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Jennifer Luppens Mahoney, John M. Brown, and Edward I. Tollerud

Abstract

Case studies of heavy snowstorms at Denver and Colorado Springs, Colorado, indicate that they occur under different meteorological conditions. The authors examine the hypothesis that there are in fact fundamental differences between the synoptic evolution of events in these two storm types by compositing a total of 28 cases, 17 (11) of which are defined as heavy snowstorms (at least 20 cm of snowfall) at Denver (Colorado Springs). These composited fields were constructed using data at three times in the history of each case. Results show distinct differences in the composited synoptic evolution of the two groups. At low levels the Denver composite shows low static stabilities, warm advection, and high values of potential temperature in the lee of the Rockies. The Colorado Springs composite, on the other hand, shows cold, stable air and cold advection in the lee. At upper levels an eastward-progressing short-wave trough is found at different longitudes in the two composites.

The implied interaction between lower and upper levels of the two composites is also very different. For the Denver composite, the trajectory of the upper-level trough brings it close to the area of low static stability and high surface potential temperature at low levels. This implies strong interaction between the upper-level system and the warm unstable air at low levels and dramatic cyclogenesis east of the Rocky Mountains, typically in southeast Colorado. In contrast, the upper short-wave trough in the Colorado Springs composite is farther north, and a layer of cool stable air is found on the High Plains of Colorado. Not surprisingly, surface cyclogenesis is notably weaker in this composite. These conclusions, substantiated by inspection of the individual cases, have obvious implications for predicting the location of heavy snow along the Front Range of Colorado.

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Lígia Bernardet, Louisa Nance, Meral Demirtas, Steve Koch, Edward Szoke, Tressa Fowler, Andrew Loughe, Jennifer Luppens Mahoney, Hui-Ya Chuang, Matthew Pyle, and Robert Gall

The Weather Research and Forecasting (WRF) Developmental Testbed Center (DTC) was formed to promote exchanges between the development and operational communities in the field of Numerical Weather Prediction (NWP). The WRF DTC serves to accelerate the transfer of NWP technology from research to operations and to support a subset of the current WRF operational configurations to the general community. This article describes the mission and recent activities of the WRF DTC, including a detailed discussion about one of its recent projects, the WRF DTC Winter Forecasting Experiment (DWFE).

DWFE was planned and executed by the WRF DTC in collaboration with forecasters and model developers. The real-time phase of the experiment took place in the winter of 2004/05, with two dynamic cores of the WRF model being run once per day out to 48 h. The models were configured with 5-km grid spacing over the entire continental United States to ascertain the value of high-resolution numerical guidance for winter weather prediction. Forecasts were distributed to many National Weather Service Weather Forecast Offices to allow forecasters both to familiarize themselves with WRF capabilities prior to WRF becoming operational at the National Centers for Environmental Prediction (NCEP) in the North American Mesoscale Model (NAM) application, and to provide feedback about the model to its developers. This paper presents the experiment's configuration, the results of objective forecast verification, including uncertainty measures, a case study to illustrate the potential use of DWFE products in the forecasting process, and a discussion about the importance and challenges of real-time experiments involving forecaster participation.

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