The forecast for next Wednesday shows a nearly west-to-east (zonal) jet stream flow over the northern US. Forecast courtesy of NOAA.
Weather Pattern Government, which lasted about six weeks and brought flooding to Texas and Oklahoma, has adjourned.
It is succeeded by a much different flow, this one featuring very hot weather in the southern parts of the US.
Decoded Science has named it Weather Pattern Hotzilla.
Weather Patterns In The Middle Latitudes
Meteorologists split the motion of air into three components, two horizontal and one vertical: The vertical component of motion, though it is responsible for precipitation and is crucial to weather, is small compared to the other two.
The horizontal movement of air is split into zonal (west to east) and meridional (north to south) components.
In the mid-latitudes, the dominant component of motion is zonal, with the maximum occurring about 15,000 feet above the surface, where wind speeds are commonly over 100 miles per hour. This is the jet stream.
What Causes The Jet Stream?
It is not obvious, just looking at the sun-earth-atmosphere system, what kind of circulation will develop in the earth’s atmosphere. The earth is a spinning spheroid with its axis of rotation tipped at a 23 degree angle to the plane of its revolution around the sun.
The result is differential heating on three scales: between pole and equator, from day to night, and from winter to summer.
The equations that describe the physics of the atmosphere cannot be solved analytically; there is no simple physical description of what the circulation will be like.
What we can say from long observation is that there is a jet stream, and that the jet stream forms in a small number of characteristic configurations. The overall end result is a transport of energy from the equator to the poles, which keeps the temperature differential between the hottest and coldest places on Earth close to 100 degrees, rather than the 500 degrees that it is on the moon.
Configurations Of The Jet Stream
Meteorologists characterize the jet stream pattern as one of three types: High Latitude Zonal; Low Latitude Zonal; and Cellular Blocking. The type of weather that occurs at the surface is intimately related to the type of jet stream pattern that whizzes above.
The Old Weather Pattern: Government
Weather Pattern Government was a Cellular Blocking pattern. Dips in the jets steam are normally carried along with the flow. When a dip ‘closes off,’ it detaches and moves on its own pace — usually very slowly. When a bulge closes off, it is called an omega block because of its shape. Weather Pattern Government had a closed low over the western United States and omega blocks to the east and west. Cellular Blocking patterns can be dangerous, as Texans saw, because if you’re in the wrong place you could have the same bad weather for quite a while.
Zonal patterns are characterized by steady west-to-east flow in the jet stream, though there can be troughs and ridges traveling through. Normally the troughs and ridges move about 20 miles per hour.
As the names imply, zonal patterns tend to be at either higher or lower latitudes.
Weather Pattern Hotzilla
Hotzilla is a high latitude zonal pattern with very little in the way of waves, so the weather is moderate along the axis of the jet and very hot to the south.The heat currently extends from coast to coast, broken only where clouds and rain along the track of Tropical Storm Bill limit the temperature.
Where Is It Hottest?
Generally the desert southwest and the southeast have been hottest because they have remained mostly cloud-free and the near-solstice sun is very strong. Daily temperature records have been set in the last two days in at least eight states; here are some of the highlights:
- Palm Springs, CA, 116
- Eureka, NV, 96
- Phoenix, AZ, 115
- Raleigh, NC, 98
- Crossville, TN, 89
- Columbia, SC, 101
- Bluefield, WV, 87
- New Bern, NC, 100
In addition to the new daily high temperature records, many more maximum low temperature records have been set. For a couple of reasons, the trend is for a narrower range of temperatures from day to night.
- Airplanes’ exhaust provides condensation nuclei and water vapor for increased high clouds.
- The warmer atmosphere can hold more water vapor.
Clouds at any level block incoming radiation and prevent outgoing radiation from escaping. Water vapor is an important greenhouse gas, and more water vapor in the atmosphere also leads to more clouds.
The result of all this is (relatively) cooler days and warmer nights.
How Long Will Hotzilla Last?
Summer heat waves are hard to displace, especially in the southern part of the United States. Current forecasts for more than a week from now show an unusual divergence of opinion. Some indicate only shallow intrusions of cooler air into the Ohio Valley and the northeast, while others show a deepening trough in the east, perhaps reaching Georgia and the Carolinas.
The weather has been remarkably persistent over the United States in the last two years, and it is hard to bet against the established pattern. But the situation is complicated by conflicting forcing mechanisms in the Pacific Ocean.
El Niño And The Weather Pattern Over The US
El Niño Eggplant is now well established in the Pacific. Normally this would lead to a low-latitude jet stream over the southern US. Weather Pattern Government actually developed out of a temporary zonal flow across the south in late winter.
The sea surface temperature anomaly chart shows the El Nino at the equator and the warm water in the Gulf of Alaska. Analysis courtesy of NOAA.
Conflicting Signals Obscure The Summer Forecast
In addition to El Niño, there is a pool of anomalously warm water in the Gulf of Alaska that tends to force a ridge in the west and a trough in the east. This has led to two cold winters in the eastern United States and has exacerbated the California drought.
It’s not clear which way the jet stream will tilt for the rest of the summer. But for now, Hotzilla rules.
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