High pressure to bring Austin familiar summer dryness as Highland Lakes fill again

Central Texas will see a return to typical summer weather after an unusually wet and stormy start to July. It’s rare to feel excited when a ridge of high atmospheric pressure is over Texas during the summer because it’s often accompanied by a heat dome generating extreme temperatures.

This high pressure system this week, though, will bring an end to the daily torrential downpours recently in the Hill Country. Many places across the region have received more than a foot of rain since the beginning of the month and they desperately need a break. Since July 1, Camp Mabry in Austin has recorded about 5.7 inches of rain, which is more than double our normal July average of about 2 inches for the whole month.

High pressure is moving in, bringing sinking air that will reduce both cloud formation and rain chances. However, we won’t experience the typical warming that usually accompanies high atmospheric pressure. When the ground is soaked from frequent rain, the sun’s energy is used first to evaporate the water, rather than warming the ground. This evaporation process cools the ground, which in turn lowers the air temperature at the Earth's surface, much like how sweat cools our body when it evaporates.

This is amazing news as we head into the middle of July with morning lows in the mid- to upper 70s and afternoon highs reaching only the lower to mid-90s for the rest of the week. However, don’t get too excited. The extra moisture in the soil, combined with southerly tropical winds coming from the Gulf of Mexico, will keep humidity levels in Austin high, making those lower 90s feel more like the upper 90s to near 100 degrees.

The one uncertainty in this weekend’s forecast is the tropical low pressure system in the northern Gulf of Mexico, which has a moderate chance of developing into a tropical depression or even Tropical Storm Dexter by Friday. Forecast models so far suggest the system will stay to the east, soaking Louisiana and Mississippi with downpours. However, a few coastal showers moving inland remain possible on Friday, primarily east of Interstate 35.

Highland Lakes Level Update

Flood-control operations have ended along the Highland Lakes, with the last of the flood gates closing on Wednesday, according to the Lower Colorado River Authority, which manages the six dam-created lakes along the Colorado River. This marks the end of the river flooding threat across the region.

Lake Buchanan, the uppermost reservoir among the Highland Lakes, has risen 20 feet since July 1 and is now 100% full, up from 61% last month. Lake Travis, the largest and most prominent lake in the Austin metro area, is 87% full, compared to 45% last month, having risen 35 feet since July 1.

According to KEYE-TV meteorologist Avery Tomasco, the amount of rain collected by the Highland Lakes is equivalent to filling Lady Bird Lake in downtown Austin 100 times over.

Today in weather history

Did you know the Guadalupe River flooded almost 40 years ago in the same location as this year’s historic flood? Overnight on July 16-17, 1987, strong thunderstorms brought heavy rainfall after the convergence of a mid-atmosphere low pressure system, a weak cold front and abundant Gulf moisture. Nearly a foot of rain fell just west of Hunt in Kerr County, causing a massive flood wave that flowed down the Guadalupe River through Ingram, Kerrville, and eventually Comfort on the morning of July 17.

“At the peak of the flood, the Guadalupe River was estimated to be two-thirds of a mile from its normal bank near Comfort, Texas, and hundreds of people had to be evacuated,” according to an account by the regional National Weather Service office for Austin and San Antonio. “The 1987 Guadalupe Flood is unfortunately known for the tragic loss of 10 teenagers and 33 injuries when a bus and van leaving a church camp encountered the floodwaters.”