A “Rex block” is a weather pattern characterized by an upper-level high-pressure system located north of a closed low.

The Rex block is an atypical weather pattern that stubbornly blocks any incoming storms that usually travel on the westerly winds. It is a strong high-pressure system setting up north of an upper-level low-pressure system.

Under the high pressure, warm and sunny weather is experienced. An omega block is so named due to the shape of the jet stream, which looks like the Greek letter omega. ... A Rex block is somewhat similar, but there are key differences, beginning with the name. A Rex block is simply named after the meteorologist who first identified the pattern, Daniel Rex.

Mother Nature is helping us explain an atmospheric phenomenon this week called a “Rex block” pattern with two excellent examples of the pattern across the U.S.. Here’s what it means for the forecast.

The reason why it stays stationary is because the strong ridging north of the low causes air pressure to move from high latitudes to low latitudes, moving vertically, causing little west-to-east movement, moving horizontally. The area near and under the low-pressure system experiences stormy, rainy weather while areas near and under the high-pressure system will experience dry, sunny weather. If a Rex Blocking pattern stays in place too long, it can cause flooding near the low-pressure system and short-term drought under the high-pressure system.

Picture the Greek letter omega in the jet stream. This is a pattern where a ridge of high pressure gets wedged between two low-pressure systems, like a semi truck double-parked on the weather highway.