Linking Short-Term Forecasts of High-Impact Weather over California to Tropical Influences

The atmosphere is so remarkably complex that determining factors that influence storm behavior can be extremely difficult. One way to attack this challenging problem is to examine it in a linear framework. Tangent linear and related adjoint versions of numerical forecast models allow us to do that. Adjoint-based tools can provide valuable insight into how distinct aspects of the initial state impact storm intensity. Here we use adjoints to understand the sensitivity of 24-h to 48-h forecasts of high-impact storms that hit the US west coast in early 2017 to changes in the initial state. These storms were associated with both widespread flooding and wind damage. The copious amount of precipitation associated with these storms was enough to end the severe drought conditions over much of California. The adjoint, tangent linear, and nonlinear models for the atmospheric portion of the nonhydrostatic Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS®) are applied to the sequence of these forecasts from early January through mid-February 2017. Because the storms resulted in impactful wind and precipitation, we consider both the sensitivity of precipitation forecasts and sensitivity of near-surface kinetic energy forecasts to changes in the initial state. For 24-h forecasts of both the precipitation and kinetic energy, the forecast sensitivity often occurs in the subtropics and mid-latitudes. A time-average plot of the sensitivity shows a maximum in the region between 25N and 40N and between 150W and 130W. When the forecast length is increased to 48-h, the maximum in the time-averaged sensitivity broadens to extend between 20N-45N and shifts westward to extend between 170W and 40W. There is very large case-to-case variability, with sensitivity extending as far west as the date line and as far south as 10N-15N on some days. Here we will examine in detail the cases with pronounced sensitivity in the tropics. These cases tend to be associated with large amounts of integrated water vapor transported northward from the tropics as well as key upper-level dynamical features such as a PV streamer extending from the mid-latitudes into the tropics. The physical mechanisms through which tropical perturbations may influence mid-latitude forecasts on these short timescales will be investigated. Implications for effective observing systems will also be discussed.