Core Rains

[Ptarmigan]

What conditions are needed for core rain events to happen? I know it usually happens to remnants of tropical lows, tropical storms, or hurricanes and happens at night. I notice it does not happen all the time with landfalling hurricanes, even the ones that move slowly, like Dennis (2005). I notice this happens a lot in Texas, like in 1921 (Thrall=38.8 inches in 18 hours), 1969 (Jamestown=31 inches in 8 hours), 1978 (Medina=46 inches in 52 hours), 1979 (Alvin=43 inches in 24 hours), and 2001 (Houston=28 inches in 12 hours). I know core rain event would look like. It is often confined with extremely heavy rain.

[wxman57]

Living in Houston, we experienced the "core dump" from Allison in 2001. The basic requirements for such an event are an abundance of tropical moisture and something to focus convergence/lift over a specific area. If this convergence mechanism is slow-moving, the potential exists for some amazing rainfall accumulations.

A decaying tropical cyclone can be the convergence mechanism for a core rainfall event. All it takes is a very weak low pressure area in deep tropical airmass that's unstable in nature. Core rainfalls are more common at night than during the day because the area of convergence/lift is confined to the weak remnant low center. During the day, daytime heating produces an irregular pattern of convergence, scattering the energy over a very large region. But at night, convection focuses over the core of the remnant low.

How much rainfall accumulates depends upon the speed of movement of the low center. In general, one can estimate the potential rainfall with any tropical system by dividing the speed of movement into 100. Allison was moving at about 3 mph. Dividing 100 by 3 gives about 33 inches. That's almost exactly what was measured in northeast and east Houston as the core of Allison passed through on Friday night, June 8th into Saturday morning.

Such core rainfalls are more common in lower latitudes (like in Texas) where remnant tropical lows may not get picked up and carried off to the northeast very quickly.

[Ptarmigan]

Living in Houston, we experienced the "core dump" from Allison in 2001. The basic requirements for such an event are an abundance of tropical moisture and something to focus convergence/lift over a specific area. If this convergence mechanism is slow-moving, the potential exists for some amazing rainfall accumulations.

A decaying tropical cyclone can be the convergence mechanism for a core rainfall event. All it takes is a very weak low pressure area in deep tropical airmass that's unstable in nature. Core rainfalls are more common at night than during the day because the area of convergence/lift is confined to the weak remnant low center. During the day, daytime heating produces an irregular pattern of convergence, scattering the energy over a very large region. But at night, convection focuses over the core of the remnant low.

How much rainfall accumulates depends upon the speed of movement of the low center. In general, one can estimate the potential rainfall with any tropical system by dividing the speed of movement into 100. Allison was moving at about 3 mph. Dividing 100 by 3 gives about 33 inches. That's almost exactly what was measured in northeast and east Houston as the core of Allison passed through on Friday night, June 8th into Saturday morning.

Such core rainfalls are more common in lower latitudes (like in Texas) where remnant tropical lows may not get picked up and carried off to the northeast very quickly.

I remember Allison well. Lots of rain. I wonder if this happens in the Southern Hemisphere. I would think that likely happens in Reunion Island. I remember Typhoon Nari being a core rain even over Taiwain in 2001, when some areas got up to 52 inches of rain. I know Virginia sees it despite being at a higher latitude, with Camille and Gaston. I wonder if there is a list of core rain events out there. Some say the 2002 Texas Flood was a core rain event even though it was caused by a upper level low, which most of the rain fell at night. I wonder if altitude plays a role too because Camille and Nari happened over a mountainous area.