Features January 2012 Issue
Case Study 1: Extrapolationís weakness
In the southeast U.S., it’s quite common to see the wind abruptly shift at some altitude between 3000 and 6000 feet. For example, a 200-mile flight during the morning from Greenwood, S. C., (KGRD) to Albany, Ga., (KABY) passes nearly midway between the Atlanta and Savannah FBWinds stations. The Atlanta winds valid at 1200 UTC are forecast to be 100 at 18 knots at 3000 feet and 280 at 26 knots at 6000 feet. That’s a 180-degree directional shift.
If your desired cruise altitude is 5000 feet, will that be a nice tailwind or an annoying headwind? The FBWinds don’t tell you where the shift will occur.
To the east of the route, Savannah is expected to have a similar wind shift with a forecast of 060 at 12 knots at 3000 feet and 270 at 31 knots at 6000 feet. Your flight planner might average this out and estimate the winds along your route to be 350 degrees at 22 knots at 5000 feet, for example. But that’s unlikely to be correct, and so is any time and fuel planning based on it.
A better approach is to visualize the wind shift with the Skew-T log (p) diagram. The diagram shown here is a one-hour forecast sounding near Macon, Ga., (KMCN). This solves all three problems with the FBWinds.
Macon is much closer to your route of flight and the forecast is valid close to the time you’ll pass through this area, so there’s less extrapolation. The Rapid Update Cycle (RUC) forecast model also provides a much higher vertical resolution of the winds. Using this diagram, it’s easy to see that the winds below 4000 feet are from the east and shift around to the west just above 4000 feet. In addition, the RUC is refreshed hourly, providing an up-to-date forecast of winds and temperatures at hourly intervals out to 18 hours in the future. —S.D.
Next: Case Study 2: A Cold Front Can Blow the Temps