As a strong disturbance pushed southeast across Minnesota on Monday, scattered thunderstorms developed by mid afternoon across eastern South Dakota and southwest Minnesota. Relatively dry air at low levels, combined with a day of strong mid-May surface heating, led to development of what is referred to as an "inverted-V" sounding, named after the upside-down V-shape of the thermodynamic profiles of temperature and dewpoint. These types of soundings typically do not contain a tremendous degree of instability. However, as rain falls into the the very dry layer below the developing storm, the enhanced evaporative cooling of the air creates accelerated sinking motions which can result in very strong winds pushing out from the storm. This was certainly the setup on Monday afternoon, as indicated from the 4 pm CDT Rapid Update Cycle (RUC) analysis sounding from near Sheldon, IA.
A map and listing of the damage reports received as of late Monday evening are given below.
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1009 PM CDT MON MAY 19 2008
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
0455 PM TSTM WND DMG 4 S SHELDON 43.13N 95.86W
05/19/2008 O`BRIEN IA EMERGENCY MNGR
HALF OF A ROOF BLOWN OFF A HOG CONFINEMENT. DEBRIS WAS
SPREAD UP TO ONE HALF MILE DOWNWIND.
0455 PM TSTM WND GST SHELDON 43.18N 95.84W
05/19/2008 M63.00 MPH O`BRIEN IA AWOS
0515 PM TSTM WND GST 5 WSW ROYAL 43.02N 95.37W
05/19/2008 E60.00 MPH CLAY IA TRAINED SPOTTER
LOTS OF FLYING DIRT
0535 PM TSTM WND GST STORM LAKE 42.65N 95.20W
05/19/2008 M62.00 MPH BUENA VISTA IA AWOS
0540 PM TSTM WND GST TRUESDALE 42.73N 95.18W
05/19/2008 E60.00 MPH BUENA VISTA IA FIRE DEPT/RESCUE
0550 PM TSTM WND DMG S NEWELL 42.60N 95.00W
05/19/2008 BUENA VISTA IA TRAINED SPOTTER
LARGE TREE DOWN ON FENCE