Brown ocean effect: Saturated soil intensify tropical cyclones

May 2015 was the wettest month ever recorded in the United States since record keeping began 121 years ago according to the National Climatic Data Center (NCDC). The main contributing factor to the nation’s “wettest-May” was the incredible rainfall totals across the Central United States. Meteorologist Chris Robbins posted that May 2015 was the wettest May for DFW with a monthly total of 16.96″ (exceeding the previous May record by more than 3 inches). Texas and Oklahoma experienced their all-time wettest month on record (“all-time” considers all months, not just May). The ground is still saturated, and rivers, streams, and lakes are still swollen in these areas; additional heavy rain, particularly from a tropical system, can easily result in more flash flooding.

Early June brought drier conditions and some relief across the area. However, ridging across the Southeast and a trough across the Central U.S. is now allowing more showers and storms to fire up across the same areas that could take a long break from the rain. To make matters worse, a system in the Caribbean is developing this weekend. Could it become tropical and impact the Lone Star state with additional heavy rainfall and flooding? One thing that will be interesting to watch is how a tropical low reacts to a warm and saturated Texas/Oklahoma soil. Could a tropical low actually intensify while over land? The possibility exists. Not only is the scenario feasible, it actually happened back in 2007 when Tropical Storm Erin intensified in western Oklahoma.

May 2015 Monthly Percent of Normal Precipitation via NOAA

2007 was a wet period for the Central United States. Precipitation was nearly 200-500 percent above normal in portions of Texas and Oklahoma by early August. Grounds were super saturated, and it was in the heart of summer. Tropical Storm Erin formed in the Gulf of Mexico and moved towards the northwest into Lamar, Texas on August 16, 2007. It slowly worked its way northward into Oklahoma. On August 19, the system began to intensify over Oklahoma. Radar indicated the formation of an eyewall. Heavy rain, damaging winds, and isolated tornadoes caused significant damage across Oklahoma, with winds gusting to 80 miles per hour.

Sixteen deaths were ultimately blamed on Erin. There were a total of nine direct fatalities and seven others caused by inland flooding. The duration of Erin’s intensification over Oklahoma was roughly three to six hours. Because of the relatively short-lived period of intensification, the National Hurricane Center did not reclassify the storm as a tropical storm. Tropical cyclones must maintain convection (thunderstorms) for a substantial length of time. The remnants of Erin did not satisfy their definition, and thus they did not deem it to be a tropical cyclone after its downgrade at landfall.

July 2007 Monthly Percent of Normal Precipitation via NOAA

The Brown Ocean Effect

Why did Erin intensify? Tropical cyclones obtain their energy from very warm ocean waters (greater than 80°F/27°C) assuming other atmospheric conditions are favorable for development. In the case of Erin, the storm intensified from very saturated soils. It derived its energy from the evaporation of the soil moisture. Dr. Theresa Andersen and Dr. Marshall Shepherd from the University of Georgia called this phenomenon the “brown ocean effect.”

A similar event occurred when Hurricane Irene made landfall in 1999 over the Florida Everglades. The higher surface friction (thick marsh vegetation) combined with the very warm, shallow water of the marshland may have caused Irene to intensify. As convection within the outer bands increased in intensity, so too did Irene’s maximum-sustained winds. Additionally, a series of destructive mesocyclone-induced downbursts occurred.

An effective “brown ocean” environment requires the following:

  • Lower levels should mimic a tropical atmosphere with minimal temperature variation.
  • The soil must contain ample moisture.
  • Latent heat from soil-moisture evaporation ≥ 70 W/m2 (i.e., 70 watts or greater per square meter).
Storm-total rainfall accumulations (inches) from Tropical Storm Erin and its remnants. Graphic courtesy of the Hydrometeorological Prediction Center.

Image Credit: NOAA/NHC/WPC

Possible Again This Week?

With all of this in mind, could something like Erin and Irene happen again over land? The answer is simple: Yes, it can.

We are only fourteen days into the 2015 Atlantic Hurricane season, and we are looking at the potential for another tropical cyclone forming. If the system organizes into a tropical storm with sustained winds of 39 miles per hour or greater, it will receive the name “Bill.” Models indicate the system could become a depression or a low-end tropical storm. Most of the models indicate the system moving into the Gulf of Mexico and moving into southeast Texas by late Monday night into Tuesday. The storm is fairly large, and it is going to take some time for the system to organize and intensify. The system will not amount to much, but it will provide more rain and potential flooding across the Houston area if the forecast track aligns towards Texas.

Houston NWS indicates the potential for 2-4 inches of rain along the coast from Houston to Galveston. The numerical prediction models have consistently indicated that the low move north into eastern Texas/Oklahoma, and eventually to the northeast around the northern periphery of an upper-level ridge across the Southeast. The low will move over very saturated soils. Could this low intensify on land as it moves over the same areas who saw excessive rainfall in May? The possibility exists, but there are still more questions than answers. Irrespective of intensification, heavy rain and flash flooding is possible for Houston, Dallas, and portions of eastern Oklahoma. Of course, this assumes that the storm track occurs as expected. If the storm tracks 100 miles to the west or east, some spots could be spared while others get hit.

"Spaghetti plot" of model-simulated tracks of surface low that is likely to form in the Gulf of Mexico by Monday (June 15). This low could become a tropical depression/storm pending the findings of a Hurricane Hunter mission scheduled for Monday morning.

Prepared Saturday 6/13 at 3 pm CDT: “Spaghetti plot” of model-simulated storm tracks. Low is likely to form in the Gulf of Mexico by Monday (June 15). This could become a tropical depression/storm pending the findings of a Hurricane Hunter mission scheduled for Monday morn.

Summary

Flooding will remain an issue across Texas and Oklahoma as a tropical low forms in the Gulf and spreads moisture northwards. The setup is there for the low to potentially intensify while over land, but it’ll depend on temperatures and the amount of latent heat released into the atmosphere. It will definitely be something to watch over the next 48 to 72 hours. Mother nature can be unpredictable, so don’t be surprised if this ends up as a chapter of the “brown ocean effect.” Either way you look at it, flooding will remain a concern across the region.

Monday 6/15: Special Update to Add Expected Rainfall Amounts

Forecast: Follow the forecast updates on the page devoted to Tropical Storm Bill.

Prepared Monday 6/15 11:20 am CDT: Our predicted rainfall amounts through noon Wednesday (6/17).

Prepared Monday 6/15 at 11:20 am CDT: Our predicted rainfall amounts through noon Wednesday (6/17).