Spring brings increasing atmospheric instability. Vestiges of winter combine with a threat of tornadoes to make spring a meteorological many-headed monster — especially this weekend and into next week.
Hydra Raises Its Wild Weather Head In South And Central US
After a short break following Severe Weather System Jon a couple of weeks ago, warm air from the Gulf of Mexico returned to the south this week.
Another pulse of energy in the jet stream will bring increasing chances of lightning, hail, and tornadoes today and tomorrow, especially in eastern Texas and the northern Gulf states, though tornadoes could occur anywhere east of the Rockies and west of the Appalachians.
And just when the first area of instability moves off the east coast, another head of Hydra will appear as a new wave in the jet stream and bring more severe weather to the same areas beginning Sunday.
A Lethal Combination: Instability; Jet Stream Wave; Warm, Humid Air
Spring brings the combination of conditions that leads to severe weather. As the ground heats up from the longer days, conduction transfers some of the warmth to the lower atmosphere. This increases the lapse rate, the decrease of temperature with height.
Normally, a lifted ‘parcel’ of air, which cools as it rises due to the decrease in pressure, finds itself colder than its surroundings and wants to sink back to where it came from. The air is stable.
However, if the lower levels heat sufficiently, a lifted ‘parcel’ of air will find itself warmer than its surroundings and want to keep rising. The air is unstable; instability leads to powerful rising and falling currents when there is a source of lifting.
What Are The Lifting Mechanisms?
There are two primary causes for rising air: Daytime solar heating; Frontal boundary movement.
When the sun heats the earth during spring and summer days, the lower atmosphere can become warm enough to create instability. Daytime thunderstorms are common over most land areas. The maximum of activity comes in the late afternoon.
A front is a boundary between colder, denser air and warmer, lighter air. Sometimes the cold air wedges beneath the warm (a cold front) and sometimes the warm air rides up over the cold (a warm front). Either way, the warm air is forced to rise.
When a front interacts with daytime heating in a humid air mass, severe weather is very likely to occur. Severe Weather System Jon produced flooding, lightning, hail, and, at its most damaging, an EF4 tornado with winds of 200 miles per hour in Illinois.
Hydra’s Severe Weather Threat
All the pieces are in place for another long-lasting bout of dangerous weather in the next several days. Very warm and humid air from the Gulf of Mexico covers the southeast as well as the south and central plains.
A pronounced dip in the jet stream has reached the southern and central plains and will move slowly eastward.
Seasonal sunshine is approaching its summer peak, and with the upper levels of the atmosphere slower to warm up, lapse rates are steepest at this time of year. Everything is in place for Hydra to produce extreme weather where the jet stream wave, warm air, and daytime heating intersect.
The Greatest Chances Of Tornadoes
The tornado danger is highest today in tornado alley across north Texas, Oklahoma, and Missouri. Tomorrow, the maximum risk is a little farther east. And later in the weekend, tornadoes could occur in the southeast or east coast.
Hydra’s Winter Head: Snow And Cold
The complex shape of the jet stream is allowing very cold air to invade the upper midwest and northeast at the same time as the warm air covers much of the south and central sections of the country.
Normally waves push eastward at a steady pace, and typically if there is a dip in the plains there would be a corresponding ridge in the east.
However, sometimes dips in the jet stream spin into closed low pressure centers and cut themselves off from the main flow. The dip (trough) in the center of the country is cut off from the flow, and another trough has pushed down into the northern tier of eastern states.
Snow has fallen in Wisconsin, Michigan, Ohio, Pennsylvania, and New York in the last few days, and is now pushing into northern New England.
Amounts are light, but most residents would agree that enough is enough.
Last night freezing temperatures dipped into the lower Ohio Valley. There are freeze warnings in effect for tonight for large parts of the east and midwest. Freeze warnings are normally issued for freezes unusually late in the spring when the growing season has begun and the low temperatures could affect crops.
More Severe Weather To Come
Hydra won’t quit when the first jet stream wave ripples to the east coast on Sunday. Another wave right on its heels will affect the southern plains even before the first outbreak of severe weather has departed the southeast coast.
Longer range forecasts indicate that a similar jet stream pattern will persist across the south for at least two weeks.