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Watching Thursday’s strong storm threat

/ February 5, 2020 at 8:16 AM

We in the Charleston weather community continue to watch Thursday evening and overnight carefully for the potential for strong to severe thunderstorms with damaging straight-line winds and perhaps a tornado or two as a cold front pushes eastward.

The ingredients

In Monday’s post, we took a look at what ingredients would need to come together for a severe weather risk on Thursday. So, where do we stand now? Let’s take a look.

Wind shear: ✅

There will be no shortage of wind shear, which is necessary for ensuring that thunderstorm downdrafts and updrafts remain separate, allowing for continued organization. The consensus is for more than enough wind shear to sustain a squall line to the coast on Thursday. Said shear will also support a small risk of a tornado on the leading edge of the line of thunderstorms if instability can develop. (More on that in a minute.)

Strong low-level jet: ✅

High-resolution ensemble mean winds around 5,000 feet on Thursday afternoon.

The guidance suite has been more than consistent in developing a very strong low-level jet on the order of 50-70 knots across SC Thursday afternoon. This will aid the straight-line wind damage threat from any thunderstorm that can get vigorous enough to tap into the jet and bring its winds to the surface.

Regardless of any thunderstorm formation, this low-level jet will make for a very breezy day across the state. Wind advisories for gusts to 40 MPH outside of thunderstorms seem rather likely. Take today to secure outdoor items, and be ready for a breezy day on the bridges tomorrow.

Instability: 🤔

Surface instability (CAPE) ensemble mean valid 7PM Thursday, ahead of the squall line.

Instability will be the big wild card here, as is so often the case in winter. It’ll be rather warm at the surface — highs in the upper 70s appear probable away from the coast — but without a cold pool aloft to help steepen the lapse rate (rate of change of temperature as you go up), the atmosphere will remain relatively stable. This, along with cloud cover and showers ahead of the line keeping surface temperatures down, will be a major governor on our severe weather threat overall.

Additionally, it’s worth noting that surface winds will shift to the south as the front approaches. This could help a more stable marine layer penetrate inland, which will be even more of a limiting factor for a severe threat in the Charleston metro proper. We see this a lot — a nasty-looking squall line crossing I-95 that turns very meager and wimpy on radar as it approaches Charleston thanks to ingesting the more stable marine air. Let’s hope this is how it plays out!

What remains to be seen is whether the wind energy can maximize what little instability will develop. This is what we in the weather world call a “high shear/low CAPE” scenario, and it is a pain to forecast. It’s just as easy to get nothing from these setups as it is to get severe storms and even tornadoes.

Severe weather outlook

The main severe weather concerns will be wind-related. Straight-line winds appear the most likely severe weather hazard, but tornadoes on the leading edge of the squall line will certainly be possible as well (especially if more instability can develop).

Overall, the greatest risk for severe weather will be along and west of I-95. I expect a few wind damage reports in the Tri-County area, but the most active weather will take place in the Midlands and into GA.

What to expect Thursday

What to do


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