By AccuWeather Meteorologist
Updated Sep. 20, 2021 6:39 PM EDT
A powerful storm that slammed into the West Coast to start the weekend will continue to be watched as it marches from coast to coast throughout the upcoming week. On top of the threat for flooding rainfall and severe thunderstorms, a blast of cooler air will sweep across the nation to welcome in the autumnal equinox, which occurs on Wednesday, Sept. 22.
The storm definitely made its presence felt as it marched into the West Coast on Saturday. An unusual early-season rain extended from Seattle southward into northern California, providing a major boost to firefighting efforts across the region. While cool and wet conditions will continued across the Northwest to end the weekend, AccuWeather forecasters say the storm isn't quite done yet.
The late-summer heat in place across the Plains, Midwest and Northeast will quickly be erased early this week as the storm marches into the Plains and Midwest.
The warm air on Sunday allowed for locations such as Bismarck, North Dakota, top out at 86 degrees Fahrenheit. While this temperature was several degrees short of the record of 94 degrees set in 1972, it was still very warm for September. Residents will notice a dramatic 24-hour shift on Monday with a high temperature forecast to dip to a seasonable 71 degrees, as the normal high on Sept. 20 is 72 degrees. Overnight temperatures are also forecast to drop dramatically in contrast to the previous 24-hour low temperature, reaching the lower 40s by Tuesday morning.
A case of weather whiplash will march eastward with this storm throughout the week, as temperatures tumble downwards of 10, 20, even 30 degrees in a matter of 24 hours time. Not only will this storm feature a powerful blast of cool air, but severe thunderstorms and flooding rainfall will also be a pressing concern on Monday.
Prior to the arrival of chilly air, many Midwestern locales can expect rain and thunderstorms to start out the week. Places like Minneapolis, Des Moines, Omaha and Kansas City will all face a threat for some feisty thunderstorm activity Monday and Monday evening.
The cold front will continue to march eastward into the day on Tuesday, shifting the wet weather along with it. Grand Rapids, Michigan, Indianapolis and Evansville, Indiana, could all experience a soaking rain and even face a threat for thunderstorm activity.
Meanwhile, places across the Plains and Midwest that face wet weather on Monday will begin to dry out and turn noticeably cooler as an expansive area of high pressure settles in.
The expected heavy rain in these areas may lead to a difficult stretch of time for area farmers across the eastern Plains and Midwest. As the harvesting of corn, soybeans and many other crops is underway, this round of unsettled conditions will likely lead to some fieldwork and harvesting delays.
By midweek, the advancing cold front may begin to run into a bit of an atmospheric roadblock as it approaches the Northeast and mid-Atlantic states. Another dome of high pressure anchored along the East Coast may be reluctant to move much, forcing the cold front to slow down. The expected slow progression of the front around midweek may further exacerbate the heavy rain and flood potential.
Many flood-weary cities such as Pittsburgh and Morgantown, West Virginia may face a renewed threat for flooding by Wednesday, and depending on the motion of the storm, that threat may extend into Thursday as well.
Another round of heavy rain is the last thing residents along the Interstate-95 corridor want to hear about after the deluge of the summer season, but that may just be the case by Thursday as rain slowly spreads eastward. Once again, places like New York City, Philadelphia, Washington, D.C. and many others may face a threat for flooding rainfall.
It may take until Friday or Saturday for the storm to completely clear out of the Northeast and mid-Atlantic states. Once that occurs, a transition towards cooler weather looks to be in store for many places by next weekend.
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