Western US cooling off after record-smashing heat wave
After weeks of abnormal weather in the western half of the country, AccuWeather meteorologists say that a cooldown is in sight for many. For some, this temperature change will be accompanied by a surge in monsoonal moisture, providing drought relief during a typically dry time of year.
In recent days and weeks, those in the western United States have proven to be no stranger to volatile, abnormal weather. With dangerous flash floods from the North American monsoon, days of extreme heat and an abnormal tropical storm in Southern California, conditions across the Intermountain West have been anything but normal.
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First responders rushed to the North Island Naval Air Station located in San Diego, on Friday, Sept. 9, after a jet slid off the runway while trying to land during rain and high wind from Tropical Storm Kay. It was not yet clear if weather was a major factor in the accident. |
As of Sept. 10, conditions in the month of September across parts of the West have been characterized by stifling, unrelenting heat. The worst of this was felt in California, where numerous locations broke all-time high temperature records by significant margins. In the state capital of Sacramento, Tuesday's high of 116 F was the hottest ever for the city, and temperatures are running an astonishing 12 degrees above normal for the month.
Elsewhere in the state, there also has not been much relief from the heat. For the month so far, San Francisco has been 6 degrees above average, while Fresno has been nearly 13 degrees above typical values. Even in Palm Springs, California, which is already among the hottest cities in the state, temperatures have been 4 degrees above normal.
Some locations in Southern California saw temperatures take a nose dive late last week, as Tropical Storm Kay brought clouds and tropical rainfall to the area. Now, a major pattern change is expected across the western U.S. that is expected to send temperatures plummeting across a more widespread area. Sacramento fell by over 20 degrees by Monday, with a Monday high of 91 F.
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"The trend to lower temperatures will begin in the upper atmosphere, as a dip in the jet stream moves ashore and becomes centered over the West. As temperatures aloft cool down, the air at ground level will follow suit," AccuWeather Senior Meteorologist Mike LeSeney explained.
In the Pacific Northwest, this change is already underway. In Seattle, where temperatures surged into the 90s on Saturday, the mercury only reached 72 F to close out the weekend, and reaching 72 F again on Monday. Through much of this week, temperatures will hover in the low 70s once again, which is near average for mid-September.
To the south, temperatures will also hover at near-seasonable levels. In San Francisco, high temperatures are expected to hover in the low to mid-70s through at least Friday, which would be near or even a bit below average. Monday's high in the city dipped all the way down to 65 F, which is much below the average. In Fresno, where high temperatures typically reach the 90s, the mercury may struggle to get out of the 80s by midweek, although Monday's high of 97 did not match the trend.
As temperatures lower this week after such a scorching heat wave, it is a reminder that summer is slowly but surely winding down.
"This time of year, the average high temperature drops a degree every two or three days. With that in mind, the likelihood of intense, abnormal heat will steadily decrease as we move closer to fall," LeSeney explained.
For some, additional heat relief will come in the form of increased showers and thunderstorms, as the North American monsoon brings an increase in moisture into the region. While this rainfall is often limited to a handful of states in the southwest, storms may spread northward into states such as Idaho, Utah and Wyoming. While Hurricane Kay brought a swath of heavy rain to Southern California, many other areas have been quite dry as of late.
"A disturbance associated with the remains of Kay will push ashore early this week, peeling off some moisture and enhancing the northward flow of monsoonal moisture," LeSeney explained.
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As is typical for monsoon-driven showers and storms, the coverage will be somewhat isolated. While some spots may stay dry on a given day, others may see rounds of very heavy rainfall and perhaps strong wind gusts. This was the case in Phoenix late Sunday evening, with an intense thunderstorm brought wind gusts of 86 mph to the city's airport. On Monday, a tornado warning was issued in two rural Arizona counties, Pima and Pinal, with a tornado indicated by radar along with strong wind gusts.
In the locations that receive the heaviest rain, flash flooding may become a concern. This is especially true in areas with mountainous terrain, as runoff can funnel into low-lying valleys and cause water levels to rise in a matter of minutes.
Meanwhile, the increase in rainfall may prove beneficial to the ongoing drought, which still covers most of the Southwest according to the U.S. Drought Monitor. Rain may also give firefighters an advantage in battling some of the ongoing wildfires in the region, however, lightning may prove problematic for the formation of new fires. Some thunderstorms in the area may produce light rainfall or even none at all, allowing lightning strikes to ignite fires in the region.
The wet, cooler pattern is expected to continue through most of the week across the West. By next weekend, a return to drier weather may be in store.
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