Wind shear (WS) is defined as a rapid change in horizontal wind speed and/or direction, with distance and/or a change in vertical wind speed and/or direction with height. A sufficient difference in wind speed, wind direction, or both can severely impact airplanes, especially within 2,000-ft AGL because of limited vertical airspace for recovery.

Forecasts of LLWS in the TAF refer only to non-convective LLWS from the surface up to and including 2,000-ft AGL. LLWS is always assumed to be present in convective activity. LLWS is included in TAFs on an “as-needed” basis to focus the aircrew’s attention on LLWS problems that currently exist or are expected. Non-convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee-side mountain effect, sea breeze front, Santa Ana winds, etc.

When LLWS conditions are expected, the non-convective LLWS code WS is included in the TAF as the last group (after cloud forecast). Once in the TAF, the WS group remains the prevailing condition until the next FM change group or the end of the TAF valid period if there are no subsequent FM groups. Forecasts of non-convective LLWS are not included in TEMPO or PROB groups.

The format of the non-convective LLWS group is:

WS Indicator for non-convective LLWS.

Hwshwshws Height of the top of the wind shear layer in hundreds of feet AGL.

Ddd True direction in 10-degree increments at the indicated height.

(VRB is not used for direction in the non-convective LLWS forecast.)

Ff Speed in knots of the forecast wind at the indicated height.

KT Unit indicator for wind.

Example:

Wind shear from the surface to 2,000 ft. Surface winds from 130° (southeast) at 12 kt changes to 270° (west) at 55 kt at 2,000 ft.

In this example, the indicator WS is followed by a three-digit number that is the top of the wind shear layer. LLWS is forecast to be present from the surface to this level. After the solidus (/), the five-digit wind group is the wind direction and speed at the top of the wind shear layer. It is not a value for the amount of shear.

A non-convective LLWS forecast is included in the initial time period or an FM group in a TAF whenever:

Forecast change indicator groups are contractions that are used to subdivide the forecast period (24 hours for scheduled TAFs; less for amended or delayed forecasts) according to significant changes in the weather.