In Depth: Flash flood threat today

Weather Blog

AUSTIN (KXAN) — The variables that most frequently cause flash flooding in central Texas are coming together as we speak, leading to the possibility of up to 6-7″ of rain in a few isolated locations.

  • Upper-level storm system
    • A powerful “closed low” is moving our way from the west, forecast to move right over Austin late today
  • Deep atmospheric moisture
    • Low-level moisture from the Gulf and upper-level moisture from the Pacific are priming our atmosphere for heavy rain
  • Convergence boundary
    • A slow-moving cold front to our northwest is forecast to move into our area late tonight through the day tomorrow

Rain totals in our area should most commonly reach 1″ to 3″ by the time the storms wrap up late tonight, but may more commonly add up to 2″ to 4″ from Austin northward. A few isolated locations could record up to 6-7″ of rain.

It is very difficult to forecast where these highest rain totals may fall due to small-scale boundary interactions as the storms evolve. 

The front is taking a very slow trek from northwest to southeast across our viewing area through the day today. This slow-moving front will cause heavy thunderstorms to develop over the same areas, potentially for several hours, causing rain totals to add up quickly. 

The rain is falling on already-saturated soil from the very wet April we have had. Camp Mabry has totaled 5.78″ of rain so far this month — more than 4 inches above normal. This above-average spring rainfall fits with what we would expect from the weak El Niño pattern in place.

With these soil conditions, it will only take 2″ to 2.5″ of rain in one hour to cause flash flooding in most areas, or 3″ to 4″ in three hours. Runoff comes more quickly in urban areas due to impervious cover (concrete, etc.), so it may only take 1″ to 1.5″ of rain per hour in Austin to trigger dangerous flash flooding.

The NOAA Weather Prediction Center is placing central Texas in a “slight risk” of excessive rainfall on today. See the following excerpts from their technical discussion below.

“The fairly slow progression of the front, along with strong surface convergence, instability (noted by surface based CAPE >2000 J/kg) and enhanced lift in the mid/upper atmosphere will help sustain widespread convection over this region.” 

“Model trends maintain swaths of 2 to 5+ inches across central/eastern Texas into far southwest Oklahoma, most focusing into the the Austin to Dallas corridor with the highest rainfall maximas.” 

“Based on the expected setup, anticipated rain totals and high rainfall rates will likely be problematic across central Texas.”

“Depending on model trends and changing soil saturation from rains expected late today and on Wednesday, portions of central Texas will likely need to be upgraded to a Moderate Risk as the threat for flash flooding rises.”

Stay with KXAN and the First Warning Weather team for frequent updates as the storms develop. 

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