During the winter months, the Ohio Valley can come under the threat of an ice storm. It takes a special setup of the weather pattern for this to occur, one that can be quite complicated to understand. In it's most simple form, an ice storm is created because a "battleground" of shallow cold air and moist/mild air collide. Cold and dry air is dense and undercuts the warm and moist air overrunning it. This sets up an area where many different types of precipitation can occur over short distances, meanwhile large ranges in temperature also occur over short distances. Refer to the graphic below to understand an ice storm in it's most basic form. Are you still wondering why these storms form, and how the many different precipitation types develop? Let's dive deeper into these storms in the graphics that follow.
How does this "Battleground" end up positioned across the Ohio Valley? To understand this question, you must begin by looking at what happens in the upper levels of the atmosphere. During the winter months, the Jet Stream ( a corridor of strong winds aloft) usually sinks further south across the United States. Occasional dips in this jet stream allow colder air to filter in from the colder climates to our north. Meanwhile, south of this jet stream conditions stay generally milder for the season. Along this jet stream is where storm systems develop, mainly due to cores of stronger winds embedded within the overall flow. These "disturbances" end up creating areas of high and low pressure near the surface and result in the movement of moisture across the country. Ultimately, all of our weather is driven by differences in temperature and moisture across the globe!
To get an ice storm across the Ohio Valley, consider this meteorological set up below. Imagine a disturbance (red jet core) moving through a jet stream that happens to be positioned like the one drawn. As a result of this jet core, low pressure areas at the surface develop on the left front and right rear portions as shown below. Meanwhile, areas of surface high pressure develop in the right front and left rear portions. Areas of rising air are associated with the low pressures, and areas of sinking air are associated with the high pressures. This graphic simply describes what is going on in the upper atmosphere, and in the lower atmosphere to provide the setup for an ice storm in the Ohio Valley. Now, lets consider what is going on near the surface in much greater detail in the graphics that follow.
Now we are zoomed into a regional view of the Ohio Valley below, keeping in mind the larger scale meteorological setup that got us to this point. In general, the low and high pressure areas are placed in about the same spot as the graphic above. To understand what is occurring near the surface, we need to first understand how air flows around high and low pressure regions. Winds flow counter-clockwise around low pressures, and clockwise around high pressures. If you imagine these wind flows like they are depicted on the graphic below, you can understand how a frontal zone is formed. Remember that our cold dry air is coming from the north, and our moist and milder air is overrunning this cold air from the south.
Now that we understand what is happening at the surface, there is one more piece of the puzzle that must be tied in. Many times, it is difficult for us to visualize what the warm and moist air "overrunning" the cold air looks like. It is important to understand this because this is the key to why we get mixed precipitation types in this setup. Refer to the graphic below, but you must look at this like you are standing in western Kentucky and looking east toward Louisville and Lexington. This is a cross section of the same setup you saw above. In this graphic, you can easily the cold air coming in from the north (left), and the warm/moist air coming in from the south (right). Remember that the cold air is more dense so it hugs the ground, meanwhile the warm/moist air is less dense and rises over the top. This creates a warm layer of air on top of cold air that is below freezing. Look at each example and try to understand why you get the mix of precipitation types, dependent on where you are located within the storm. Now consider what small shifts in the track of the storm can do to the precipitation type you might encounter. This is the challenge that forecasters have to face when trying to decide which precipitation type, and how much is going to fall with a storm. This is just a snap shot, now imagine the storm moving!!! As you would expect, it is quite difficult to forecast these storms, and small changes in the track result in big changes to what you experience.
Hopefully, you have a better understanding of why these storms form, and the difficulty of how they are forecast. This meteorological scenario (or one close to it) has potential to set up late this weekend into early next week across the Ohio Valley. There is still some question as to where the actual "frontal zone" will set up and below are 3 potential scenarios that could play out across the Ohio Valley. Scenario 1 would cause the lowest impact from a winter weather standpoint, but could bring heavy rainfall to the region.
Scenario 2 would bring a more impactful winter event across southern Indiana and north central Kentucky. Meanwhile, heavier rain would fall across south central Kentucky.
Finally, scenario 3 would bring a significant winter event to much of the region. Stay tuned to your latest forecasts as the week progresses.