Orographic Precipitation on Oahu with Operational Forecasting Application

The Hawaiian island's main source of precipitation is from orographic precipitation, driven by the prevailing trade winds and the moisture rich marine environment. Rain gauge observations of precipitation and stream gauges for hydrological comparison are used alongside the Smith and Barstad Linear Model for orographic precipitation. Weather balloon soundings are used to initialize the model. The work has two primary goals: 1) to determine how well the linear model can capture precipitation observations on the islands, and 2) to generate maps of rainfall probabilities which depend on upstream conditions.

This work is done in tandem with the National Weather Service (NWS) Honolulu Forecast Office to help identify island regions that are especially problematic for forecasters, and have important social impacts. Accurate forecasting of Probability of Precipitation (PoP) is a daily need for forecasters and depends strongly on island terrain which is not well resolved in typical forecast models. From this work we find that some environmental conditions and microclimates are more easily and accurately forecasted than others. The difference can usually be attributed to a combination of elevation, the direction the slope is facing, the wind speed and direction, atmospheric moisture content, and atmospheric stability.