Day 39: Winter Surprise!?!
Last night I mentioned in my post that it looked more and more like the winter storm for Oklahoma was going to end up being a non-story. For most of Oklahoma, this turned out to be the case. Whereas there were places that received several inches of a wet fluffy snow, most places didn’t see any accumulations. Accumulations that did occur were primarily confined to grassy surfaces and travel issues were minimal.
This was not the case in far eastern and southeast Oklahoma. Nor was it the case for a large portion of Arkansas. The forecast when most residents went to bed called for rain, possibly mixing with snow at times, throughout the night. No accumulations were forecast for places like Fort Smith, Little Rock, Memphis, etc. Talk about a surprise when residents in these areas awoke to anywhere from 2-4″ of snow on the ground with more snow on the way. So what happened? How did snow fall in southern Kansas and eastern Oklahoma, and miss central Oklahoma?
First, as all my former synoptic students should be able to tell you, we need to diagnose why the precipitation developing where it was. Normally, I would make them first tell me where there is precipitation, but I’ll provide that answer this time.
Above is a water vapor image valid at 0845 UTC (3:45 AM CST). Below is the same image, annotated by me. In the image above there are two areas in the southern plains that have brighter colors (more moisture aloft). And as explained in previous posts, more moisture aloft is a good indicator that there is some sort of upward vertical motion transporting the moisture upward. We’ll start our journey in these two areas (circled in the image below).
So why is their upward motion (and precipitation falling) in the area circled in red? The answer here lies in the transport of warm, moist air about 5,000 feet (1500 meters) above the ground. In the image below, areas that are shaded in various shades of red indicate where warm air was being advected (blown around) toward cold air. The darker the red, the stronger the advection.
As the warm (and more moist) air begins to interact with the colder air, the warm air begins to move over the top of the cold air. This is a common form of atmospheric lift in winter (known as isentropic lift, or, in slang, “overrunning” because the warm air is overrunning the cold air). This atmospheric lift was enough to produce precipitation during the night in northcentral Texas, southeast Oklahoma, and much of Arkansas.
To the northwest of this area, we have another area of atmospheric lift occurring in southern Kansas and northern New Mexico (shaded in green in the water vapor image mentioned previously). If we look at the warm air advection map (above) we would see there is none of this occurring at that level of the atmosphere and so that isn’t the source of lift. However, similar to how a surface cold front can sometimes provide the focus for atmospheric lift, fronts aloft can do the same. If we look at a portion of the atmosphere typically around 10,000 feet (3000 meters) above the ground we can see that a relatively strong front was draped across southern Kansas (the brighter the colors in the image below, the stronger the front). It turns out that convergence along the front was strong enough to induce vertical motion and, ultimately, precipitation. As you can see, there really wasn’t much of a focusing mechanism for precipitation across much of Oklahoma, which is why we didn’t see widespread heavy snow (or rain). Just a drizzle, mist for much of the day.
So, this explains why there was precipitation where there was, but why did the forecast rain actually fall as snow in southeast Oklahoma and western Arkansas – especially with the surface temperature above 32F? The answer lies in what the temperature did above the ground. Normally, when temperatures are above 32F we should expect rain, right? Well, not always.
In places like Fort Smith, AR and Little Rock, AR, the warm air (above 32F) at the surface was very shallow – no more than about 1000 feet deep. As the initial precipitation fell across the area, it fell in the form of light rain – at the surface. It was actually snow aloft, but it melted back to rain as it fell through the warm layer just above the surface. Most forecasters thought that the precipitation would be light enough that the snowflakes would have time to melt as the fell through the warm layer and thus little to no snow would fall throughout most of the night.
However, as heavier precipitation began to develop around midnight, the precipitation began falling so fast that it did not have time to completely melt as it fell through the warm layer near the ground. Normally, this would continue while the heavy precipitation fell and then transition back to rain as the heavier precipitation moved away. Unfortunately for meteorologists, the precipitation remained heavy for a longer time than expected. This allowed more and more big, wet snowflakes to make it to the ground before completely melting. This had two effects: 1) it allowed snow to begin to stick to the ground as it was falling – there wasn’t enough time for a snowflake to melt before the next snowflake landed and 2) as the snow fell into the warm layer, the warm layer began to cool – similar to how a drink cools as you put ice into it. The cooling of the warm layer allowed for less melting of the snow which meant more snow reached the ground. As more reached the ground, the surface temperatures began to cool, allowing more snow to stick, and we’re off to the races. This heavy precipitation induced snowfall is why when the precipitation was less intense, there was a tendency for it to mix with or switch back to rain across southeast Oklahoma and much of Arkansas.
Now, in southern Kansas, the temperature was below 32F throughout the entire atmosphere and melting of the snowflake was not an issue. Thus, the snow wasn’t as much of a surprise (in fact it was well forecast) as it was for places to the south and east.
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