Flooding chances in the Western U.S.

Last Monday, March 20, was the first day of spring across the Northern Hemisphere. For most years, the first day of spring arrives on March 21, but the “vernal equinox” happened at 2:24 p.m. on that date.

The word “equinox” comes from the Latin words meaning “equal night.” This is an astronomical event when the Sun is directly overhead at the Earth’s equator, and we experience 12-hours of day and night. On June 21, 2023, our first day of astronomical summer, the Sun will be directly overhead in the Northern Hemisphere at 23.5 degrees North, the Tropic of Cancer. Residents here in North Idaho will never see the Sun directly overhead as we are too far to the north as our latitude is approximately 47.7 degrees North.

The first day of “meteorological spring” was March 1. The months of March, April and May are used for the spring calculations because it’s easier to compare the seasonal and monthly statistics. They are also based on the temperature cycles rather than the Earth’s position relative to the Sun.

Coeur d’Alene experienced its first 50-degree day March 17. The last time we had readings in the 50s was Nov. 4, 2022, when the high temperature was 52 degrees. Five days later, March 22, the mercury topped out at a very mild 58 degrees at Cliff’s station. As we move farther along into the spring season, officials will be carefully watching the levels of Lake Coeur d’Alene and other lakes across the region. Lake Coeur d’Alene currently stands at just over 2,122.7 feet, about 6 feet below the summer maximum level of 2,128 feet. Flood stage is reached when the lake hits 2,133 feet. Moderate flooding is at 2,136 feet and major flooding is at 2,138 feet.

By the way, since we’re talking about Lake Coeur d’Alene, some have asked me if this lake was created from an ice age. Well, Lake Coeur d’Alene is one of many of our natural lakes in North Idaho. Scientists believe that our lake was formed due to the melting of the glacial ice from the last ice age. The event is called the Missoula Floods and occurred about 12,000 to 15,000 years ago. The floods occurred as an ice sheet from Canada carved out Lake Pend Oreille and dammed the Clark Fork River. The water built up to form Glacial Lake Missoula and then eventually broke through the ice dam that resulted in the massive flooding and the formation of Lake Coeur d’Alene.

Most of the time, the Inland Empire sees its greatest risk of high waters during the spring season. The big floods typically result from torrential thunderstorm downpours, gusty southerly winds plus warm rains falling on melting snowpacks in the higher elevations. With the recent snowfalls in the mountains, many locations are currently reporting near or above 100 inches on the summits.

Currently, the National Oceanic and Atmospheric Administration (NOAA) states that the chances of minor flooding in our part of the country is very small. However, there is the potential for moderate flooding across much of California, due to the record snowpacks in the Sierra Nevada mountains. The desert regions in the southeastern part of the state are not expected to have any flood issues.

When looking at the comparison of moisture totals at some the stations in California, the difference between this year and 2022 is remarkable. For example, at Blue Canyon, a mountain station located northeast of Sacramento, only had 4.23 inches of rain from Jan. 1 through March 24, 2022. By contrast, over 50 inches of moisture has fallen since the beginning of 2023.

Despite the heavy amounts of precipitation in California, the reservoirs in the Colorado River Basin that include Lake Powell and Lake Mead continue to be at record low levels as most of the heavy moisture stayed to the north. Lake Mead’s current water level is about 1,046 feet, which is 15 feet lower than at this time in 2022.

In addition to the flood potential in California, there is also the risk for floods across much of the eastern half of the U.S., except the Northeast. It’s possible that we could be hearing about moderate to major floods along the Mississippi River. The main areas for concern are from Minneapolis, Minnesota southward to St. Louis, Mo.

In terms of our local weather, more 50-degree temperatures are expected later in the week. We should also see occasional rain showers with still the chance of some snow in the lower elevations, but mainly during the overnight hours. Despite the recent decrease in moisture in our region, it still looks like an above-normal precipitation pattern in April.

The long-range computer models continue to forecast more storms for California through at least the first week of April. This will mean more snow in the Sierra Nevada mountains that will add to an already record snowpack.

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Contact Randy Mann at randy@longrangeweather.com.