Heat — Polk, Iowa

2006-07-17 · Polk, Iowa

Wider weather episode

A very unstable airmass was in place over Iowa during the afternoon of the 17th, though it was also quite capped. The 700 mb temperature was in the 14 to 15 C. range, making it difficult for thunderstorm activity to initiate. Plenty of moisture was available with CAPE values in the 6000 to 8000 J/kg range based on the surface, and near 5000 J/kg in the mixed layer. Lifted indices fell to near -14 C. by the late afternoon as well. With the exception of the surface, the lower levels were fairly dry. The LCL was in excess of 2100 meters by late afternoon. There was plenty of potential for hail with -10 to -30 C layer CAPE in excess of 1000 J/kg, though the freezing level was in excess of 15,500 feet. Low level lapse rates were in the 8 to 9 C. range in the zero to 3 km layer. The shear environment was fairly favorable by late afternoon with 30 to 40 kts of effective shear. A cold front moved southeast across the state during the afternoon. Ahead of it, a hot and very moist airmass was in place. Afternoon highs were in the mid 90s to around 100 with dew points in the mid to upper 70s. Thunderstorms were not able to fire until the late afternoon hours and only over the far eastern part of the CWA. Once the storms did form, the mode of severe weather was hail. Hail of golf ball, upwards of 3 inch diameter fell in Black Hawk County at Waterloo. The hail path began near the airport and then tracked southeast through downtown Waterloo, causing extensive damage. Tama, Wapello, and Davis Counties were also hit with large hail. Hail of nearly baseball size pelted the countryside south of Bloomfield. One brief tornado touched down in Tama County near Tama. It touched down in an open area just west of town and caused little damage. During the afternoon hours, temperatures in Des Moines reached 100 degrees for the first time since August of 2003. An 83 year old woman was overcome by heat and found dead in her back yard around 1530 in the afternoon.

View location on OpenStreetMap → (42.4667, -92.2833)

Source: NOAA Storm Events Database, event_id 5516095. Narrative written by the NWS forecast office that issued the report.