WeatherWeather Blog

Actions

When Weather Patterns Get "Stuck"

What is blocking? And how does it affect our weather?
Posted at 3:08 PM, May 24, 2023

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

What's more unusual, is this weather has been around for the past week, and is expected to continue through at least Memorial weekend. Once again, given the tendency for the Midwest to experience rapidly changing weather, this has been wonderful.

There is a reason for this, our weather pattern has become stuck over the last few days, not only in Omaha but across much of the country. This stuck weather pattern has a name, blocking. What is blocking? How does it develop? What are the different types? And how long do they last?

WHAT IS BLOCKING?

Weather across the world acts much like a fluid, and it moves. Weather is driven by the jet stream, a belt of fast moving air high up in the atmosphere. This jet stream moves generally west to east across North America, fluctuating from north to south at times. Generally speaking, the jet stream sets up over the United States during the winter, allowing cold Canadian air to settle in; and moves north in Canada during the summer, allowing the warm tropical air to move in.

Jet Stream.PNG
Simplified look at the Jet Stream across North America. The jet stream forms between the gradient of warm, tropical air to the south and cold, Canadian air to the north. This fluctuates constantly pulling in both 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 west-to-east, others it curves 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 up. Troughs are often associated with low pressure systems, while ridges are associated with high pressure.

Troughs and Ridges.PNG
Simplified look at the troughs (dips, eastern half) and ridges (humps, western half) which define the jet stream. These can vary from day-to-day, location-to-location.

Usually, the jet stream helps to propel these weather systems across the country, which is typically why our weather can vary from day to day. However, there are times when the right conditions can set up where things grind to a halt. This can happen for a variety of reasons, but the results are mostly the same where the weather becomes stagnant. This is known as blocking, where the overall weather pattern becomes misaligned and stuck, leading to the same weather happening again and again.

Blocking is not rare, it happens several times a year, most notably in the summer months. We experienced a blocking pattern in late April, remember that string of warmer days at the end of the month? That was a result of blocking.

THE "OMEGA BLOCK"

While there are multiple types of blocking, the one which had dominated our weather for late May and late April is known as an "omega block". An omega block occurs when an area of high-pressure builds northward through the central part of the country. This high-pressure forces the jet stream to bend far north into southern Canada. The generally weak flow with the jet stream being stretched that far north means it is difficult to push the high-pressure away.

Omega Block.PNG
The jet stream on May 24, notice how the jet stream resembles the greek letter Omega?

On the flanks of the high-pressure, two areas of low-pressure build over the western and eastern half of the country. This is the reason the name "omega block" exists, as the jet stream makes the shape of the Greek letter Omega.

What does this typically mean for Omaha weather? Well, the same we have been feeling the past few days. Warmer than average temperatures, dry weather, low humidity, and lots of sunshine. We are under the dominance of the high-pressure, which is responsible for this kind of weather.

In western Nebraska, and the rest of the High Plains, the story is slightly different. Being closer to the area of low-pressure, these zones see a daily chance of thunderstorms stretching from Montana to western Nebraska to the Texas Panhandle.

When will this pattern break down? Signs point to next week, possibly as early as Memorial Day. The high-pressure will weaken as a low-pressure system in the northeast breaks off the main flow and stalls over the southeast United States. To the west, the low-pressure system over the western US will work its way eastward.

Next Week.PNG
A look at the jet stream possibly into next week, active weather to the south, drier weather to the north.

This means that Omaha could expect a slight uptick in rain chances next week, as well as the warm weather to continue.

7 Day.PNG
7 day forecast as of May 24. The increased rain chances next week, plus the warmer weather, represents the small pattern change.