Developing a Fog Severity Index For the National Weather Service’s Hanford County Warning Area
Kris Mattarochia, Science and Operations Officer, NWS Hanford/San Joaquin Valley, Hanford, CA
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Abstract
Dense Fog is the number one cause of weather related fatalities across the Hanford National Weather Service (NWS) County Warning Area (CWA). Predicting the duration and coverage of Tule fog, is one of the biggest forecasting challenges at NWS Hanford. Reduced visibilities create strain on many transportation sectors across central California, including CalTrans, local school districts and the general public. The legacy dense fog advisory product is the primary means by which reduced visibilities are messaged. However, not all fog events are similar and the current means to communicate the potential and/or the existence of fog doesn’t allow forecasters to address specific impacts or confidence in a succinct manner. A 12 mile stretch of Highway 99 in the central valley of California through Fresno and Tulare counties is one of the largest areas of concern. California Highway Patrol’s Pace Program utilizes officers to lead vehicles at a safe distance when visibility is reduced to less than 500 feet in heavy traffic. Dense fog which forms around midnight and lasts several hours into the morning along Highway 99, is of greater significance than an isolated fog event in a sparsely populated location. Model guidance performs poorly predicting visibilities in this region, and the implied deterministic nature of the dense fog advisory product creates a fear of false alarm among forecasters, which can lead to missed events or little lead time. Thus a multi-tiered Fog Severity Index is proposed, which attempts to stratify and categorize these very different events by assigning an index of 1 to 5 and a color coded scale. For example, some roadways are more prone to chain reaction crashes because of their configuration compared to others during dense fog events. Another important consideration is that impacts can be experienced even when the fog isn’t considered dense, the definition of which is a quarter mile or less. The orientation of mountainous roadways can lead to travel difficulties even when visibilities are near a mile. Previous research has shown that 700mb heights from the Oakland, CA sounding and afternoon dewpoint depressions at Fresno show some correlation to dense fog events in the central valley of California. A Fog Severity Index which is scaled, can better meet the specific needs which arise with all of the above considerations. This approach will facilitate comparisons to previous events, which can assist our core partners with planning and preparation by increasing lead times. The Fog Severity Index also allows forecasters to alert users of impacts to travel during events which may not be considered “extreme” or “dense”. Unlike the dense fog advisory product, the fog severity index can be calibrated to impacts specific to urban, valley or mountainous regions of the CWA. For this project, fatalities due to fog related automobile accidents and meteorological conditions were researched over the past 15 years.