Description
Atmospheric rivers (ARs) are long and narrow corridors of enhanced vertically integrated water vapor (IWV) and water vapor transport (IVT) found within the warm sector of extratropical cyclones. Landfalling ARs are responsible for approximately half of the annual precipitation along the U.S. West Coast and are linked to extreme precipitation events and flooding. In order to mitigate flooding concerns and potential water shortages, water managers in California rely on accurate AR forecasts in order to make decisions regarding reservoir storage. This study quantifies the skill of week-2 control and ensemble-mean IVT magnitude forecasts by the Global Forecast System Reforecast (GFSR) for a 20-water year period 1997-2016 for a grid point near Bodega Bay, California. Trends in atmospheric flow patterns associated with teleconnections are studied for the best-forecasted AR days and the worst-forecasted false alarms to investigate synoptic-scale influences on forecast skill. Results illustrate that the GFSR ensemble-mean forecasts typically underforecasted IVT magnitude on days with a landfalling AR near Bodega Bay, with mean percent errors 40-70% below observed values at 7-to-14-day leads, signaling a reduction in the detection of ARs. The best forecasts of AR days at 10 days lead time were commonly associated with flow patterns that persisted in the positive phase of the Pacific-North American pattern (PNA) and weakened or the positive phase of the Eastern Pacific Oscillation (EPO). The worst forecasts (e.g., false alarms) were commonly associated with flow patterns that trended towards a strengthening positive phase of the PNA or the negative phase of the EPO. The trend in the PNA with decreasing lead time associated with the worst forecasts suggests the likely role of ridge amplification over the Northeast Pacific leading to AR landfalling in British Columbia instead of California.
Identifier
psu-etd-150
