A blocking pattern is a situation in which troughs and/or ridges (the upper level flow pattern) become fixed in place. It can cause weather to repeat for several days. In a way it makes forecasting easier since the position of the flow pattern stays fairly fixed. Depending on where a location is relative to the troughs and/or ridges, long periods of dry weather or wet weather can occur at a location. The Rex Block is a situation in which a large ridge in the western U.S. is locked into place and squeezed on two sides by a cut-off low to the south and a trough to the west/north. This can bring many days of dry weather under the ridge. The cut-off low prevents the ridge from progressing to the east and the flow pattern of the jet stream north and west of the ridge is such that it has difficulty progressing the ridge also. The flow pattern of the jet stream can be forced around the ridge in a backward S-shape. The image below is of a developing Rex Block. The features to notice are the ridge over the west coast, the cut-off low over northern Mexico, the right side of the trough over the Pacific Ocean, and the flow pattern of the upper level wind wrapping around this pattern in a backward S-shape. Rex Blocks can have varying strengths and the positions of the features can vary although the overall look will be similar to the image. In a stronger Rex Block, a greater amount of flow will wrap around the cut-off low.  |