Fast-moving thunderstorms were zipping across the Mississippi Valley on Thursday afternoon, as an upper-level storm accelerated through the region. NOAA’s Storm Prediction Center placed a large part of the Mississippi and Ohio Valleys and lower Great Lakes under an enhanced risk of severe weather for Thursday afternoon and evening (see Figure 1). The day’s first tornado watch was posted for northeast Missouri, southeast Iowa, and far northwest Illinois, effective until 8:00 pm CDT, with a second watch in effect until 11:00 pm CDT for most of northern Illinois and parts of extreme southern Wisconsin and far western Indiana. The strongest and longest-lived tornadoes typically form within discrete supercell thunderstorms, as opposed to squall lines or large thunderstorm clusters. If the line of storms in Iowa and Missouri becomes a solid squall line, the threat of strong tornadoes will diminish along it, but residents in its path should still be prepared for a burst of very heavy rain, high winds, and large hail, with brief tornadoes possible. Other storms could become supercells ahead of the line, particularly in northern Illinois.Figure 1.
Much of the central and eastern U.S. was at risk for severe weather on Wednesday afternoon and evening, April 9. Image credit: NOAA Storm Prediction Center.
Heavy rains and severe weather are again plaguing the Ohio Valley, which has endured several days of large thunderstorm complexes called mesoscale convective systems (MCSs) moving along a persistent east-west frontal zone. By Wednesday afternoon, the warm front had shifted to the southern Great Lakes, with temperatures ranging from 40s to its north to 60s and 70s just to its south. However, the final push of this week’s upper-level storm system may still bring one more round of severe storms and heavy rain across parts of Kentucky and West Virginia as well as southern Indiana and Ohio. A severe thunderstorm watch was issued Wednesday afternoon for the upper Ohio Valley, and a solid swatch of flash flood watches extend from the St. Louis area eastward to the Virginia/West Virginia border.
We’ll be covering the severe weather on Thursday afternoon and evening in our experimental Weather Underground live blog
Preliminary data from NOAA’s Storm Prediction Center as of late Wednesday is overlaid here on the SPC convective outlook issued on Wednesday afternoon. A supercell thunderstorm in south-central Kansas produced eight tornado reports, with two others in southeast Missouri and western Oklahoma. Thanks to wunderground member TropicalAnalystwx13 for posting this overlay. Additional reports that came in after this graphic was produced can be found on the day’s SPC summary page
. Data and imagery: NOAA Storm Prediction Center.Wednesday’s storms: few tornadoes, but plenty of hail and high wind
The nation was spared major damage on Wednesday despite an extensive arc of severe weather from Texas to North Carolina. The most impressive storms were along the dry line from western Oklahoma into south-central Kansas. One long-lived supercell near the intersection of the dry line and the nation-straddling warm front produced several tornadoes near Medicine Lodge, KS. Storm chaser Mike Prendergast captured this impressive cone-shaped tornado
near Deerhead, KS, with a faint rainbow visible. Another supercell dropped hail up to 3” in diameter in west-central Oklahoma. Large hail was the favored mode of the day’s severe weather, with more than 200 reports nationwide. Baseball-sized hail was reported in Missouri, Kentucky, and Indiana, as well as Oklahoma, Figure 3.
This hailstone in Sullivan, Missouri, was literally baseball-sized! Image credit: Hanna Findley
Forecasters had correctly anticipated that the dry-line storms would be sparse but intense, although the coverage was even less than some had expected. Thin high clouds that overspread much of Kansas and Oklahoma cut down on daytime heating and reduced the available instability, which weakened the day’s severe potential somewhat. In addition, a layer of very warm, dry air atop the moist surface air served as a formidable cap for any thunderstorms attempting to draw on the surface air (although some less severe “elevated” thunderstorms did develop above the cap).
Jeff Masters will be posting an update on Friday covering NOAA’s latest El Niño projections and the seasonal hurricane forecasts issued this week by Colorado State University and Tropical Storm Risk.
Bob HensonFigure 4.
Mammatus clouds fill the sky near Slaton, TX, as thunderstorms prowl the Texas South Plains near Lubbock on Wednesday, April 8. Wind gusts as high as 76 mph and heavy blowing dust accompanied the storms. Image credit: Matt Mahalik
, Texas Tech University.