STARCASTING

The summit of Mauna Kea is arguably the best site on Earth for astronomical observations and the capital investment in telescopes on Mauna Kea has exceeded $600 million. The success of astronomical observations on Mauna Kea isstrongly influenced by weather conditions. During prevailing clear periods, astronomical observing qualityvaries substantially due to changes in the vertical profiles of temperature, wind, moisture, and turbulence. Cloud and storm systems occasionally cause adverse or even hazardous conditions. To facilitate the best possible use of good atmospheric conditions and to support operation safety on the 4200m mountain summit, a new interdisciplinary research program has been initiated that provides custom weather forecasts/nowcasts and meteorological data to the Mauna Kea Observatories. An operational mesoscale numerical modeling effort provides crucial forecast guidance for astronomical image quality, or seeing, during prevalent fair weather and for adverse weather. Of the existing telescopes on Mauna Kea, several commonly have multiple instruments or detectors mounted simultaneously, increasing the observational choices and thereby also increasing the utility of the custom weather forecast products provided by the newly established Mauna Kea Weather Center. Summit temperature forecasts allow mirrors to be set to theambient temperature to reduce image degradation. Precipitable water forecasts allow infrared and submillimeterobservations to be prioritized according to atmospheric opacity. Forecasts of adverse weather protect the safety of personnel, mitigate the hazard to telescope facilities, and allow for scheduling of maintenance when observingis impaired by cloud. This paper provides an overview of the unique forecast requirements and challenges faced by Mauna Kea weather forecasters. Progress toward meeting these challenges and opportunities for future research are discussed.