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.


14 September 2018

Florence makes landfall but life-threatening storm surge and rainfall lingers

Radar image from the time of landfall.
Florence made landfall around Wrightsville Beach NC at 8am on Friday as a Category 1 hurricane. As expected and feared, the flooding from both storm surge and rainfall has been terrible and will continue throughout the weekend. Widespread catastrophic wind damage is the only aspect of the storm that was avoided because of its pre-landfall weakening.

Satellite image from the time of landfall.
PRELIMINARY TOTALS: From Thursday morning through Friday morning, Morehead City up near Cape Lookout has received 30" of rain, and down by Cape Fear, Wilmington is at 17" so far and Wrightsville Beach is at 18.5".  Unfortunately, there's lots more to come. Additionally, the storm surge up in the New Bern area has been reported at 10-15 feet, just to give an idea of the severity of this.

24-hour rainfall totals, spanning Thursday morning through Friday morning (https://nc.water.usgs.gov/realtime/rainfall.php)
The storm surge and rainfall will continue today and into tomorrow.  Coastal areas will experience multiple peaks in the storm tide (that's the total observed water level which is a combination of the astronomical tide and the storm surge) because the strong onshore winds will persist through several high tides. Florence is just drifting to the west at 6 mph -- at that rate, it will move less than 100 miles by tonight.

Storm surge warnings still cover a lot of the NC coast and the northern SC coast. Inland flooding due to excessive rainfall is likely from today through the weekend.
After a slow, drawn-out landfall from southern NC to northern NC, Florence will eventually start pulling out of the Carolinas by Monday morning.  The storm surge will have ended by then, but not the flash flooding. Beyond that, it should turn toward the northeast and dissipate... but still bring gusty winds and heavy rain to the northeast on Monday and Tuesday.

A key phrase and message that needs to be repeated over and and over is "there's more to the story than the category!".  The category rating of a hurricane only refers to the peak sustained winds in the eyewall, so it does not account for the storm's size or the tremendous impacts from storm surge and rainfall.  There are many examples of Category 1 and 2 hurricanes being far more destructive, deadly, and costly than the rarer, stronger "major hurricanes" (Category 3+).

These non-wind-related risks have been emphasized for many days... pulling from some of my posts over the past 8 days:

Sept 6: "an inland track would bring multiple hazards, including torrential rain and damaging winds over coastal and some inland areas, and a substantial rise in water above normally dry land in coastal zones. A storm hugging the coast offshore would result in heavy rain and strong winds near the coast, dangerous surf, beach erosion and coastal flooding" 

Sept 7: "coastal areas will deal with a damaging wind threat, flooding rain and a substantial storm surge — which is a rise in ocean water above normally dry land. Some areas further inland could also contend with damaging winds and flooding rain."

Sept 8: "impacts such as rainfall and storm surge will blanket a large portion of the coastline regardless of if and where it makes a direct landfall."

Sept 9: "threat in the Carolinas and Virginia: a life-threatening storm surge at the coast [and] life-threatening freshwater flooding from a prolonged and exceptionally heavy rainfall event” from the coast to interior sections."

Sept 10: "storm surge and coastal flooding will almost certainly be a big problem from North Carolina up into the Long Island area. ... models suggest ... a stalled tropical cyclone producing incredible and dangerous amounts of rain over the mid-Atlantic region."

Sept 11: "This could be an unprecedented disaster for North Carolina ... enormous rainfall totals could be in store for coastal areas as well as inland ... Storm surge is a very big concern ... large swells, elevated water levels, and coastal flooding will affect Florida through Massachusetts over the coming days"

Sept 12: "Florence is still expected to reach the coast early Friday morning as a major hurricane, but could actually drift southward along the coast, delivering a destructive storm surge to hundreds of miles of coastline, as well as feet of rainfall at the coast and inland."

