As Decoded Science predicted last Sunday, the severe weather system named Aardvark persisted most of this week, after it became associated with a cutoff low pressure swirl in the middle levels of the atmosphere.
It is instructive to look at how Aardvark formed, fumed, and finally fizzled – and then to ask if this is a harbinger of things to come.
Aardvark Was So Destructive Because He Moved So Slowly
Aardvark was characterized by both severity and longevity. The four-day event was the result of a formation in the jet stream known as a cutoff low. A cutoff low develops when a wave in the jet stream curls all the way around and spins into a separate eddy that has a life of its own.
Normally such features are reluctant to move, and Aardvark certainly took his not-so-sweet time. Cutoff lows are not particularly uncommon, but they normally occur over the open ocean, where storms become deep, vertical low pressure systems. A cutoff low over the central United States is rather rare.
The First Stage Of Aardvark: Tornadoes, Hail, And Lightning
As the cutoff low associated with Aardvark crossed the Rocky Mountains, the stage was set for violent weather. Previously starved for moisture, the system took advantage of a humid southerly flow from the Gulf of Mexico.
Above this, as the cutoff low moved east, was a dry westerly wind. The combination of moist air overlain by dry, and a twisting of the wind with height, is a lethal combination, meteorologically. The air is unstable, meaning that any lifting creates a general overturning, with violent up and downdrafts; thunderstorms with lightning and hail erupt. The turning of the wind with height (wind shear) causes individual circulations to spin off — tornadoes.
These conditions persisted through Sunday and Monday, and set off tornadoes as powerful as EF4 (winds up to 200 miles per hour). Dozens of people were killed (37 confirmed as of Thursday night) and thousands of homes and businesses were leveled. And then Aardvark changed his mood.
The Second Stage Of Aardvark: Massive Flooding
By Tuesday, the cutoff low had undergone a minor, but significant, change. The winds at jet stream level became more aligned with the wind near the ground. There was still shear in the form of a change of wind speed with height, but the change of direction was not so prominent. Most of the action was confined to thunderstorms, rather than tornadoes. But now, with all the winds aligned and Aardvark still mostly grazing over the same pastures for long periods of time, flooding was massive.
A line of thunderstorms normally moves with a substantial component of motion perpendicular to the line. The storms pass over; the deluges move on, and everyone gets a substantial, but manageable, amount of rain. When the motion of the precipitation is parallel to the line of storms itself, a condition called training, huge amounts of rain can be deposited in the unfortunately-located places. On Tuesday and Wednesday, trains occurred from the Gulf coast to New England. Significant flooding was reported in Baltimore and Philadelphia.
The unquestioned champion of Aardvark’s rain competition was the Gulf coast of Alabama and the Florida panhandle. A line of storms trained over Pensacola, FL on Tuesday and dumped over twenty-two inches of rain, including an amazing 5.6 inches in a single hour. To understand just how much rain that is, consider the following comparisons:
- It normally takes Pensacola, one of the wetter cities in the lower 48, about four moths to measure 22 inches of rain and about a month to receive five inches.
- Los Angeles has had less than 22 inches of rain in the past two and a half YEARS.
Will There Be More Aardvarks?
Decoded Science is retiring the name Aardvark as his remnants move out to sea, but there may be more severe weather ahead: May is the height of tornado season. The cold front associated with Aardvark will stall over the Florida peninsula, and there is still the possibility of thunderstorms with minor flooding — even a small tornado — in the area around the front through Saturday. However, Aardvark himself has left the building.
The next outbreak of wild weather will be named Beaver; this animal is not yet clearly in sight, but there are indications that another dip in the jet stream will reach tornado alley by the middle of next week. Warm, humid air is poised to move north again from the Gulf of Mexico, and the stage will be set for another round of severe weather.