Unless you’ve been trapped in the depths of a D.C. Metro underground station by inoperative escalators and elevators, you know the several consecutive days last week of positively delightful spring weather has been replaced over the weekend by less than totally pleasant conditions this week.

In his post last week Jason asked “what’s the cause of this lovely stretch of San Diego-like weather in Washington, D.C.?” He answered “we can thank an atmospheric pattern known as an omega block”. The question now is what’s the cause of the prolonged stretch of less than perfect (icky, lousy, unsettled, crumby, etc.) conditions this week? Perhaps surprisingly, the answer is the same: an omega block. How can that be?

An omega block is characterized by a north/south oriented almost stationary ridge of high pressure which disrupts (“blocks”) the prevailing westerly wind flow (think jet stream) in mid-latitudes. The area of tranquil high pressure generally sits between two areas of stormy low pressure regions. As Jason described last week much of the eastern U.S., including D.C., lay to east of the high pressure system where northerly winds kept warm, moist air at bay.

The evolution of the flow pattern between last Wednesday (5/11) and today (5/17) (NOAA)

In an extremely rare sequence of events heading into the weekend, the blocking high and flanking low pressure systems retrograded (i.e. redeveloped) westward. The high pressure’s push to the west allowed southerly winds to transport moist unstable air, conducive to rain and thunderstorms, into the local region. So the region is now stuck with stormy low pressure rather than tranquil high pressure. These “blocky” patterns are stubborn to break down once they generate.

Complex flow pattern this Friday as the high and low pressure systems retrograde.

This image above, which shows the block in transition last Friday, is as complex and unusual a circulation pattern one will ever see. Notice how different the flow is in the image above compared tothe average May flow (shown immediately below). Instead of the more typical west to east (or zonal) flow, the flow is much more north to south or south to north (or meriodonal).

Average mid-May flow (NOAA)

The physics and dynamics of developments like this are a major unsolved problem in meteorological science. I’d argue this is another of example of extreme weather we’ve seen so much of over the last several years. I dare not tread (at this time) into the morass of verbiage on whether these instances of extreme weather are related to climate change.