Joe’s Weather Blog: About those once in a xxx year rains (WED-8/23)

Good morning…today’s blog will be somewhat shorter as we’re now going to enjoy some nice weather for a few days…our next chance of at least some rain (not a repeat of what we’ve been dealing with lately) will come later in the weekend. Once again more below average highs are expected to linger for awhile. I’m also keeping an eye on the tropics which may get very active over the next 10 days or so. There was a tropical storm called Harvey over the past weekend…it fell apart but is trying to come back to life…it map impact TX and LA heading towards the weekend and next week.


Forecast:

Today: Sunny and pleasant with highs around 80°

Tonight: Mostly clear and cool with lows back down into the 50s

Thursday: Partly cloudy and pleasant with highs around 80°


Discussion:

Random rain trivia IF I’ve done my math correctly…KC has about 319 sq miles of land…let’s say an average of 6″ of rain fell Monday into early Tuesday morning cross those 319 sq miles…how many gallons of water would that be?

Well according to the USGS website…a LOT!

33,262,563,840 gallons…that’s 33.2 BILLION gallons of water!

Roughly 10 BILLION gallons of water flow over Niagara Falls each 3.7 hours…so this would be the equivalent of roughly more than 10 hours of water flowing over Niagara Falls

Picture via niagarafallslive.com

Man I wish I was working today just so that I can share that information in a graphic with you and geek out on the air about it…alas hopefully a day off.

So now you know…

So about the “this is a once in 200…300…500…1000 year rain” that might be spilling out the KC media…

Don’t believe it.

It’s not what it sounds like…but boy it sounds good doesn’t it. I totally get it too. When you have 10 or 11″ of rain…it makes you wonder surely that will never happen again. Odds are it may…and in your life time…not every 500 years or whatever.

This gets messy so bear with me.

Back in the 1960s the United States Geological Society was creating flood maps of the country for the National Flood Insurance Program (NFIP). This has been also extended into the world of rain amounts as well.

Here is some information directly from the USGS.

“In the 1960’s, the United States government decided to use the
1-percent annual exceedance probability (AEP) flood as the basis
for the National Flood Insurance Program. The 1-percent AEP
flood was thought to be a fair balance between protecting the
public and overly stringent regulation. Because the 1-percent AEP
flood has a 1 in 100 chance of being equaled or exceeded in any
1 year, and it has an average recurrence interval of 100 years, it
often is referred to as the “100-year flood”

Now the USGS is trying desperately to get away from this phrasing…

Think about it…we had few weather records 100 years ago…we had even fewer records 200 years ago and we had no real records going back 500 years ago. Think about the way the terrain has been changed in the last 50…100…200+ years…from farmland everywhere to big cities with pavement, buildings, sewer systems etc. That rain no longer drains into the soil…it runs off creating flooding events.

Also think about how we’ve altered and changed the various creeks and rivers…how their waters have been channeled creating additional flooding chances. Water does NOT like to be contained. So the phrase “land-use” is important.

In reality what they mean and want to convey is that the risk of a 100 year rain/flood REALLY means that the chance of that occurring EVERY year is about 1%. The chance of it occurring when they say a once in 200 year flood is REALLY 1/2 of 1% and finally for something that supposedly occurs only once in a 1000 years…REALLY has a 1/10 of 1% chance of occurring EVERY year.

Back to the confusion part…and a direct quote from the USGS…

“Admittedly, use of such terms as the “100-year flood” can confuse or unintentionally mislead those unfamiliar with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval
terminology. For example, one would discuss the “1-percent AEP flood” as opposed to the “100-year flood.”

So they see the confusion…

There is a whole calculation involved in determining the “return period” of a certain event…for example…let’s say a 100 year flood chance for a person who lives for 75 years (gets confusing)…check out the true information.

So all of a sudden that once in 100 year event has a better than 50/50 chance of happening in a person’s lifetime…confusing I know.

Actually here is a website that will do the calculation for you.

So when you hear about the 1 in xxx year storm/hurricane/flood/rain whatever…you will know that’s really #fakeweather

Let’s finish this with a nice shot of the Sun’s Corona from the other day. This sent in from Calvin Barrett in Higginsville, MO

Joe

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