Posted at: 06/11/2013 10:20 AM
Updated at: 06/11/2013 10:26 AM
There is a threat for severe weather tonight.
The possibility of strong thunderstorms is
different than the “normal”
severe weather we experience for a couple of reason. First, the timing of tonight’s thunderstorms is in the overnight hours and not during the late afternoon/ early evening. In fact, the storms will likely not reach Minnesota/Iowa until after midnight. Second, the thunderstorm activity will arrive as a cluster of storms, known as a “mesoscale convective system” or MCS. The leading edge of an MCS is where most severe weather occurs. The main threat is usually confined to severe wind gusts measuring above 60 mph, but small hail cannot be ruled out.
You may have heard us talk about thunderstorm fuel, or instability, in the past. As today’s warm air moves north, instability will increase giving thunderstorms plenty of “fuel”. So, then why no thunderstorms in the afternoon? As the warm air moves in it will also warm the middle levels of the atmosphere. When the middle levels become warm, it suppresses thunderstorm growth and prevents thunderstorm formation.
Today’s severe weather is expected to originate in western South Dakota with supercell thunderstorms. The middle levels of the atmosphere will cool off by this point. As these storms progress eastward they will form an MCS and quickly move across South Dakota, Minnesota and Iowa. The exact path of MCS will be difficult to determine until storms form this afternoon. One weather model has the storms arriving in our area between 4 AM and 10 AM. Some severe weather warnings may be needed. We will be tracking this cluster closely once it develops.