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The upper-level low that was spinning along the California Coast, Thursday October 15th, 2015, was around for a long time. The chart below shows the location of the low at 500 mb (or roughly 18,000 ft) that morning. This low actually formed over western Canada on Friday October 2nd. It dropped south through California, meandered around the Desert Southwest, and then it dropped into Mexico, ejected back over the Pacific Ocean, performed a loop, and ended up at its location along the California Coast, affecting the area a second time. The red dots and connecting line indicate its approximate path over the past two weeks. The longevity of this low was pretty amazing and it's indicative of a couple things: 1) The jet stream pattern did not change much over that two week period. 2) The jet stream remained far enough to the west, north, and east of the low to largely leave it alone. While lows decouple from the jet stream quite frequently, they typically don't last this long over the United States. Much of the time, dips or "troughs" in the jet stream will exert enough influence to break them up and lift them to the north, where their remnants are absorbed by the jet stream. After remaining intact one more day, that did occur the weekend of the 17th-18th as the low (and trough) on the left edge of this image moved ashore into the U.S. These lows occur most frequently during the transitional seasons of spring and autumn, when wild fluctuations in the position of the jet stream occur.

Below is a two-week animation of water vapor imagery from satellite. At the beginning of the loop, note the arc of orange over southwestern Canada. This is where the low initially formed. This loop shows the progress of the low from the morning of Friday, October 2nd, to Thursday morning, October 15th.

Water vapor imagery is extremely useful to meteorologists as it shows the flow in the middle-upper portions of the atmosphere, as well as the waves embedded in the flow. The color enhancement helps better define features. Dark or darkening areas (including oranges and reds) indicate sinking, warming, and drying air. Light or lightening areas (including the blues, greens, and reds) indicate rising, cooling, and moistening air. The blues, greens, and red are typically areas of clouds that are composed of ice. There is a lot of information available in this imagery.




This page was composed by the staff at the National Weather Service in Hastings, Nebraska.