When weather patterns become ‘stuck’

It’s no secret that the weather in Omaha this week couldn’t be better for late May. Given the range of weather we can see at this time of year, warm weather, low humidity, sunny skies and light winds are the better sides of the spectrum.

What’s even stranger is that this weather has been around all of last week, and is expected to continue through at least Memorial Weekend. Again, given the Midwest’s tendency to experience rapidly changing weather, this was great.

There’s a reason for that, our weather pattern has been stuck over the past few days, not just in Omaha but across most of the country. This stuck weather pattern has a name, blocking. What is a ban? How to develop? What are the different types? How long do they last?

What is a ban?

Weather around the world behaves like a fluid, moving. The weather is affected by the jet stream, a belt of fast-moving air at the top of the atmosphere. This jet stream generally moves from west to east across North America, and sometimes fluctuates from north to south. Generally, the jet stream rises over the United States during the winter, allowing cold Canadian air to settle; It moves north in Canada during the summer, allowing warm tropical air to move in.


A simplified look at the jet stream across North America. The jet stream forms between a gradient of warm tropical air in the south and cool Canadian air in the north. This fluctuates constantly pulling air masses into the United States.

The jet stream can form a variety of shapes as it crosses North America. Some days it can move straight from west to east, and on other days it bends like waves. These waves can be divided into two regions, troughs where the jet stream dips down, and ridges where the jet stream is pushed upward. Depressions are often associated with low-pressure systems, while hills are associated with high pressure.

Basins and hills.PNG


A simplified look at the troughs (ridges, eastern half) and ridges (ridges, western half) that define the jet stream. These can vary from day to day, and from place to place.

Normally, the jet stream helps push these weather systems across the country, which is why the weather can vary from day to day. However, there are times that can create the right conditions where things stall. This can happen for a number of reasons, but the results are mostly the same as when the weather becomes stagnant. This is known as blocking, where the overall weather pattern becomes skewed and off, causing the same weather to occur over and over again.

Blockages are not rare, occurring several times a year, most notably in the summer months. We saw a crippling pattern in late April, remember that string of warmer days at the end of the month? This was a result of the ban.

“omega block”

While there are multiple types of blocking, the type that dominates our weather in late May and late April is known as the “omega block.” An omega block occurs when a high pressure area builds north across the central part of the country. This high pressure forces the jet stream to bend northward into southern Canada. The generally weak flow as the jet stream extends farther north means it is difficult to push the high pressure away.

Omega Block.PNG


Jet Stream On May 24, notice how the jet stream resembles the Greek letter Omega?

On the sides of high pressure, two low pressure areas are formed in the western and eastern half of the country. This is why the name “omega cluster” exists, since the jet stream forms the shape of the Greek letter Omega.

What does this usually mean for Omaha weather? Well, the same way we felt the last few days. Warmer than average temperatures, dry weather, low humidity, and plenty of sunshine. We are under the control of high pressure responsible for this type of weather.

In western Nebraska, and the rest of the high plains, the story is a little different. Being closer to the low pressure area, these areas see a daily chance of thunderstorms extending from Montana into western Nebraska to the Texas Panhandle.

When will this pattern break? Signs point to next week, perhaps as early as Memorial Day. High pressure will weaken as a low pressure system in the Northeast cuts off the main flow and stalls over the southeastern United States. In the west, a low pressure system over the western United States will make its way east.

Next week.PNG


A look at the jet stream possibly next week, active weather in the south, and drier weather in the north.

This means Omaha can expect an uptick in rain chances next week, as well as continued warm weather.

7 days.PNG


7-day forecast as of May 24. Increased rain chances next week, combined with warmer weather, represent a slight change in the pattern.

(tags translatable) KMTV Weather

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