An atmospheric pattern that remains stagnant for days on end is known as a "blocking pattern." One example of this is a "Rex Block."
A Rex Block happens when high pressure is located north of low pressure, as seen in the image below.
Air flows around high pressure and down to the low pressure, as shown in this image.
That flow results in a transfer of energy from north to south, but very little from west to east. The blue arrows in the following image show the "normal" west-to-east flow across the United States.
During a Rex Block, though, that normal west-to-east flow is not possible.
The Rex Block results in little movement of storms from west to east. Instead, in the example image at the top of this story, the low pressure storm will stay stationary for several days. This pattern eventually breaks, but while it is present, areas seeing nice weather will continue seeing nice weather. Areas seeing stormy weather, on the other hand, will continue being stormy.
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