Blocking the Spring warmth - WNEM TV 5

First Warn 5

Blocking the Spring warmth

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It hasn't exactly been the most typical year so far here in Michigan. The winter season repeatedly offered up above-average temperatures, sometimes coming in 30 degrees warmer than we should have been. Early Spring proved to offer little difference, with temperatures even taking on a Summer-like feel in the upper 70s and low 80s as recently as the end of April!

Yet almost exactly in time with the turn of the calendar to May, we've seen the wheels completely come off. Highs that should easily be reaching the middle and upper 60s have been remanded to little more than the low 50s for much of the month so far. So, why all of the chilly weather, and is this a sign of what the rest of May has to offer?

The Omega Block

What might sound like the title for a low-budget action movie, is actually a significant weather pattern that can develop periodically within the jet stream. When it does, it can result in massive deviations from normal in everything from temperature to precipitation, wind patterns, and overall storm tracks. 

As part of its name implies, an Omega Block pattern blocks the normal progression of our weather and depending on where the feature sets up shop, it can be great news for some, and very bad news for others.

It's important to note that the jet stream serves two key roles in how our day-to-day weather behaves. This high-speed river of air serves as the primary driving force for moving storm systems and regions of fair weather around our planet, and occurs in both hemispheres. At the same time, the jet stream also serves as a barrier between cold polar air and the milder air of the mid-latitudes and tropics.

So, how do we get a blocking pattern in the jet stream, and where does the Omega come into all of this? Well sadly, it isn't added to make the pattern sound more intimidating. It actually has to do with what the jet stream looks like when drawn on a map.

At the center of the pattern, the jet stream retreats suddenly northward in an upside-down 'U' shape. To the immediate east and west of this ridge, the jet usually levels out into a pair of depressions. When viewed as a whole, the orientation of the jet stream resembles the Greek capital letter, Omega (Ω).

Why is this Significant?

The basic ingredient in an Omega Block pattern may sound a bit more familiar; a ridge of high pressure. Generally characterized by quiet, warm weather, high pressure ridges can cause warm air to surge much farther north than it otherwise would. As the ridge deepens, it becomes harder and harder to move and can become stuck in place.

To compensate, the jet stream curls up around the base of the ridge on either side, forming troughs of low pressure. These regions behave opposite the ridge, allowing colder air to spill down from the poles and generating stormier conditions overall. Storm systems steered around the periphery of the ridge can also become stuck here.

These troughs working in tandem with the ridge create the Omega pattern, which can linger in place for days or even weeks. The pattern breaks when either the ridge weakens, or one of the surrounding troughs intensifies enough to overpower and break the ridge.

What Does this Mean for Us?

Since the beginning of May, we have become more and more trapped within an Omega Block that has developed across the continental United States. Unfortunately for us, the high pressure ridge has set up almost directly over the central United States, leaving us stuck in the eastern trough of low pressure.

The storm system that left Mid-Michigan split between chilly rain and mild sun late last week took so long to move away due to this blocking pattern. Even though we shook the rain off over the weekend, we remain in the chilly grip of abnormally cold air being pulled south from the Arctic by the trough. Those below-average readings have in turn resulted in our repeated bouts with frost and freeze conditions.

Will the Omega Block linger through May?

If you're hoping to get back to weather more appropriate for the month of May, you're in luck! The catch is, you will need to be patient. In the near term, our First Warn 5 7-Day Forecast shows a trend back into the low 60s during the second half of the work week, combined with a little in the way of rain. We will still be running slightly below average, as highs should be reaching the middle and upper 60s. Still, it's better than 40s and 50s.

Looking a bit farther down the road, forecast trends suggest that we will tend to run slightly below average through the middle of the month. This does not necessarily mean 50s or worse, though.

Looking out into the second half of the month, there is hope for a more substantial warmup! Long-range trends point to the blocking pattern finally breaking down, allowing the warm air trapped in the nation's midsection to spill east. We'll continue to monitor these trends for you in the coming weeks!

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