Interesting Weather Information

Thursday, July 6, 2017

Why Does the NWS Forecast Office in Wilmington, OH Issue So Many Areal Flood Advisories?

Below is the number of Areal Flood Advisories issued by NWS Wilmington, OH per year since 2008. Notice the high False Alarm Rate and the great increase through the end of June 2017.  Why the increase? I do not know.

It seems the False Alarm Rate is independent of the number of Areal Flood Advisories issued. My interpretation of this is too high a reliance on automatic algorithms and too low a level of critical human thinking.

I received justified criticism because how do we know  NWS ILN is over issuing Areal Flood Advisories if we do not know how other NWS offices compare. Take a look below.

Then I received one more criticism of my claim - again justified AND a good example of how honest, professional discourse can move the understanding of an issue along.

The individual asked if all NWS County Warning and Forecast Areas (CWFA) were the same size.  They are not.

So the graph below normalizes the number of Areal Flood Advisories by area.

[(Advisories issued) / (CWFA area sq. mi.)]  x 10000 = Number Issued per 10,000 sq. mi

NWS ILN is so far out in front here that it makes me wonder if there is a problem with the data source.

But the other 19 offices are consistent with each other in the number of Areal Flood Advisories issued and with terrain considerations.

For example, Jackson, KY and Morristown, (Knoxville area), TN have a great deal of steep terrain which increases runoff and the chance of flooding.  Northern Indiana is flat and not so prone to flooding so you would expect the first two NWS offices to issue more Areal Flood Advisories per 10k sq. mi.than Northern Indiana.

An Areal Flood Advisory verifies if there is at least one Local Storm Report of flooding, no matter how minor, within or on the border of the defining polygon. If there is a Local Storm Report of flooding outside the polygon, no matter how close it does not count for verification.

Advisory Data: Iowa State University
Iowa Environmental Mesonet, VTEC Browser

NWS Office  CWFA Areal Coverage:
in  SQ MI from NWS Service Manual 10-507
NOV 10, 2009

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