Why Such Catastrophic Flooding with Hurricane Helene?

 Hurricane Helene has brought terrible destruction to the southeast U.S., with the worst damage and loss of life in the higher terrain of western North Carolina.

The death toll is now well over 100 and the total damage is estimated to exceed 100 billion dollars.

As we will see, localized rainfall totals for the storm were extreme, with some locations receiving as much as 30 inches.

To understand what happened, first consider the terrain of the region (below).  You can clearly see the high terrain of the Appalachians over the northwest portion of the domain.

A map of rainfall totals for the event shows values reaching 15-25 inches on the eastern side of the Appalachians, where the hurricane pushed air against the terrain.

In fact, one location (Busick RAWS) had a 3-day total of 31.33 inches of rain, more than the annual total of many locations in the U.S.

The track of the hurricane was perfect for creating a very strong, moist, southeaserly flow that was forced up the eastern side of the barrier, dropping immense amounts of precipitation.

Northwesterners are very aware of the dramatic effects of geographic enhancement of precipitation:  as air rises on terrain it cools and is forced to give up its water vapor, which is converted into precipitation

Hurricane Helene at 5 AM PDT on Friday still had a very strong circulation (see pressure forecast for that time below).    Winds rotate counterclockwise around lows, so that the winds were from the southeast over western North Carolina

The hurricane entrained a plume of very moist air right into the eastern side of the Appalachians, something shown by a simulation of the winds around 5000 ft (850 hPa pressure) on Friday morning.  The purple color indicates the strongest winds...and the wind pennants show the direction.  The hurricane center was in the perfect location to push air into the North Carolina mountains.

The NOAA/NWS High-Resolution Rapid Refresh  (HRRR) model predicted the heavy precipitation in the right location  (see below).

Now the editorial portion of this blog.

The ability to forecast severe weather like this has gotten immensely better, with weather modeling technology enjoying profound improvement.   

But even with far better weather forecasts, there are still events with large death tolls, like this event and the Lahaina wildfire on Maui.    

Society must learn to better use advancing weather prediction to reduce the large death tolls in such events.