AUSTIN -- Why did it flood?
The flood event early Thursday morning produced two to nine inches of rainfall in Travis County, allowing for Onion Creek to reach its highest level in recorded history.
This flooded neighborhoods in Southeast Austin. In the KVUE viewing area alone, rainfall amounts ranged from zero inches over many spots in the Hill Country, to a foot of rainfall in Central Hays County.
So why did it rain so much in some spots, while others were left dry?
To answer this question let’s look at the weather setup for Wednesday night and Thursday morning:
Southeasterly winds at the surface fed moisture inland from the Gulf of Mexico. Meanwhile moisture from what was Hurricane Raymond in the Pacific Ocean was on the increase in the mid-levels of our atmosphere.
The last ingredient was a trough of low pressure moving in from the west. This is what induced lift in our atmosphere, allowing for the available moisture to turn into heavy showers and thunderstorms.
Speaking of moisture, ahead of the flood on Wednesday evening our atmosphere had approximately two inches of available moisture. This is also known as “precipitable water." Precipitable water is the amount of rainfall that would fall if all moisture in the atmosphere was squeezed out.
Forecast computer models ranged from one to five inches of rainfall for this system, a far cry from the 12 inches that actually fell over Hays County. But models work on mathematical calculations. It takes the current setup in the atmosphere, looks at available moisture and spits out a calculation.
Before we curse the computer models for being wrong, they led us to believe that flash flooding could be a high threat 12 hours before the rain even started to fall.
On KVUE News Midday on Wednesday, we showed you a model that produced a line of storms that would not move from west to east, but from southwest to northeast. This is what we call "training."
Just like train cars going over a track, storms would roll over the same area for multiple hours, dropping several inches of rain. This gave us our first indication that the flood threat was growing.
By late evening Wednesday the training of showers and storms started, and the flood threat became real. The meteorological explanation on why the training occurred was because of two things: A weak upper-level disturbance coming from the southwest, and little to no steering currents in our atmosphere ahead of a front that was still in West Texas.
Looking back, the hardest part of the forecast was knowing exactly where that line of training would occur.
Keep this in mind: If the line was 40 miles west of the city, Austin’s rainfall totals would be much smaller and the lakes would be going up. If it was 40 miles east, Austin would be more like the Hill Country and some parts of the city would record very little rainfall.