A week ago, it was looking like the cold front passage that came Sunday would be a quick-moving front, behind which, we’d be graced with the presence of cool, dry air. We got the cool air (record low temperatures were set in several locations on Monday), but not the dry. In fact, moisture from Hurricane Sergio (from the East Pacific) got caught up in the cold front, and now we are in the midst of some historic flooding across the state.
The graphics above show the worst flooding underway in the Hill Country (particularly along the Llano and Colorado Rivers) with significant flooding also underway along the Trinity River in North Texas. A widespread area of more than five inches of rain has fallen from the Hill Country all the way to the Red River, with localized bullseyes of 10+ inches near Llano and 15+ inches near Huntsville. The flooding in the Hill Country is particularly severe. The Hill Country contains very shallow bedrock and steep terrain, making it particularly susceptible to flooding. Infrastructure is already being damaged, and evacuations are underway in parts of the Hill Country.
The FM 2900 bridge in Kingsland over the Llano River has collapsed.
— Avery Tomasco (@averytomascowx) October 16, 2018
So when will it stop? While the rain rates may diminish somewhat over the next few days, unfortunately, it looks like it will be the weekend before we really start to dry out. Upper-level cutoff lows over the Southwest are notoriously stubborn, and usually do not dislodge until another longwave trough comes in and lifts them out. Looking at the long-range guidance, it will be at least this weekend before that happens. As long as this upper-level low remains in place, the stream of Pacific moisture will continue. Before the end of the week, it looks like the areas already impacted will pick up at least another three inches of rain, with more than six additional inches of rain possible in some places. It goes without saying that North and Central Texas are very saturated right now, and any additional rainfall will lead to worsening flood conditions.