Predictability of Various Dynamical Features during the 13–15 February 2019 Atmospheric River Event

The “Valentine’s Day” atmospheric river (AR) event affected much of the U.S. West Coast from 13–15 February 2019. This event was ranked as an AR3 (Ralph et al. 2019) along most of the California coast, reaching AR4 intensity along the southern-most region. Strong winds and heavy rainfall were associated with this storm; some locations experienced rain rates exceeding ~10” in 24-h, which consequently resulted in several road closures and evacuations throughout the state of California.

Not only was the Valentine’s Day AR an impactful event, it was also dynamically diverse. A sequence of Rossby wave breaking, the presence of multiple low-pressure systems in the North Pacific Basin, and the intensification of a mesoscale frontal wave all played a role in the evolution of this event. Examination of the 10-member Global Ensemble Forecast System (GEFS) Reforecast v2 data highlights the lead time at which the ensemble members begin to diverge, and allows us to compare the dynamical features between the ensemble members at this time. Additionally, we simulate this event using the Model for Prediction Across Scales (MPAS) using a high-resolution (10 km horizontal grid spacing) variable-resolution mesh for a more in-depth investigation into each feature associated with the Valentine’s Day AR and the larger-scale environment. By removing the effects of latent heating and applying potential vorticity (PV) inversion techniques, we can quantify the contribution of diabatic processes, and confirm which aspects were most sensitive to latent heating.

Preliminary examination of the control and dry MPAS simulations shows that even in the absence of diabatic heating, an AR forms and reaches Southern California, which indicates that some aspects (e.g., the landfall in Southern California) of this event were potentially more predictable than others. Further analysis will provide additional evidence and testing of this hypothesis.