The GFS model runs for 384 hours, but do not trust the model beyond 240 hours: here is why
The American Global Forecast System (GFS) model is one of the most reliable models used by meteorologists to predict the tracks of tropical cyclones. It runs on six-hour intervals at 00:00, 06:00, 12:00 and 18:00 UTC, for 384 hours (16 days). The GFS is the only major tropical weather model that runs for 384 hours: the CMC and ECMWF models run for 240 hours (ten days), while the HWRF hurricane model runs for 126 hours (just over five days). The GFS model past hour 240 is extremely unreliable, and is almost never right. In very rare cases, there are exceptions, however, such as Hurricane Matthew which the model picked up on over ten days out.
During the hurricane season, the GFS model is notorious for forming “ghost storms” (storms that typically are dropped after one run or change in track and intensity greatly each run). Most of these “ghost storms” occur after hour 240. In fact, the 06z Tuesday GFS run developed a strong 993-mb tropical storm over 300 hours out in the northeastern Gulf of Mexico, as noted above. Ghost storms often occur throughout the season, and later in the season is it not uncommon to see “ghost” major hurricanes on the GFS model. However, it should be noted that if a model indicates the formation of a “ghost storm,” it could indicate that the environment for tropical cyclone development could at least be more favorable than normal. There are certain “warning signs” of ghost storms, and if a tropical cyclone displayed on a model displays any of the following characteristics, it is probably a ghost storm. They are listed below.
WARNING SIGNS OF GHOST STORMS
- The storm appears on the GFS model after hour 240
- The storm greatly changes in intensity from run to run
- The storm is frequently dropped by the model and re-added in later runs
- The storm’s timeline shifts back in time, or does not move forward in time
- The storm does not develop on any other model
People should simply not trust the models too far out. This is the reason why the National Hurricane Center does not issue tropical weather outlooks for longer than a five-day period. I will be back for another blog post tomorrow.