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Overrunning

by Steve Templeton

KMOV.com

Posted on November 28, 2007 at 7:56 PM

Updated Wednesday, Nov 4 at 3:30 PM


Overrunning is a term that describes why we are thinking there's a possibility of freezing rain or sleet Saturday morning as this wet weather begins.

Overrunning is when warm and moist air runs over on top of a shallow dome of cold air at the surface. I'll probably over simplifiy this, but the analogy of a speed bump comes to mind. Your tires have no choice but to run up and over the speed bump. Similarily, warm air is running up and over a shallow dome of cold and below freezing air. So when rain from the warm air falls down into the cold below freezing air it might freeze into an ice pellet (sleet) or freeze once it hits the surface (freezing rain). This is the situation that makes freezing rain and sleet possible Saturday morning before we see a change to all rain in the afternoon.

Part of what makes such a forecast so difficult is the extent and depth of the cold air. For example, as the warm air runs over the cold air, it mixes with and warms the cold air. If the air near the surface warms to above freezing, it's all rain. Also, forecasting the exact location of the shallow cold air is critical. If the whole storm system shifts to the North, then we're in the rain. Shift to the South and we're in the snow.

With that in mind, the shifting trend in the latest models has been to the North. This wraps enough above freezing air into the Bi-State for mainly rain.

The forecast as of tonight (Wednesday night) looks similar to last night: A mix of rain, freezing rain and/or sleet at the beginning of this storm Saturday morning. This is more likely in Western to Northern Missouri where the moisture will reach first. Then as the moisture gets into the St. Louis metro area by mid to late morning, there may be that brief wintery mix before changing to rain in the afternoon. Off and on rain showers and very windy conditions can be expected through Sunday morning. By Sunday afternoon or evening the cold air catches up to the rain just as the storm is winding down. This means some light snow or flurries is possible with little to no accumulation.

It's nice to see the forecast somewhat consistent the past two evenings, but there's still time for drastic changes in the models. We'll keep you posted as always online and on the air on Channel 4.

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