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FOUR SEASONS BREEZE | NOVEMBER 2020 11 As we get into November, the transition from the summer heat to cooler days is well underway, and the chances of stormy weather increases. So, the question typically pops up: "Will this be a wet year or a dry year?" Weather computer models, which are reasonably good in the short term, are not very good in the longer term (beyond 15 days or so.) So, it seems the answer to the question is like a flip of the coin, or using a ouija board, or going to a psychic. Well, you get the idea. But, actually, we can do better by looking at the Pacific Ocean surface water temperatures. The Pacific Ocean is a remarkable source of energy input into the atmosphere. Warm surface temperatures increase evaporation rates of moisture from the ocean to the atmosphere. The moisture rises and condenses into clouds, and that condensation process releases heat energy, which, in, turn affects the weather patterns across the northern hemisphere. Hurricanes, for example, of which there have been many this year, derive their energy from the very warm Gulf of Mexico and the equatorial ocean regions. There are two notable Pacific Ocean sea surface temperature patterns of which you may have heard: El Niño and La Niña. The circulation of both the Pacific and Atlantic Oceans has the warmest waters moving northward along the east coast of the continents, and cooler waters moving southward along the west coast of the continents. In the Pacific, ocean temperatures are much warmer off the Asian continent, and comparatively much cooler off the North American continent. Periodically, there is a change in the normal ocean water temperatures such that there is a rise in temperatures in the eastern Pacific equatorial regions, and the water temperatures cool off the Asian coast. This phenomenon is known as El Niño. With warmer waters in the equatorial eastern Pacific, there is therefore much more energy put into the eastern Pacific atmosphere which drives storm systems into California and produces above- normal rainfall. Conversely, the water temperatures in the eastern Pacific can get colder than normal, and not enough energy is available to steer storms into California on a regular basis. Under these conditions, persistently dry conditions typically occur over extended periods of time. This is the La Niña event. For the coming rainy season, the National Weather Service's Climate Prediction Center (CPC) recently issued the following statement: "La Niña conditions are present and are likely to continue through the Northern Hemisphere winter (~75% chance)." Further, the CPC indicates that Southern California will have about a 35 percent chance of having below normal precipitation during October, November, and December, and a 55 percent chance of above normal temperatures during these three months. These conditions can result in a prolonged fire season in California. For January, February, and March, the precipitation probabilities are over a 50 percent chance of being below normal, with a 45 percent chance of above-average temperatures. (See precipitation probability maps below.) There is typically a very distinct and stable weather pattern across the Pacific Ocean and the North American Continent associated with La Niña episodes. Major low pressure circulations occur near the Gulf of Alaska and Hudson Bay in Canada. These two stable low pressure systems produce an area of high pressure and dry conditions across much of southwestern states (and usually very cold weather in the eastern half of the U.S.). This does not mean that we won't have any storms. We will. But the frequency and intensity of storms will generally be less than in normal years. No crystal ball is absolutely perfect, and there have been a few deviations from the long-term projections, but in general, the bottom line of all of these analyses is that there is a relatively high probability of a drier than normal fall and winter, with above average temperatures. If you have a question about any weather topic, please feel free to email me at mzeldin45@ gmail.com. Periodically, I will devote a column to answer your questions "Understanding the Weather" – A Series by Mel Zeldin, Retired Meteorologist Will It Be a Wet or Dry Rainy Season?

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