Key Points:
- South Carolina will be on the fringe of a major severe thunderstorm and tornado outbreak through Sunday.
- The risks include damaging wind, isolated tornadoes, damaging hail, and flash flooding as the thunderstorms impact us tonight and Sunday.
- There is still uncertainty about the timing. The latest model guidance looks like Friday's with respect to timing, but we remain concerned that storms could arrive earlier than expected.
- Before the storms, take some time to review tornado safety tips (see the last paragraph and the links therein).
- Those traveling into Tennessee, Georgia and Alabama this afternoon and tonight must be on high alert for severe thunderstorms with damaging winds, violent tornadoes, and large hail.
The Ides of March has historically been a rough day, not just for Julius Caesar. A once-per-decade extreme outbreak of severe thunderstorms and tornadoes is underway over the Deep South, centered over Alabama and Mississippi. We can relate; a terrible tornado outbreak hit South Carolina and neighboring states on this day in 2008. We will see a share of the severe storm action tonight and Sunday. Here are the outlooks for now through Sunday night from the Storm Prediction Center (SPC):
SPC has a high risk for today and tonight over in Alabama and Mississippi, y'all. That's the top of the scale for these outlooks and something you see maybe once per year on average.
So, the people to our west are about to have an awful afternoon and night of severe weather, including violent tornadoes, damaging wind, and large hail. It's likely to be even worse than last night in the Midwest and Mississippi Valley; there were over 400 reports of severe weather, including 24 tornado reports (the actual number of tornadoes will be lower because some of the reports were for the same long-tracked tornadoes). All this sounds scary, and it is, so let's be grateful that we're not ground zero for this outbreak (like we were in 2008).
Thunderstorms underway over the Tennessee Valley and points south will consolidate into a squall line that rakes across South Carolina after midnight tonight through Sunday.
The forecast for the timing remains challenging. Squall lines like what we expect to see normally plow eastward rapidly, but all our guidance tells us that the line will slow down while it's moving through South Carolina. It still looks like the squall line will arrive in the Upstate as early as 2 a.m., though the higher ground northwest of I-85 will see some rain and drizzle at times well before that (It may already be misty in some places up there). The leading edge of the storms will likely stretch from Rock Hill to Aiken by 6 a.m., crossing the Central Savannah River Area, Midlands, most of the Lowcountry, and the upper Pee Dee during the morning. The rest of the state gets hit during the early afternoon. The caveat emptor here is that, as implied above, these squall lines often move faster than expected, and you'll want to remain mindful of this possibility.
Simulated weather radar (from today's 1200 UTC run of the HRRR model) from 8 p.m. today through 8 p.m. Sunday shows the model's forecast progression of the squall line through South Carolina. Keep in mind that the model's prediction will be imperfect and this is only provided to give a general idea of the timing of thunderstorms and their intensity as they cross the state through Sunday evening.
Image Source: WeatherBELL
Another uncertainty is the potential for a second line of thunderstorms to form behind the first. The model output above shows it will start along I-77 on Sunday afternoon, but others show it farther to the west. The severe storm risk with this second line will be lower but not zero.
All three severe storm threats will be present: damaging wind, isolated tornadoes, and large hail. Locally damaging wind gusts will likely occur as the line of storms moves through. The tornado threat is the second concern. It helps that much of the state will have the storms pass through during the coolest and least unstable part of the day. However, that models show a broken line of storms moving through rather than a solid squall line increases my concern for tornadoes. If that's how it goes tonight and Sunday, we'll see individual thunderstorm cells in the line ("discrete cells" is the weather nerd jargon). They are more likely to become supercells (the type of thunderstorm that spawns most tornadoes) when you have a sheared (winds changing direction with height) lower atmosphere, as we will tonight and Sunday. As for the hail, supercells can generate a lot of it, so isolated damaging hail is a concern, too.
Also, there is a risk for isolated flash flooding due to training thunderstorms or slow-moving downpours. This may be only poor-drainage flooding if the storms maintain a faster pace.
The forecast is on track for Monday and beyond: cool Monday with a chance for a shower in the Pee Dee, dry and pleasant Tuesday, nice and warm Wednesday, there's a chance for showers and maybe a thunderstorm Thursday, then chilly mornings on Friday and Saturday.
So, what do you do to lower your risk? For starters, it's essential to prepare in advance. Decide on your best shelter (interior room, basement, under a staircase, etc.) ahead of the storm so that you can react when you get the alert for a tornado warning; you may not have time to evaluate shelter options when you get a warning. That's especially true tonight because parts of the state could see a tornado in the middle of the night or early in the morning when most people sleep. Be sure you have at least two ways to receive warnings (mobile device emergency alerts, mobile apps, a weather radio, SCEMD's CodeRED alerts, etc.), and they need to be LOUD so they wake you if you're sleeping. Move fast to your shelter if a tornado warning is issued; get yourself and your family to shelter first and evaluate the situation when you get there. Also, it's good to have some protective items in your shelter, such as a bicycle or motorcycle helmet (head trauma is common in tornadoes), and work gloves, sturdy clothes, and sturdy shoes are a necessity in the aftermath. Finally, take some time to review tornado safety information from the National Weather Service and SCEMD to get more prep tips.