Anybody reading this from Columbia and Jefferson City to St. Louis, listen up. Winter Storm Warnings (colored in magenta) are in effect from 6 PM tonight until 6 PM on Thursday while Winter Weather Advisories are posted for everyone else. The brunt of the storm looks to be along the I-70 corridor between Columbia and St. Louis and along U.S. 50 between Jefferson City and near Union where 50 merges with I-44. While the warning goes in effect at 6 PM tonight, I'm expecting snow to actually start falling between 7 and 8 PM in the St. Louis area and all the snow will wrap up around 7 PM Thursday night.
Here is a map of where the system will be at 12 Z on Thursday (that'll be 7 AM for Missouri). While the low is well off to our southwest, it's strong enough to funnel in moisture over the warm front and into the cold air mass overtaking the state right now. All the levels of the atmosphere are below freezing, so everything that falls will fall as snow. Again, the snow should wrap up before or around 7 PM Thursday as the system continues to scoot off to the east.
How much snow could the St. Louis area and other areas in Missouri see? To get an idea, I look at the QPF (quantifiable precipitation forecast) to see how much liquid will end up falling as the storm system moves through the region. At St. Louis, we're expecting 0.37 inches of liquid falling from the system while areas further west will see 0.47 inches of liquid. To turn that into possible snow amounts, we like to use a snow to liquid ratio. For example, there's a snow to liquid ratio of 15:1. That means for every 15 inches of snow that fell, that would equal 1 inch of rain or liquid precipitation after all of it is melted. This number changes depending on the setup of the storm system and many dynamics affecting weather at the surface and upper levels.
With plenty of cold air in place and little dry air in the upper levels when the snow will begin to fall, a 15:1 or 20:1 ratio isn't too far off with this system. With .37 inches of liquid to work with, that'll mean snowfall accumulations around the St. Louis area between 5.5 and 7 inches. Further west you go towards Columbia and just outside of Kansas City, with a QPF of .47 inches, don't be surprised to see snowfall totals between 7 and 9 inches. This is an impressive storm system and shouldn't be taken lightly. Don't be surprised to hear school closings early tonight and be careful if travel plans are in your immediate future. Stay safe, folks.
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