Sunday, September 9, 2018

Some Straight Talk About the Flood Potential in the Middle Atlantic States

As more data has come in today, the potential for catastrophic flooding (similar to Hurricane Agnes' flooding or worse) exists for the Middle Atlantic Region. 

The bottom line: it is likely some areas will flood that have never flooded before. Prepare NOW to evacuate you and your family. If you are given an evacuation order, please comply immediately before roads get clogged and chaos commences. Please read on.

Meteorologists hate being wrong and I would love to have the luxury of waiting for more data to write this (meteorologists always like "more data"). But, given the magnitude of the disaster potentially ahead of us, and given the increasingly limited time to prepare, I am writing this now. Please take this seriously. I have been doing this for five decades and I wouldn't write this and put my reputation at stake if I weren't seriously concerned about what may occur. I am also trying to put myself in the position of a family living in the threatened area.

If I had to pick a single weather forecasting model, it would be the ECMWF. It, and the other models, are forecasting giant amounts of rain. Here is the latest ECMWF:
click to enlarge
The maximum amount forecast (in between individual weather stations) is more than 30 inches!! Yes, it is entirely possible the rain pattern could shift a bit north/south/east/west and it might "only" be 20 inches. Regardless, it appears a major flood event is on the way in the Middle Atlantic region.

What makes this even worse is that during the last 30 days more rain than usual has fallen over most of the area where Florence's heaviest rains are forecast to fall. The map below is the number of inches of rain above (green/blue) or below (reds/yellows) normal over the last 30 days.

In much of western Virginia, Maryland and eastern West Virginia, it has been wetter than normal.
With already wetter than normal conditions, the excessive rainfall from Hurricane Florence will...
  • Run off more quickly than usual into...
  • Rivers that are already higher than usual.
All of this may equal tremendous or even catastrophic flooding.

For perspective, I went back and looked at the ECMWF four days before Hurricane Harvey's torrential rain began in Houston. The maximum amount forecast for the region was 29 inches. The heaviest actual amount was 60.58 inches! The model's peak rainfall forecast was less than half of what actually fell. With Harvey more than 13,000 people had to be rescued in metro Houston.
Hurricane Harvey flooding in southeast Texas, 2017
If thirty or more inches of rain fall with Florence, more than 13,000 may need to be rescued over a much larger and more remote area (as opposed to concentrated in a major city) which means far more resources will be required. If I were an emergency manager, I would be looking to bring people in from outside of the region NOW, especially expert applied hydrologists.

By comparison, 1973's Hurricane Agnes caused record flooding in the Middle Atlantic region with less than 15 inches of rain over Virginia and West Virginia. So, it can happen even if much less than the ECMWF's 30 inches fall.
Richmond flooding from Hurricane Agnes, 1973
So, What Do I Do Now?
  • My suggestion that emergency managers bring in extra help in advance is above. 
  • My preparation suggestions for individuals and families are here
  • Health suggestions pertaining to waterborne illness (from Ascel Bio) are here
I will likely update all of this tonight if there is enough extra data to justify it. After all of this research and writing, I'm going to watch my Chiefs for a while. 

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