Expected fall weather changes

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The official start of Fall on Saturday, September 23, marks the start of a season where the weather is jostling between the remnants of summer and the old winter trying to claw its way into the picture.

The first freeze and first snow often arrive in the fall, but on the other hand, hurricane season can still be active and severe weather, including tornadoes, can be a concern at times. The West also underwent multiple changes.

1. The first freeze of the season has arrived for many: The first low temperatures of 32 degrees are typically experienced before or around the time objects officially fall in the higher elevations of the West and near the Canadian border.

By the end of October, the first average freeze had occurred from most of the Northeast into the Midwest and into the northern parts of the South. First freezes are common in November from the mid-Atlantic coast to most of the Deep South.

Average time of first freezing.

(Data: NOAA)

2. The first measurable snow has arrived for many: The mountain highlands in the west are likely to see snowfall by the end of September or earlier.

By October, areas from the Western Highlands to the Northern Plains and Michigan’s Upper Peninsula see their first measurable snowfall, defined as at least a tenth of an inch.

Much of the country sees its first measurable snowfall by the end of November or early December, including from the Northeast to the Midwest and southern Rocky Mountains.

(More: Dig deeper to find out when the first snow is expected to fall)

The month with the first average accumulated snowfall (0.1 inch or more) of the season, according to a 30-year average.

(Data: NOAA)

3. Hurricane season may still be active, but its end is in sight: The Atlantic hurricane season is still in full swing as fall begins.

Four additional named storms typically originate in the Atlantic after the autumnal equinox, based on the 1991-2020 average.

Since 2016, we have seen Hurricanes Matthew, Nate, Michael, Delta, and Zeta all make landfall in the United States in October.

Last year, Hurricane Nicole made landfall in Florida on November 10. Hurricane Nicole was the fourth November hurricane to reach the mainland United States in records dating back to the mid-19th century, and the first to do so since Hurricane Kate struck the Panhandle. In 1985.

Autumn is also when the area we monitor for tropical development in the Atlantic Ocean begins to shrink in size.

By October, we typically see tropical storm and hurricane-forming areas shift westward toward parts of the Caribbean Sea, Gulf of Mexico, and far western Atlantic Ocean as the “Cabo Verde” portion of the hurricane season — with African easterly waves developing in the eastern Atlantic — fades away.

4. Severe weather and tornado outbreaks possible: Just like spring, fall is a battle season as warm, moist air from the Gulf of Mexico meets increasingly powerful cold fronts and jet stream winds that typically sweep across the country.

When this combination of ingredients comes together, it can ignite numerous severe thunderstorms that produce damaging winds, large hail, and tornadoes.

November 2022 was a reminder with an above-average 68 tornadoes per month, most of which were in the South. The deadly tornadoes struck Texas and Oklahoma on November 4, then Alabama early on November 30.

One of the nation’s worst tornado outbreaks occurred just days before Thanksgiving in 1992 when a swarm of 105 tornadoes tore through parts of 13 states from Texas to the Carolinas. The severe weather killed 26 people and injured 638 others.

5. Two changes to watch in the West: Southern California’s notorious offshore winds, known as Santa Ana winds, typically kick in by October, according to a 2017 climatology study.

Fueled by a combination of high pressure in the Great Basin and the southward return of the jet stream, these intense hot winds push through the passes and valleys of the Los Angeles Basin on occasion, most commonly from September through March.

What makes it especially dangerous in October is that soil moisture is at its driest after the summer dry season. Santa Ana winds can burn either an existing wildfire or a small fire that has just developed into an inferno.

Autumn usually brings major changes in the weather pattern in the West.

One weather pattern that emerges in the fall is characterized by a strong area of ​​high pressure over the interior of the West, causing offshore winds, including Santa Ana winds, from the northeast or east to move through canyons and pass into southern California. These winds can also bring hot temperatures to the area.

Santa Ana winds are most common from September through March and can increase fire danger when combined with low humidity levels or persistent fires. Wildfire risk is typically higher in the fall due to vegetation drying out as California ends the dry season.

As fall progresses, the jet stream tends to push low-pressure systems further south than in summer, resulting in increased precipitation to the west.

This includes Los Angeles, which sees a significant increase in average precipitation starting in November.

Rainfall amounts begin to increase in October in the Pacific Northwest, while November is the wettest month of the year, on average, in Seattle and Portland, Oregon, with 6.57 inches and 5.63 inches, respectively.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment, and the importance of science in our lives. This story does not necessarily represent the position of our parent company, IBM.

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