CLEVELAND - Tracking our potential late weekend winter storm using computer guidance models. Lets revisit the computer guidance that is trying to move a major winter storm into Ohio late this weekend into early next week. What is it saying today compared to yesterday?
Look at the snowfall output from the American Model, the GFS, from the 8 p.m. Tuesday evening run below. It shows lots of snowfall for Ohio Sunday night thru Monday. The pink color is 8 inches or more! WOW! Now, if you see this posted on Facebook, you'll be rushing to the nearest grocery store for bread and milk!
Here's the Canadian Guidance Model, the CMC. Its moved the storm farther north into Ohio from its earlier runs yesterday. But the results are similar: heavy snowfall, in pink and purple, across parts of Ohio. Just a little farther south. If you believe this, some of you might need bread, milk AND pudding!
BUT WAIT JUST ONE MINUTE! There are some problems with these images. Notice at the top of both pictures where it says Snow Accumulation 10:1 Liquid to Snow ratio. The computer assumes that all the snow will accumulate at a ratio where 1-inch of liquid water would produce 10 inches of snow. That occurs with temperatures between about 28 and 32 degrees. But what happens if the surface temperatures are warmer or colder? Then the snowfall ratios and snow fall totals would change.
It also assumes that all types of frozen precipitation are snow. But, what happens if warm air works in above the ground surface and produces sleet or freezing rain, instead of snow? Well, the above snowfall products don't differentiate and, consequently print it all out as snow.
Now, if we take a look at another GFS guidance product for precipitation type, we instantly see the point I was making above. Instead of snow, the models see this storm producing lots of sleet and even freezing rain across parts of Ohio Sunday and Monday. Obviously, freezing rain and sleet will drastically cut back on any snowfall accumulations. Mixed precipitation also basically makes the above snowfall prognostications BOGUS and inaccurate for many areas in the Buckeye State!
That's why, whenever you see images from Computer Guidance Weather Models on the Internet, hopefully they come with some expert explanation or some caveats. Before you retweet or share weather images like these, make sure they come from a source you trust. Otherwise, you'll be stuck with lots of bread and milk...and no big storm to enjoy it with.