Performance of MOS statistical weather forecast guidance over the tropical western Pacific warm pool

The National Weather Service (NWS) statistical forecast guidance system, produced by the Meteorological Development Laboratory (MDL), has been an aid to forecasters since the early 1970's.  This guidance, called the Model Output Statistics (MOS) forecast system, provides an objective interpretation of the underlying numerical weather prediction (NWP) model.  The MOS system tunes the forecasts at specific stations to the observations, and it provides estimates for many of the weather elements the forecasters must include in their products, such as probability of precipitation (PoP), wind speed and direction, and temperature and dew point.  The current operational MOS system is based on the model output of the Global Forecast System (GFS).  GFS station-based MOS guidance is available for the contiguous United States (CONUS), Alaska, Hawaii, Puerto Rico, and tropical western Pacific islands.  Progress is also being made in the development of a gridded statistical guidance system but the project is beyond the scope of this abstract.   

For the MOS forecast system for the tropical western Pacific Ocean, the wind and probability of precipitation (PoP) forecast guidance were implemented in 2005 and 2007, respectively.  The forecast guidance for temperature and dew point is being developed.  The Pacific MOS system provides weather forecast guidance for island sites in the area from 15° S to 30° N and from 130° E to 170° W.  Many of these stations are located over the tropical western Pacific warm pool which covers the area approximately from 20° S to 20° N and from 90° E to 180°.  The sea surface temperature (SST) over the warm pool is the highest in the world ocean and a major heat source for the global atmosphere.

The tropical western Pacific warm pool oscillates and has spatial and temporal variability.  The atmospheric boundary layer over the warm pool couples with the ocean and has spatial and temporal variations in heating and moistening in the layer.  Its temporal variability has multiple time-scales ranging from diurnal to intra-seasonal and longer periods.  The impact of this variability shows in the rainfall, clouds, and intensity of the trade wind.  The intensity of the trade wind impacts the temperature variations.  A previous study on the skill of GFS-based Pacific MOS forecasts shows that the MOS wind speed and direction forecasts can predict diurnal variations better than the direct GFS model forecasts, especially true for the wind speed.  In this paper the skill of the new temperature and dew point forecast guidance in handling the diurnal, monthly (intra-seasonal) and inter-seasonal variations will be addressed.

This paper will present the comparison of western Pacific MOS forecast skills for island sites in the interior (e.g., Pohnpei) and near the edge (e.g., Midway Island) of the warm pool.  This verification study will focus on the newly developed temperature and dew point guidance.  New independent data will also be used to evaluate the operational MOS forecast guidance of wind and PoP.  The evaluation of Pacific MOS forecast guidance will be used as a reference in the design of future gridded MOS forecast guidance development for the western Pacific islands.