One of multiple types of blocking patterns, the omega block features a low/high/low arrangement from west to east. This leads to persistently cool & wet conditions where the upper lows are situated while warm & dry air prevails in the region under the influence of the ridge sandwiched between the two.
The jet stream takes on an amplified or meridional flow in such a regime, presenting a more north-south component as opposed to the typical west-east disposition. This meandering structure can often result in slow-moving and stalling systems whereas a more zonal flow often results in a brisk & progressive pace to the action. Sensible weather is frequently on the incessant side in the affected regions, albeit glaringly juxtaposed between them.
Blocking patterns can be the equivalent of dropping a stone into a moving stream and forcing the flow to be curbed as it finds a path around the obstruction. They can also be stubborn to fully decay, frequently lasting multiple days or even occasionally weeks at a time in some extreme cases.
This is a graphical example:
The moniker comes from the pattern’s overall resemblance to the Greek letter omega: