What’s in a hurricane name? When it comes to flood potential in southeast Texas, not much. Some of the most disastrous flooding in that region has come from storms that were no longer hurricanes like Tropical Storm Allison and the remnants of Hurricane Harvey. Allison meandered around for multiple days causing nearly $5 billion in damage and killing 23 people. Hurricane Harvey made landfall as a category 4 storm near Corpus Christi, Texas and then stalled over southeastern Texas for days. According to NOAA, it ranks as the 2nd costliest hurricane in U.S. history at $125 billion in damages. Much of the property damage and fatalities occurred well after the storm was downgraded from a hurricane or tropical storm. As I write this, another potentially “nameless” storm could threaten the same region with excessive rainfall and flooding. Here’s what we know.
The 2 pm NOAA Tropical Weather Outlook on September 16th f only gives the storm a 20% chance of further development within the next 5 days. However, the focus on whether the storm may actually become a named system would be misguided and irrelevant. According the National Hurricane Center Outlook:
Disorganized showers and thunderstorms over the northwestern Gulf of Mexico are associated with a broad area of low pressure. Some slow development of this system is possible before it moves inland along the northwestern Gulf coast Tuesday tonight or Wednesday. Regardless
of development, this system is expected to produce locally heavy rainfall along portions of the central and upper Texas coastal areas
later this week.
The NOAA Weather Prediction Center (WPC) 1 to 3 day precipitation forecast is for up to a foot of rainfall in some parts of the region (Click this link to see the map). What’s going on? The tropical disturbance just offshore of Louisiana at the time of writing is projected to move westward by Tuesday morning. Meteorologists like me use something called precipitable water to quantify how much moisture is in the atmosphere from the surface to the “top of the atmosphere. Research in my group at the University of Georgia led by Dr. Amanda Schroeder, now at the National Weather Service, has found that extreme precipitable water values can be a good predictor of flooding. Values exceeding 2 inches within the column of the atmosphere are particularly worrisome, and WPC is forecasting values of 2 to 2.25 inches of precipitable water in the middle and upper Texas coasts. Another meteorological indicator of concern is Convective Available Potential Energy (CAPE). This is a metric that we use to determine how unstable the atmosphere may be in a storm environment. If the atmosphere is unstable, there is plenty of energy for air parcels to rise if lifted. It looks as though certain values of CAPE will be conducive for heavy rainfall.
The word “intense” is particularly important. According to the WPC discussion,
….this environment could support 3″ an hour rain totals should cells move/form ashore and merge or train, which would be problematic outside of wetlands, with urban areas the most sensitive.
There are a couple of alarming statements in that sentence. 3 inches per hour is a lot of rainfall in short amount of time. The word “train” is a term used in meteorology to describe rain cells that essentially form or move over the same geographic location for significant periods of time. “Training” was a big problem with Hurricane Harvey and its remnants.
Ironically, there is a “Jekyll and Hyde” story in the approaching rainfall for this part of Texas. The region has been very dry and even the WPC discussion noted that “much of the region has been dry lately, so initial rainfall should be welcome.” However, the combination of intense rainfall rates Tuesday to Thursday coupled with the urban impervious surfaces of the region and “stormwater engineering for last century’s rainstorms” make for a risky scenario.
This region is certainly prone to flooding, but it is dangerous to assume that your previous experience with a storm is an accurate predictor of how the next one will be. It is my hope that we are slowly inching people towards the understanding that the name or category of a storm is not as important as its potential impacts.