The worst recorded incidents of U.K.standing rotor activity have been west the of Mid Wales coast in easterly winds.In 1987 a Hawk out of Valley recorded accelerations of one to seven 'g'.
The conditions for rotor streaming and standing rotors are mutually exclusive.
-Rotor streaming;stable air,stronger winds (typically more than 25 knots),semi organised type of mechanical turbulence in the friction layer (not above 2.5 times the ridge height)
-Standing rotors;winds as low as half that required for streaming,can extend much further downwind,worst turbulence occurs below ridge height.
If moisture is present roll clouds may form for both types.If not the only indication will be rapid fluctuations in surface wind speed when general flow is expected at around 20 knots or more.