What is an upper-level blocking pattern?

 The last week of May will feel more summer-like as temperatures climb into the mid and upper 80s Tuesday and Wednesday, and then in the upper 70s/80s for the rest of the week. The warmup, the result of an upper-level blocking pattern that'll hold strong through the weekend. You've likely heard us talk about blocking patterns, but what exactly are they?

An upper-level blocking pattern occurs when either high pressure or low pressure develop over an area, preventing other weather systems from moving through - hence a block. Blocking patterns can last for several days which can often result in a persistent weather pattern for those locations that end up under the block. For example, blocking high pressure in the summer months can lead to warm -or hot- and dry conditions for many days in a row. There are several blocking patterns that can occur, but the two meteorologists often reference the most are the omega block and rex block.

Omega Block:

An omega block often occurs in the summer and is responsible for hot and dry conditions that tend to lead to heat waves as precipitation usually occurs around the outer edges of the blocking high. This blocking pattern gets its name because it resembles the Greek letter Ω.

The higher pressure aloft will cause the air to sink, warming temperatures near the surface. The sinking motion also prevents any air from rising, limiting the chance for any precipitation to develop underneath the ridge. This is why these types of blocking patterns can worsen drought conditions, while at the same time cause temperatures to soar. Often times heat and drought will go hand-in-hand. Overtime, the blocking pattern will start to break down as smaller scale disturbances move over top of the ridge, causing it to weaken and eventually breaking it down.

Rex Block:

Rex blocks occur when high-pressure becomes locked in place from low-pressure to the south. The cut-off nature of the low prevents the high from moving and typically the jet stream winds to the west are not strong enough to initially move the high itself. These types of patterns will hold until one of the pressure systems changes intensity, causing the high and low to become unbalanced.

Dry conditions can be found near the blocking high while an unsettled pattern will occur near the low.