Analysis of Windstorm Criteria

From Hopkins (1994) ...

Colman and Dierking (1992), three criteria for windstorms:

1. strong cross-barrier flow at mountain-top level
2. an inversion at or just above the ridge top
3. a critical level (the cross-barrier flow decreases to zero) above the ridge top

The windstorms are driven by downslope winds, gap winds, or a combination of the two. The force balance in this situation becomes nearly antitriptic, with the wind at a large angle to the isobars.

For the first criteria, this requires a strong pressure gradient across the mountain axis. The pressure gradient increases with a deepening low approaching from the west, and a quasi-stationary high in the eastern Gulf of Alaska.

Relating this to the mid and upper level pattern, a sharpening wave pattern is favorable. A deep trough, tilting negative into the Aleutians supports a rapidly deepening surface cyclone. The downstream ridge must remain nearly stationary, and amplify. Downstream of the ridge axis, falling heights then sharpen / amplify the ridge, and increase upper level confluence supporting an intensifying surface high locked over the eastern GOA.

Comparing the 24-hour change in 500hPa heights for the strong cases versus the weak cases, the strong class exhibits all of these characteristics. The weak class amplifies the trough and ridge system, but it increases heights slightly over western Canada, keeping the ridge a little more broad, with weaker upper level confluence. As a result, surface pressures in the eastern GOA increase more in the strong class than in the weak class.

--> The second criteria point is related to stability. An inversion ... layer of high stability ... is required at or just above the ridge top. The layer is found between 800hPa and 500hPa, and must be near the critical level (third criteria point). One measure of stability that applies in this situation is the lifted index, which is the difference between the temperature of a surface-based parcel lifted to 500hPa and the environmental 500hPa temperature. A higher LI indicates higher stability.

Composites of the strong and weak classes shows that the strong cases feature an area of much higher LI over eastern Alaska toward ANC.

--> The third criteria point is related to vertical wind shear. Strong cross-barrier winds are required at mountain-top level (per the first point), and a level with zero cross-barrier flow is required further above the mountain-top (per the third point). The SW-NE orientation of the Chugach Mountains to the east of Anchorage require southeast winds at mountain-top, and southwest winds at the critical level. So strong WAA - veering winds with height (90 deg optimal) - are necessary ... with a resulting westerly shear vector.

The WAA also acts to decrease surface pressure on the west side of the mountains. In addition, the WAA helps amplify the downstream ridging, supporting strengthening surface high pressure on the east side of the mountains. Both of these results increase the cross-barrier pressure gradient

The strong class features a much stronger north-south temperature gradient than the weak class, which by the thermal wind balance supports stronger westerly shear.

850hPa temperature

One necessary note about the two classes:
The month of each of the weak cases averages +/-2.5 months from January, whereas the strong cases average only +/-1.2 months from January ... so the strong cases occur more frequently during the favorable winter season, which makes sense.