1. Anticipation, Avoidance & Forecasting
Low-Level Wind Shear
Anticipation, Avoidance & Forecasting

2. What is it?
“a change in horizontal wind speed and/or direction, and/or vertical speed with distance, measured in a horizontal and/or vertical direction.”
Could be speed shear, directional shear or a combination of both
Occurs within 2000 ft AGL

3. What about low-level turbulence?
Wind shear
Fluctuations in airspeed (only momentary fluctuations in vertical speed)
Prevailing winds at the top of the low level and the surface are similar
Change in air speed accompanied by rapid change in vertical speed
Wind speed and/or direction changes significantly from the top of the shear layer to the surface

4. What are the dangers? Headwind at low level to tailwind at surface
With the loss of headwind, nose pitches down,
Pull nose up and further reduce air speed
Could lead to the plane landing short
Tailwind at low level to headwind at surface
Once headwinds are encountered, nose pitches up, air speed increases
Could lead to the plane landing long
Headwind or Tailwind at low level to crosswind at the surface
Much more of an impact to GA aircraft than larger commercial aircraft

5. Convective vs. non-Convective
Generally short-lived
Radar is the best tool for detection, though increasingly difficult further away from the radar and in complex terrain
Can last for hours
Soundings, profilers, and radar can all be used for detection

6. Convective LLWS Very hard to detect even with radar
Microburst, macroburst, and squall lines are most often associated with convective LLWS, but almost any storm has the potential to produce an outflow boundary
Not explicitly stated in the TAF, however, it is to be assumed whenever TS is forecasted
If storms are expected to strong outflow winds, a variable convective wind group may be included (VRB25G40KT)

7. July 22, 2011 case
20:03Z – 20:54Z

8. July 22, 2011 case

9. Non-Convective LLWS Causes: Explicitly stated in the TAF
Frontal Passage
Inversion
Low-Level Jet
Sea breeze front
Other local effects
Explicitly stated in the TAF
Slightly easier to detect and forecast than convective LLWS, but still one of the most difficult TAF elements

10. Non-Convective LLWS in a TAF
If it is included it will be after any cloud groups
Often the last group in a line (only PROB30 could come after it)
Format
WShhh/dddssKT
Height of the top of the WS layer (WS should be expected below this altitude) in hundreds of ft. AGL to 020
Wind direction at top of the WS layer
Wind speed at top of the WS layer

11. Non-Convective LLWS in a TAF
KAMA Z 1100/ KT P6SM SCT050
FM KT P6SM SKC WS010/15050KT
FM KT P6SM SCT030
FM KT P6SM BKN050=
WS010/15050KT
Top of the wind shear layer is 1000 ft.
The wind at 1000 ft. is southeasterly (150˚) at 50 KT

12. From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear
Frontal Passage
Can occur with a warm, cold, stationary, or occluded front
The slower and more shallow the front, the more impact of LLWS
From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear

13. Frontal Passage case

14. Dec 31, 2010 Amarillo, TX

15. Inversion and Low-Level Jets

16. From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear
Other local effects
Generally terrain influenced (Gap winds, valley, canyon and mountain effects)
Knowledge of the surrounding terrain and climatology is key for understanding and anticipating
From: DLAC 2 Writing TAFs for Winds and Low-Level Wind Shear

17. Data Resources for LLWS
Wind Profilers
VAD wind profiles
Soundings
Aircraft and Pilot observations (ACARS/MDCRS/TAMDAR/PIREPS)
TAF and AIRMET ( )

18. Wind profilers
Available at:
Very few in the Southwestern region

19. Wind Profiler Data

20. Image courtesy of Radar Operations Center
VAD Wind Profile
Derived from WSR-88D (weather radar) data
More extensive coverage than wind profilers
Data may be limited in areas of mountainous terrain or with few echoes on radar
Data may be unrepresentative if a wind shift is present
Image courtesy of Radar Operations Center

21. VAD Wind Profile

22. Soundings
Available at:

24. PIREPS: Help us help you!
For most airports, PIREPs are the only direct verification of LLWS
Often times, LLWS can occur an non-peak times, and thus you may be the first to experience the LLWS

25. LLWS forecasting
Arguably the least understood TAF element among forecasters
So how do we increase forecaster situational awareness of LLWS?
Training
In practice?

26. LLWS forecaster decision aid

27. Initial Feedback Overwhelmingly positive feedback from forecasters
This, combined with feedback from local GA pilots, has posed the question, are we over forecasting LLWS in low level jet scenarios?
Traditional “rules of thumb” may not accurately express LLWS conditions
“I had to put in some pretty extreme winds to get any response”
Increased awareness of LLWS with fronts
Another question that has been raised is how to handle LLWS associated with outflow boundaries of storms that are far away or have already dissipated

28. References
Arkell, R. 2000: Differentiating Between Types of Wind Shear in Aviation Forecasting. Natl. Wea. Dig., 24:3,
National Weather Service, 2008: National Weather Service Instruction Available at
National Weather Service, 1979: NOAA Technical Memorandum NWS FCST-23; “Low-Level Wind Shear: A critical review”
Sinclair, P. and Kuhn, P., 1991: Aircraft Low Altitude Wind Shear Detection and Warning System. J. Appl. Meteor., 30, 3-16.
University Corporation for Atmospheric Research, 2008: “Writing TAFs for Winds and Low-Level Wind Shear.” Available at