Development of a New NAM-based MOS Precipitation Type System

The Meteorological Development Laboratory (MDL) has recently completed an updated suite of Model Output Statistics (MOS) guidance for the North American Mesoscale (NAM) model. To date, statistically post-processed guidance for precipitation type has only been available to the public as part of the Global Forecast System (GFS)-based MOS. This paper describes the development of a new MOS precipitation type system for the NAM and its performance when compared to climatology and corresponding MOS forecasts from the GFS.

Equations for the conditional probability of freezing, frozen, and liquid precipitation types were developed at METAR sites from present weather observations and ~9 cool seasons of NAM model output. The development sample was comprised of a mix of retrospective output from the current version of the NAM as well as output from previous model versions. Equations valid every 3 hours on the hour were developed out to 84 hours in advance for the 0000 and 1200 UTC model cycles. A conditional best category is also produced by applying statistically-derived thresholds to the probabilities. Results from a k-fold cross-validation test indicate the new NAM MOS precipitation type guidance is comparable in skill to corresponding GFS MOS forecasts out to 84 hours in advance, with improved discrimination of the best category relative to GFS MOS. On September 29, 2015 the new precipitation type guidance was incorporated into the operational NAM MOS text message, and has also been made available to forecasters experimentally in gridded format.