Crisp Canadian air will collide with the outer fringes of the scorching heat in the Southern Plains Thursday afternoon, producing scattered severe thunderstorms across parts of the Midwest. All modes of severe weather are possible. Powerful straight-line wind gusts and very large hail will be the most likely sources of damage, but a few tornadoes cannot be ruled out.
An intensifying shortwave trough alongside its associated cold front and surface low will plow southeastward across the northern Plains Thursday. This trough will amplify and broaden as it treks toward the Great Lakes and Ohio River Valley, where it will be halted by strong high pressure over the western Atlantic Ocean and stall, This will bring several days of unsettled weather to the East. Throughout its journey, the trough will wreck havoc across much of the country’s midsection. Iowa, Missouri and Illinois will be targeted Thursday, followed by the Ohio River Valley and Mid-South Friday.
Between the trough’s cold front and leading warm front resides a steamy, warm sector ripe for severe thunderstorms. Temperatures across Missouri will climb into the 90s Thursday afternoon. Closer to the boundary of the warm front, temperatures will only reach the mid to upper 80s. But with moist southeasterly surface flow from the Gulf of Mexico, dew point temperatures in the 70s with contribute to extreme instability building across the entire warm sector.
Warm, moist southeasterly winds will make a clockwise 180° turn aloft through a vigorous northwesterly jet streak Thursday afternoon. This strong directional and speed shear, or change in wind direction and speed with height, will be more than sufficient to generate rotation and to separate storm updrafts from downdrafts. Rotating updrafts always signal the existence of a supercell thunderstorm, which could be capable of producing very large hailstones and sometimes tornadoes. The separation of updrafts and downdrafts prevents cool air from blocking the upward motion that feeds thunderstorms, helping them organize into multicellular clusters or squall lines. The strongest wind shear across the warm sector will develop over most of Iowa and Missouri, becoming less intense over western Illinois.
Morning thunderstorms will gradually dissipate as they track eastward throughout the morning. After the debris makes way for sunshine, scattered but discrete thunderstorms will fire by mid-afternoon ahead of another warm frontal boundary in central Kansas and Missouri. There is an isolated risk for some of the storms in Kansas to produce strong wind gusts and large hail, but the moisture and forcing will be more favorable for severe storms further east.
The initial storms will gradually organize into clusters and spread northeastwards, developing over Iowa by early evening. Many of these storms will be supercellular. They will be capable of producing powerful wind gusts on their southeast and southwest flank,s along with the potential for very large hail with diameters in excess of 2″ and possibly a tornado. Cities most at risk for these supercells include Kansas City, Jefferson City, St. Louis and Des Moines. Wind gusts will be the primary threat further east in cities like Davenport and Springfield.
The storms will not reach Illinois until close to sunset, at which point the loss of daytime heating and widespread outflow boundaries will have caused the storms to merge into clusters and linear segments. This will largely reduce the hail and tornado threat but will increase the chance for wind damage. The storms will cross Illinois throughout the overnight hours.
Wind shear and strong moisture inflow will support the linear system of thunderstorms through early Friday afternoon as the storms cross the Ohio River Valley and the Mid-South. Residual outflow boundaries and the approach of the shortwave aloft will spark new severe thunderstorms over these areas later Friday afternoon. Some of these storms could be capable of producing large hail and tornadoes.
The upper-level trough responsible for Thursday’s severe weather will continue to cause problems as it migrates across the country and eventually stalls. Be sure to check back for updates as we know about impacts from the system from the Midwest to the Northeast.