The winds and temperatures aloft forecast, or FB, is made twice daily based on upper air observations take at 0000 and 1200 Zulu.
The forecast header instructs the user on the valid period for the forecast.
The header of this example winds and temperatures aloft tells us this forecast was based on data from 1200 Zulu on the ninth day of the month.
It is valid on the nine day of the month at 1800 Zulu for use between 1400 and 2100 Zulu.
Included in the header is a statement reminding us the temperatures are always negative above 24,000 feet.
Winds and Temperatures Data
The winds and temperature are given as two numbers describing direction, followed by two numbers indicating wind speed, and then the temperature.
For example, winds and temperature forecast for 12,000 feet at Abilene, ABI, shows the winds from 200 degrees at 24 knots, with a temperature of plus six degrees Celsius.
At 18,000 feet the winds are expected to be from 230 degrees at 31 knots, with a temperature of minus ten degrees Celsius.
Since the direction is given in ten degree increments, the zero can be dropped from the direction.
Why Is Some Data Missing?
Winds are not forecast for altitudes within 1,500 feet above station elevation, and temperatures are not forecast for altitudes within 2,500 feet above the station elevation.
This is why no winds or temperature are given for ABI at 3,000 feet, and why no temperature is given for ABR at 3,000 feet.
“9900” is used on the winds and temperatures aloft forecast to indicate calm winds.
Calm winds are expected at 3,000 feet over wichita, ICT.
Winds Over 100 Knots
Notice the winds over abilene at 39,000 feet,
This coding indicates wind speeds of more than 100 knots.
To determine the winds and temperature, subtract 50 from the wind direction, and add 100 to the wind speed.
In this case, the winds and temperatures aloft over Abilene at 39,000 feet are forecast to be from 250 degrees at 104 knots, with a temperature of minus 56 degrees Celsius.