Short Range, Low-level HYSPLIT Trajectory Model Forecasts for Prediction of Coastal Stratus and Fog

The prediction of low stratus and fog is an important forecast problem for aviation and surface transportation operations. A critical element in these forecasts is the presence of low level flow from sources of moisture into the forecast area. In transition situations, such as a reversal from offshore to onshore flow in coastal areas, rapid changes in wind direction and speed sometimes make it difficult for forecasters to visualize the origin of the air mass over the area. The National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory's (ARL) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model can provide forecasts of low level trajectories out to 24 hours using data from a variety of numerical prediction models generated by the National Centers for Environmental Prediction (NCEP). While the HYSPLIT model is most often used in Hazardous Materials (HazMat) events such as nuclear accidents, smoke plumes from wildfires, and volcanic eruptions, meteorological applications are also possible.

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.