A strengthening frontal boundary extended from Lake Superior to the western High Plains on Tuesday, September 9th, while a surface warm front extended east from developing low pressure in Kansas east to central Illinois. A vigorous mid-level short wave was moving across western and central Nebraska at the time resulting in the strengthening low pressure and cyclogenesis. Deep column moisture was located to the south of the warm front with surface dewpoints in the lower 70s and precipitable water values of 1.5 to 2 inches. Moderate to strong instability was in the frontal zone along with moderate effective bulk shear.
Several strong thunderstorms developed during the early afternoon in the vicinity of the warm front. A line of thunderstorms moved across southwest Iowa toward Des Moines during the early afternoon. The below image from 1816z shows this line of thunderstorms to the southwest of Des Moines. The CIMSS Prob Severe product was indicating a 69 percent chance for this line of thunderstorms to become severe in the following 60 minutes. The thick cirrus anvil from this thunderstorm prevented the Vertical Growth and Glaciation Rate information from being determined.
An examination of the KDMX all-tilts products and digital VIL indicated more of a threat of strong winds over large hail. However RAP forecast soundings showed a strong thermal inversion around 1500 feet. The KDMX velocity imagery indicated approaching 50 to 60 knot winds approaching Des Moines along the radial. However, the CIMSS Prob Severe product lowered the severe probability to around 30 percent as the line of storms approached Des Moines. A severe thunderstorm warning was issued for the counties to the west of Des Moines. The thunderstorms moved through the Des Moines area around 1915z uneventfully as the inversion prevented the strong winds aloft from mixing down to the surface.
Another batch of thunderstorms strengthened over eastern Nebraska during the early afternoon. These thunderstorms approached the Omaha area and produced a wind gust to 55 knots at Fremont Nebraska (KFET) at 1955z which is about 30 miles west of KOMA. The CIMSS Prob Severe product from 1948z indicated only an 8 percent chance for this thunderstorm to become severe in the next 60 minutes. Again, the Growth and Glaciation Rate information was not available.
Marc Kavinsky
NWS Milwaukee/Sullivan
John Cintineo
UW-CIMSS