Short Range, Low-level HYSPLIT Trajectory Model Forecasts for Prediction of Coastal Stratus and Fog
Some coastal stratus and fog cases in California and other regions of the U. S. were identified by the use of GOES visible images, validated by airport observations. Then a matrix of 18-24hr forecast HYSPLIT trajectories were generated, based on data from the Global Forecast System (GFS) or North American Mesoscale (NAM) model with a starting point altitude of 50m. Trajectories were then compared to stratus and fog coverage in visible satellite imagery near the verifying time.
Within 24-h of summer (May-August) California coastal stratus events, trajectories showed strong north to northwest flow, with paths that intersected the coast line, sometimes with pronounced curvature into coastal bays such as San Francisco and Monterey. The trajectories typically crossed progressively cooler waters as they neared the coastline. For stratus events specifically in the San Francisco Bay area, mean 18-h GFS trajectories with a starting point of 37.5N and 123W flowed eastward directly into the Bay region by 1200 UTC the following morning, while for non-stratus days, the flow was parallel to the coast. Over Southern California, cyclonic eddies with relatively weak flow were observed in the Los Angeles bight during significant onshore stratus and fog events.
Additional cases show the potential forecast applications of HYSPLIT in other coastal areas such as the Gulf of Mexico, the South Atlantic states, and New England. In some instances, it was possible to estimate the inland extent of stratus or fog the following morning. Along with potential forecast applications, HYSPLIT could be useful for training purposes by showing conditions associated with past significant fog events.