Diagnosing the Role of Atmospheric Rivers, Past and Present, In Snowfall Events on Mt. Shasta, California
AuthorHansen, Cassie Hansen
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Extreme weather events on the West Coast of the United States are rare yet pose significant socio-economic impacts making their study of the utmost importance. We have examined the synoptic features associated with a historic snowfall during February 1959 on Mt. Shasta, California. During the 13-19 February 1959 Mt. Shasta received 480 cm of snow and set a single snow event record for the mountain, and at the time, set the record for the single greatest snowfall event ever recorded on planet Earth. Analysis of this event is challenging because of the sparse and coarse resolution nature of atmospheric observations at the time and the absence of satellite imagery. Nevertheless, while considering the limitations of the data, analysis of this event has contributed to our understanding of synoptic and meso-scale dynamics associated with extreme snowstorm events. This analysis relies heavily on National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, analysis of regional sounding and precipitation data, archived newspaper articles and reminiscences from long-term residents of the area.Through comparative analysis and simulations two additional contemporary storms were identified, both seemingly possessing the key synoptic elements for producing massive amounts of snowfall on Mt. Shasta. The first event occurred in January of 2010 and resulted in 304 cm of snow. The second event was in December of 2010 and was an under performing "moderate" snowstorm, for the Mt. Shasta region, that seemingly had all the right attributes, however at 120 cm accumulation failed to yield snow accumulation totals comparable to the totals observed in the other two storms. In this research we examine synoptic commonalities and differences between the two aforementioned 2010 cases using the Global Forecasting System (GFS) model analysis combined with Atmospheric Infrared Sounder (AIRS), Geostationary Operational Environmental Satellite (GOES), Special Sensor Microwave Imager (SSM/I), and Tropical Rainfall Measuring Mission (TRMM), along with CALIPSO and CLOUDSAT to identify the optimal phasing of the synoptic elements that influence storm behavior. This research finds that not one single mechanism is responsible for the amount of snowfall observed in any case. Results indicate that mid-level moisture, combined with a low-level AR, are necessary to transport the massive amounts of moisture-laden air to Mt. Shasta that can result in extreme snowfall events. Analysis of the contemporary storms result in the identification of additional factors that must be sequenced in a specific order and timed so as to coincide with the geographical location of Mt. Shasta in order to produce comparable events. Synoptic components that phased several days prior to these events were as follow: 1) amplification and breaking ofRossby waves, 2) availability of extra-tropical moisture that included enhanced mid-levelmoisture in the 850 hPa – 600 hPa layer, 3) the transition from a meridional to zonal polar jet, and 4) an active subtropical jet stream. The timing and phasing of the northern and southern branches of the polar jet stream led to an idealized long-term juxtaposition of moisture and cold air. The optimal phasing of these factors within the circulation pattern is key to the production of extreme snowfall on Mt. Shasta.