Severe weather looks likely in the DC Metro area on Thursday, as a powerful cold front approaches the region. The storm is still taking shape right now in the Midwest and the northern Plains states, and the Storm Prediction Center anticipates a widespread outbreak of severe weather in places like Chicago, northern Indiana, and portions of Michigan and Ohio this afternoon through tonight.
I've been hearing the dreaded "D" word a lot today- derecho- and I want to assure you that the dynamics are not in place for this type of weather in our area today or tomorrow. However, the elements are in place for a large supercell thunderstorm complex in the Midwest today, where derechos are much more common.
As heat and humidity build in places like Illinois and Iowa this afternoon, the stage will be set for a type of supercell thunderstorm called an MCS, or mesoscale convective system. "Mesoscale" means that the size of the system is in the mid-range for weather patterns; mesoscale systems can be hundreds of miles across. The word "convective" is a reference to the strong upward motion in these systems. Hot and moisture-laden air is very buoyant, so it quickly rises and helps to produce these large thunderstorm complexes. An MCS will likely form in the Midwest today, with the remnants of that system rolling through the DC Metro area (and especially points north of the Beltway) in the wee hours of Thursday morning. Most of the area will get a strong gust of wind, followed by a period of heavy rain that tapers off to showers sometime before dawn.
The moisture from this overnight system will increase tomorrow afternoon's severe weather potential. Thursday afternoon looks to be the "main event" for severe weather in the DC Metro area. A trough in the upper atmosphere will dig south toward us, and the trailing cold front will trigger widespread thunderstorms and a high potential for storms reaching severe limits. A Severe Storm has winds of 60mph or greater, and/or hail at least 1" in diameter.