Severe weather is back in the forecast. The Storm Prediction Center has highlighted the area with a 15-percent risk for severe weather. That would translate to a Slight Risk, or a two out of five, on their one-to-five scale where five is the most severe.
This weekend is a big deal. I’ve heard from – at least – a dozen different people during the last 36 hours asking about this weekend’s forecast and the threat for severe weather. There are Boy Scouts are going to be earning badges south of Hattiesburg, the French Quarter Fest in NOLA is this weekend, folks are planning fishing trips in Alabama, and everyone else seems to want to go golfing (pretty much) everywhere.
I’m all about it! Glad people are getting out and enjoying all that the Gulf Coast has to offer. But when everyone looks at their weather app they see the little thunderstorm icon for Saturday. And, rightfully, double-check their plans. No one wants to get caught outside in severe weather.
So, taking a page out of my Hurricane Season write-ups, and borrowing a line from former United States Secretary of Defense Donald Rumsfeld, we are going to look at the known knowns. The known unknowns. And the unknown unknowns.
Known, knowns
Speaking of borrowing a line, I think that when looking at Saturday’s forecast, Ollie Williams, probably said it best: It’s gon’ rain!
Storms will move through the region starting early Saturday morning, moving from west to east. At a baseline, these storms will be capable of producing heavy rain, frequent lightning, gusty wind, and small hail.
The specific timeline is not a total “known known” but generally, Saturday is looking like a soggy day for Louisiana, Mississippi, and Alabama if you live south of I-20.
The atmosphere will also be setup to offer the chance for severe weather. That means the possibility wind in excess of 60mph, hail bigger than the size of a quarter and the possibility for a tornado across Louisiana, southern Mississippi, and southern Alabama.
Historically, this type of setup has been relatively potent. The CIPS Analogs out of Saint Louis University shows that when looking at the top 105 analogs (similar atmospheric setups in the past) it produced severe weather nearly half the time.
And that has been steadily increasing during the past five days.
Known, Unknowns
At this distance in time, there are probably more of these than most people are comfortable with. But we are still a ways out (as far as specifics from weather models are concerned). But here are a few things that we know we don’t know yet…
1. The localized severe weather parameters
While the NAM computer weather model may be showing some high numbers for the Significant Tornado Parameter, the GFS computer weather model may show a ton of instability, the localized numbers are still unknown. Generally, at this distance in time I like to use these computer models to give me a general sense of what to anticipate.
While the NAM SigTor values may show a 3.9 for Vicksburg, MS and a 0.7 for Mobile, Alabama, I don’t get hung up on specifics. To me, I’ll take that as anticipating values – generally – between 0.5 and 4. Okay.
While the GFS shows CAPE values of 1500 for Hammon, Louisiana and 400 for Meridian, Mississippi, I don’t get caught up on those specifics. I’ll read it as look at CAPE values – generally – between 500 and 2000. Noted.
But what does that mean?
Exactly. At this time, not as much as some people would lead you to believe. Generally, those types of numbers would mean that the setup is conducive to produce severe weather, but that’s about all.
The bullseye of high SigTor values don’t mean as much as where the gradient from low to high is the sharpest. And we won’t start to know that for another 36 hours.
High CAPE values are fine. But down here, the knowing the values for “Most Unstable” CAPE isn’t as valuable as knowing the values for “Mixed Layer” CAPE. And we are still 12 hours away from starting to get an idea about that.
Then we also need to look into shear, helicity, LCLs, downdraft CAPE, and a whole host of other parameters that we won’t start to know about for another 12 to 36 hours. BEcause of that, this is still a known unknown.
2. The speed of the line of storms.
The wind fields are going to be SW to NE with mid-level wind speeds between 40 and 60 knots. Historically, that is pretty normal. That would mean storm motion may be pretty quick.
But because we are still looking at coarse model data (we are still about 72 hours out) the specific storm motion and storm speed is unknown.
Also, some of the shorter-range models are starting to suggest that there is a chance that the line of storms get hung up somewhere in their journey from Louisiana to southern Alabama. If/When they may hang up could – for a short period – decrease the severe threat while increasing the flooding threat. Imagine 1″ to 3″ per hour rain falling over the same areas for three to six hours. Not ideal.
But when / if / where this happens is still a known unknown.
3. When will it hit ________ ?
We know that we don’t know this. For sure. We are still too far out for specifics like that. I know everyone has weekend plans (I do, too) but the science just isn’t there yet to give town-by-town hour-by-hour forecasting 72 hours out.
Plus, since we don’t know how fast storms are going to be moving yet, we can’t know when they will be at a particular place. Imagine asking a friend in Baton Rouge when she was leaving to drive to your house in Mobile – but not asking how fast she was going to drive. How would you know when to expect her? It is the same with these storms, we don’t know how fast they are going to be going nor do we have the scientific ability to offer a site specific forecast, so we can’t offer hour-by-hour town-by-town forecasts yet.
Unknown, unknowns
This one is tough, because we don’t know about what we don’t know. Sometimes when these systems come out of the plains they open up rather than close off. Sometimes when the occlude out, it puts a lid on the severe weather threat. Other times they negative tilt and that raises the severe weather threat.
We won’t know these things until the weather balloons start to sample the area of low pressure as it moves ashore. And that starts to happen late this afternoon and evening.
