1. printed by www.postersession.com The Mechanisms and Local Effects of Heavy Snow in Interior Valleys of Northwest Californi a Matthew Kidwell, Senior Forecaster National Weather Service, Eureka, California In moderate southwest flow snow falls at lower elevations in inland valleys that are protected by the terrain from the relatively warmer maritime air mass. There are a few notable conditions that must be met for this to occur. 1. 1.Coastal snow levels around 5500 feet or lower. If the incoming air is too warm it will fall as rain or freezing rain. 2. 2.Sustained precipitation occurring through the period. If there is a break, it may allow other processes to take over. 3. 3.The absence of a vigorous system with strong mechanical mixing. 4. 4.The absence of Conditional Slantwise Instability(CSI). CSI might allow more marine air to mix into the interior valleys. Impacts: 1. 1.Heavy snow caused significant problems on the roads. This included a major accident closing highway 299 west of Weaverville, CA for several hours. 2. 2.Extensive utility outages occurred due to the heavy snow in Weaverville. The Phenomenon Case Study December 20 and 21, 2012 Hypothesis Forecaster Tips Conclusions Future Work Synoptic Setup Snow falls at lower levels in inland valleys than at the coast. This varies from valley to valley and makes forecasting difficult. In Southwest flow the terrain blocks the warmer maritime air allowing the cooler continental air to remain in place. This forces warm moist air to flow up and over the terrain and cooler air. The configuration of the local terrain, the atmospheric stability, and the distance from the coast drive the character of these events 1. 1.Hi-Resolution model simulation to better resolve the terrain and resultant effects. 2. 2.Compare this event with a more vigorous system that mixes out the cold air in the inland valleys. 3. 3.Launch soundings around the area during an event to get a more accurate picture of the atmosphere. 4. 4.Opportunities exist for a more rigorous study of this challenging phenomena. Heavy Snow Impacts for this Case A time series of NAM model vertical cross sections including relative humidity (image), temperature (green lines), and potential temperature (orange lines), at 18Z on the 20 th, and 00Z and 06Z on the 21st. The cross sections follow a line running from southwest to northwest (shown in yellow in the synoptic setup figure on the left of the poster). The GFS model showing the general synoptic pattern at 00Z on Friday December 21. This event began around 12Z (4AM local time) on the 20 th of December 2012. Heavy snow persisted for about 24 hours. This is a good example of the type of event that drops significant snow in the interior valleys, but at similar elevations closer to the coast the precipitation is all rain. NAM model sounding at 00Z on the 21 st shown in BUFKIT. Located near Weaverville around 3000 feet above Mean Sea Level. The green arrows represent the southwest flow. The circled area highlights the high terrain blocking the valleys. The star is the eastern Trinity Valleys and our area of interest. An upper level low sets up off the coast with the associated cold front slowly approaching the area. This brings moist southwest flow over the west coast through the event. Twenty two inches of snow fell at 2750 feet along Trinity Lake north of Weaverville in about 30 hours. Storm total snowfall reports from CoCoRAHS, Co-op observers, the NWS on the 20 th and 21 st. These are 48 hour totals, although most of the snow fell in 30 hours. References Historic Cases from NWS Spokane and NWS Eureka. The figure on the left shows a classic warm front. The figure on the right shows the terrain acting as a boundary to keep the cold air in place. Acknowledgements: Mel Nordquist for advice on this on this project..