The Storm Prediction Center is highlighting parts of Louisiana, Mississippi and Alabama for severe weather this weekend. As a potent storm system emerges into the Southern Plains and Mid-South it will drag a warm front to the north on Saturday across the region. That will be followed by pushing a cold front through the same area on Sunday.
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But that isn’t the only story here. It could be argued that the do-nothing front on Thursday-Friday (associated with the blizzard in the upper Midwest) may play a role in the overall evolution of the severe threat this weekend.
But in order to understand why, we need to go back to school…
Time to Science
Today across the Gulf Coast, it won’t be terribly humid. Sure, there will be humidity, but you know what real humidity feels like. That starts to change on Thursday. As the area of low pressure associated with the blizzard opens up into the upper mid west, it will pull a decent fetch of moisture to the north from the Gulf of Mexico.
Because that system will be so far to the north though, a real cold front won’t really make it through the Gulf Coast region.
You can see on the 850mb dew point map, that the moisture (green shading) surges north ahead of the system in the plains and midwest. But no cold front kicks the all of moisture back south of the area. There is a frame or two of not-as-green in the Gulf Coast area, but not enough of a kick to dry out the lower levels.
This may be highlighted better by looking closer to the surface.
The above map is the surface Theta-E map. I could get into the math regarding it – but I’ll save everyone (for today) the long-winded explanation – but suffice it to say that it is a good measure of “thunderstorm fuel” available at the surface. And that “fuel” is just sitting around waiting for the next system to push through and use it.
And the next system shows up on Saturday and Sunday.
MOAR SCYENZ!
Well, as the next system shows up on Saturday, the baseline available moisture is already a bit higher. And that is bad news for those who like tranquil weather because the temperature in the low levels ahead of this next system are going to be a few-degrees-Celsius warmer than recent systems.
This is where we get to take one prediction, and follow the chain, and watch it lead to other predictions. To start, though, I’m going to use an analogy.
Imagine you want to wash a car. And you have a bucket of water. A five gallon bucket. You filled it up 4/5ths of the way. So, you have four gallons of water to wash the car. Now imagine your friend comes over to help. But he brings his own five-gallon bucket with four gallons of water, too.
If you combined your water into one bucket, you would still only be able to have access to five gallons of water.
But imagine, if he brought a 10-gallon bucket. And it had four gallons of water in it. You would then be able to add your four gallons to his four gallons and have eight gallons of water to use to wash the car. Because the bucket can hold that much more water.
The analogy to the warmer atmosphere being able to “hold” more water is a pretty rough one, and not completely accurate for physics purists – but it is applicable here.
So, we have the first system that pooled higher dew point values (water in a bucket) but didn’t use it. the dew point values are just sitting there. Then the next system moves in, with its own batch of high dew point values (water in its bucket). So the second system now has access to more moisture. But recall, the dew point can’t get any higher than the temperature. Recent systems have had lower temperatures, so no matter how much recent systems wanted to have more moisture, they couldn’t. There was no room!
But this next system will be warmer. It will have room. And it will have leftover moisture from the previous system, too.
It is, in a way, carrying a bigger bucket, to a friend who already has water.
Okay, okay. I know we got into the weeds pretty quick there. But that is taking one projection, and following the daisy-chain of physics, to lead to a forecast.
The place where forecasting becomes tough: Just because the system has access to the extra moisture and is using more heat, doesn’t definitely mean the system will use it to make severe weather. There needs to be other things happening, too.
Then What Happened…
J-Live album references aside, now that we have the foundation in place it is time to look at some of the other things happening in the atmosphere and how things are aligned.
Sadly, model guidance suggests that things are aligned pretty well for – at least – a handful of severe storms across the region. The sounding posted above shows a reasonable amount of instability, good shear and helicity as well as Omega (though not much) within the Effective Inflow Layer. and that EIL is also an area where there is sufficient turn of the wind with height.
This is why it looks like this round may be a bit more potent than the last few rounds of severe storms. This one has better access to more heat, more moisture, and an atmosphere that can promote more organized thunderstorms.
For those hoping to see a “futurecast” map with where the rain and storms will be at certains times, I am going to have to disappoint you. It is still too early to talk about specifics like that. I know everyone wants to know exactly where the rain will be at a certain time because of plans, but we just can’t accurately know that yet.
The Bottom Line
There isn’t an abundance of anything that scream “severe weather outbreak” at this point. But there is enough of data here to suggest that – at least some – severe weather is possible. And unfortunately, it looks like all modes of severe weather will be possible: Flooding rain, wind gusts greater than 60mph, hail, and tornadoes. That is why the SPC is highlighting the area with a risk area for this weekend.
Both Saturday and Sunday will feature a chance for showers and storms from Louisiana through Mississippi and into Alabama. Better chances for storms west on Saturday and tot he east on Sunday.
For now, if you have outdoor plans this weekend, make some back-up plans. And no matter what you are doing, make sure you have access to weather information and alerts this weekend. And keep tabs on the forecast this week as meteorologists continue to wrestle with the data to figure out exactly what will happen.