Sept 13: "The storm has grown in size over time as they typically do, so although the peak winds are weaker than they were, the storm surge threat has actually increased for more of the coastline. Rainfall has been and continues to be a significant danger.  A tropical cyclone of any intensity can dump feet of rain if it moves slowly, and Florence is no different. 20-40 inches of rain could fall over parts of eastern North Carolina which would easily induce tremendous flash flooding. Even further west into the mountains, 6-10 inches of rain could fall. The rain will persist for a few days in this region."

The public must get beyond the "it's only a Category 1" mentality.  Wind is one of the three primary hazards associated with tropical cyclones, and in the US, it's by far the least deadly and costly.

The rest of the tropics can be summarized in a single map... Helene is quickly losing its tropical characteristics over the cold eastern Atlantic but could bring stormy conditions to the Azores, Isaac has entered the eastern Caribbean and weakened to a tropical depression, and Joyce is a tropical storm that will orbit around and get obliterated by Helene in the near future.  This surge in activity will come to an abrupt end this weekend.

13 September 2018

Florence's wind and rain reaching eastern North Carolina; also watching Helene, Isaac, Joyce

It's been two weeks since Florence formed, and early Thursday morning, the first rainbands and tropical storm force winds from this long-track hurricane have reached Cape Hatteras and Cape Lookout (lots of radar loops).  At 8am, the storm was centered 170 miles east-southeast of Wilmington NC and moving toward the northwest at 12 mph.  Maximum winds in the eyewall fell to 110 mph, making it an upper-end Category 2 hurricane.  It is not expected to strengthen anymore, even as it passes over the warm Gulf Stream today.

However, THERE'S MORE TO THE STORY THAN THE CATEGORY. While the peak winds have fallen, they are still strong enough to be destructive to houses and trees, and the water-related threats (storm surge and rain) remain a very big concern.  Tropical storm force winds extend up to 200 miles from the center.

The storm has grown in size over time as they typically do, so although the peak winds are weaker than they were, the storm surge threat has actually increased for more of the coastline. The storm surge warning from Cape Fear to Cape Lookout still includes a potential for 9-13 feet of storm surge, on top of the normal tides (flooding will be worse around high tide, and a slow-moving storm will stick around for at least one tide cycle). Now that the landfall trajectory is fairly locked in, here are some storm surge inundation maps for the Cape Lookout and Cape Fear areas, but I encourage you to peruse it in more detail at https://www.nhc.noaa.gov/refresh/graphics_at1+shtml/093018.shtml?inundation#contents
You can also monitor relevant tide gauges at https://tidesandcurrents.noaa.gov/quicklook/view.html?name=Florence

Rainfall has been and continues to be a significant danger.  A tropical cyclone of any intensity can dump feet of rain if it moves slowly, and Florence is no different. 20-40 inches of rain could fall over parts of eastern North Carolina which would easily induce tremendous flash flooding. Even further west into the mountains, 6-10 inches of rain could fall. The rain will persist for a few days in this region.  The latest forecast does not indicate quite as much of a prolonged stall, but rather a gradual drift westward then turning northward out of the Carolinas by Monday.

Only 29 hurricanes have made landfall within 100 miles of Wilmington since 1851... the most recent was Arthur in 2014, and the most intense was Hazel in 1954.  Florence will become the 30th tonight.

A brief overview of the rest of the tropics, which are bursting with activity.  Hurricane Helene is quickly disintegrating as it heads north over cool water, Tropical Storm Isaac remains disorganized as it passes over the Leeward islands, Subtropical Storm Joyce is spinning innocently well west of the Azores, and that pesky disturbance in the Gulf of Mexico is still a rainfall threat for Texas this weekend.

Since Joyce formed yesterday, the season is up to 10 named storms, 5 hurricanes, and 1 major hurricane.  In an average season by this date, we'd have 7 named storms, 3 hurricanes, and 1 major hurricane. In terms of ACE (Accumulated Cyclone Energy), the season is at 127% of average for the date.