this cold front and produce a steep pressure gradient along the Central Coast. This pressure gradient will produce moderate-gale to fresh-gale (32- to 46-mph) northwesterly winds along the coast Monday afternoon through Tuesday afternoon.

An upper-level low-pressure system will pass through the area Tuesday night into Wednesday morning.

The placement of the system is uncertain. If this low tracks along the coastline, we will get rain. If it tracks farther inland, the Central Coast will remain dry. In either scenario, this does not look to be a major storm.

Gusty northeasterly (offshore) winds are set to develop later Wednesday and persist through Friday. This offshore flow will produce dry conditions with plenty of sunshine Thursday and Friday.

Increasing chances of rain are expected next weekend.

Today’s surf report

Today’s 7- to 9-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 14-second period) will decrease to 6 to 8 feet Sunday and will remain at this height and period through Monday morning.

Moderate-gale to fresh-gale (32- to 46-mph) northwesterly winds will generate an 8- to 10-foot northwesterly (315-degree deep-water) sea and swell (with a 5- to 15-second period) Monday afternoon. This northwesterly sea and swell will peak Monday night into Tuesday morning at 10 to 12 feet (with 5- to 14-second period), decreasing to 7 to 9 feet (with an 8- to 15-second period) Wednesday.

A 6- to 8-foot northwesterly (300-degree deep-water) swell (with an 8- to 16-second period) is forecast along the Pecho Coast on Thursday through Friday.

Preliminary extended surf analysis

A series of storms in the North Pacific will continue to produce a series of medium- to higher-energy swell events through February.

Seawater temperatures

Seawater temperatures will range from 54 to 56 degrees through Sunday, decreasing to 52 to 54 degrees Monday through Tuesday.

Environmental note

More than half of the electricity that PG&E delivers to its customers comes from clean, carbon-free sources. Visit www.pge.com for more information about clean energy.

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 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, send him an email at pgeweather@pge.com.