My next blog shall be one about the incredible flooding of October 1994 in Southeast Texas. The massive rainfall event set many records on the area rivers, as well as records for precipitation of like-sized storms and even possibly maybe setting a new world record for point rainfall in the 6 hour timeframe!
The event all began when the moisture from the remminants of Pacific Hurricane Rosa entered SW Texas on October 15th. Rosa along with a low level flow of moisture off the Gulf of Mexico ahead of a trough combined to produce widespread pws in excess of 2.25 in. Meteorologists recognized that the only thing preventing a large rain event at the time was a lifting mechanism. At the same time the same trough helping to produce the moist flow was bringing a cold front and accompanying low pressure into the area.
The rain began to fall in Southeast Texas on October 15th as the remminants of Hurricane Rosa began to pass through the area. At about the same time, a warm front ahead of the low moved north out of the Gulf of Mexico. On the 15th and 16th, much of the area recieved about half a foot of rain, with some areas between Houston, College Station, and San Antonio accumulating almost 10 inches! The Rosa/warm front combo produced enough rain to cause significant flooding and would have been memoriable if these were the only rains from the storm system. However, for some areas, The heaviest rains were yet to fall.
On October 16th, the cold front associated with the low began to move into the area. Incredible amounts of moisture were in place due to the passage of Rosa and the warm front the previous two days and this set up a very large flooding event. Moderate rainfall totals were recorded during the day as the front moved through the counties between Austin and Houston, but when night fell, the raingates were opened like they rarely do. Nocturnal radiation (cooling of cloud tops due to radiation of heat into space at night) led to increaced convective activity along the front. The biggest storm cells of the entire rain event occured that night. From Southeast to Northwest, they were located over the Lavaca River Basin, West Fork San Jacinto River Basin, and the Kickapoo Creek Basin.
The Lavaca Storm Cell dropped almost 20 inches of rain over the river basin, and resulted in a flood that shattared a 54 year old record and was estimated to have a streamflow over two times that of the 100 year flood. As big as the Lavaca storm was, it was actually the least extreme storm cell of the night.
About 50 miles Northwest of the Lavaca Storm was a severe thunderstorm located over the West Fork San Jacinto River Basin. The storm was in the form of a line segmant and dropped tremendous rains between 20-25 inches in a 8 hour pereod over the area. The hardest hit areas were Upper Cypress Creek, Spring Creek, Lake Creek, and the West Fork San Jacinto River, with a flood in excess of the 100 year flood on all the above mentioned river basins which resulted in new record heights. The West Fork San Jacinto Storm can be seen below:
The Greatest rainfall of the entire rain event occured over Lake Livingston and Kickapoo Creek. Located about 30 or so miles Northwest of the West Fork San Jacinto Storm, the Kickapoo Storm was an extremely efficant rainmaker. Unlike the Lavaca or West Fork Storms, the Kickapoo Storm Cell had what is called a Low Echo Centroid (LEC) which was tropical in nature and is much more efficant at producing rain (although I am not sure how). Fed by both moisture from the West Fork Storm as well as a LLJ directly off the Gulf of Mexico, the storm produced almost unheard of rain totals in a 6 hour time frame. The offical amount states that almost 25 inches of rain fell in 6 hours, but runoff data from Kickapoo Creek suggests that an entire foot more fell in the same time frame. If verified, the 36.7 inches (933 mm) of rain in 6 hours implied by runoff data would be a new world record for that time frame! The massive runoff from Kickapoo Creek was 3 times greater than that of the 100 year flood and helped to fuel record flooding downstream on the Trinity River. The Kickapoo Storm Cell can be seen below:
Although the most extreme flooding was done, it kept raining. Late in the day on the 17th, The front stalled near the coast and remained there until October 19th. 10-20 more inches of rain accumulated during this time period in an area from Baytown to Beaumont, aggrivating the record/near record flooding on the lower San Jacinto and Trinity rivers, as well as causing record flooding on the Lower Neches River. By the time the front finally finished passing through Southeast Texas, an average of 10-20 inches was consistant across most of the area.
Although the October 1994 Flood was extreme, it actually could have been worse. The city of Houston recieved less than a foot from the rain event, despite having totals twice as large not too far away to the North, West, and East. Had a rain bomb such as the Kickapoo Storm developed directly over or just upstream of the city, the resulting flooding would probably be worse than that experienced during Tropical Storm Allison!
The setup that produced the October 1994 Floods (Remminants of a Pacific Hurricane + trough) was one of the most efficant flood setups possible in Texas. A similar flood event occured just 4 years later in Central Texas on October 17th and 18th of 1998, although this one was more removed from Gulf moisture and shorter in duration.
An alternate senerio that could produce as much or even more rain would be the stalling of a landfalling tropical cyclone. Most recently experienced with Tropical Storm Allison, tropical cyclones can easily bring excessive rainfall to Southeast Texas. Famous examples include Allison (2001 and 1989), Claudette (1979, holds the US 24 hour rainfall record at 43 inches), and an unnamed tropical cyclone from 1960. Hovever, if a large and strong tropical cyclone were to stall out over the area, even more rainfall can result. Hurricane Wilma Stalled over Mexico's Yucatan Penninsula for a day and ended up dropping 62.05 inches of rain in the Cancun area, a world record for non-orographicly enhanced rainfall in 24 hours. If a similar situation set up in Southeast Texas, all flood records would be shattared (along with resulting damage as severe or possibly even more severe than that experenced with Ike). However, such an event has very little chance of happening, due to enhanced steering currents for stronger hurricanes. Therefore, Southeast Texas's greatest rainstorms still originate in the Pacific and with potent autum troughs.