Frank's Weather or Not: Rainfall Rule of Thumb

By Frank Billingsley - Chief Meteorologist

HOUSTON - When I arrived in Houston in March of 1989, I also brought with me one of the busiest tropical seasons we'd ever had!

On June 27 we had our first Tropical Storm Allison (not the bad one of 2001) which brought in 4-8" of rain, isolated amounts of 20", 3 Texas fatalities, and $200-400million in damages. There was also a Barry that year (!) which did nothing, but then Hurricane Chantal hit High Island on August 1st producing 11 fatalities, $100million in damage and damaged 3,000 homes!

Finally, Jerry on October 15th moved into Galveston with 85mph winds killing three people who drove off the seawall. Jerry is the latest storm on record to ever hit Texas, so I use that as a rule of thumb for when our hurricane season is usually over.

Speaking of Rules of Thumb, it was during that summer that National Weather Service Meteorologist Ron Stagno taught me one of his go-to formulas that I've shared with you all many times as this rule has never failed me:

100 DIVIDED BY HOW FAST THE STORM IS MOVING = INCHES OF RAIN POSSIBLE

So if a hurricane is moving 20mph, then 100/20 = 5 inches of rain. If a hurricane is moving 3 miles per hour, like our 2001 Tropical Storm Allison then 100/3 = 33 inches of rain and that is a good approximation of what many people got:

TS Allison extreme rain measured 36" to 42"

Hurricane Harvey in 2017 moved 2mph and 100/2 = 50" of rain. And that is what fell on some locations.

Why am I bringing this up? Because Barry today is moving 5mph and the forecast is for as much as 20" of rain. After all, 100/5 = 20. Here is the  FV3-GFS forecast for Accumulated Rainfall through Monday:

This rainfall on top of storm surge and a Mississippi River that is already within 10' of overtopping will potentially be devastating for parts of Louisiana. Please send a prayer for our neighbors.

Have a safe weekend!

Frank

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