18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
An examination of the operational predictability of mesoscale terrain-induced features in eastern Colorado from several models
Edward J. Szoke, NOAA/ERL/FSL, Boulder, CO; and B. L. Shaw
Poster PDF (134.1 kB)
As radar and surface observations have improved, forecasters have become increasingly aware of the importance of mesoscale circulations. Such features can have a significant impact on the sensible weather, but in the past were seldom captured by operational numerical models. With the increase in grid resolution of operational models, and the availability of locally run smaller-scale models, the possibility of resolving and predicting features on the mesoscale has become a reality. In this study we will look at the predictability of two well-known mesoscale features that occur in northeastern Colorado by examining forecasts from the latest operational versions of the Eta and RUC models, as well as from a local model being run operationally at FSL for the Boulder Weather Forecast Office (WFO).

The features of interest are known locally as the "Denver Cyclone" and the "Longmont Anticyclone," and both have been well-documented at conferences and in the literature, through observational studies and numerical modeling using various research models. Both features are induced by the interaction of the synoptic flow with terrain in northeastern Colorado. The resultant weather that can arise in association with these features ranges from dramatic variation in the wind field to mesoscale distribution of precipitation and localized occurrence of severe (winter and convective season) weather. Clearly there is high interest in trying to make operational forecasts of these features, which before the advent of finer grid resolution models were done by generally using forecasts of the synoptic flow and an understanding of the potential mesoscale features that could develop. The potential to forecast these features directly with numerical models gives the possibility for more accurate predictions of the occurrence of these important phenomena.

Although local models have been running at FSL (and made available to the Boulder WFO) for a number of years, there has not been any consistent verification effort aimed at these two flow features. Also, a recent change of the local model to a version of the MM5, with somewhat better resolution of the lower levels, has improved the overall ability to forecast the circulations. Meanwhile, the reduction of the grid resolution of the operational Eta to 22 km has resulted in better forecasts of both features with this model, and an initial look at forecasts from the new version of the RUC, at 20-km grid resolution, has also shown good predictability. In this paper we will look at all three models, examining individual predictions as well as the potential value in an ensemble approach in a subjective examination of the predictions offered by the various models for occurrence of the Denver Cyclone and Longmont Anticylone. Comparison will be made with sensible weather using detailed observations (METAR and local mesonet) and radar data, coupled with the LAPS analysis for the verification times.

Supplementary URL: http://laps.fsl.noaa.gov/szoke/mesoconf/cases_homepage.html