Last Friday, Decoded Science named Weather Pattern Government and forecast that it would not move much overall for a couple of weeks. After a respite during which the pattern relaxed a little, this weather system has now become even more pronounced, and is affecting the entire United States.
Weather Pattern Government’s Shape
The jet stream is a circumpolar belt of fast-moving wind in the middle of the atmosphere. The winds blow from west to east, with undulations big and small.
These undulations are waves of different lengths that correspond to certain kinds of weather at the surface beneath them. A long wave with a large amplitude can sometimes bring movement of the pattern almost to a standstill (you can guess why we chose the name).
Much of the winter was characterized by a powerful wave with a crest (ridge) in the west and a trough (the opposite of a ridge) in the east. This trough brought a very cold and snowy winter to the northeast and the ridge gave the west record high temperatures.
In April the flow reversed: a trough set up in the west and a ridge in the east. The first phase was Severe Weather Event Jon, which lasted about a week. After a couple of weeks of ‘normal’ weather, the pattern returned and forecasters expected it to be more persistent. Weather Pattern Government was born.
Waves of all lengths travel along in the jet stream flow. Shorter waves move around the periphery of the large troughs and ridges. When last week’s shorter length trough rippled through the plains, it caused an outbreak of tornadoes. As the short wave entered the east coast ridge, the whole pattern flattened a little, but now a new shorter wave has reinforced the long wave, brought rain to California, and threatens the plains and midwest with another round of tornadoes this weekend.
Southern California Rain
Yesterday, 1.63 inches of rain fell in San Diego. Residents along the Gulf Coast wouldn’t even raise an eyebrow at the appearance of such a ‘shower.’ But flooding was widespread in the San Diego area, where drainage is not sufficient to deal with even such a modest amount of rain.
Drainage was insufficient because it doesn’t rain much in San Diego. To realize how unusual this 1.63 inches of rain is, consider:
- Normal rainfall for May is .12 inches.
- Normal rainfall for the three months April, May, June is less than an inch.
- Normal rainfall for the five months April through August is less than what fell in a single day this week.
Snow In The Rockies Today And Saturday, North Dakota Sunday
Though the snow connected with this branch of Government will not be as heavy or widespread as in last week’s storm, which dumped a record 13.5 inches on Sioux Falls, South Dakota, heavy snow will fall in the Mountains today and tomorrow, and North Dakota on Sunday.
Tornado Threat High In The Plains Saturday And Midwest Sunday
The most extreme weather associated with Government this weekend will be severe thunderstorms producing hail and heavy rain, and the likelihood of tornadoes. The areas affected will, unfortunately, be those hit last weekend:
- Today: The central plains from north Texas to Nebraska.
- Saturday: The entire area of the plains from south Texas to the Dakotas.
- Sunday: The midwest, mainly Illinois and Wisconsin.
Though there will be severe thunderstorms today, the threat increases on Saturday because of an influx of drier air at jet stream level. This seems counterintuitive, because precipitation is associated with moisture in the atmosphere. However, the moisture near the surface streaming northward from the Gulf of Mexico is sufficient for copious rain.
Dry Air And Conditional Instability
Consider a column of air that extends from the ground up to ten thousand feet. If a parcel of air is lifted from the ground to the top of this column, it will cool by an amount determined by the decrease in pressure (pressure decreases as you ascend in the atmosphere because there is less air pushing down from above you). If the air is unsaturated, a rising parcel will cool at a rate of about five degrees per thousand feet.
But if the air is saturated, moisture condenses because colder air cannot hold as much water vapor as warmer air.
When water condenses, it releases heat. A rising parcel of air will only cool at about three degrees per thousand feet because of the heat added by the condensation process.
When the air above is dry and the air below is saturated, the top of a lifted column cools more than the bottom. As the process proceeds, the actual decrease of temperature with height increases, and a rising saturated parcel is more likely to find itself warmer than its surroundings and keep rising. This is conditional instability.
Conditional Instability And Thunderstorm Formation
When the air is conditionally unstable, it is waiting for something to lift it. That something can be a cold front or daytime heating. This weekend, the air east of the Rockies will be characterized by saturated air near the ground and a dry flow above — it will be conditionally unstable. The sun’s daytime heating will set off powerful updrafts. Thunderstorms will form throughout the unstable air, and tornadoes are likely where the change of direction of the wind with height begins a spinning motion.
More To Come?
The weather pattern of trough in the west and ridge in the east is now well established. Another short wave is looming to cross the long-wave trough next week with weather very similar to this weekend’s.
Government Affects Everybody
Government reaches all citizens, some more significantly than others. In the southeast, some record high temperatures might be set any time in the next week, perhaps longer. What effects of Government do you see?