Good Morning all, I thought I’d go ahead and throw up a short-term update over the next 24 hours as location will be very important in terms of weather and precipitation type and placement. The event we are dealing with will be called overrunning, or the term isentropic lift. And to quickly define that all that really says is how the moisture is being lifted into the atmosphere. Main principles you need to understand about today will be that cold air is more dense(heavier) than warm air. Thus, if warm air is forced into an area of dense cold air it will first lift up and over as if there is a “ramp” in place. That is what we will have over Indiana going into this evening. Cold dense air is already in place. Go outside or take a look at the thermometer if you don’t believe me! Anyways, on to the forecast!

Lets take a look first at the pieces to this puzzle:
The above map shows the mean(average) upper level show, and for the most part it’s actually fairly zonal (west to east) with a very strong and fast flow going from west to east, however within this flow are waves of energy (vorticity) Red is positive and Blue is negative. We’re specifically focusing on the red approaching the Midwest. And that is because this will be associated with the life that will ultimately occur over the next 24 hours. Before spreading East, where I think things eventually turn for the worse for the Upper Ohio Valley. The map I got from Oklahoma State University: http://hoot.metr.ou.edu/upperair/vadvec/305 It was the least ‘busy’ map I could find to describe this event. It is a very good resource if you want another tool to assist in weather forecasting! Next factor will be that stationary front that will be across the upper Tennessee Valley that is the separation between the warmer moist air to the south and cold dense air to the north. It will meander a bit north over the course of today as we get the warm return (red arrows) as the disturbance (dashed line) moves in from the west. As the warm air moves in this evening it will run right into this cold air that is in place, and it will move up and over the cold air, thus the cloud cover this afternoon, and when the disturbance arrives it will become precipitation. Here is the tricky part.

Easiest way to describe this is by use of a “Skew-T” below I will highlight what you need to know.

This is a snap shot of 00z Model run last night for the GFS model. Data is from KIND in Indianapolis for about 6:00 PM EST. Today. And shows by this point the overrunning will be underway by this point. The SURFACE temperature will remain below freezing through the entire duration of this event. However, if you look on the chart you’ll notice I pointed out “Warm Air Aloft”, when you go up to about 2 -3,000 ft in the air it is ABOVE FREEZING! How is this possible. Remember, the first paragraph, I mentioned the one principle you will need to understand for this whole event. This is what it looks like on a Skew-T diagram. This set up will either produce Sleet or Freezing rain. The depth of the cold layer in this case suggests Sleet across The I-70 corridor.

Here is a horizontal map view of today’s distribution.

Keep up with Interactive Radar for the latest advisories and forecast if you’re in between the black lines going into the evening hours it will be very dangerous and slick this evening. Do not travel unless you absolutely need to.

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