Upper-Level Blocking Patterns
Omega Blocking Pattern
Extreme heat has been the story for the end of June and the beginning of July 2021 on the west coast, with some cities breaking over 100° multiple days in a row. One of the reasons for the record-breaking heatwave is an upper-level blocking pattern called the “Omega Block.”
The blocking pattern is caused by a wavy pattern of the jet stream. The jet stream is just a band of strong wind in the upper levels of the atmosphere that can help drive weather patterns at the surface. Because of the pattern, it creates upper-level ridges and troughs associated with high and low-pressure areas. A large dome of high pressure in the upper levels set up across the western U.S. and went as north as Canada. During these types of patterns, you have really dry patterns and light winds. This keeps everything pretty stagnant for a period of time. Add on top of that, that many areas to the west are already extremely dry and under drought conditions. A dry ground will heat up quicker than a ground with any moisture in it.
This type of pattern is also known for warmer than normal temperatures and in this particular case record-breaking temperatures. Cities like: Portland, OR; Salem, OR; Vancouver, WA; Medford, OR; Olympia, WA; and Seattle, WA all broke all-time record high temperatures on June 28, 2021. An all-time record high is the highest temperature recorded in any month of the year within that city. Another notable all-time high temperature was broken in Canada. Lytton, British Columbia reached 121° degrees on June 29, 2021, which breaks the all-time high-temperature record for the country. Compare that to Iowa’s all-time record high temperatures of 118° in Keokuk set back on July 20, 1934.