This weather model predicted SLO County rain storm — more than a week ahead of time

Earlier this year, I wrote about a new and improved version of the National Oceanic and Atmospheric Administration’s Global Forecasting (GFS) model, also known as the American model in the weather forecasting community.

You may hear a forecaster write or talk about weather models. What are these models?

They are not people who display the latest fashion trends on a runway. These numerical models are collections of mathematical formulas that replicate the physics of the atmosphere.

They perform billions of calculations to simulate weather patterns’ motion in the Earth’s chaotic atmosphere on some of the most powerful computers in the world.

Over the decades, these models have vastly improved. Today, numerical weather models are practically indispensable in giving guidance to forecasters.

One of the models I rely on the most is the Wavewatch III, which provides globally state-of-the-art ocean wave forecasts. I have used different versions of this model since the early 1990s.

As you can imagine, it is also used by surfers and others who enjoy or rely on the ocean for their livelihood.

This latest version, Wavewatch III, a one-way coupled model with the Global Ensemble Forecast System (GEFS), has improved the prediction of ocean waves generated by the surface winds. This new version’s forecast was extended from 10 days out to 16 days.

Earlier this month, on Oct. 6, this model successfully forecasted moderate gale-force to fresh gale-force northwesterly winds — 32 to 46 mph with gusts to 55 mph — along the coastline on Monday, Oct. 11.

Later that day, the Diablo Canyon metalogical tower reported sustained northwesterly winds of 41 mph with gusts to 56 mph at 5:30 p.m.

This was one of the strongest northwesterly wind events we have seen this year and caused seawater temperatures to plummet due to the enormous amount of upwelling.

On Saturday, Oct. 16, the model predicted the storm that slammed into San Luis Obispo County on Monday morning.

On Sunday, Oct. 25, a record-breaking 942 millibar low-pressure system developed off the Pacific Northwest, breaking the previous record low of 943 mb recorded in the same area from Post-Tropical Cyclone Harriet in October 1977.

The associated cold front, strengthened by vigorous upper-level winds, moved through the Central Coast with moderate gale-force to fresh gale-force (32 to 46 mph) southerly winds and between 2 and 4 inches of badly needed rain.

Naturally, the model also predicted the arrival of a 14- to 16-foot westerly (275-degree, deep-water) swell with an 18- to 20-second period from the Gulf of Alaska on Monday.

This updated version accurately predicted the storm well more than one week out.

I have been forecasting weather along the Central Coast since 1991, and this is a considerable advancement.

The timeless waves that break on our beaches are an excellent proxy measurement of the winds that blow across the ocean and the storms that generate them.

So far this year along the Central Coast, from my observations of atmospheric pressure, winds and precipitation, the GFS has outperformed the European Centre for Medium-Range Weather Forecasts (ECMWF) model.

The ECMWF model is considered by many to be the premier model in the weather forecasting community and is an outstanding achievement for those who developed it.

I would like to thank Jessica Meixner, physical scientist at the National Centers for Environmental Prediction, who has always been so helpful in directing me to the atmospheric and oceanographic data needed to write my weather forecast.

The output of the Wavewatch III numerical model can be viewed at polar.ncep.noaa.gov/waves.

PG&E safety message

Monday’s storm showed the importance of being prepared. To learn about preparation and response, please visit www.pge.com.

This story was originally published October 26, 2021 at 5:05 AM.