Anomalously Cold Icing Conditions within Atmospheric River Winter Storms along the West Coast of North America

Across North America aircraft icing is typically expected to occur at temperatures warmer than -25^oC. In the absence of deep convection (lightning observed), the National Weather Service (NWS) Current Icing Product (CIP) and Forecast Icing Product (FIP), operational flight guidance tools, don’t allow cloud layers colder than -25^oC to be considered as regions for aircraft icing. Severe icing by large commercial aircraft is sometimes observed along the West Coast of US at temperatures colder than -25^oC, especially during so-called atmospheric river (AR) type storms. The narrow bands of anomalously high precipitable water associated with AR storms can impact regions from Southern California to as far north as the Pacific Coast of Alaska. These severe icing events can be missed by the NWS guidance tools.

In this study a set of AR case studies along the West Coast over northern California with anomalously cold aircraft icing are analyzed using icing pilot reports, mountain top web cameras, NWS NEXRAD radar, satellite imagery, mountain top weather stations, NWS soundings, and special high-resolution (1-km) runs of the Weather Research and Forecasting (WRF) numerical weather prediction model. The results of the analysis are used to outline a new potential ‘Atmospheric River’ icing module for possible inclusion into the CIP and FIP icing guidance tools. The new module will allow the area effected by the atmospheric river to be identified and the icing threat to be maximized even within cloud layers where temperatures are cooler than -25^oC.