Model output from the operational 40km Rapid Update Cycle (RUC) has been available since April 6, 1998. The RUC generates fields useful to severe weather forecasting every hour, including convective available potential energy(CAPE), convective inhibition (CIN), lifted index (LI), and helicity.

A new version of the RUC with 20km horizontal resolution is being implemented at NCEP this spring. Improvements are not limited to just a reduction in grid length. A more sophisticated land surface model with higher resolution vegetation and soil data should improve important interactions in the lowest layers of the model, and updated microphysics and convective parameterizations will improve the RUC precipitation forecasts. Testing of the 20km RUC has shown a better capability for forecasting convective precipitation, both in detail for individual cases and in overall threat scores and bias.

We will take a severe weather case from May 24, 2000 and compare the performance and biases of the old 40km RUC with the new 20km RUC. May 24 was a severe weather day for much of the east coast, with large hail, severe winds and heavy rain reported from New York southward to North Carolina.

The authors will illustrate the use of the RUC in severe weather forecasting with this case study from the spring of 2000. They will also assess the utility of CAPE, CIN, LI and helicity in anticipating correctly the character of later convection. An examination of the output from both versions of the model will help educate forecasters in differences between the old RUC and the new RUC.