-It was shown that abrupt changes in the large-scale structure of atmospheric flows may lead to the rapid decay of blocking. Analysis of phase diagrams made it possible to identify when sharp changes occurred in the dynamics of the system. The connection of these changes to the decay of blocking was estimated for three blocking events in the Southern Hemisphere. In addition to phase diagrams, enstrophy was used as a diagnostic tool for the analysis of blocking events. From the results of this analysis, four scenarios for the decay mechanisms were determined: (i) decay with a lack of synoptic-scale support, (ii) decay with an active role for synoptic processes, and (iii − iv) either of these mechanisms in the interaction with an abrupt change in the character of the planetary-scale flow.

The synoptic-and planetary-scale forcing in two blocking anticyclones occurring over the southeast Pacific Ocean was examined using potential vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short-lived. Some studies of NH blocking dynamics have also shown that the extent to which the planetary-and synoptic-scale and planetary-synoptic-scale interaction forcing that contribute to the genesis and maintenance of Pacific and Atlantic region events can be different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH was carried out in order to determine whether or not these events are associated with similar dynamic mechanisms to those in the NH. Using the National Center for Atmospheric Research and National Centers for Environmental Prediction (NCAR-NCEP) reanalyses dataset and applying a low-pass filter to the relevant variables, the authors examined the scale interactions associated with two blocking events that occurred during July and August 1986 and applied potential vorticity diagnostics. Results demonstrate that blocking in the southeast Pacific was associated with similar synoptic features, and the forcing mechanisms on the planetary, synoptic scales, and interactions were more similar to North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrated that blocking events in the NH were associated with synergistically interacting synoptic-and planetary-scale waves, while in the SH, blocking events resulted from the superposition of synoptic and planetary waves. This result may explain the paucity of blocking occurrences and their tendency to be weaker and less persistent over much of the SH.

or several years, the U.S. Federal Highway Administration (FHWA) Office of Transportation Operations Road Weather Management Program has been involved in the development of a guidance tool for winter road maintenance decision makers. The tool, known as the maintenance decision support system (MDSS), is the culmination of work by a consortium of national laboratories, directed by FHWA with feedback from both the state departments of transportation and the private sector. The MDSS combines advanced weather and road condition prediction with automated rules of practice to produce a customized and optimized set of treatment recommendations for winter road maintenance managers. The potential payoff is higher mobility and safety on roadways, and more efficient and cost-effective use of staff, equipment, and chemicals. The latter would produce a reduced impact on the environment. This paper describes the history, development, and status of the MDSS project. Preliminary results and lessons learned from a field demonstration, future development efforts, and the technology transfer plan is discussed.