Thursday, August 27, 2020
Hurricane Laura and the Wind Speed Dilemma
Last night, Hurricane Laura made landfall on the southwestern coast of Louisiana, bring heavy rain (6-8 inches), strong winds (gusting to 132 mph at one location), and a coastal storm surge (roughly 10 feet at the most vulnerable locations).
The NWS Lake Charles radar image at midnight central time showed a well defined eye as the storm was making landfall.
Now the dilemma and interesting part. Based on reconnaissance aircraft and other information, the National Weather Service’s National Hurricane Center had estimated that Laura was a Category Four hurricane just prior to landfall, and according to the official Saffir-Simpson scale, that means the sustained surface (10-m) winds, averaged over a few minutes, were between 130 and 156 mph (see below). Not gusts, sustained winds.
Saffir-Simpson Hurricane Categories
But here is the issue. What were the maximum sustained winds that occurred last night as Laura made landfall? Looking at all available stations, the highest sustained wind was 98 mph at Lake Charles Airport. The map below shows the sustained winds at 1 AM, when the storm was just moving inland (wind barbs show sustained winds, with gusts in red). The blue arrow indicates Lake Charles Airport.
Looking at the sustained winds, one would conclude that Laura was only a weak category two hurricane (96-110 mph).
And then there are gusts. Gusts are not used as part of the Saffir-Simpson hurricane scale, but, let’s face it, gusts are very important. The big damage in most storms are done by the gusts.
Below are the maximum gusts of Laura. Two locations are extreme: Calcasieu Pass on the coast and Lake Charles, a few miles to the north (127 and 132 mph gusts, respectively)
Such strong gusts are consistent with the destruction of the NWS radar dome at Lake Charles Airport–they are rated to handle up to about 135 mph. (see the before and after below).
So what is going on? How strong was the storm? Category two or four?
A key issue is friction and drag, which is much greater over land (with trees, hills, buildings, etc) that over the aerodynamically smooth water. As a result of this surface drag, winds decrease VERY rapidly over land, even if the hurricane remains relatively intact aloft.
Let me illustrate this visually, by showing you a forecast by the state-of-the-art NOAA/NWS HRRR model as Laura made landfall. These plots show surface (10-m) surface wind in knots (1 knot=1.15 mph)
Before landfall (9 PM PDT), a nice hurricane structure is apparent, with some winds getting to 90 knots in the eyewall.
But then as the storm makes landfall (1 AM PDT), you can see a profound weakening of winds over land.
And by 5 AM PDT, with the storm completely over land, the fastest winds are gone.
So even if the storm had category four sustained winds near the surface while it is offshore, the sustained winds decline precipitously when the store goes onshore.
But yet the storm can still remain very, very dangerous in the hours after landfall.
First, even the reduced sustained winds (e.g., 90-100 mph in this case) can produce great damage.
But there is more. Gusts don’t necessarily decline as rapidly as sustained winds as the storm moves over land.
To illustrate this, here is a plot of the predicted gusts as the storm made landfall. Not as much a decline over land as for sustained winds. Gusts are caused by the intermittent mixing down of faster (higher momentum) air from aloft down to the surface. So even if winds are slower down lower, sometimes air from aloft…where the winds are still blowing hard…can be mixed to the surface. So gusts can hold out longer than sustained winds as a storm makes landfall.
The bottom line: a storm that was category four over water can still maintain a real “punch” over land, even after it nominally declines to a category two. Strong, damaging gusts can remain, even when the sustained winds decline.
Some excellent articles on the surprisingly low wind speed over land during hurricanes, by meteorologist and writer Bob Henson, can be found here:
Will do a weather forecast and discussion tomorrow-Friday (video on my blog)
My blog on the KNKX firing is found here.