Location of Hill Country in Texas |
The heaviest rain fell between October 14 and October 16, with lighter rain the remainder of the week. In those three days more than a foot of rain fell in the Hill Country, with 7 to 10 inches of the rain falling in the 24-hours ending at 7:00 a.m. on October 16. The CoCoRaHS stations shown below in Llano and Mason Counties tallied the highest totals during the three-day period and for the week.
The persistent rain last week over Hill Country and central Texas was caused by what is called an "overrunning" pattern. In this case a cold front, marking the leading edge of a large cold air mass that spread across the western two-thirds of the U.S., lie along the Texas coast. Warm, moisture-laden air from the Gulf rode up and over the dense, cold air in the lower layers of the atmosphere producing clouds and rain. In addition, several waves of low pressure moved along this frontal boundary enhancing the upward motion and the flow of air from the Gulf.
Surface weather map for Monday, October 15 at 10:00 a.m. CDT. |
Here is the upper air sounding from Corpus Christi, TX the evening of October 15. You can see the cold layer of air near the surface and northerly winds, and the southerly winds aloft along with the warmer, saturated air. During the winter an overrunning setup like this could produce significant freezing rain.
Atmospheric sounding plot (Skew-T) for Corpus Christie, TX at 7:00 p.m. CDT October 15. The solid red line is the temperature plot, the solid green line is the dew point. |
A wet September and rain earlier in October meant that any rain last week was going to run off into the rivers and streams. The rise on rivers in the Hill Country was spectacular, rising 10 feet or more in just a few hours. These tributaries eventually feed into the Colorado River (the Texas Colorado River, not the Colorado River). The Colorado River flows southeast through a series of lakes used for flood control and recreation (Buchanan, Inks, LBJ, Marble Falls, Travis, and Austin) and eventually to the Gulf of Mexico.
Watershed of the Texas Colorado River. |
Hydrographs for rivers in Hill Country during the flooding, compiled by NWS San Antonio/Austin. |
Lakes Buchanan, Inks, LBJ, Marble Falls and Travis were closed to recreational use last week and were expected to remain closed until at least until today. On Monday, a boil order was issued by Austin Water for all of its more than one million customers due to elevated levels of silt from the flooding. The boil order is expected to be in effect until at least Sunday.
This week the remnants of Hurricane Willa, which made landfall on the west coast of Mexico on Tuesday, interacted with an upper level trough moving through central Texas triggering additional rain yesterday. The heaviest amounts fell between Galveston and Houston where 2 to more than 5 inches fell. Much of the Dallas/Fort Worth metroplex received 1 to 3 inches of rain. Most reservoirs in central Texas are now near or over capacity.
Texas reservoir status as of October 25, 2018. Source: Texas Water Development Board |