Correlating Return Intervals, Forecast Anomalies and Probability of Extremes to Customer Impacts Across the National Weather Serviceís Hanford County Warning Area
Kris Mattarochia, Science and Operations Officer, NWS Hanford/San Joaquin Valley, Hanford, CA
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Abstract
Average return intervals and forecast anomalies continue to play a significant role in the decision making process of a meteorologist. This information is becoming more readily available and easier to incorporate into operations, because of more user friendly applications like the National Weather Serviceís (NWS) Western Region (WR) Ensemble Situational Awareness Table and the Extreme Precipitation Forecasting Tool, developed by NWS Las Vegas. However, what isnít quite clear yet is how certain ranges or values of indices from these applications correlate with specific customer impacts. Matching rare or climatologically anomalous values of meteorological elements such as Precipitable Water, Integrated Water Vapor Transport or Specific Humidity to impacts across NWS Hanfordís County Warning Area (CWA) can improve the communication of risk and the delivery of decision support services to our core partners. Analyzing return intervals, forecast anomalies and probability of extremes can also help forecasters categorize events and provide a frame of reference to our customers, by comparing expected impacts to similar weather events that are memorable from the past.

Research completed at NWS Hanford has determined some thresholds which have correlated with customer and core partner impacts across the CWA. These thresholds are broken down into values in the short and long term portion of the forecast and for sensitive areas like burn scars. For example, in the short term, during the abnormally wet winter of 2016-2017, 75% of the 6 hour rainfall required for a 4 percent chance annual event, was sufficient to create evacuations of a mobile home park in Madera County, California. In the long term portion of the forecast, 50 % of the 6 hour Quantitative Precipitation Forecast required for a 5 percent chance annual event, triggers forecasters to notify core partners outlining the potential for heavy rainfall.Some of these criteria are used in a locally developed playbook, which advises forecasters when to notify core partners, who need longer lead times to prepare for events to mitigate risk. For example, organizing employees for overtime or accumulating resources needed for sandbagging becomes more expensive when there is less time to prepare. Incorporating new technologies which improve forecastersí capabilities to more clearly communicate impacts can save local communities time and money, which is a key goal with regards to establishing a Weather Ready Nation.

This presentation will explore a few instances when specific anomalies and return intervals relative to impacts could have increased confidence and lead time for our core partners. Communicating risk can also be aided by finding specific ranges of return intervals and forecast anomalies which can trigger low, medium or high confidence. However, there remains some uncertainty in this project, since large anomalies donít always equate to unusual events, thus some of these cases will be examined. Future research will also explore how these thresholds can be adjusted considering pre-existing conditions like antecedent rainfall.