## 5 Discussion  
The persistent strong updraft seen in WCR data some 25 km downwind of the crest is referred to here as a hydraulic jump. The WCR transects are relatively short, and the radar echoes deteriorate with distance in the lee, for lack of scatterers. Thus, the data do not exclude the possibility that this updraft is part of a non-linear, high-amplitude trapped lee wave, as suggested by the WRF simulation. The transition from a trapped lee wave to a hydraulic jump is ill-defined, even in model output (e.g., [12]). Certainly, this wave involved a supercritical downslope flow that suddenly transitioned into a strong, deep updraft. Abundant turbulence was generated by the wave breaking in the hydraulic jump, but also by shear around the margins of the plunging jet. Two counter-rotating vortices were observed and simulated as part of this strong updraft. Both observations and simulations show reverse flow in these vortices, at least in one of the transects. Turbulence near these vortices is not surprising given the tremendous wind shear associated with them. [PERSON] [19] and [PERSON] and Kuettner [20] describe two types of rotors, whose formation depend on the upstream environment: in both cases an inversion is present near mountaintop level, but the wind shear across this inversion varies. Under weak wind shear the rotor produces moderate or severe turbulence. Under stronger shear the rotor is associated with trapped lee waves, in which case the turbulence may be severe [4; 20; 22; 51; 52]. Both the WRF simulations and WCR observations suggest that the case presented here was predominantly of the second type. This observational study adds to the growing body of literature describing the fine-scale structure of downslope wind storms and associated turbulence. While the MBM may be rather prone to  
Figure 13: Zoomed-in view of the lee portion of the last WCR transect (distance matches with Figure 12d) highlighting the hydraulic jump and rotors. (**a**) Radar reflectivity; (**b**) hydrometeor vertical velocity; and (**c**) along-track wind speed with hydrometeor streamlines (solid lines) and schematically drawn counter-rotating vortices (dashed lines). The flight level is shown as a dashed line in (**a**), and flight-level vertical velocity and zonal wind speed are shown in (**b**) and (**c**), respectively. downslope wind storms [21; 22; 31], even under marginally strong wind at mountaintop level, they do occur elsewhere (as mentioned in the Introduction), including in more densely populated areas. Most severe wind events in the Boulder, Colorado area, for instance, are associated with downslope wind storms [53]. The present study builds on previous research over the MBM in Wyoming in that it documents the upwind conditions leading to a persistent downslope windstorm. A better understanding of upstream conditions conducive to downslope wind storm formation should result in a better predictability of these potentially hazardous events. In the event studied here, the upstream conditions all appeared conducive initially, but later on several conditions no longer applied (Section 3.2), suggesting that the persistence of the downslope windstorm was affected by a self-induced critical level. In an effort to evaluate predictability based on upstream conditions, we examined the cross-mountain acceleration and downslope windstorm formation in 19 other winter storms over the MBM. These storms were sampled by the UWKA with WCR in four different projects during the winters of 2006, 2008, 2009, and 2013. The cases were selected based on one other criterion, i.e., the availability of upwind vertical profile data, from aircraft soundings conducted near the Saratoga airport (in 2006) or radiosondes released from Saratoga during the UWKA flight. As in the present case study, the objective of those 19 flights did not relate to orographic wave dynamics, but rather to cloud microphysics, so they present a reasonable depiction of flow dynamics across the MBM in winter storms.