Posted: 6:39 pm Thursday, September 13th, 2018
By Kirk Mellish
Let me take you a little behind the scenes, behind the forecast curtain if you will.
Meteorology (science) often gets lost on the hype train of the general public, news media, and much worse on social media. Everyone becomes a meteorologist because they saw something on YouTube, Periscope, ____Live, Reddit etc. or someone shared a map on Facebook they aren’t qualified to understand or interpret. (They are media-ologists lol)
Whenever there’s a big weather system it becomes amateur hour, the wrong things I see people say on social media or overhear out in public is horrifying and leaves me shaking my head about how little most people actually understand about weather or forecasting.
I’ve been beating the dead horse of “it’s the water not the wind” for a week and have done so in past years for past hurricanes, but it falls on deaf ears I guess.
Tropical Meteorology is a specialty, a sub-specialty of the field. (BTW I am not a hurricane expert).
The “Maximum” wind near the storm’s center may have weakened but Florence the storm SYSTEM did not weaken, it merely redistributed its energy (structural change) in a way not reflected in an ancient and over-simplified category classification system.* The SIZE of the storm has actually GROWN even while those category levels have fallen.
So it doesn’t matter what category it has the flooding has been and continues to be the main concern, aka Harvey in Texas last year. The storm continues to put on the brakes slowing down to make the storm surge flooding and rain flooding the big issue for property damage and threat to life over a multi day period (not instantaneously).
North Carolina state 7-day rainfall records may well fall and records go back 150 years in places like Wilmington. Amounts in some spots could be 1000 year event levels. Florence is set for a prolonged storm surge, battering waves, tropical storm force wind gusts for a couple days and devastating coastal and inland flooding and river flooding in the week ahead. 5-7 day rain totals continue to look tremendous at a foot or two in some areas in the Carolinas even for some inland spots.
* Greg Masters of Weather Underground points out calculations by Mark Powell indicate:
In other words near the core of the storm there will still be 100+mph winds, but despite the weakening from its CAT 4 peak out in the ocean, the growing size of the cyclone means impacts will actually increase from when it was a “stronger” but a more compact storm.
Florence hasn’t weakened much from a total energy perspective merely spread that energy out over a greater area so for areas further away from the eye of the storm the change in the systems structure has actually increased the risk for wind and rain impacts especially up and down the coast line.
There’s nothing to say the wind max can’t still go up and down more times before it gets fully on land.
Calculations indicate rainfall totals just across the Carolinas of over 18 TRILLION gallons when all is said and done over a 7-day period. About 5 million people will get about 10 inches of rain.
For example, already today 89 mph winds have already been reported 100 miles away from the center of the storm, that’s why it’s a bad idea to overly focus on that thin forecast track line. It doesn’t tell the full story, which is why we have the “Cone of Uncertainty”. It’s also important to know and remember that negative impacts can and do also occur OUTSIDE of that cone.
It is perfectly normal for the intensity of the winds near the core (center or eye) of the hurricane to fluctuation, sometimes significantly and sometimes over short periods of time.
Track/path prediction accuracy has improved markedly over the past decades, but neither humans nor models are very good at forecasting these wind intensity changes.
“They’re always wrong”.
Here is the forecast from 6 days ago for this Thursday morning in the NOAA/NWS collaboration:
Here’s what the real world looked like this Thursday morning at verification time:
Looks pretty dang good to me especially for six days in advance. It got better:
So to recap, a wind drop in a tropical cyclone of a category or two, of 5-10 mph or even 20 or so does not change the impacts especially with regard to storm surge, particularly if the size of the storm has actually grown. Water kills people in these weather events more than anything else.
Follow me on Twitter @ MellishMeterWSB.