By: John Dissauer
CAPE GIRARDEAU, Mo. -- A Freezing Rain Advisory is in effect for most of the Heartland from 6am to 6pm Tuesday.
There are going to be two important factors as to if and how much freezing rain we get.
Temperatures will slowly start to rise from the lower 20's Tuesday morning. Clouds will move in overhead keeping temperatures from climbing quickly. Temperatures are expect to remain near the freezing mark (32° F) by early afternoon.
Dry air is currently in place over the Heartland. Dew points, a measure of the amount of moisture in the air, range from 6° to 12° as of 6:00pm Monday. (That is an indication that the air is dry. Although, not as dry as it was this morning as dew points were below zero!) In order for the rain to make it all the way to the ground we need the amount of moisture in the air to increase.
It is similar to holding a dry sponge under a drippy faucet. The drops of water will keep going in to the sponge, but the water won't go through the sponge and end up on the floor. However, once the sponge becomes wet enough, the water will pass right through the sponge.
Something else that will happen while the moisture falls in to the dry air. Something called evaporative cooling. As the drops of water fall and evaporate, the air around the drops will get cooler.
This is where forecasting temperatures for tomorrow morning through early afternoon becomes tricky. Just how much evaporative cooling will there be? I think around noon temperatures will be sitting around 30°. That is cold enough for whatever moisture that comes out of the atmosphere to freeze.
Bottom line... I think we could see a two to three hour period late Tuesday morning through early Tuesday afternoon where we could see light freezing rain. I do no think this will amount to an accumulating ice event. It will be brief.
Then the freezing rain will change over to all rain by late afternoon. There could even be a few thunderstorms passing through the Heartland Tuesday night. Severe weather is not expected for Tuesday night or Wednesday morning.
Weather Factoid: The official Monday morning low of 6° was the coldest temperature in 687 days. The previous cold day was Feb. 4, 2007 when the temperature dropped to -3°.