16 September 2018

'Florence the Terrible' rewrites North Carolina's history books

As of Sunday morning, Tropical Depression Florence is located just 230 miles from where it was on Friday morning when it made landfall... that's an average speed of 4.8 mph.  This slowing to a drift right near the coastline was well-predicted several days prior to landfall, so while it's a success for the weather models, it's a disaster for the affected areas.

Estimated three-day rainfall totals, from Thursday morning through Sunday morning.
At 5am Eastern on Sunday, Florence was centered near Columbia SC and had peak winds of 25 mph.  Of course, it's not the winds that were or are the concern, it's the RAIN. "Florence the Terrible" has smashed the record for North Carolina's wettest tropical cyclone... Floyd (1999) held the record at 24.06 inches, but Florence's maximum so far (it's still adding up) is at 30.59 inches as of Saturday night.  Large swaths of eastern NC have received over 20 inches since Thursday morning.

And this is not over yet.  The outlook for the next three days includes even more rain focused on North Carolina.  And even after the rain departs, the rivers will keep rising as they feed into each other and accumulate. (If you are near a river in the affected areas, check https://water.weather.gov/ahps/ for forecast flood stages... the crest could still be several days away).

Three-day rainfall forecast, valid from Sunday morning through Wednesday morning.
It's worth repeating again today that "there's more to the story than the category".  What does that mean exactly? The category rating we hear about refers only to the peak wind in the storm -- it says absolutely nothing about the size of the storm, the storm surge potential, or the rainfall potential. So while you may still have a roof on your house because "it's only a Category 1", you may also be desperately hoping to get rescued from that same roof because of the flooding.

Tornadoes are always a threat in rainbands, and so far there have been two tornado reports in southeast NC, with the threat still active today. Tornadoes embedded in rainbands are typically on the weak end of the scale, but move extremely fast and with little to no warning. Plus, with saturated ground, it doesn't take much wind to topple trees.

Locations of the two tornado reports as of 6am Sunday (left) and tornado risk for the rest of Sunday (right).
This long regional radar loop starts on Wednesday night and runs through Sunday morning... it gives an idea of how slow the motion has been, and how the warm ocean provides an endless fuel source to keep intense rainbands energized that can dump rain at a rate of 2-3 inches/hour for hours at a time over a given location.  The latest version is available at http://andrew.rsmas.miami.edu/bmcnoldy/tropics/radar/

Elsewhere, Helene and Joyce will both be "terminated" today as they become extratropical cyclones, and advisories were ended for Isaac on Saturday morning, though the remnants are being monitored for potential redevelopment.



  1. On the radar mosaic, what are the artifacts of radial explosions to the south and west of the hurricane? Particularly near the beginning of the loop, in the Southern corner of South Carolina.

    Thank you for blogging...absolutely fascinating.

  2. Great question! They are very prevalent in the summer... it's bugs and bats! You see radar echoes swell with activity shortly after sunset in warm humid places, then that clutter drops off quickly by sunrise.

  3. Bugs and bats?! What are the bugs and bats hitting? I had assumed radar was taken from satellites, and cannot imagine satellites being low enough to get bugs and bats hitting them. Can you give more details of how bugs and bats create those radial blips that look like they are hundreds of miles across?

  4. These radars are on the ground (technically there are some in space but that's not what you'll ever see for routine weather coverage). The National Weather Service operates 152 of them in the US: https://radar.weather.gov/. Radars work by sending out pulses of energy, and when a portion of that of that pulse hits something, a fraction of the energy bounces back and is received by the radar antenna. That "something" doesn't have to be just rain. The more power that's returned, the higher the "reflectivity" on a radar image. And since radars sit right near the ground, the lowest scan angle is tilted above the horizon, so the further away from the radar site you go, the further above the ground you're seeing.