Today I am going to discuss blocking and wave patterns in the atmosphere. We sometimes find ourselves stuck in one weather pattern or another which can last for days, weeks or even months. These patterns can bring cool and wet or hot and dry conditions which we try to forecast months out but often fail because of the size of the global patterns.
Open Waves
Whether longwave or shortwave, by far the most common pattern seen in upper air charts are just plain troughs and ridges. These waves and troughs are considered ‘open’ as, for the most part, there is no closed circulation associated with the waves.
They are progressive meaning they move from west to east. Low-pressure troughs are identified by brown dashed lines while ridges of high pressure are identified by brown zigzag lines.
The majority of inclement weather occurs between the trough and the downwind (eastward) ridge while fair weather occurs between the ridge and the downwind trough.
Positive Tilted Troughs
A trough’s axis is usually not directly in line from north to south but have some tilt relative to the poles.
Positive tilted troughs will extend from the lowest pressure northeast to southwest in the Northern Hemisphere (southeast to northwest in the Southern Hemisphere).
In respect to severe weather, positive tilted troughs produce the least amount.
Negative Tilted Troughs
Negative tilted troughs usually begin as positive tiled troughs. As the short-wave energy races east though the longwave it distorts its shape from positive to neutral (north-south) orientation to a negative (northwest to southeast) orientation.
These types of troughs produce the most severe weather. This is because there is strong southerly surface wind with its warm air underneath the incoming cold air in the upper atmosphere creating unstable conditions.
Also, there is a large change in wind direction from the surface into the upper atmosphere (called wind shear) which aids in the formation of supercell thunderstorms. In this example, the New England states would be under a threat of severe weather.
Zonal Flow
When the air flow is parallel (or nearly parallel) to the latitude lines then it is considered to be a zonal flow. Surface level storm systems, and associated cold fronts, move very fast from west to east in zonal flows but have very little north to south (or south to north) movement.
As a result, locations to the pole-ward of a zonal flow will remain cool or cold, while equator-ward, the weather remains mild or warm. Usually there is a positively and negatively tilted trough at each end of zonal flow.
Cut-off Low
“Cut-off low, weatherman’s woe.” These are persistent low-pressure areas that have become isolated or ‘cut-off’ from the main airflow.
They usually result from a strong short wave moving south on the west side of troughs that extended the trough toward the equator. The momentum of the short wave pulls the trough out of the main airflow and forms a closed, low pressure, circulation.
The ‘woe’ comes for their agonizingly slow motion as they can drift for many days. While modern weather computer models forecast their drift rather well they still tend to forecast the closed low pressure to ‘open up’ and rejoin the main airflow aloft too quickly.
They can occur any time of the year and just about anywhere on the planet. Unsettled weather occurs over the eastern half of cut-off lows though there can be some precipitation wrapping around the north end of the low affecting the northwest quadrant.
Blocking Patterns
Blocking patterns occurs when centers of high pressure and/or low pressure set up over a region in such a way that they prevent other weather systems from moving through. When the blocking pattern is in place other systems are forced to go around it. Blocking patterns can remain in place for several days, resulting in long spans of persistent weather for locations under the block.
Blocking High
Typically, a summertime occurrence, blocking highs are responsible for major heat waves. Any precipitation is usually shunted around the periphery of the high-pressure area.
High pressure aloft causes the air to subside or sink. This downward motion compresses and warms the air in the lower atmosphere while simultaneously trapping heat rising from the earth’s surface, leading to heat waves.
The skies are usually clear due to the downward motion of air. Eventually blocking highs will weaken when a short wave moves over the top of the high causing it to decrease with an end to the heat wave.
Omega Block
Omega blocks get their name because the upper air pattern looks like the Greek letter omega (Ω). Omega blocks are a combination of two cutoff lows with one blocking high sandwiched between them.
Because of their size, Omega blocks are often quite persistent and can lead to flooding and drought conditions depending upon one’s location under the pattern. Cooler temperatures and precipitation accompany the lows while warm and clear conditions prevail under the high.
Rex Block
This is the time of year we have expectations of warm weather which would cause humanity in Michigan to have a hankering to flock to our copious coasts and inland lakes to play in the water – not so much this year thus far. Below are the current Great Lakes water temps.
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Chances of rain increase as we go through out the day – however we will be increasing our time between wet and dry periods this week. After two full days of sunshine we will see increasing clouds today with chances of rain later this afternoon through tomorrow evening. After this system moves through we should happily welcome three full days of sunny skies and warmer temperatures. Do I hear a yippee from all the bloggers out there?
Seven Day Forecast
Pollen Forecast
5 months in a row and counting! So far June is well below normal temps!
This breezy day is helping out the farmers dry out there fields. How many weeks are they behind in planting this season versus others ?
https://www.accuweather.com/en/weather-news/more-weather-woes-for-farmers-there-will-be-a-lot-of-acres-not-planted/70008442?utm_source=facebook&utm_medium=social&utm_term=&utm_content=&utm_campaign=accuweather
Yeah it’s not good. The farmer that does our fields is almost a month behind on most of his planting. I did notice today he was out planting some of the fields, but being the 4th of June is pretty late. Best thing to hope for now is an extended growing season into later September or October. Odds are though July or August is probably going to end up super hot and or dry, then that will just make things worse.
WOOD now has four consecutive days in the 80’s in their forecast!
Good post MV a lot of good information. It looks like we are store for a week or more of “don’t have to” weather that is don’t have to have the heat on and don’t have to have the air on. The truth is much of Michigan has a lot of don’t have to weather in the summer time into the start of the fall season. Right now hazy blue skies (still smoke from the fires?) with a temperature here of 54° it is also 54 at the airport.
Slim