A Rare Ice Storm in the Intermountain Western U.S.: Storm Morphology

A winter storm on Monday, 09 January 2017 caused freezing rain to develop across several valley locations in western Colorado and eastern Utah. This storm was connected to an anomalously strong atmospheric river that continually advected subtropical moisture into the region, surging precipitable water values to over 200% of the climatological normal for early January. The influx of warm, moist air aloft was concurrent with significant clearing over portions of western Colorado and eastern Utah during the early morning hours. Clearing skies reinforced sub-freezing planetary boundary layer conditions resulting in strong temperature inversions over many valley locations. The resultant atmospheric profile was favorable for freezing rain to be the dominant precipitation type to reach the surface. Forecast confidence of valley precipitation type was low prior to the event given discrepancies between computer simulations and observed upper air soundings launched from Grand Junction. Inconsistencies were found in how strong and deep the layer of warm air aloft would be and if it would be sufficient enough to erode any surface inversions. Additionally, short-term model guidance struggled with the aforementioned significant clearing Sunday night and therefore the depth of cold air in the lower levels. A detailed and exhaustive look into the physical forcing mechanisms that triggered this rare event will be presented along with the ability of numerical models to quantify precipitation type in extremely complex terrain.