An implementation plan for cool season quantitative precipitation forecasting.

"This report was commissioned and produced under the Office of the Lead Scientist, United States Weather Research Program, National Center for Atmospheric Research, Boulder, CO, USA." -- from title page.

Technical Report

The Cool Season Quantitative Precipitation Forecasting (CSQPF) workshop was convened in February 2004 to develop an implementation plan for improvement of cool season quantitative precipitation forecasting (QPF). The overall goals of the workshop were to define critical research activities required to advance short-term (0-48 h) QPF during the cool season, to consider the development of numerical weather prediction (NWP) and data assimilation systems relevant to the cool season problem, to propose and assess various methods for observing the atmosphere on scales necessary to provide improvements in cool season precipitation forecasts, and to examine the value of the improved forecasts and how the improved information is ultimately used. Four working groups undertook these tasks. The working groups were charged with examining (1) research and field studies that will contribute to advances in CSQPF; (2) numerical weather prediction and data assimilation advances required to address the CSQPF problem; (3) current and future observing systems and test-beds as a means of advancing CSQPF; and 4) the CSQPF needs of users, and the roles of the public, private, and academic sectors. This document presents a detailed summary of the findings and recommendations of the workshop. To improve 0-48 hr cool season QPF, workshop participants concluded that greater understanding is needed in five key areas: 1) 4D mesoscale structure and dynamics of weather systems, particularly above the boundary layer; 2) the structure and mesoscale dynamics of the rain-freezing rain-snow transition region; 3) regional mesoscale boundary layer forcing, particularly orographic and lake effects; 4) moisture sources and transport into winter systems; and 5) the predictability of cool season precipitation in all of its forms. The workshop recognized that many important phenomena contribute to the difficulty of cool season QPF, including snowstorms with convection, banded precipitation, freezing drizzle and rain, supercooled clouds, flooding winter rain and snowmelt, cold-air damming, lake-effect snow, and orographic precipitation. Subtle problems, such as an incorrect forecast of surface temperature, can dramatically impact wintertime forecasts. Scientific questions related to each of these key areas and phenomena were posed and test-bed approaches to addressing these questions debated. The workshop recognized in this debate that there are major regional differences in the user needs for cool season QPF and that forecasts should be tailored to those needs.