Hi cf6-80c2b5f,
The MET forecaster have a pretty good idea about the expected ISO Bar pressure gradient with the passage of fronts etc. They can guesstimate the geostrophic wind speed at 1,000 m agl (3300ft) from the expected pressure gradient. The surface wind will be slowed down by friction, so there will be wind shear between the two levels and 2,000 feet is approximately half way between the two.