Rocky Mountain Gap Flow Being Channeled into Calgary - Dec 7, 2017

Calgary weather never fails to be interesting - which is exciting to observe, but tricky to forecast. Especially since most processes that make a significant difference in terms of temperature and wind for instance over small spatial distances, happen on the mesoscale. Before the recent Pineapple Express ate all of our snow, it was interesting to watch the mild temperatures slowly eating away at the edge of a firmly entrenched Arctic air mass, which led to temperature differences approaching 20C across the city at times.

Today, a tremendous, building upper ridge has taken hold along the west coast of the continent, leading to a blocking pattern that will hold for several days to come, bringing us stagnant weather conditions. For us in southern Alberta, this will be a good thing in terms of sunshine and mild temperatures. In the BC interior however, a slack surface pressure gradient means light winds, with persistent valley fog and stratus (as well as air quality issues) beneath the strongly subsiding air, which is destined to take hold for several days in some communities. The only remedy...get up high and/or go skiing!

500mb height anomaly for Thursday Dec 7 at 18Z.

Now, we focus on the task at hand. A strong subsidence inversion has developed (and continues to develop) as a result of large scale sinking air beneath and just downstream of the upper ridge, with the inversion layer occurring a very short distance above the ground. This inversion layer intersects the slope of the Rockies west of Calgary a short distance up, thereby preventing winds from flowing over the tops - especially given the absence of high momentum mid-level flow. However, a strong pressure gradient has developed between air in the mountain valleys (with SLP values approaching 1050mb in the BC interior) and that of the Alberta plain, where there is lower surface pressure. This leads to air having the tendency to squeeze through gaps in the terrain from high to low pressure - otherwise known as gap flows.

12Z NAM3km prog sounding valid at 17Z Dec 7, along the front range of the Rockies west of Calgary. Note the inversion a few hundred metres up.

IR SAT with SLP overlay (since few good hi-res surface analyses exist publicly for this area), at 1630Z Dec 7

12Z Dec 7 500mb analysis, for reference

A relatively large geographical gap in the terrain (or technically the merging of two gaps) exists along the Trans-Canada Highway just east of Exshaw, at the confluence of the Bow and Kananaskis River Valleys. Flow that squeezes through this gap is then channeled by the terrain eastwards toward Calgary along the Bow River Valley. The valley initially trends northeast, and then southeast at Cochrane into the city. Winds are lighter at the surface on the higher terrain above the valley, but breezier within the valley itself - which leads straight into downtown. YYC is situated some distance to the northeast of downtown, outside of the valley, and somewhat blocked from channeled flow owing to Nose Hill just west of the airfield. Where YYC has been consistently been reporting light, sustained winds out of the southwest on the morning of Dec 7, downtown has been getting blasted by a cool, gusty WNW wind.

YYC metars between 15-17Z, on the morning of Dec 7.

My trusty kestrel was consistently reading about 12 gusting 19 knots out of the WNW during the same time frame as the above metars (and all night for that matter! It was noisy!)

17Z surface obs via Wunderground. While some home station readings may be somewhat suspect (like my kestrel), the overall sense is that winds are stronger westerly near Cochrane, turning to more northwesterly as they channel down the valley into Calgary - with lighter winds observed on the surrounding higher terrain.

Even the NAM3km has been consistently and accurately modeling this channeled gap flow. When comparing a topographical map to model output, it is clear that this feature is being resolved at the model's resolution. So while parts of the city may experience light surface winds, areas in lower elevations along the valley in northwest and central parts of the city will likely remain much breezier - a pattern that could very well continue for several days.

Topographical map of the Bow Valley

Note the strong surface pressure gradient along the edge of the Rockies.

Note that the wind maximum follows the shape of the Bow Valley into Calgary. Looks like several more wind maxima/gap flows are being progged further south as well.

In sum, the strong surface pressure gradient that has developed between the BC interior and the Alberta plain, coupled with a strong, low subsidence inversion has favoured the development of gap flows along the front range of the Rockies. The inversion prevents flow over the mountains, so it is instead squeezed through cuts in the terrain - and can be seen channeled through terrain features in a shallow layer over the plains to the east.