Wx Info: That wasn’t snow flurries, South Mississippi, it was something even more rare

I’ve been informed that this article is too verbose with facts. So, if you aren’t interesting in the Scientific Method of the discovery, but instead just the “tl;dr version” then here you go: A specific set of atmospheric events led to a lower amount of very small raindrops to fall. it looked like snow, but wasn’t. This setup is even more rare than seeing snow. .

Last night I got a handful of reports of light snow flurries falling across parts of southern Mississippi. Most of the videos and pictures came in between about 9p and 11p. And while most folks thought it was snow, a few others asked, “what is this ?” because, as they noted, the temperature was in the mid 40s and they’d never seen snow at temperatures that warm.

You skeptical few were right to be skeptical. And you folks who reported snow flurries were right to report snow flurries. Because it did look like a snow flurry! But you skeptical few were right to look at the thermometer and think, “wait just a second!”

This is why I always laugh and say the phrase “seeing is believing!” is one of the most flawed phrases in human history. Seeing is not believing. Seeing is, well, just seeing. It is one piece of evidence. But it needs to stack up with the rest of the evidence before we can jump to any conclusion.

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THE REPORTS

I received a few videos of some very lightly falling precipitation. The precip was pretty small and seemed to float around like snow would.

Then I received a few pictures of the same precipitation. A few featured street lights in the background, others were a sheen on a rooftop with some sort of accumulation.

That was followed by reports, with no pictures or videos, from folks in different towns saying, “yes, I saw the same thing!”

And then a few more reports from people saying, I saw it too, but I’m not sure what it was.

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CHECK THE DATA

Was it snow? Well, the first thing we want to look at is the weather balloon data from the area to see how cold it was above the ground. In order to get snow to fall, it needs to be below freezing pretty close to the surface.

And to get flurries to fall, it needs to be below freezing really, really close to the surface (more on that in a second).

Here is the weather balloon data from Jackson. It might be tough to see exactly, because we are looking toward the bottom, but the air is above-freezing until about 8,000ft up.

If we zoom in really close, we can see that a bit better. Any time the red line is to the right of that slanted blue line, the temperature of the air is above freezing. And you have to get up to around 2500km (about 7,500ft) before it flips back to the other side.

We get close down around 1km (about 3,000ft) but it is still above freezing there, too. And we have to be below freezing in order to make a snow flake. And every where it is below freezing the atmosphere is really, really dry.

But maybe Jackson is the anomaly, right? So lets check the other two weather balloons that get launched in the region! New Orleans and Birmingham both launch weather balloons.

But, I will say, they show very similar situations. New Orleans is a bit warmer and Birmingham is a bit cooler. But neither show an atmosphere cold enough to produce snow flurries.

But you might ask, “Well, could it be different for south Mississippi? Maybe there was a cold bubble!”

Let’s check. We can look and see what the computer models suggest the atmosphere looked like between these sites based on the weather balloons and satellite-derived data.

Nope, same thing goes. Too warm for flurries.

You may say, “You’re wrong. I’ve seen it snow above-freezing before!”

I’m certain you have. I have, too! But those snowflakes you saw were probably pretty big. Because physics won’t allow for small snowflakes to exist in temperatures too much above freezing – because they melt faster!

Think of it like boiling a pot of water. How much heat do you need to boil a big pot of water to cook a big batch of mashed potatoes? Probably a lot. It may take 10 to 15 minutes to get that pot of water to boil. But if you are making soup for just yourself and boiling a smaller pot of water, that may turn to a boil in a matter of just a few minutes.

Water has a very high specific heat. It takes a lot of energy to change the temperature. So the more water, the more resilient it is to change. The less water, the less resilient.

So, big snowflakes last longer than tiny snowflakes. So you can get bigger, wet (half melted) snowflakes falling at temperatures as warm as 38F to 40F, but little flurries need temperatures much closer to freezing.

“But Nick, what if these were big snowflakes that just melted into tiny ones?”

That’s a fair question. But then I would asked, what happened to the rest of the water in the bigger snowflake? The atmosphere was very moist, so the excess water wouldn’t just evaporate. It would have to fall as rain. And based on the videos, I didn’t see any of that happening.

So it had to be something else.

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SO WHAT WAS HAPPENING?!

Well, I got to thinkin’. Just what is happening.

I went and looked at all levels of the atmosphere, but all I could find is a very diffuse boundary at the surface.

There isn’t enough there to do much. And I am looking at the weather balloon data (those are Skew-Ts!) and going, that sure is a familiar looking shape. Where have I seen that before?

Then all the sudden I felt like Bert in Mary Poppins. I’m sitting here drawing outlines going, “wait, a second… wait just a second!”

That is the sounding you normally see in the deformation zone behind an area of low pressure! In the winter, this is the area where you see very heavy snowfall. Here is an example of one below:

As pointed out by the NWS in Louisville, the deformation zone is the place to be if you are looking for big snowfall in these events due to the mechanics that shake out in the atmosphere based on the placement.

So I went and looked at the data.

I found a very weak boundary at the surface and BAM found the area of low pressure in the Gulf. And that little boundary lined up underneath the deformation zone.

But looking at the skew-t data above, there wasn’t much there that would indicate precipitation was possible. We didn’t have the normal forcing higher up. And higher up it was really dry.

Until I looked really, really close.

While we didn’t have any forcing in the mid-levels we had barely measurable omega closer to the surface. And I mean really, really close – we are talking between about 500ft to 1,500ft off the ground.

The little boundary that slid through the area didn’t use any of the mechanics of the deformation zone. Instead it was the little bit of forcing oh-so-close to the surface that was able to squeeze out some very teeny-tiny rain drops.

You can think of it as the difference between wringing out a sponge that was just dunked into water versus wringing out a sponge that was dunked in water three days ago.

Because these raindrops weren’t very large, they were floating around and drifting in the wind like a snow flurry would do. But they were liquid.

Normally these small little liquid drops would evaporate, but the atmosphere was pretty close to saturated. So we reached a very delicate balance between saturated enough to allow tiny raindrops to exist without evaporating but not saturated enough to make normal-sized raindrops. And the amount of forcing wasn’t so much that we had mist or drizzle, but it was just enough that we created little individual raindrops.

This is a similar phenomenon to what often happens when we have ice crystal flurries, but this time it was about 10F to 15F warmer. And usually when this happens at these temperatures we don’t reach the right balance between all of the forces and temperatures to create such an event.

So, if you saw this last night, you saw something that doesn’t happen very often around here at all!

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