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US Weather Research Program (USWRP) and
NOAA/NEXRAD Radar Operations Center (ROC)

On July 14-15, 1998 a very unstable airmass over the upper Midwest led to a severe weather event over Minnesota and northern Iowa. The synoptic conditions were weak and a low level jet of southerly flow brought very moist (mid 70s dewpoints) air into central Minnesota. The system first developed over the higher terrain of southwestern Montana and became loosely organized into a N-S line as it propagated eastward. Upon encountering the enhanced moisture over Minnesota the convection intensified and became highly organized into an E-W line stretching across much of Minnesota. For a period of about 12 hours, as the system sagged southward into Iowa, the NEXRAD level II data exhibited characteristics typically associated with severe weather: reflectivities in excess of 65 dBZ (indicative of heavy rain and hail), small scale Doppler velocity couplets in excess of 50 m/s differential (rotation), and strong low-level Doppler velocities in excess of 30 m/s at the leading edge of the storm (gust front). Numerous ground reports throughout Minnesota and Iowa noted flash flooding, large hail (up to 10 cm size), strong straight line winds (40 m/s) and a couple of F0 tornadoes. The tornadoes were coincident in time and space to the Doppler velocity couplets observed. The ground reports confirm that the NEXRAD level II data were able to identify and track the severe weather associated with this event.

Multi-day reflectivity swath

Reflectivity swath consists of the maximum reflectivity value that passed over each grid point, accumulated over a 48-hour time period beginning on 0 UTC 14 July. Note N-S orientation and eastward propagation of system over Montana and North Dakota followed by a reorientation (E-W) and intensification over Minnesota.

Surface damage reports from Minnesota and Iowa

Surface damage reports from Minnesota and Iowa for the event.

University Corp for Atmospheric Research National Center for Atmospheric Research Mesoscale and Microscale Meteorology Division