Weather at Coe Park
The Big Picture-California Climate
California's climate is called Mediterranean, which means that we tend to have hot, dry summers (with occasional morning stratus) and cool, wet winters. In the summer and early fall, California's weather is generally dry because of the Pacific high, which can be envisioned as a mountain of heavy air sitting off of the coast and on top of our state. The Pacific high deflects Pacific storms to the north, where they may drop their moisture on Oregon and Washington instead. Because the Pacific high is composed of stable air with low humidity and relatively little vertical movement, summer thunderstorms, common over much of the rest of the country, are rare. Along the coast during the summer months there is frequently fog, and in the mornings particularly, this fog often blankets the western hills of Coe State Park. The fog, however, seldom contains enough moisture to result in appreciable amounts of precipitation, and it usually burns off by mid-morning. Occasionally, although not every year, some summer rain will fall as the result of subtropical air moving up from the south and providing moisture with concomitant instability. (Because of condensation, moist air cools less than dry air when it rises, and being warmer than the surrounding air, it continues to rise and to have more of its moisture condense voilà! cumulonimbus cloud formation!) This rare summer rain is often accompanied by a few pyrotechnic displays with appropriate commentary by Jove.
In the late fall, the Pacific high tends to move farther south, thereby allowing entry of trans-Pacific depressions, or lows, which originate in the tropical waters of the western Pacific and over the large Indonesian islands in particular. As they pass over California, usually on the track of the jet stream, these depressions bring rain, with snow in the higher elevations. Although the annual rainfall in Coe Park is not great, there are places in California along the northern coast and between 4,000 and 8,000 feet elevation in the Sierras that receive on average over eighty inches of precipitation per year (about six times the amount of precipitation of San Jose).
Coe Park Climate
Nighttime lows and daytime highs tend to be more extreme in the canyons throughout the park. In the Narrows, where in winter there is little or no sun, ice on small pools may remain for weeks at a time.
Ridges are more exposed to the wind and hence may yield fairly low wind-chill readings during and after winter storms. For example, on Pine Ridge, with a temperature of 35 degrees and a wind of 30+ miles per hour (not an uncommon situation), the wind-chill factor would be close to 0 degrees on unprotected skin! Snow falls a few times per year above 2,500 feet, dusting some of the higher ridges.
In the eastern part of the park, rainfall is less and temperatures more extreme, with 100 degree highs not unusual in the summer (not the place for a long bike ride with only one water bottle!).
Signs Of An Approaching Storm
An approaching front (a band of cold air spawned by a depression) is usually signaled by a gradual lowering and thickening of the cloud layer and a shift in the wind from the northwest to the southwest. As the front approaches, rain may begin, and the wind will continue to increase. The cloud mass may lower enough to cover the ridges of Coe. Mountains (particularly ridges perpendicular to the storm winds) receive more precipitation than surrounding flatlands. That is because they force the stormy air to rise, the air cools three to five degrees Fahrenheit for each increase of 1,000 feet elevation, and the cooling causes moisture to condense and precipitate.
After the front has passed, the wind will shift, often suddenly, to the northwest, and the temperature will probably begin to fall. Bands of showers with accompanying winds and falling temperatures may continue to bring rain for another 12 to 24 hours.
A stationary depression spawning many fronts or a series of depressions passing on the jet stream may result in days or even weeks of wet unsettled weather, particularly from January through March. Sometime in late April or May, the Pacific high will move back over California and the waters to the west, and the long, dry season will begin again.