Appendix D. Low Level Wind Shear Monitoring

Low-Level Wind Shear (LLWS) monitoring has been implemented since AvnFPS 3.0. The WSR88D VWP tabular information, wind profiler data, and Aircraft Communications Addressing and Reporting System (ACARS) data along with surface observations at TAF sites allow vertical wind shear to be calculated. With appropriate thresholds, AvnFPS can alert the forecaster to the presence or the possibility of LLWS in the vicinity of their airports. As defined in [NWSI 10-813], Section 1.2.8, LLWS is wind shear of 10 kts or more per 100ft in a layer more than 200 feet thick. Only winds within the lowest 2000 feet are used for LLWS calculations. LLWS monitoring alerts appear on AvnFPS Monitoring GUI, in a manner similar to the lightning alerts.

Remember that vertical wind shear is not a scalar quantity, but a vector. Using just the difference in magnitude, i.e. speed shear, will, in the majority of cases, underestimate the amount of shear present. Direction of the horizontal winds must be considered as well.

On a benign weather day, wind shear values typically are less than 0.08 s-1. Wind shear meeting official criteria is 0.169 s-1. The LLWS monitoring algorithm uses two threshold values which provides three wind shear regimes. These thresholds separates the wind shear spectrum into normal (green) and caution (yellow) and warning (red) regions. The placement of the thresholds is set via the AvnFPS configuration GUI and are customizable for each TAF site, see Section 4.1: “Editing Monitoring Rules”.

As you move your mouse over the LLWS alert light, the current TAF is displayed along with the source identifier producing the greatest shear and a "best-guess" (based on available data) wind shear group, suitable for placing into the TAF. The LLWS alert light will only change if elevated wind shears are detected and there is no WS group in your TAF forecast valid at that time. If there is a WS group in the TAF, the LLWS Alert light will not change, i.e. it will stay green, regardless of the amount of wind shear detected.

It is the WFO's aviation focal point's judgement to decide whether to use a radar's VWP product or profiler's data for a particular TAF site. Physical promixity and elevation of the data sources--radars and profilers--need to be considered as to whether LLWS monitoring is practical (or effective) for a particular TAF site. Placement of a radar on a mountaintop would be a disqualifying factor for use in LLWS monitoring since winds from such a location are likely to be unrepresentative of conditions over an airport that is situated at lower elevation. Keep in mind that wind shear impacting aircraft operations is limited to altitudes of 2000 ft AGL and lower. For a given TAF site, one can configure multiple upper-air sources to be used with the airport's surface observations. In such cases, the alert level on the AvnFPS Monitoring GUI is based on the maximum shear found.

To begin LLWS monitoring for your TAF sites, see Section 4.2: “Editing TAF Site Information”.

In the event that the LLWS monitoring icon goes yellow or red, the forecaster should consult the VWP product of the profiler, radar, or ACARS that triggered the alert.

If LLWS alert is based on radar data, the forecaster should examine the radar's VWP text product, not the graphic that's displayed in D-2D. The radar VWP text product shows greater vertical resolution in the lowest 2000 feet than what is displayed in the graphic.

With these data, the forecaster can judge the validity of the wind data and susequently the LLWS alert. If there is corroborative evidence from other sources that strong, non-convective wind shears are occurring, then amending the TAF to put in a WS group should be considered.