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Randy Mann weather: Halloween forecast: Dry but cold

| October 25, 2021 1:07 AM

UPPER-LEVEL WINDS ARE DIRECTING BIG STORMS TO OUR SOUTH

Rain returned to the Inland Northwest last week as the storm door popped open. Much-needed moisture has been falling southward into California as up to 7 inches of rain is possible in the Sacramento Valley from the current series of storms. Some of the foothill locations to the east of Sacramento could see a total of nearly 15 inches of rain from these systems, very unusual for October.

Rainfall totals in Coeur d’Alene and surrounding regions have been lighter as the bulk of the moisture has been falling to our south. As of Sunday, Cliff has measured about an inch of rain for the month of October. Our normal for this month is 2.22 inches, so it looks like we’ll end up with another month with below-normal rainfall. Once these systems move out, conditions are expected to briefly turn drier, just in time for Halloween. Temperatures around trick-or-treat time should be in the 40s, but likely drop into at least the upper 30s later that evening.

The storm systems that have been moving into the western states since last week have been warmer. Therefore, snow levels have been high. However, colder air may move southward late in the week leading to a chance of snow in the mountains. Despite the chance of some snow in the mountains, it now looks like that this will not be the third October in a row with measurable snowfall in Coeur d’Alene. Except for the last several years, we typically don’t see any measurable snowfall until around the middle of November. During the big snow year of 2007-08 when we had a record 172.9 inches, there was a tenth of an inch of snow measured on Nov. 14, but the bigger snows didn’t arrive until the end of November. The long-range computer models are showing a chance of snow in the lower elevations toward the middle of next month.

Our storms are directed by the upper-level winds, also known as the jet streams. Recently, the strongest portion of the jet stream has been to our south, which is where the heavier rains have been falling. These narrow rivers of fast-moving air will usually follow the boundaries of hot and cold air. Our planet has four main jet streams, two in the Northern Hemisphere and two in the Southern Hemisphere. In our part of the world, we are mostly influenced by the “polar jet stream.” When the flow is out of the Gulf of Alaska and coming in from a northwesterly direction, our chances in the winter are much higher for snow.

North Idaho can also see heavy rainfall from the polar jet stream as the southern branch becomes strong and will tap into moisture near Hawaii and send it northward into the Pacific Northwest. This scenario has often been referred to as the “Pineapple Express,” and there is often widespread flooding from this type of event.

The other jet stream in the Northern Hemisphere is the “sub-tropical jet stream.” This river of air is often located south of the U.S., in Mexico. However, during the warmer El Nino years, this jet stream will often migrate northward and will occasionally bring very heavy rainfall to the southern U.S., including the central and southern portions of California.

In the Southern Hemisphere, their version of the polar and sub-tropical jet stream is similar. Despite high and low-pressure systems rotating in different directions south of the Equator, the upper-level winds in that part of the world also travel in a west-to-east direction.

The upper-level winds from about 30,000 to 39,000 feet can be strong. On Feb. 18, 2019, the National Weather Service clocked a wind speed at 35,000 feet of 231 miles per hour above New York City. This was the fastest jet stream speed on record, breaking the old mark of 223 miles per hour in 1957. Thanks to the strong tailwind, a Boeing 787 flew at a record speed of 801 miles per hour. That’s higher than the speed of sound, which is about 767 miles per hour.

There is uncertainty on how the northern jet stream has varied in the past, or how it may change in the future. According to the National Science Foundation, new research from the examination of ice cores over the last 1,250 years in the Greenland ice sheet suggests that the natural variability of the northern jet stream could be changed by 2060 if the Earth’s temperature continues to warm, especially over the eastern U.S. and western Europe. Strong variations in the jet stream winds could have larger impacts that lead to more weather extremes, like floods and droughts.

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Contact Randy Mann at randy@longrangeweather.com

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