NORTH TEXAS (CBSDFW.COM) – With the word now out about a Winter Storm on the way for North Texas by mid-week, I know it stirs deep emotion and excitement of seeing the snowflakes fly and planning out where your snowman will be built and how tall it may be. Unfortunately, the pesky atmosphere has thrown a wrench in those plans thanks in part to a layer of warm air, not too much higher up than the tallest downtown building that will melt all those snowflakes before they make it to the surface.
With bitterly cold temperatures expected Wednesday night through Saturday, it’ll be far too cold for the rain to remain liquid and therefore will freeze on contact. This is what we call freezing rain and it will make a mess of everything and cause widespread travel problems on area roadways along with possible power outages.
The nature of arctic air is to be extremely shallow—sometimes only a thousand or two thousand feet thick. To put that in comparison, the tallest building in Downtown Dallas, the Bank of America building that lights up green, is 921 feet tall. So the depth of this extremely cold, arctic air may only be two to three times as thick, or “tall”, as that building.
Above that, the temperatures sharply rise into the 40s which is the death nail for any snowflake.
So what exactly does this notch look like? Let me show you.
Above is called the Skew T, Log P Diagram. That’s a very fancy way of saying it’s a temperature cross-section of the atmosphere. The bottom of the chart is where we live. That’s the surface of the Earth. The very top of the chart is over 40,000 feet in the air, higher than jet planes fly.
On this chart are a number of things. Most importantly, is this blue line. This is the freeze line in the atmosphere. If you follow that blue line down to the bottom of the chart, you’ll see you’ll arrive at 0°C. All the numbers to the right of that are above freezing and all the numbers left of that are below freezing.
You’ll notice a red and a green line going from the bottom of the chart to the top of the chart. The red line is what the temperature is doing as you ascend into the atmosphere and the green line is the dewpoint as you ascend into the atmosphere. The further those lines are spaced apart from each other, the drier the atmosphere is (as you see at the very top of the chart). The close together they are, or in this case, overlapping, the more saturated it is. When the lines overlap, it is 100% saturated and this is where it is precipitating (snowing, raining). Picture these red/green lines as the highway the snowflakes will take on their way to the surface. That’ll be important in our understanding of this ice event.
Notice that red and green lines at the top of the atmosphere is far, far to the left of that blue, diagonal freeze line that I pointed out first. The further to the left you are of that blue line, the more below freezing you are. As you would expect, at the top of the atmosphere it is WAY below freezing (in this case more than 60° below zero!) As you follow those lines down to the bottom of the chart, two things begin to happen.
1. The red and green lines get close together and eventually overlap. This means it is 100% saturated and is snowing hard many thousands of feet above your head.
2. As you move down those lines, it gets closer and closer to the blue freeze line and eventually crosses over.
That little area to the right of the blue line (above freezing) but to the left of the red/green line is what I call the “notch.” That’s an area of the atmosphere that is above freezing a few thousand feet above the ground. Going back to my highway analogy, these snowflakes that are falling through the atmosphere along the red/green lines crosses over the blue line into an area of the atmosphere that is above freezing beginning at about 13,000 feet (650 millibars).
The red/green lines (overlapped) remain to the right of the blue line until the snowflakes, which by now have melted into rain drops, until about 2,500 feet above the ground (925 millibars). So according to this chart, the part of the atmosphere that is above freezing is very, very thick (more than 10,000 feet thick) which way more than sufficient in melting any snowflakes to raindrops.
Here’s the catch. As the rain drops continue to fall, you’ll notice that the red/green lines once again cross back to the left of the blue freeze line 2,500 feet above the ground. Temperatures quickly fall into the 20s which will either refreeze the now raindrops and or will freeze on contact of cars, roads, trees, powerlines, etc.
Usually to get quick and effective freezing, you need temps around 28 degrees or lower. This time however, the wind will be blowing strong which will help remove any residual heat from both rain drops and surface objects making for quick glazing even if temperatures are as warm as 31°.
Bottom line: significant icing is expected with this system with the outside chance of the precipitation switching over to all snow before the even comes to a close on Thursday evening.
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