11 September 2018

Florence closing in on Carolinas; watching Helene, Isaac, and Gulf of Mexico

Florence regained Category 4 hurricane status on Monday afternoon, and is not forecast to weaken much if at all prior to landfall.  This could be an unprecedented disaster for North Carolina. Hurricane and storm surge watches now blanket much of the South Carolina and North Carolina coasts, and enormous rainfall totals could be in store for coastal areas as well as inland. Check the NHC website for the most recent watches, warnings, and forecasts.

As of 8am EDT on Tuesday, Florence is centered 940 miles southeast of Wilmington NC and is moving toward the west-northwest at 15 mph.  It is expected to turn slightly more to the north and pick up speed in the coming days, reaching the coast late Thursday night.  Impacts such as storm surge, rainfall, and strong wind will start to arrive at least a day before the eye does, so preparations and evacuations should be complete by Wednesday night.  The current maximum sustained winds are 130 mph and are expected to increase. It could become a Category 5 hurricane (155mph+) at some point, though not necessarily maintain that through landfall.

[Technical note: Overnight, Florence underwent what's called an "eyewall replacement cycle".  ERCs are normal periodic restructuring events that major hurricanes go through. The original eyewall contracts, a new larger eyewall forms outside of it, the inner eyewall dissipates, and the new eyewall takes over.  While this is happening and shortly after, the storm tends to weaken slightly and look more ragged on satellite images, but the end result is almost always a stronger storm with a broader wind field.  We will see more of these take place in the coming days.]

Storm surge is a very big concern with intense hurricanes, and Florence will be no different.  Storm surge is extremely sensitive to the track, so this far out, it's not worth trying to pinpoint where and how much the maximum will be. But large swells, elevated water levels, and coastal flooding will affect Florida through Massachusetts over the coming days (i.e. a BIG portion of the east coast).

Models are still in disagreement on the stall. Some have Florence making landfall and moving inland like "normal", while others paint a scenario where shortly after landfall, the dominant steering features in the atmosphere leave the storm without anywhere to go, so it sits for days in the same place. That would certainly be the worse of the two options. The image below shows the 7-day rainfall forecast on the left and the 3-day flash flood potential on the right.

Hurricane Helene is nearly a major hurricane now, which would be the second of the season.  It is located west of the Cabo Verde islands and is forecast to turn northward into the open Atlantic. There's a chance it could reach the Azores over the weekend, bringing rain and gusty winds even it's not technically a tropical cyclone by then.

Isaac has been battling dry air and losing, and has been downgraded to a strong tropical storm.  But, it is forecast to at least maintain that intensity and impact the Lesser Antilles in a couple days.  Tropical storm force winds should reach the Leeward islands by Wednesday night, and the storm could cross the islands on Thursday night possibly as a Category 1 hurricane (coincidentally, the same timing as Florence and the Carolinas). I'd guess that hurricane watches will be issued later this morning. There is significant disagreement in the models regarding what happens to Isaac beyond Friday, so there's almost no point in even discussing it yet.

Finally, the disturbance in the western Caribbean remains disorganized and heavily sheared. Models do not favor development into a tropical cyclone, but as it oozes toward the northwest, the chances for heavy rain in Texas still exist through the end of the weekend.


  1. Thank you for these, Brian. Will you be able to post another one of those graphics that show the European vs American models of the three storms? I really believe the more people see how much better the European models are, the harder they'll push for the U.S. to spend money upgrading our own systems.

  2. I cannot claim to either understand all the visuals or the many terms used by our wonderful weather scientists (I still don't quite get the diff between 'our' and the 'European' models). Nonetheless, I still value you all so much. I wish anything like this had been available before the 54-ish hurricane that wiped out my small NE fishing town.

    1. Hurricane Carol (one I'd never heard about). Read on Wiki, the storm surge in Narragansett exceeded the Hurricane of 38.

    2. Hurricane Edna also impacted New England in 54 (another one I'd never heard about)...

  3. What a concise and useful pursuit in which you diligently remain engaged. Many thanks to you as my sister and niece are in the way of Florence, along with their respective families and friends. Alas, the best they can do is to follow your updates as they batten down the hatches. Cheers, mate. Please carry on!

  4. Brian -- I've just discovered your blog (via a link from the New York Times website) and wanted to thank you for sharing your expertise and insight. I will bookmark! Question: has there been an effort to compare predictions made for hurricane tracks through the US and European models, and the actual outcomes?

  5. Thank you for the feedback. I will include the ensemble comparisons, but not in every post... I agree they are useful, especially in the longer range outlooks when uncertainty is so high. I am also happy to hear that you find these updates helpful! Regarding the model evaluations, yes, NHC does a detailed verification of their own forecasts as well from each model at the end of the season.
    Cheers, and stay safe!