Although the cold front in the central U.S. will bring snow to the western Great Lakes today and though we still have more than two weeks of winter (vernal equinox: March 20), we’re dealing with a major springtime — rather than wintertime — storm system.
Actually, you won’t find the definition “Springtime Storm System” in the weather dictionary. A strong storm system is a strong storm system no matter what the calendar says. What really defines a storm is the contrast of air masses behind and ahead of the front. The greater the contrast between hot and cold, and dry and humid, the stronger the storm system — be it Christmas Day, the first day of spring or the last day of September. However, the classic lineup of severe weather ingredients often comes together in the springtime as the last of winter’s cold surges battle southward into the beginnings of summer’s eventual heat and humidity.
Such is the case today. The storm system moving across the Tennessee River Valley at this moment first gained national attention Wednesday when strong, long-track and, unfortunately, deadly tornadoes ripped through Harrisburg, Illinois early in the morning. The clash of hot/cold, humid/dry was classic and the likelihood of severe thunderstorms with tornadoes had been identified by meteorologists earlier in the week. As the storm system moves eastward today and tomorrow, additional severe thunderstorms and strong tornadoes are expected along, and ahead of the cold front.
The contrast between air masses is key to any strong storm system but rarely does such a strong system come to the table without additional meteorological features. In addition to the marked contrast, another unusually strong feature has been identified: significant turning of the low-level winds. Like a screw being unscrewed and lifted upward from a piece of wood, the low-level winds swirl in a broad counter-clockwise fashion across the cold front lifting moisture and warmth aloft where it violently mixes with colder and drier air. Strong updrafts and downdrafts within the storm cloud push the cloud tops to higher levels of the atmosphere allowing the contrast of cold and warm to become even more extreme, while causing a strong rush of downward air that can cause widespread damage when it spreads out from the base of the storm.
Finally, any unusually strong storm system will typically have strong jet stream winds above the storm complex. Like an exhaust fan above the stovetop, the jet stream winds help accelerate upward vertical motion within the storm and further enhance the overall “torque” or twist of the storm. Today’s winds from Arkansas to West Virginia between 34,000 and 40,000 feet are 130-140 knots — one of the classic severe weather signature items.
Fortunately, next week will begin like a lamb for almost the entire country as strong high pressure dominates nearly coast to coast.