The 2003 Pacific THORPEX Observing System Test is the first in a series of Pacific and Atlantic observation campaigns in support of the WWRP/USRP THORPEX Program. THORPEX - a Global Atmospheric Research Program, is a 10 year international research program under the auspices of the World Meteorological Organization/World Weather Research Program (WMO/WWRP) to accelerate improvements in short range (up to 3 days), medium range (3-7 days) and extended range (two week) weather predictions and the societal value of advanced forecast products. THORPEX will examine predictability and observing system issues, and establish the potential to produce significant statistically-verifiable improvements in forecasts of high impact weather. The program builds upon and coordinates advances being made in the operational forecasting and basic research communities.