October 22: Borderline Severe Risk Today



Let’s get the easy part of today’s forecast out of the way: you’re pretty likely to get some free lawn-watering today.

Now the complicated part: will we see any severe weather? To evaluate that, let’s start with the severe thunderstorm “recipe card”.

The storm system moving in from the west is the same one that produced the tornadic storms in Dallas and Memphis over the past couple of days, so we definitely have a trigger to get storms going.

We’ll also have enough humidity to fuel some stronger storms this afternoon, with dew points likely to climb into the 60s. That’s not an oppressive humidity level, but it’s sufficient for the storm threat.

One thing we’ll definitely have is a lot of wind energy in the atmosphere overhead. We measure that with a statistic called “storm-relative helicity” — it measures the change in wind speed and direction in the lowest 3 kilometers of the atmosphere. Those SRH values are forecast to be quite high across central North Carolina by early afternoon.

That’s three check marks on the recipe card…leaving us with two big ol’ question marks. What’s really in doubt is how unstable the atmosphere will be — high instability occurs with warm air near the ground and cold air in the upper atmosphere.

This part of the forecast could go either way. The big wild card is this morning’s shower activity — if we don’t get a break between the morning showers and the afternoon storms, the atmosphere won’t have time to warm up and become unstable.

Unfortunately, the HRRR model (one of our best tools for diagnosing short-term weather) shows just enough of a break for the atmosphere to charge up. Here’s its radar simulation from 8:00am through 8:00pm, showing morning showers, an early afternoon break, then scattered strong/severe storms from mid to late afternoon.

Everything depends on how much we warm up during that short break (if the break happens at all). The HRRR model shows temperatures reaching the mid to upper 70s (maybe even near 80° farther south) right before the storms move in.

That’s warm enough to produce a decent amount of energy in the lower atmosphere — we measure that with a statistic called Convective Available Potential Energy.

The combination of humidity and warmth results in CAPE values that aren’t nearly as high as what we’d see in late spring or summer, but “high enough” for an elevated severe weather threat.

The Storm Prediction Center’s ensemble model shows the best combination of ALL of the severe weather ingredients shaping up from the Triangle to the south and east this afternoon.

The SPC has included most of central North Carolina in a “Slight Risk” (level 2 of 5) for severe weather. That reflects the potential for damaging winds and isolated tornadoes, but also takes into account our uncertainty about whether the storms will even get going in the first place.

If all of that has you a little confused…

…let’s break into the best-case and worst-case scenarios.
BEST CASE: The rain and clouds stick around long enough to keep temperatures in the 60s. No severe storms…in fact, hardly any lightning and thunder at all.
WORST CASE: Temperatures warm up to the upper half of the 70s, southerly winds bring even more humidity into the area, and numerous severe thunderstorms develop this afternoon.

I really think there’s a significant chance — probably better than 50/50 — that we end up with that best-case scenario. But in situations like this, “better safe than sorry” is the operating rule. So just stay weather-aware this afternoon, and we’ll be here keeping a close eye on things for you.

The rest of the week looks calm and pleasant! A chance of showers returns to the forecast Saturday and maybe Sunday, but the long-range data is all over the place right now. I think the best chance of rain will hold off until early next week — we’ll keep you updated on that once today’s storm threat is off the board.


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