Seasonal Predictability of Vapor Pressure Deficit in the Contiguous United States

Vapor pressure deficit (VPD), a measure of the deficit of lower atmosphere moisture, is highly correlated with the annual area burned by wildland fires in the western United States. Consequently, it is possible that forecasts of VPD anomalies could be used to assess future seasonal wildland fire danger potential. However, sources of VPD predictability on seasonal (> one month) timescales have not been heavily studied. We use a linear inverse model (LIM) to assess western US VPD forecast skill and evaluate the contributions from a long-term warming trend, sea surface temperatures (SSTs), and VPD variability unrelated to SSTs or its trend (‘internal’ VPD variability). In LIM forecasts, the warming trend contributes substantially to VPD forecast skill in summer and early fall for lead times up to 24 months, with a secondary contribution from internal VPD variability at lead times of one to two months. Conversely, SSTs contribute to VPD skill for forecasts that verify during the months from December to June at lead times of up to six months. Forecasts that are especially skillful and display high confidence, ‘seasonal forecasts of opportunity’ (SFOs), are associated with SSTs and internal VPD. SSTs contribute to forecast skill over California, the Southwest, and Texas while VPD anomalies contribute to skill over the Great Basin and western parts of the Northern Great Plains. SFOs have initial conditions that favor the development of the El Niño-Southern Oscillation, with La Niña (El Niño) SSTs associated with positive (negative) western US VPD anomalies and consequently, enhanced (reduced) wildland fire risk.