Atlantic hurricanes are slowing down at landfall

[EricPeterson]

There are a couple papers on slowing hurricanes.  One is about Atlantic hurricanes https://www.nature.com/articles/s41612-019-0074-8 and seems reasonable by my analysis.  They compute the mean rate of motion and show a drop in the slowest 5% of storms (fig 1b) and corresponding changes in direction (fig 1d).  However I do not believe the first part of the title "Hurricane stalling along the North American coast ..." is quite accurate.  I believe that stalling, where steering currents break down, and the hurricane drifts, then stops or almost stops, then drifts in another direction, is a rare weather event.  I believe those weather events are determined by natural cycles.  The slowing is real though, in the 4-6 mph hurricanes, and the reasons in the paper are plausible.

The analysis I did uses the same HURDAT2 data they used.  However I did not use HURDAT2 landfall markings because I do not believe those are consistently applied.  In particular landfalls at non-6-hour intervals are captured in newer but not older data.  Instead i used the NOAA GLOBE data which contains elevation data in which any elevation greater than -500 meters is non-ocean.  The gist of my algorithm is finding all  landfalls for each storm and then finding the slowest.  In the case of Claudette the second landfall was slowest.  For Wilma the first of three landfalls was the slowest. Dorian had only one point of landfall (7 meters).  Thus I will miss some landfalls on very small islands.  But those misses will be consistent through the record and not affect the trend other than possibly lowering its statistical validity a small amount.

The code is linked on the webpage: https://virtualcoinclub.com/wx/slow/   Here are the 70 year trends  The 0-3 mph landfalls ("stalled") do not have a statistically significant trend.  There is too much natural variation in those types of weather patterns.  But the 4-6 mph landfalls have increased  significantly.   The 0-6 landfalls as a whole have risen from 40% to 50% of total landfalls.  That means hurricanes and tropical storms in the Atlantic basin are generally slowing down at landfall.

[EricPeterson]

I rewrote the code.  Same results but much cleaner Python.  The per-storm logic is all in one location, about a page of code.  The year summary logic is about a half page of code.  The biggest difference is instead of separate copy/paste versions for each analysis, the code is consolidated.  Another feature is to automatically generate the HTML table of slow moving storms.  Yeah, that's 20 year old generation and done better in table generation code available on the server side or in javascript.  But nothing's quite as good as roll-your-own if you have the time since data is being pulled from various dictionaries (e.g. storm metadata).

Now I need to re-implement the rest of the plots like rapid intensification, slower weakening after landfall, etc.  I'll add a rapid weakening plot (obviously not including storms over land) to see if that's change.  Rapid intensification was flat in the old code, should be the same with the new code.

[EricPeterson]

Actually the results aren't quite the same.  Storm motions in storms like Dorian are now calculated more precisely.  Dorian was special because it only hit one Bahamas island according to the lat/lons.  I calculate the motion in 6 hour intervals before and after and find the slowest 6 hour segment (1.5 mph).  The bottom line is there is also an increase in the percentage of the slowest-of-the-slow storms.