Heavy rain followed by Helene's impacts could cause major flooding in the southern Appalachians

AccuWeather

After Helene makes landfall, it will slam the Southeast, striking the southern Appalachians just a day after a separate rainstorm, which may lead to extreme flooding.

Transcript

00:00Joining us right now is AccuWeather hurricane expert Dan DePodwin and Dan we have a strengthening

00:04storm on our hands in the area of the western Caribbean. We're looking at a very likely

00:10intensification to not just a category one but probably a category two hurricane by this time

00:14tomorrow. So Dan in advance of this it's not just going to begin dry in parts of the Carolinas.

00:22Yeah you're right Jeff and we wanted to really focus in here on obviously there's many risks

00:26from hurricanes, storm surge, damaging winds, but the flooding rain the inland flood threat can be

00:30sometimes overlooked and unfortunately it causes on average the highest amount of fatalities from

00:35tropical systems and that is one of our significant concerns with Helene and that is sort of

00:42amplified by the fact that we have a significant rain on the way before Helene even reaches the

00:47area. You can see on Wednesday afternoon here heavy rain over Atlanta metro area,

00:51upstate South Carolina, the southern Appalachians and that's well in advance of Helene circulation

00:55which is still well south in the Gulf of Mexico by Wednesday evening again heavy rain

01:00that sort of stays in the same place there over the southern Appalachians and

01:04northeastern Georgia. So we are very concerned here what could turn into a very significant

01:09life-threatening flood risk especially as you head into Thursday and Thursday night as Helene's

01:14center of circulation moves northward and we bring all of that tropical moisture on top of what could

01:20be three four inches of rain that's already fallen on Wednesday and Wednesday night. Yeah it's a very

01:24dangerous situation to get three or four inches of rain in advance of a hurricane and Dan the

01:28upslope flow there coming from the lower elevations to Piedmont in North Carolina up into the high

01:32ground that's a huge storyline as that's going to drive a lot of extra moisture out of the

01:39atmosphere there wringing it out so we're looking at how much rain in these areas. We're looking at

01:43eight to twelve inches of rain across a lot of the southern Appalachians and that is and some of

01:48that three four or five inches that could fall in just a course of several hours so we think in

01:52a few areas there could be a flooding disaster because of these high rainfall totals. We are

01:57flinging a ton of moisture in fact some record amounts of moisture for the southeast in September

02:03up the terrain which is always a recipe for very heavy rainfall. And Dan just in general if you get

02:09eight or nine inches of rain in the mountains compared to eight or nine inches in the coastal

02:13plain how does that behave differently? Yeah it behaves very differently Jeff if eight inches in

02:18a lot of Florida and a lot of the Gulf Coast places like Houston they can handle six to eight

02:22inches of rain as long as it comes in a several hour time frame but even three or four inches

02:27of rain in complex terrain that includes the southern Appalachians we've seen it happen many

02:32times in those locations as well as parts of the northeast just recently this summer in Vermont

02:36and that's what's caused our concern for this flooding in areas of terrain here in the southern

02:41Appalachians. And we may be introducing a high risk there we have introduced a high risk into

02:46western parts of North Carolina big concerns in these areas and the wind is also a huge story on

02:51the east side of the eyewall. It is a concern we'll be talking more about that here as we head

02:55through the rest of the afternoon and evening that wind risk can really extend far inland

02:59uh not just 40 to 60 mile per hour gusts but 60 to 80 mile per hour gusts even maybe getting

03:03close to Augusta Georgia. And a lot of trees in that area so it's the eastern eyewall there we'll

03:08see the most tenacious winds but the winds will expand inland as well even separate from the

03:12center of that storm. AccuWeather hurricane expert and senior director of forecast operations Dan

03:17DePodwin thanks again Dan. You're welcome Jeff. All right and a quick look at what else is going

03:22on out there with this storm just the general big picture timeline we want you to be aware

03:27that Helene will continue to strengthen uh into and through the northwest Caribbean late today

03:31it's been named this is only going to strengthen from here on uh and it will likely rapidly

03:36intensify during the day tomorrow as we move through an area of very high moisture content

03:41in the atmosphere and overall we're looking at some big concerns with the again abundant

03:47abundant warm water into the eastern gulf and overall there is not much wind shear in the

03:53path of this storm there may be some wind shear right now that's going to be decreasing

03:56as time progresses so you can see that warm water in place here mid-80s. Here's another depiction

04:02of the ocean heat content and we realize that this map may look a little different there's

04:06some grainy edges to the data and so forth but we wanted to give you an indication of some of

04:11the raw data here ocean heat content we're looking not just at the water temperature but also the

04:15depth of that warm water and it's the eastern gulf where we do have some high oceanic heat content

04:21compared to other areas as we close in closer to the coastline that variable will be decreasing

04:27but there will be a period of time until maybe 12 hours before landfall where it's over some very

04:32high oceanic heat content we still have some winds here in the path of this storm over southwest

04:36florida and all the way down to the straits between cuba and cancun but look at this the

04:41lighter shade of lavender begins to expand a little bit farther north with the storm so there will be

04:47a decrease in wind shear that will allow the storm to strengthen and there will be no shortage of

04:52moisture here abundant moisture into the eastern gulf that this storm is going to be transporting

04:57in that direction so short term damien the eyepath is going to wrap it around the west side of cuba

05:03but we want to take it forward in time and we have nudged back the time of landfall

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