Meteorological conditions behind Thursday’s active t-storms
Here are a collection of analyses depicting the meteorological conditions that fostered some of Thursday’s active t-storms
By Tom Skilling (Via Facebook)
Moisture within Thursday’s warm, buoyant air mass—which included low 80-degree high temps and low to mid 60-degree dew points indicating moderately “humid” air—ascended into “divergent” upper-level winds at the jet stream level (18,000 to 39,000 ft.)
If you look at the wind shear and upper air panel posted below, you can see from the wind streamlines, showing the direction of air flow aloft, diverging over northern Illinois and Indiana. Diverging upper winds are often a proxy for upward air motion. This occurs because air movement is moving into the vertical in the updrafts rather than moving horizontally which we measure as wind.
This upwelling of air helps build towering cumulonimbus clouds (i.e. thunderheads) and it’s within these towering thunderheads that hailstones form as the upwelling moist air coats hailstones with layers of rain which freeze in the colder, sub-freezing reaches of these clouds–building the size of the hailstones which are held aloft by the updrafts until they finally grow large enough to fall—or until updrafts slow or cease. This process of hail creation is what’s known as “accretion”.
Here’s how the Midwestern Regional Climate Center describes the hail formation process:
“Hail forms when a thunderstorm updraft lifts a water droplet above the freezing level in the atmosphere. The frozen water droplet then accretes super-cooled water or water vapor, which freezes once it comes in contact with the frozen droplet. This process causes a hailstone to grow.” Read more here: https://mrcc.geddes.rcac.purdue.edu/living_wx/hail
The fact these storms can ascend into strong upper level winds allows them to “mix” some of this upper wind energy down to the surface in t-storm wind gusts.
IT’S THE WARM SEASON—and rainfalls have been widely varied with some areas missing out on rain altogether.—HERE ARE SOME OF THE HEAVIER RAINFALL TALLIES THURSDAY based on preliminary reports:
0.82″ Dyer, IN
0.68″ Crete
0.68″ Riverside
0.63″ Morris
0.59″ Burr Ridge
0.55″ Woodridge
THURSDAY AFTERNOON’S UPPER AIR FLOW—Note the strongest upper winds sit to our west at the 500 mb level shown here. That’s likely why Thursday’s afternoon’s t-storms didn’t grow tornadic on a broad scale.
LOTS OF MOISTURE AVAILABLE–This Thursday afternoon analysis of the presence of moisture indicates the heaviest concentration of moisture occurring over eastern Iowa, Wisconsin, Illinois part of northern Indiana, and western Michigan.
The areas shaded yellow and orange are where the greatest concentration of moisture is available in a deep layer of the atmosphere extending from ground level aloft to nearly 40,000 ft. Little wonder we’ve managed to get some healthy downpours out of some of the t-storms that have developed Thursday. BUT—t-storm coverage varies—so some areas between these storms were able to escape afternoon downpours—while other areas were drenched.
Some of the PEAK WIND GUSTS reported by observation sites in and around Chicago
Some of the HEAVIER 12-hour rainfall totals Thursday
HIGH TEMPS THURSDAY–It was a warm spring day across the area.
Dew points in the low and mid 60s indicate the presence of humid air.
For the latest news, weather, sports, and streaming video, head to WGN-TV.
