After a dry summer, it's looking like a wet fall

It’s been another very dry summer season in our region. The June through August period was drier than normal in North Idaho and other parts of the Inland Northwest. For the three months beginning June 1 and ending Aug. 31, a total of 2.53 inches of rainfall was recorded in Coeur d’Alene. The normal for this period is 4.07 inches.

In 2023, the June 1 through Aug. 31 period was also drier than normal with 2.92 inches of rainfall. The majority of the moisture from June through August of 2024 arrived in the first three weeks of June, as 1.71 inches fell. Since June 20, the first day of “astronomical summer” only 0.82 inches of rain was measured in Coeur d’Alene. If we don’t receive any additional moisture between now and Sept. 22, this would be the driest astronomical summer season since 2006 when we had 0.83 inches of moisture. According to Cliff’s records, this would be the third driest period in history.

While most of us observe the fall season later this month, the “meteorological summer” season ended Aug. 31. Sept. 22 is the beginning of the “astronomical” fall in the Northern Hemisphere, which is based on the position of the Earth relative to the sun. In just over three weeks, the sun will be at 90 degrees, or directly overhead, at the equator. The next time the sun’s rays will be over the equator will be the first day of spring in 2025, which is called the vernal or spring equinox.

Although we base our seasons on the calendar, most meteorologists and climatologists break down the seasons into groups of three months. For example, the meteorological summer includes June, July and August. The meteorological fall begins Sept. 1 and ends Nov. 30. The meteorological winter is from December through February and the spring includes March, April and May.

I prepared a study showing the amounts of precipitation received during the astronomical summer that often began June 21 at Cliff’s station in Coeur d’Alene since 2004. During our spring season, our region will often pick up showers and occasional thunderstorms through the early-to-mid-June period. Therefore, I did a comparison of the astronomical summer seasons that began around June 21 and ended around Sept. 20. I also computed the astronomical fall or autumn precipitation in Coeur d’Alene from Sept. 21 through Dec. 20.

Since 2004, most of the dry summer seasons often led to a wetter-than-normal fall across the Inland Northwest, especially in the last 10 years. In 2010, we were close to normal for summer and fall precipitation, but in 2011, we had a dry summer followed by a wet fall. The following year, in 2012, the summer was wetter than normal with a drier-than-average fall. However, 2012 was a record year for moisture as 43.27 inches of rain and melted snow was measured.

Since 2015, there has been a consistent trend of drier-than-normal summer seasons, followed by wetter-than-average falls. In 2016, we were only slightly below average (0.02 inches), but the fall of 2016 was the wettest in the last 20 years. During that three-month period, 14.61 inches of rain and melted snow were reported, compared to the normal of 8.26 inches.

The long-range computer models indicate that the strong high-pressure ridge will continue to be situated over the western states for at least the next one to two weeks. More dry weather along with 90-degree temperatures are possible this week, and perhaps into next week as well. By the way, the latest September day with a 90-degree high was during the torrid summer of 1963 on Sept. 26 with a reading of 90 degrees. Also, with very little moisture expected, we will likely fall well below the normal 3.67 inches for the astronomical summer season. Back in 2006 when only 0.83 inches fell, an above-normal 10.71 inches was measured the following fall. In 2022, only 1.05 inches of rain was reported during the summer with another above-average total of 9.72 inches the following fall.

Assuming these recent trends continue, after a dry summer, the odds are very good for above-normal moisture for the Pacific Northwest. This new pattern could begin as early as late September, but more likely in October and November. December is another month that we could see higher-than-average moisture. With a new La Niña expected by late this year, as we’ve been mentioning, the chances for higher-than-average snowfall, especially in the higher mountains, are much better this upcoming winter season.

Randy Mann can be reached at randy@longrangeweather.com.