Monday, October 5, 2015

Historic Flooding in South Carolina

Monday, October 5, 2015
2:49 pm

Flood waters in Charleston at intersection King Street and Huger. Credit: US Army National Guard

The Pacific Northwest is no stranger to flooding. I believe that Washington has had the most federal disaster declarations of any state, which may be surprise you due to our seemingly-mundane weather. But we get these massive floods from time to time, and we also have a tendency to get strong winds, especially on the coast. We have extremely heavy snows and avalanches in the mountains as well. Our natural disasters aren't just limited to the weather... we've got earthquakes, volcanoes, and landslides too. Let's not forget about those wildfires either. When you think about it, we live in a pretty disaster-prone place, even though it might not seem like it when it rains an entire day and we've only got a tenth of an inch of liquid sunshine to show for it.

But there are times when we are reminded that no matter how hard we try, we just can't outdo some of the severe weather found in other parts of the country. And this is one of those times.

The stalled rain band over South Carolina on 16:10z October 3, 2015. Notice Hurricane Joaquin to the southeast. Credit: NASA Worldview Terra MODIS Satellite

South Carolina is just recovering from an extraordinary rainfall event, and North Carolina got hit pretty hard as well. The rainfall was primarily caused by two things: strong easterly flow due to a low pressure center over the southeast and a high pressure system way up north by Hudson Bay and convergence due to air circulating around the southeast part of the low. Converging air rises and forms clouds and precipitation, and in this case, there was a lot of convergence, and therefore a lot of rain. Additionally, once this stream of moisture went ashore and interacted with terrain, the air was forced to rise more, resulting in even heavier rainfall. This rain was NOT directly associated with Hurricane Joaquin, which was just to the southwest. However, Joaquin did provide a tropical moisture source which further contributed to the heavy rainfall. In the end, the strength of the convergence, the uplift from terrain, the stationary nature of the rainband, and the influence of nearby Hurricane Joaquin all combined to create a massive flooding event for Southern Carolina.

Rainfall accumulations across the Carolinas and surrounding states from October 1–4, ending at 6:24 p.m. EDT (22:24 UTC). Areas in white indicate accumulations in excess of 20 in (510 mm). Credit: Raleigh, NC NWS Twitter Page

The picture above shows 4-day rainfall totals from October 1-4 ending at 6:24 pm EDT on the 4th. As such, it does not represent final rainfall totals, but it represents most of what they got. The rainfall is off the charts. The chart only goes up to 16 inches, but those areas of white that you see in South Carolina represent areas where over 20 inches were estimated to have fallen! The Pacific Northwest has had very impressive rainfall totals before, but I can't remember anything like this, especially for lowland areas. For South Carolina, many locations got over 20 inches, and while this was measured over four days, the majority of the rainfall occurred over the weekend. Here are some South Carolina rainfall statistics I found on (I'm sure they got them from the National Weather Service). Amounts are in inches, and these are through October 4th.

Mount Pleasant: 26.88
Kingstree: 21.80
Dalzell: 21.66
Georgetown: 20.95
Sumter: 19.43
Summerville: 17.75
Charleston Airport: 17.29
Moncks Corner: 17.02
Charleston (Downtown): 16.29
North Myrtle Beach: 13.84
Columbia (Downtown): 11.93
Greer: 6.30

By the way, the previous state record for most rain in 24 hours was 14.80 inches, set in 1999 with Hurricane Floyd. Numerous places smashed this record.

Mt. Rainier National Park: Left pic, November 5, 2006. Right pic: same location two days later. Credit: National Park Service

Washington has had many major flood events, and I remember one back in November 2006. An incredible amount of rain fell throughout Western Washington and the Cascades from November 5 to November 7. I went to Snoqualmie Falls on November 7 - a Tuesday - and it was as muddy and high as I had ever seen it. It was something of a spiritual experience for me. Still, Seattle got 3.29 inches on the 6th, good for our 4th rainiest day ever (the rainiest day was October 20, 2003, when Sea-Tac picked up 5.02 inches of rain!). Meanwhile, Charleston got 11.50 inches of rainfall on Saturday, breaking their old record of 10.52 inches set back on September 21, 1998. We can get very wet here and have catastrophic flooding both in the mountains and very rarely in Seattle (the December 14, 2006 flood comes to mind, where a woman in Madison Valley died), but being in such a moderate climate, we tend not to see the flash flooding that South Carolina got this past weekend.

You'll see many media reports saying that this is a "once in a thousand years" flood event. I decided to take a look at this, and it turns out that while this event is unprecedented in many areas, the "once in a thousand years" might be a bit overstated. Technically speaking, "once in a thousand years" means that an event of this magnitude has a .1% chance of occurring any given year, but headlines like "South Carolina Reeling From ".1% Chance Of This Bad Of A Flood Occurring Any Given Year" Flood" just don't, well, make that much money.

There's also talk of climate change, but keep in mind that heavy rainfall events have always happened and always will. There is evidence that heavy rain events will increase in severity (for example, more "Pineapple Express" flooding events are expected here in the Pacific Northwest in a warming climate), but it is far from conclusive. Besides, the global warming signal is fairly weak throughout the globe right now with the exception of the far northern latitudes. Long story short... let's talk about the "new normal" in 2100, when our climate will be significantly different from what it is today. The climate is not much different today than it was 20 years ago.

Historic rainfall in SC -- only happens once every 100-200 years. Credit:

The National Weather Service compiled a neat little graphic showing the return period of events like these, and it looks like it's more of a 100 to 200-year event for most places. There are some places where it looks to be off the charts and that the "1000-year event" label is justified, but these places look few and far between. The media can be very misleading... don't let it fool you!

Enjoy the nice weather here! We'll have a couple weak systems coming through this week, but nothing major. However, I don't think it's misleading to say that storm season is right around the corner!

Thanks for reading,

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