Winter Storm Watch: Heavy snowfall and the Memphis Low benchmark

ST. LOUIS — A Winter Storm Watch is now covering Metro St. Louis Tuesday night through Wednesday morning with the possibility of heavy snowfall of 4 inches or more.

There are all sorts of rules of thumb for forecasting different types of weather in different locations. In St. Louis we have the so-called “Memphis Low” benchmark for forecasting heavy snowfall.

Simply put, most of our heavy snow events have a ground low pressure center that passes over or very close to Memphis, Tennessee. If you browse through the archives of old weather maps, you will see it again and again. Almost all major winter storms have this surface low over Memphis.

Courtesy: Chris Higgins

Of course, the air must be cold enough to support snow as a type of precipitation. But if this condition is met, most of the mature lows that sweep over Memphis in winter will produce their heaviest snow near St. Louis and the I-44/I-70 corridor.

There is also a second feature we are tracking and that is the extension of this low pressure center to about 5,000 feet (850MB level). The benchmark for the 5,000 foot low pressure center is Cape Girardeau. If the low passes at 5,000 feet above or very close to Cape Girardeau, that in turn puts St. Louis in the sweet spot for heavy snowfall.

Let’s see what the forecast looks like for our weather system expected Tuesday night through Wednesday morning. The current forecast shows a near-ideal Memphis Low Track carrying the surface low pressure from southeast Arkansas through the Memphis metro area and then to the southwest tip of Indiana. This is a classic Memphis low track. The 5,000 ft (850 mb low) reflection of the surface low pressure is expected to pass very close to Cape Girardeau.

There are other key features to track as well, but if those two key features meet the heavy snow guideline, it means the St. Louis area should be right in the middle of the heavy snow as the storm sweeps across the region Tuesday night through Wednesday morning .

how much snow Well, that still depends on the exact track of the system and also on the surface air temperatures as well as the temperature of the ground itself. This system is moving into a slightly cold atmosphere for January, but has plenty of cold air in the air.

So the storm must suck out that cold air to turn the cold rain into wet snow, which the latest data says is likely to occur around midnight Tuesday night, with the heaviest rates of snowfall expected between 2 a.m. and 8 a.m. early Wednesday morning. Early indications are for widespread snowfall of 4 inches or more with this system. Winter Storm Watch: Heavy snowfall and the Memphis Low benchmark

Sarah Y. Kim

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