20 FOUR SEASONS BREEZE | FEBRUARY 2023
Many of you may have heard the term "atmospheric river" on TV weather segments. But what is it? Simply stated, an atmospheric river
describes a long fetch of moisture across the eastern Pacific Ocean, often originating from the subtropics, which provides huge amounts
of moisture into the atmosphere. The recent string of storms from late December into early January producing significant rains were the
byproduct of the atmospheric river. When these do occur, substantial rains impact California, more in northern and central California, but
often Southern California as well. When the moisture originates near Hawaii, the term often used to describe this river is the "Pineapple
Connection."
These "rivers" are generally about 250 to 375 miles in width and can be more than 1,000 miles in length. (See graphic below.) The moisture
brought to the West Coast lifts, cools, and creates rain in lower elevations and snow in the higher elevations. Upper level dynamics can
amplify the lifting, and these conditions then can produce great amounts of rain and snow.
During the prolonged drought conditions in California, persistent high pressure blocked and deflected any such rivers toward the Pacific
Northwest. A huge dome of this high pressure during Christmas produced near record warm temperatures in Southern California, which, in
turn, caused arctic air to move southward into the central and eastern states, causing the frigid and snowy conditions there. But right after
Christmas, that high pressure finally gave way to westerly upper level winds and the onset of the atmospheric river over a three-week period
affecting all of California. The state definitely needs the rain and mountain snow, and it is possible that it can redevelop further into January,
but as of this writing, it is hard to predict with any certainty.
"Understanding the Weather" – A Series by Mel Zeldin, Retired Meteorologist
ATMOSPHERIC RIVERS
