Low Level Wind Shear - National Weather Service



Low Level Wind Shear

(Joseph Calderone, WFO CHS)

Low-level wind shear is a REQUIRED entry into TAFs should we receive reports via PIREPs or a valid measuring mechanism indicating LLWS is occurring at or near either of our TAF sites. According to NWSI 10-813, which uses the 1979 NOAA Technical Memorandum NWS FCST-23, “Low Level Wind Shear: A Critical Review,” the following definitions of LLWS are:

1) When non-convective vertical wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick are expected or reliably reported with 2000 feet of the surface.

2) When one or more PIREPs received show (or other tools detect/indicate) non-convective LLWS within 2000 feet of the surface at or in vicinity if the TAF report, causing an air speed gain or loss of 20 knots or more.

For our purposes, we have the use of model soundings, the actual radiosonde, and the VWP product off of the KCLX radar. We may also use BUFKIT, RAOB or ACARS soundings via AWIPS or the internet to determine the shear difference.

The most simplified way of determining LLWS is to check the difference in wind speed AND direction between the surface and the 1000ft and 2000ft winds. If using the actual radiosonde winds while it ascends, 1-minute winds are roughly 1000ft (about 330m) and 2-minute winds are roughly 2000ft (660m). Otherwise, check the other available sources and use your best meteorological judgment.

To help with directional differences, use the Wind Shear Computation Table. This chart originally came from NOAA Technical Memorandum NWS FCST-23 but was in m/s. The chart included here uses speeds in knots, which is what is received on our radiosonde, is what is produced on model soundings, and is what goes into the TAF. This calculation is done because LLWS is not just a scalar value but is a vector value. Therefore, wind direction difference in elevation plays a role!

If there is a significant directional difference, then assign the surface wind speed to w1 on the chart and assign the 1000ft and/or 2000ft wind speed to w2 on the chart. Approximate as needed. Then, check the difference in degrees and find the appropriate column. Where your 1000ft and/or 2000ft wind speed goes across and meets the associated directional difference is the LLWS value. Look at the color-coded entry where your difference ends up. If green, LLWS is not likely (and, is instead, low level turbulence). If yellow, marginal LLWS is possible. If red, LLWS is likely.

Remember that this is just a rule-of-thumb. If you have valid meteorological reasoning to include LLWS, then do so! Use the Aviation AFD to explain your reasoning!

** NOTE: You may feel free to collaborate LLWS with any surrounding office but there is no requirement. **

|WIND SHEAR COMPUTATION TABLE GIVEN THE WIND SPEED w1 AT ONE LEVEL, |  |

|WIND SPEED w2 AT THE OTHER LEVEL, AND THE ANGULAR DIFFERENCE BETWEEN |

|THE TWO WIND VECTORS |  |  |

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