Bill the Inland Cyclone

   It is a day after landfall, and Bill has since moved up to the Dallas/ft. Worth area. Amazingly though, Bill is still a tropical depression with fantastic structure for a storm that has been over land for the past 24 hours. In fact, Bill looks to have better structure and convection that it did when it came on shore yesterday morning. 
Yesterday morning Bill made landfall packing 60 mph winds and a minimum central pressure of 997mb. Currently it’s a tropical depression with winds of 35 mph and a minimum central pressure of 1002mb.

24hr past precipitation

   The winds are not the main threat from this inland cyclone, but the immense amounts of rain are. Up to 11 inches of rain have fallen across Texas since the storm made landfall. Unfortunately there is more to come. For Texas and Oklahoma 3-7+ inches of rain are forecast over the next 5 days.

5-day precipitation forecast

   Interestingly enough this phenomenon of inland cyclones was studied in 2013 by Theresa Andersen and J. Marshall Shepherd of the University of Georgia in Athens with NASA funding. The ability for an inland cyclone to maintain its strength has been dubbed the “Brown Ocean” effect where "The land essentially mimics the moisture-rich environment of the ocean, where the storm originated”

   The researchers combed through NOAA archives, analyzing every land falling tropical cyclone from 1979 to 2008.  Storms had to meet the criteria of retaining a measurable central pressure by the time they tracked at least 220 miles (350 kilometers) inland, away from the maritime influence of the nearest coast to be part of the study.
Their research found that of the 227 storms they looked at, 45 maintained or increased strength while over land. They found that of the 45 storms that had deviated from the norm, they did so through three different processes. NOAA categorizes storms that lose tropical characteristics as “Extratropical” systems. This term is broad in the sense that it does not define the different types of extratropical systems that can exist. Of the 45, 17 of the inland storms fell into the cold-core extratropical cyclone category where the storms derive energy from clashes between different air masses. Another 16 of the inland storms were part of a group classified by the study as TCMIs (tropical cyclone maintenance and intensification events). These storms retain their tropical characteristics and warm core over land, allowing the storms to deliver higher amounts of precipitation.  The rest of the storms exhibited hybrid characteristics, combining features of a cold core system and a TCMI to varying degrees.

Image Credit: 

NASA/Kathryn Hansen

   For these storms to exist and at times thrive, three observable conditions needed to be present. “First, the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature. Second, soils in the vicinity of the storms need to contain ample moisture. Finally, evaporation of the soil moisture releases latent heat, which the team found must measure at least 70 watts averaged per square meter. For comparison, the latent heat flux from the ocean averages about 200 watts per square meter.”
In regards to Bill, it appears this storm may be a TCMI, as it is still a warm core system with immense amounts of precipitation. It will likely shift to a cold core system as it swirls north into less tropical air, but the precipitation amounts will still be large.

   The Northeast will be this storms last stop, as it draws its moisture all the way of to Massachusetts. It will likely have lost all of its tropical characteristics by the time it arrives, but it will still have plenty of rain with it. From West Virginia all the way up into Maine, a swath of 2-5 inches of rain can be expected. This rain will arrive for the weekend, so be prepared, pack an umbrella, and be wary of flash flooding.



Information was gathered from the article "Brown Ocean' Can Fuel Inland Tropical Cyclones" By Kathryn Hansen