ST. LOUIS, Mo. – Heavy snow is always the most talked-about weather event in St. Louis because of its widespread impacts.
“It has the most impact, the most widespread impact,” explains Jim Sieveking, meteorologist at the National Weather Service. “A tornado will only impact a small area, right, it can be devastating in the area, but an extreme snowfall, like a foot or more of snow can have a widespread impact. We feel the weight of the forecast on us.”
Joe Monroe, an engineer at the Illinois Department of Transportation watches the forecast before a winter weather event.
“I probably check it too much,” Monroe says.
Knowing one tweak can cause a snow forecast to boom or bust.
“The biggest challenge for forecasting snow is the forecast track,” Sieveking warns. “If you are off by just a few miles that can mean the difference from St. Charles County getting a foot of snow and St. Louis County from getting a foot of snow.”
If the heaviest band of snow alters it’s track by only a few miles, like from Main Street in St. Charles to Downtown St. Louis then plows must play catch up.
“With getting more snow, it’s just a ramp-up. Normally you can get the people back in and slowly chip away at it to get caught up,” Monroe says. “Where when you get ice and it snap freezes. Just getting people in and then getting them through to traffic and getting chemical down to start unwinding the situation is a challenge.”
That’s where temperatures come in.
If a forecast is predicted for 74 degrees in the summer but lands at 71, most people won’t notice. In the winter, a change from 34 degrees to 31 causes the roads to change from wet to frozen.
“You might have 3 or 4 degrees different in temperature, so it could be snowing here, it could be sleet where the I-55, I-70, and I-64 come together, and it could be rain in Belleville,” Monroe explains. “So you try to put a plan together.”
Meteorologists help plan by predicting temperatures at the surface and through all levels of the atmosphere. A difference in a few degrees at any location, any height, spread along hundreds of miles, can make the forecast tricky, even for the experts. That’s why you’ll see ranges of predicted snow for your home.
Other factors like snow banding, caused by converging moisture and wind, create a concentration of heavy snow in one specific area. The placement of where the bands will land, is difficult to predict days in advance.
As the event draws closer it’s easier to nail down more specific totals, but remember, weather can change in an instant.
“We try to update our products and forecast as soon as possible,” Sieveking says. “But we also like to call MODOT and IDOT and get to the people making the decisions on who’s going to be actually cleaning the snow off and give them the latest information.”