Monday, October 1, 2018

CoCoRaHS Observers in Right Place, Right Time in Western NC

When I pull up the CoCoRaHS web site each day one of the first things I check is the national map for any unusually high values that might indicate an error.  On Sunday September 30 the first thing I noticed was that the scale showed a top value over eight inches. There was a red dot over western North Carolina. "Perhaps an observer entered observation time as precip or made a typo entering the amount", I thought. It's a very common error.

Zooming in, it was clear this was no error. The southeast corner of Buncombe County, North Carolina got a very large amount of rain, and heavy rain also fell in surrounding counties.

Rainfall amounts of 1 inch or more from Sunday morning, September 30.
The largest amount of 8.11 inches was recorded by the observer at NC-BC-115, Black Mountain 5.5 SE. The comments entered by the observer helped tell the story of what happened overnight. Comments are very helpful and important in events like this.

What was really interesting was that this was an isolated storm for the most part. Out of 33 CoCoRaHS observers reporting in Buncombe County that morning, 22 reported amounts of 0.10 inch or less, and about half of those reporting zero.

Radar showed a nearly station storm over southeastern Buncombe, western McDowell, and western Rutherford counties. The center of the white circle is the approximate location of NC-BC-115. The first image in the sequence shows two strong cells just to the south. The middle image is three hours later and the heavy rain has been falling for two hours.

Radar images from

The precipitation accumulation map from the NWS Advanced Hydrologic Prediction Service (AHPS)  indicates 6 to 8 inches over southeast Buncombe County, and just how small the area of heavy rain was.

The heavy rain resulted in flash flooding with debris flows, roads and bridges washed out in the affected counties. The National Weather Service issued an Areal Flood Watch at 4:46 a.m. EDT.

It is possible that that more than 8.11" of rain fell in the area near NC-BC-115, but were it not for that observation and that of the observer at NC-RT-6 Chimney Rock 0.3 with 5.01" there would not have been ground truth for this event. Right place, right time.

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