26 August 2017
I’ve forecasted the weather for 37 years and I’ve never seen consistent model output forecasting rainfall amounts of over 30 inches before. Astounding and astonishing are the only words I can use to describe what I’ve been seeing from the numerical models. Not only that, but it’s the same with nearly every usually reliable model. Could all of the models be wrong? Could this storm fizzle rather quickly once it makes it inland? The answer is yes, and it’s good to remember that models often do not do a great job of forecasting extreme events. Tropical systems themselves are also more difficult than the cold core low-pressure systems we usually deal with.
Numerical models run on physics, but there are things that the models cannot forecast directly and must be parameterised, and we do not know if these parameterizations will hold in this event. We often see models predict huge rain amounts when a known issue called convective feedback. That said, we can usually spot this because the next run will not show it, or other models will not depict it either. However, in this case, ALL of the reliable guidance is forecasting tremendous amounts of rain; in many cases over 25 inches over 120 hours, with much of that in 72 hours!
The hero of this storm so far has been our new weather satellite, GOES-16. The 1-minute images in 16 different bands of light have made forecasting Harvey much easier. The taxpayers got their money for that satellite on just this storm. The rest is a bonus.
As I type this late Friday, Harvey has made landfall near Rockport Texas as a category 4 hurricane. The last Cat 4 to hit Texas was Carla in 1961. In those days, we had no satellites and weather radar was rather new. A cub reporter for KHOU TV in Houston talked the U.S. Weather Bureau in Galveston into letting him show a radar image of Carla’s eye on TV as it approached. That likely got a lot of folks to evacuate, and it got that reporter noticed as well. A young Dan Rather was soon working for CBS, and the rest is history. 56 years later we have amazing technology, but forecasting tropical cyclones remains difficult (especially intensity) and all we can do is tell folks that the potential for a real catastrophe is high. It’s not certain though, and this is one time when I really hope all of those models are dead wrong.
If they verify, the flood that will result will be remembered far longer than that night Carla came ashore. If it impacts the entire city of Houston, the insurance companies will take a big hit. Flooded refineries will impact your pocket book even if you live a thousand miles away from Texas.
If you want to read a really excellent book about hurricanes, I have two recommendations. Isaac’s Storm is about the great 1900 hurricane that remains the greatest natural disaster in U.S. history. For a modern look at the science, Kerry Emanuel’s Divine Wind is your next read. Both of these books are by far the best I know of about hurricanes, and every meteorologist I know has them on their book shelf.
One last thing to mention. The ocean is higher now than when Carla came through and it’s also warmer (Gulf temps. were about 1ºC above normal as Harvey exploded into a major hurricane). Every storm system is impacted now by our changing climate. Climate change is not noticeable much on the day to day, but during storms, and extreme weather, make no mistake, it’s playing a significant role.
More on Harvey Saturday afternoon.