Strong cooling with rain, snow and wind ends monsoon in the southwest of the USA

The last showers and thunderstorms of the North American monsoon are upon us as a major shift in weather patterns will soon end the influx of tropical moisture over the Southwest U.S. and drive summer heat out of the deserts, AccuWeather meteorologists say.

The clock is ticking for the North American monsoon and the annual shift in winds that transport moisture to the western United States

Vehicles in the Phoenix area navigate a flooded roadway that accompanied severe storms on Monday, July 9, 2018. (AP Photo/Ross D. Franklin)

On Sunday, Tropical Storm Ileana lost its tropical characteristics as it moved northward through the Gulf of California near the western coast of mainland Mexico. However, some tropical moisture will still reach the deserts, valleys and mountains of New Mexico and Colorado.

When this moisture comes into contact with the aftermath of the North American monsoon and its scattered showers and thunderstorms, rainfall and thunderstorm activity will increase starting early this week.

The result will be an increase in rainfall, which can trigger dangerous flash flooding and severe thunderstorms. Flash flooding may occur in remote areas, but also in Phoenix, Flagstaff and Tucson, Arizona; Durango, Colorado; and Gallup, New Mexico, to name a few cities and towns in the region. Hikers venturing into the canyons should be cautious. Aroyyos can fill with water quickly. Mudslides and other debris flows may occur.

Tropical humidity is not the only factor contributing to heavier showers and thunderstorms.

There will be a big shift in the jet stream early in the week. The jet stream will drift unusually far south for September, creating a weather pattern more typical of March or April. This means there will be a big surge of cool air, offset only by the effects of the continued warmth of the landscape and the mid-September sun.

This summer has brought a record number of consecutive days of triple-digit temperatures in Phoenix. As of Monday night, that number was 113. The old record of 76 consecutive days of temperatures above 100 degrees Fahrenheit, set in the summer of 1993, was broken weeks ago.

By the middle of this week, highs in Phoenix at or above 100 will be replaced by highs in the low 90s. The last time this was consistently the case was in early May.

In Palm Springs, California, highs of 100 degrees or more will be replaced by highs above 80 degrees for the first time since May. Temperatures could even drop to record-breaking lows above 50 degrees on Monday night.

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By the middle of this week, it may be so cool that temperatures in Las Vegas won't rise above 70 degrees for a day or two, which is more typical for early to mid-April.

Los Angeles will see highs of 21 degrees, about 10 degrees below historical average. With occasional breezes, AccuWeather RealFeel® temperatures could drop to 15 degrees during the day.

Stronger than usual winds will add to the chill in the San Francisco area, so temperatures in the 15s could feel more like 10s or a June gloom, albeit with some sunshine in between.

The cool pattern looks even better in the mountains.

Although these are not the first snowflakes of the season thanks to a few cool days in late August, this eruption could be even more impressive in terms of the amount of snow produced over the Sierra Nevada.

It is possible that several centimeters of slush will accumulate in the higher elevations by the middle of the week.

Most roads will be just wet, but visibility may slow travel as the snow will likely be accompanied by gusty winds. Although a few cool, wind-driven showers may occur over Grapevine, no snow is expected at this time.

A common price for the changing weather pattern will be strong gusts over the Southwest region, especially through Monday. In extreme cases, gusts between 60 and 80 mph are possible over the passes and may reach nearly 100 mph over some of the ridges and peaks. Gusts of 30-50 mph are likely over the interior valleys, but are unusual for mid-September in a non-Santa Ana environment with south-west shifting winds.

In areas where there is little or no rain, such as many deserts in California, Nevada and Arizona, the gusts will raise dust and pose a risk of vehicle rollovers.

Strong winds will also make firefighting extremely difficult due to the ongoing fires. Even though humidity will increase as temperatures drop, the persistent windy conditions can still pose a huge challenge.

Parts of Arizona, New Mexico and Colorado will be hit by showers and thunderstorms through Monday, but lightning strikes on the fringes of the storms could spark new fires.

As the strong southwest winds move westward and subside somewhat, the North American monsoon with its moist air and thunderstorm activity ends and a more gradual fall weather pattern sets in.

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