Had a nice, local chase yesterday of ~150 miles across northeast Kansas. Departed Lawrence around 530 pm and met up with Jared Leighton and company outside of Topeka. We stopped to observe the first views of the initial supercell west of Topeka around 630pm. Decent structure was observed, albeit a little elongated with the new flanking line development stretching from south of the frontal boundary into the main updraft just north of the boundary. With time, a RFD clear slot cut into the slightly elevated updraft base, north of Interstate 70. Westerly winds increased, with some dust becoming lofted underneath the RFD gust front.
LEFT image shows rapidly developing storm east of MHK. MIDDLE image shows the left half of the updraft/flanking line region. Note the RFD clear slot can be seen in the far right of the image. Even though most of the updraft was behind the frontal boundary, surface-based parcels were likely reaching the storm via flanking line, aiding in its persistance. RIGHT image shows main updraft base with beginnings of a RFD clear slot. With the surface boundary so close to RFD gust front, it's likely the shallow nature of the cool side of the boundary allowed for severe winds to penetrate down to the surface.
We repositioned along Hwy24, east of Topeka by 7pm. The highlight of the day occurred after I just pulled over near Williamstown, KS at the intersection of Hwy24 and Hwy59 at approximately 720pm. A strong surface vortex rapidly developed just south of Hwy24, ingesting copious amounts of dirt and debris in Williamstown. This feature developed along the intersection of the frontal and RFD boundaries, near the southern periphery of the reflectivity appendage. There was apparently enough low-level convergence in a very concentrated area at these boundary intersections to support such a cyclonic surface system. There was no apparent vertical connection between the RFD gust front updraft base and the surface feature. Additionally, outflow seemed to certainly help dictate its forward propagation. Hence, the best term to classify the phenomena is a strong gustnado. The gustnado was characterized with intense inflow jets and well-defined corner-like regions during the initial vortex phase, before the feature gradually enlarged and became multivortex in nature. During its peak intensity for approximately 5 minutes, the gustnado was 1/4 mile wide and contained estimated winds of 80mph. NWS TOP storm survey confirmed these types of speeds with the gustnado, discovering two center pivot irrigation systems toppled, a large shed destroyed, and numerous tree limbs snapped in its path. Sequence of photos showing the evolution of the gustnado vortex from stationary location. Note the 5th image depicting an enhanced outflow jet propagating the vortex. Also note the bow in RFD gust front.
The remainder of the evening was spent sampling hail sizes in three different supercells along Hwy59, south of Lawrence. We observed 1.50” hail in Pleasant Grove at 835pm, 1” hail on the north side of Ottawa around 9pm, and 1.50” hail south of Princeton at 925pm. It was a convenient beginning to my 2011 season.
Radar image near the time of the gustnado pictured above (717pm). Both the cold front and RFD boundaries are depicted in a dash line. Surface wind direction obtained from local mesonet sites, denoted by arrows. Cyclonic circle shows area of main mesocyclone. The "X" denotes the exact location of the gustnado vortex.