Post Mathew: Do We Know What a Hurricane is Anymore?
October 10, 2016
For those that have bought the “dodged a bullet” theory of Hurricane Mathew.
In a sea level, water vapor, heat content transformed world, storms have multiple pathways to destructive power.
With North Carolina reeling from more than 17 inches of rain from Hurricane Matthew, it’s time to face the fact that the way we measure hurricanes and communicate their likely impacts is seriously flawed.
We need a new hurricane intensity metric that more accurately reflects a storm’s potential to cause death and destruction well inland, rather than the Saffir-Simpson Wind Intensity Scale, which focuses on the potential for coastal damage from high winds and storm surge flooding.
Right now, the Saffir-Simpson Hurricane Wind Intensity Scale is what we use to communicate how strong a storm is. Developed by engineer Herb Saffir and meteorologist Bob Simpson in the early 1970s, it is based on the maximum sustained winds in a hurricane.
But this ignores the multitude of other threats that hurricanes pose to coastal and inland areas. In the U.S., Hurricane Matthew, which was the longest-lasting Category 4 and 5 storm on record during the month of October, will be remembered more for its water damage — both storm surge and inland flooding — than its winds, which is what the Saffir Simpson Scale communicates.
With Matthew, the storm did some of its worst damage when it was “just” a Category 1 storm, slinking along the coast of the Carolinas. The storm’s category may have suggested to people that it was not a huge threat to North and South Carolina, when in fact the opposite was true.
The storm combined with a preexisting frontal system and record high amounts of atmospheric moisture, pumped into the air from unusually warm sea surface temperatures, to crank out torrential, flooding rains.
It’s long been known that water is the biggest killer during hurricanes. So why do we use a scale that only communicates the wind and coastal storm surge threat?
The answer is that we just haven’t come up with a better one yet. Or rather, several have been proposed, but none have become a consensus view within the weather community.
Total precipitable water (TPW) is a measure of the amount of water vapor in an entire atmospheric column. Envision a one-meter-square tower stretching from the surface to the top of the atmosphere’s weather-bearing layer. Condense all this vapor instantly and you get a precipitated water depth. Data collected by weather balloons are routinely used to compute TPW. In at least two locations — Jacksonville, Fla., and Charleston, S.C. — air feeding into Hurricane Matthew contained record-high levels of TPW: 2.93 inches and 2.85 inches, respectively.
Rarely does the East Coast see values this extreme, even during the height of summer. But the waters over the western Atlantic, including the Gulf Stream, have been exceptionally warm, nearly record-breaking in their own right. This drove evaporation into high gear, rendering the overlying air sopping wet. The figure below shows a bull’s eye of 2.9 inches TPW in the very core of Matthew at 8 p.m. Friday. Most of that moisture was being converted into rain.