Weekend Storm Rainfall Totals
by KDLT Weather
December 16, 2012 8:50 PM
The second straight wet weekend is another welcome sight for many that might finally be our start to ease this severe to extreme drought. What made this event unique is pretty obvious, it was rain not snow that was falling in many areas in the middle of December. This storm rolled northeastward out of the desert southwest transporting plenty of warm air northward leading to plenty of rainfall versus the typical December snow. Enough rain in fact to set a daily precipitation record for the city of Sioux Falls. This storm will also put an end to our dry streak as December will now go down as wetter than average for almost every city in our area, the first time since May. A list of all of the precipitation totals for not only this storm, but for the month of December, are listed below along with some new tools KDLT uses to determine precipitation type in warm winter storms like this one.
With a variety of precipitation types observed throughout the region, we were able to try out some of the new products that came with a radar upgrade we received over the summer at all three radar sites that cover our region. You may remember several months ago, we covered a story about how the National Weather Service radars in Sioux Falls, Aberdeen, and Rapid City were being upgraded with "dual pol" capabilities. Well, with very little precipitation of any type over the past few months, we haven't had many opportunities to use these new products; but today, we finally had the chance to work with these new features to better examine what exactly is happening across our region. Keep in mind, however, that with new technologies; there is still a lot for us as meteorolgists to learn, so it will take us a little time to fully understand how to apply this new data to our forecasts.
Compared to last weekend's storm, this weekend's event is obviously very different and it doesn't take a meteorologist to figure that out. Overnight Friday into Saturday morning, as this system began moving in; we have seen several different types of precipitation from rain to snow thanks to a little more warm air drawn in from the southeast.
Now that the snow and rain is beginning to move out of here, I wanted to go back and show off a couple of these features and show you how they handled today's event. I really tried to only focus on using two of these new products today: Differential Reflectivity (ZDR) and Correlation Coefficient (CC). Both of these are useful during severe weather, but they are also very helpful during winter weather events like today.
DIFFERENTIAL REFLECTIVITY (ZDR): The difference between the horizontal and vertical reflectivity factors. This is measured in units of decibels (dB).
The primary use of this particular product is to give us a better idea of what type of precipitation is actually falling at a certain level. To make it even more useful, it can even help us break down the types of precipitation into different textures or intensities. For years, we've been able to analyze precipitation horizontally, but these newer products look at it vertically as well. Just being able to look at precipitation in a whole new dimension makes us that much more confident in our forecasts.
Let me attempt to break this down and explain how it works. Think about what you already know about rain, snow, or even hail. They all come in different shapes. Now, think about raindrops in general and how they are shaped. The small, microscopic raindrops that fall as drizzle are much more round in shape than a large rain droplet that falls from a severe thunderstorm. Similarly, the smallest hailstones are more spherical than the largest ones, which usually have jagged edges. Obviously, we aren't dealing with hail today in South Dakota, so we can eliminate that one all together.
So what does the radar do after it has measured the size of an object in the vertical and horizontal? It simply subtracts the height of the object from the width, assigning a value somewhere between -4 and 6 dB.
For example, if you had a perfectly spherical droplet of drizzle or even a small hailstone; you would likely find that the length and width of the droplet is about the same. Well, what happens if you subtract a number from the same number? You get zero no matter how big your measurements are. If you subtract 1 from 1, you get zero the same way you would if you subtracted 1,000 from 1,000. Unfortunately, we use math in meteorology, too. Luckily, we have a radar that does it for us.
Now, take a large rain droplet for example. Because they're heavier, these droplets tend to flatten and stretch horizontally as they fall to the ground. When this happens, our measurements in the horizontal increase while the vertical decreases. Break out the calculators one more time and see what happens when you subtract any small number from a larger one. You'll get a number greater than 0, right? Well, the only other precipitation type that would give a similar ZDR value would be a melting hailstone, and we already ruled that one out; so if you see positive ZDR values, you have rain. Now, to get a little more specific with the rainfall; we can also look to see how heavy it may be. The larger the ZDR value, the heavier the rainfall would likely be.
Obviously, the only group of ZDR values left are those negative values. These droplets are obviously taller than they are wide, and the only type of precipitation we can associate with negative ZDR values is vertically oriented ice crystals, or sleet.
