A cold front produced over an inch of rain in many San Luis Obispo County locations Halloween evening into Saturday morning. San Luis Obispo hasn’t seen over an inch of rain in a 24-hour period since the 28th of February. So what is a cold front and how does it produce unsettled weather?
In 1919, a team of Norwegian meteorologists, led by 57-year-old Vilhelm Bjerknes, came up with the theory of weather fronts, an analogy for the First World War battlefronts.
Fronts are long boundary lines that can stretch for hundreds or even thousands of miles across the Earth between two air masses of different densities, usually caused by temperature or humidity differences. They often bring precious rain.
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. Fronts nearly always are accompanied by clouds and, if strong enough, by rain, snow or thunderstorms.
Cold fronts, like Friday night’s Halloween tempest, contain colder and denser air that push the lighter and warmer air upward — somewhat like snow moving up the shoveled surface of a fast-moving snow plow. The warm air cools as it rises, and it condenses into clouds.
As the water vapor transforms into clouds or liquid/frozen water, it releases tremendous amounts of latent heat, further enhancing the amount of lift. When the size of the water droplet or ice crystal has grown to a point that it can no longer be supported in the air column by updrafts, it falls as rain, hail or snow.
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 meteorologists as the indication that a cold front has passed a particular location. On weathersurface charts, a cold front is represented by a solid line with triangles along the front pointing in the direction of movement.
The Halloween cold front passed Diablo Canyon Power Plant’s meteorological tower at 7 p.m. as the wind shifted out of the northwest. 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 unstable, which can lead to heavy showers, hail and the sound of thunder.
An 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.
Along the Central Coast, these cold fronts usually move in from the northwest, in a southeasterly direction. 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.
The cold front on Halloween had tremendous upper-level wind support, which strengthened the upward motion of the winds at the frontal boundary and enhanced the amount of rain. This storm also brought heavy snow to the Sierra above 5,000 feet.
PG&E safety tip: If you see a downed power line, assume it is energized and keep yourself and others away. Call 911 immediately to report the location of the downed line then 800-743-5002, PG&E’s 24-hour emergency and customer service line.