Thursday, October 25, 2018

More Rain for Texas

Location of Hill Country in Texas
Last week persistent torrential rain fell over the Texas Hill Country north and northwest of San Antonio/Austin causing record and near-record flooding on rivers and lakes in the area. This area is particularly susceptible to flash flooding and is known as Flash Flood Alley. It wasn't just the amount of rain, but the rainfall rates that helped cause the problems this week, as well as a record wet September.

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.
Many of the tributaries to the Colorado River quickly flooded, with all of the water feeding into the Colorado River. The Llano River at Llano came close to setting a new record flood level but fell just short of the record of 41.5 feet set on June 14, 1935. Flood stage is 10 feet. The flooding washed out a highway bridge over the Llano River outside Kingsland, TX on Tuesday, October 16. Here is video of the bridge washing out. Damage to homes and other infrastructure was extensive.

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

Rain is still needed in west Texas and in the panhandle, but much of the state could use a break for a while. Precipitation is much above normal since September 1, more than 20 inches above normal in some places.

Tuesday, October 9, 2018

So Much Weather...

Trying to decide what to write about is a difficult task this week because there is so much to talk about - summer-like heat in the east, hurricanes, snow in the Rockies and Plains, and heavy rain from Texas to Iowa. A number of , so Here is a quick summary of the significant weather events taking place across the U.S.

To start with, here is the latest watch/warning map for the give you an idea of the variety of weather ongoing.

Watch /Warning/Advisory map as of 9:23 p.m. CDT 10/9/2018. The latest map can be found at

Tropical Weather

The major news this week is Hurricane Michael, destined to make landfall along the coast of the Florida Panhandle on Wednesday. Dangerous winds gusting more than 100 mph and catastrophic storm surge are expected as Michael approaches and crosses the Florida coast. As I write this Michael is a strong Category 3 storm and is expected to reach Category 4 before landfall. 

Hurricane Michael as the sun was setting over the eastern Gulf of Mexico at 6:07 p.m. EDT.
Image from

Infrared image of Hurricane Michael at 10:07 p.m. EDT. Image from

Hurricane Leslie has been looping around the Atlantic for 17 days, and reached hurricane again tonight. I it expected to maintain tropical status as it moves east toward Africa this week.

This afternoon, Tropical Storm Nadine in the eastern Atlantic was named and is forecast to move northeast before weakening by Friday.

For the latest information on Hurricane Michael visit the National Hurricane Center web site.

Winter Weather

Snow has been falling on and off in the Rockies the past several days as cold air spilled into the region and a low spun up over the southern Rockies. Snowfall from Monday to Tuesday accumulated from Montana south through Colorado, and from Colorado northeast through the Nebraska panhandle and into the Dakotas. More than 5 inches of snow fell west of Denver in the 24 hour period ending Tuesday morning, and 6 to 8 inches accumulated northeast of Rapid City, SD. Tonight winter weather advisories extend from northern Minnesota south into western Kansas.

24 hour snowfall ending the morning of October 9, 2018.

Heavy Rain

It has been a stormy set up on the boundary of the cold air in the west and warm air in the east, and rain fell this week from Texas northeast to the Canadian border. Rainfall accumulations this week ranged from 7 to 9 inches in parts of Texas, 6 to 8 inches in western Oklahoma, and 7 to 10 inches in eastern Kansas and in the Kansas City metro area.

Big changes are ahead later this week. Hurricane Michael will make landfall, weaken, and move into the Atlantic. Cold air that has been pooling in Canada and the western U.S. will spread to the east and Gulf coasts bringing an end to summer-like weather for most.

Monday, October 1, 2018

CoCoRaHS Observers in Right Place, Right Time in Western NC

When I pull up the CoCoRaHS web site each day one of the first things I check is the national map for any unusually high values that might indicate an error.  On Sunday September 30 the first thing I noticed was that the scale showed a top value over eight inches. There was a red dot over western North Carolina. "Perhaps an observer entered observation time as precip or made a typo entering the amount", I thought. It's a very common error.

Zooming in, it was clear this was no error. The southeast corner of Buncombe County, North Carolina got a very large amount of rain, and heavy rain also fell in surrounding counties.

Rainfall amounts of 1 inch or more from Sunday morning, September 30.
The largest amount of 8.11 inches was recorded by the observer at NC-BC-115, Black Mountain 5.5 SE. The comments entered by the observer helped tell the story of what happened overnight. Comments are very helpful and important in events like this.

What was really interesting was that this was an isolated storm for the most part. Out of 33 CoCoRaHS observers reporting in Buncombe County that morning, 22 reported amounts of 0.10 inch or less, and about half of those reporting zero.

Radar showed a nearly station storm over southeastern Buncombe, western McDowell, and western Rutherford counties. The center of the white circle is the approximate location of NC-BC-115. The first image in the sequence shows two strong cells just to the south. The middle image is three hours later and the heavy rain has been falling for two hours.

Radar images from

The precipitation accumulation map from the NWS Advanced Hydrologic Prediction Service (AHPS)  indicates 6 to 8 inches over southeast Buncombe County, and just how small the area of heavy rain was.

The heavy rain resulted in flash flooding with debris flows, roads and bridges washed out in the affected counties. The National Weather Service issued an Areal Flood Watch at 4:46 a.m. EDT.

It is possible that that more than 8.11" of rain fell in the area near NC-BC-115, but were it not for that observation and that of the observer at NC-RT-6 Chimney Rock 0.3 with 5.01" there would not have been ground truth for this event. Right place, right time.