Tropical Depression Bill Still Posing Major Flood Risk

Although it was downgraded to tropical-depression status by the National Hurricane Center at 1:00 am Wednesday, the system known as Bill remains a serious threat for flood-producing rain over a belt extending through central Texas across eastern Oklahoma and into the Midwest. NHC issued its final advisory on Bill at 4:00 am CDT, when the center of circulation was located about 45 miles south of Waco, TX. Top winds were at 35 mph, and Bill was moving north at 13 mph with a minimum central pressure of 1000 mb. National Weather Service radar showed the center of Bill approaching the Dallas-Fort Worth area at 10:00 am CDT, with a large shield of moderate to occasionally heavy rain north of its center. Responsibility for monitoring TD Bill has been handed from NHC to the NOAA’s Weather Prediction Center, which is monitoring Bill’s rainfall and the resulting flood threat.


Figure 1. Radar display from WU’s Storm app at 10:05 am CDT Wednesday, June 17.

A persistent inflow band extended from Bill’s center to the Texas coast, with large rainfall totals accumulating between Corpus Christi and Houston (see Figure 2). NOAA/WPC warned on Wednesday morning that storm totals of a foot of rain could occur beneath the band extending south from Bill to the coastline. This morning, one rain gauge near El Campo, TX, maintained by the Lower Colorado River Authority reported 11.40” over the past 24 hours. “These rainfall amounts would produce flash flooding in an average year...but flash flooding during this event may be very widespread and particularly dangerous given the super-saturated soils in parts of Texas,” noted WPC in a 9:23 am CDT advisory.


Figure 2. 24-hour rainfall estimates from 7:00 am CDT Tuesday, June 16, through Wednesday, June 17, derived from raingauges and radar data and supplemented by satellite information. Image source: NOAA/NWS Advanced Hydrologic Prediction Service.

Tonight: Red River under the gun
As Bill continues sliding northward, it’s expected to maintain its identity as a coherent low-pressure center for at least the next day or so. Some models continue to suggest that Bill could even restrengthen slightly tonight over southeast Oklahoma, a possibility raised in recent studies of the “brown ocean effect.” As discussed in our post on Monday, there’s an increasing amount of research on the process by which a tropical cyclone can re-intensify well inland if soils are sufficiently moist. Both Texas and Oklahoma saw the wettest month in their weather histories during May, and soils remain saturated across much of the area. Since the brown-ocean reintensification effect is a rare beast, we might not expect forecast models to latch onto it. The 1200 GMT Wednesday run of the high-resolution HRRR short-range model suggested a slight deepening of Bill’s central pressure late this evening near the Texas-Oklahoma border, while the longer-range NAM model from 1200 GMT Wednesday indicates that Bill will at least maintain its current level of organization into Thursday, when the system is predicted to slow to a crawl over southeast Oklahoma. Eventually, Bill will be get pulled into an existing frontal boundary and should begin to lose its definition near southern Missouri by Saturday.


Figure 3. Projected three-day precipitation totals from the NOAA Weather Prediction Center, for the period from 1200 GMT June 17 to June 20, show the expected track of Bill around the high-pressure center in the southeast U.S. Image credit: NWS/WPC.

The NAM model predicts widespread 5” – 10” rainfall amounts from the DFW area across eastern Oklahoma to southwest Missouri from Wednesday through Saturday morning. A large arc of flash flood watches remained in effect on Wednesday morning from the Texas coast to southern Illinois, and Bill’s remnants are likely to produce heavy rain for days to come as they traverse the preexisting frontal boundary. Models indicate a very sharp gradient along the northwest side of the rains associated with Bill, so Oklahoma City and Tulsa may be close to the edge of the heaviest amounts. However, the NWS offices in both cities are warning of a potentially life-threatening flooding event where the rains do fall. Rivers and reservoirs still engorged from Texas and Oklahoma east to Arkansas and Louisiana will not be able to handle much additional rain from Bill, so in addition to flash flooding, mainstem flooding will likely emerge as a major concern toward the weekend, including along the Arkansas and Red Rivers and their major tributaries. Major flooding was already being reported on Wednesday morning along several rivers in the region monitored by the NWS’s Arkansas–Red Basin River Forecast Center.


Figure 4. The circulation around Tropical Storm Bill dwarfed that around Hurricane Carlos as of late Tuesday, June 16. Image credit: @NOAASatellites.

Carlos hangs on to tropical storm status
After briefly regaining hurricane strength yesterday, Carlos weakened dramatically overnight as its circulation hugged the northwest coast of Mexico. The small size of Carlos and its location near the coast has given forecasters headaches and led to large variations in intensity. The 10:00 am CDT advisory from NHC gives Carlos only a few more hours of life as a tropical cyclone, with the system projected to become a remnant low by Thursday morning, if not sooner. Satellite loops this morning showed an rapidly disorganizing system. Heavy rains will persist as Carlos and its moisture interact with the Sierra Madre del Sur mountain range, which will cause flash flooding and mudslides. On the heels of Hurricane Blanca, which made landfall over Baja California on June 8 a month earlier than any previous landfall on record for the peninsula, Carlos will be the second tropical cyclone of at least tropical-storm strength to make landfall this year on the Pacific coast of Mexico. According to the NOAA Historical Hurricanes website, there have been only two other years since accurate record keeping began in 1951 that the Northeast Pacific has seen two landfalling tropical cyclones so early in the year: 1951 (by June 1) and 1971 (by June 17.)


Bob Henson and Jeff Masters