Freezing rain does not fall anywhere on the ZDR scale because it cannot be detected by radar. A common misconception in meteorology is the difference between sleet and freezing rain. If a snowflake melts completely as it is falling through a warmer layer of the atmosphere, it cannot refreeze until it reaches the surface. Once it reaches the surface, it may refreeze on contact only if the temperature of the surface is below 32 degrees. Sleet, on the other hand, is a partially melted snowflake that is able to refreeze if it falls through a much colder layer below 32 degrees. Since freezing rain does not freeze until it reaches the ground, the radar beam is unable to pick that out because the radar beam does not analyze precipitation near the surface.
See how much more complicated winter weather forecasting can be? It gets even better.
What about snow? Well, there really isn't a ZDR value that represents snow very well because flakes tend to tumble around in several different positions as they fall. The radar cannot determine how the flake is positioned, so the radar assigns ZDR values throughout the entire scale when looking at snowfall. What does that look like on radar? Well, it looks like a mess. You'd probably think this would make things a little more difficult, but it actually makes it easier on us because when we see an area of inconsistency, we can automatically jump to some sort of conclusion without wrecking our brain trying to solve the puzzle.
Now, compare this radar image to the one we typically would show on television. You can see that the blues and greys representing snow and sleet match up with the blues and pinks that represent snow and sleet.
While radar is a helpful tool, it doesn't always give us the best idea of what is actually happening at the surface. Because of that, we can't use this tool as a crutch. It's always important to turn to our ASOS network of observing stations to see what is actually happening. It's also important to consider the temperature observed at the surface to determine whether rainfall will freeze on contact or if it will remain in liquid form.
But in meteorology, it's never that simple. One isolated location may be observing snow, but it doesn't tell us what is happening a mile away. In fact, with only a few of these stations across our region; you may find it surprising that meteorologists get the forecast right as often as they do. That's where the next dual pol product comes in.
CORRELATION COEFFICIENT (CC): Measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume. CC is shown as a percentage.
As if ZDR wasn't complicated enough, this feature gets a little more complicated. I'll try not to go into too much detail, but the CC is actually a little less complicated when it comes to actually using it. There are a few different things that CC can tell us, but to make this easier; it compares two separate radar pulses to see if there is a mixture of precipitation types or if it is purely rain or snow. It is given as a percentage and broken into three different categories.
I'll start with the first category, which is considered non-meteorological objects. Just like rain and snow, radars are able to detect things like birds, bugs, tall buildings that may surround the radar site, or even debris that may be found flying in the air during a violent tornado. With today's situation, the most reliable is found a little farther away from the radar site because trees and buildings near the radar site are interfering with the radar beam.
The greens and yellows represent CC values between 80% and 97%. These values help us identify hail or wet aggrigates detected by the radar beams. Since hail usually falls during heavy rain, these are the colors you'd find in the hail core of a supercell thunderstorm. In today's event, it would represent a mixture of sleet, snow, or rain in one particular area.
The reds and dark purples represent CC values higher than 97%. Rain and snow both fall in this category, so wouldn't that make things more confusing. Remember that with CC, we are only looking to see how consistent a particular precipitation type is; not the actual type.
With the radar image below, we can determine that the greens and yellows southwest of Sioux Falls are showing where rain and snow are mixed together over McCook and Turner Counties. North and east of Sioux Falls, where the reds and purples are found, leads us to believe that the precipitation has switched over to pure rain or snow. To find out whether this is rain or snow, you would want to go back and look at your ZDR values.
Shortly after these images were taken, snow briefly switched back over to a mixture of rain and snow over much of the city even though the observing station at the Sioux Falls airport was showing only rain as you can see in the image below.
What you may find even more interesting about this particular radar image is that miles away from the city, where there are no buildings; radar was giving CC values of less than 80% picking up some sort of non-meteorological object in northern Rock County, Minnesota between Jasper and Hardwick. If you're familiar with these backroads, you probably already know what this is; but after seeing this same phenomena several months ago, I was stumped. I later found out that the radar beam was picking up on the tall wind turbines along the Buffalo Ridge.
As the snow and rain continues to move off to our east during the overnight hours, we will continue to see partly cloudy skies through the night and even into the first of the week. By Wednesday, it looks like another system will be passing nearby giving us another shot at seeing some snowfall.
With these new technologies now being used by our meteorologists, you can rest assured that you're getting the best coverage around. No matter what kind of storm we are faced with next, you can count on us.
As always, we will have the very latest on KDLT and right here on KDLT.com.