Toward a gridded MOS system

Model Output Statistics (MOS) has provided guidance to forecasters, both public and private, for over three decades. This guidance has primarily been produced at locations where observations were taken and the data were available for two or more years. In MOS, we relate the observations to variables forecast by numerical models, "persistence" observations, and geophysical variables such as terrain elevation.

With the implementation of the Interactive Forecast Preparation System (IFPS), guidance is needed on a gird commensurate with the map projection used in the contiguous states (Lambert Conformal), Alaska (polar stereographic), and Hawaii and Puerto Rico (Mercator). Because observations are generally not available at gridpoints, MOS must be adapted to meet this requirement.

Two approaches are being pursued. One is the so-called generalized operator method whereby observations for many stations are pooled to produce relationships that can be applied to any point within the area, be it observation location or gridpoint. Geophysical fields such as upslope, upslope winds, and terrain height can be used.

Another approach is to use the MOS guidance at the observation locations and analyze these to produce a grid of values. This can be done with the Cressman analysis method modified to take into account terrain elevation and expected lapse rates.

These two methods will be discussed and examples shown.