A tornado-like vortex at Portage Lake, Alaska
10 March, 2005

Video was taken of a tornado-like vortex by an Alaska DOT camera at the Anton Anderson Memorial Tunnel toll gate near the northwest end of Portage Lake. According to Carven Scott, SOO at the NWS WFO Anchorage office, South Central Alaska was in the middle of an high wind event. A nearby station in the Portage Valley recorded 109 mph gusts 4 times this day. This vortex left behind numerous downed trees. What is impressive is that these trees grew in an environment where 100 mph winds occur relatively frequently.

I believe the video is pointing roughly to the west-southwest. I've left a normal speed video, as provided by Carven from the DOT, and a version at 10% speed. Note the video is time stamped to the second. Normal speed is still a little fast relative to true life speed. The 10% is taken relative to 'normal speed'.

Click on the map to expand the view
click on the map to expand

This wind event occurred, as many do, north of a deep surface cyclone in the Gulf of Alaska (see this surface plot). But the question is, what is the forcing the vortex stretching? The 12 and 00 UTC Anchorage soundings does show large cooling aloft but no production of any significant CAPE. Note that the vertical wind shear at Anchorage is very large, however, much of the low-level shear could be topographically induced. Certainly the high background surface winds in Portage Valley doesn't support strong vertical speed shear, at least in the middle of the valley. However, there are a variety of flow configurations around the steep topography to induce local vertical shear effects and potentially plenty of sources of horizontal vorticity and vortex tilting. There are other documented cases of vortices in similar situations but I haven't heard of any this intense.

Another question is what is the maximum near surface winds in this vortex? I'll be getting some measurements of landmarks in the frame of the video soon.