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.

Environmental Factors Contibuting to the Grassfire Outbreak of Oct 17

On Tuesday October 17,^th 2017, a powerful wind event wreaked havoc across much of the southern half of Alberta during the afternoon and evening hours. The main problems came from the ignition and rapid spread of grassfires, which impacted numerous communities across the south – with many residents facing mandatory (albeit temporary) evacuation orders. Even though widespread structural damage was avoided, several buildings including homes were destroyed, along with other losses of property, livestock and wildlife. Unfortunately, one fatality involving a volunteer firefighter and another serious injury occurred as a result of the grassfires near Hilda.

Here is an unofficial map of the grassfires created using Google Earth. These were the main fires or areas of fires (since several were clustered in the Siksika/Gleichen/Carseland area) that impacted communities in some way. Note that only Alberta fires are shown (since others occurred to the east); the fires near Hilda and Empress led to the evacuation of several communities in southwest Saskatchewan.

The wind also caused widespread power outages that affected thousands of customers across Alberta, led to the derailment of two trains (one near Wainwright and another near Trochu), flipped several big rigs on major roadways, and caused damage to numerous trees during the latter half of the day on Tuesday. There would have undoubtedly been more damage to deciduous trees had more foliage remained, which would have caused even more trees to uproot that could cause more damage to property and powerlines. As well, parts of city streets in downtown Calgary were blocked off due to concerns of debris flying off of skyscrapers. While Wind Warnings had been issued for the affected areas several hours in advance, I’m not quite sure anyone anticipated the scale of problems the winds would create. The storm would rapidly progress across the prairie provinces overnight and into the next day, where high winds would continue to cause damage and help spark wildfires.

Photo: Artur Pyzalski. One family lost their home and two dogs in a grassfire near Balzac, just north of Calgary, due to what was determined to be a carelessly discarded cigarette butt.

Photo: Lethbridge News Now. Grassfires in the coulees of Lethbridge came perilously close to communities in the city's south.

The main culprit of the wind was an intense mid-latitude cyclone, that would rapidly deepen as it departed the lee of the Rockies of western Alberta – being supported by a highly energetic upper wave that was transiting through westerly flow aloft across western Canada. During the spring and fall, greater thermal contrasts typically exist in closer proximity compared with the rest of the year, which, in a nutshell, can act to intensify the dynamics of such weather systems. Indeed, very strong mid-level jet streams were associated with this particular system, and a rapidly deepening surface low would contribute to the strengthening pressure gradient that would ultimately be responsible for the widespread, tremendous surface winds – both within the warm sector, and following cold frontal passage of this frontal system.

RAP 21Z Oct 17 500mb Mesoanalysis. A ~110kt H5 speed maximum over northern Washington reveals the potency of this highly progressive shortwave trough. Courtesy COD.

00Z Surface analysis (3 hours after the previous image) reveals a ~982mb surface low over western Saskatchewan. The low is the surface reflection of the strong upper divergence associated with the shortwave trough, and is still deepening during this time. Since fronts are auto-analyzed, the thick blue line has been added to show the more accurate position of the cold front at 00Z, when most fires were ongoing. Of particular note here is the tremendous MSLP gradient, which is responsible for the widespread strong winds. Courtesy COD.

Now, we take a closer look at environmental factors that led to the ignition of the numerous, rapidly spreading wildfires across southern Alberta – in order that we may better anticipate future occurrences, with the hope of being able to enact some preventative measures as a result.

One of the main factors that led to this wildfire outbreak was the time of year. Earlier, it’s already been postulated that certain types of weather systems can often be more intense during the spring and fall. But here in Alberta, we’re also often quite “brown” at those times of year – especially in spring, before “green-up” occurs. In April and early May, most or all snow has melted, the rapidly intensifying sun is drying out the ground, precipitation is sparse prior to the “June monsoon” (later spring rains), and the sap has yet to begin flowing up into the trees. In forestry, this is known as the “spring dip”, which is characterized by the low foliar moisture content of the trees during this time. Thus, on warm, dry, gusty days in this environment, we typically see the highest fire hazard. These factors are of course less in fall (aside from brown, non-snow-covered grass), and this year in particular saw great fire hazard persisting throughout the summer, due to being anomalously hot and dry.

However, had a wind event like this occurred over the greener fields of early summer, or the frozen, snow-covered earth in winter, we wouldn’t have seen a rash of grassfires (though we could have seen other types of problems). Early spring, possibly late summer (when widespread wind events like this are uncommon), and fall are really the main windows of time when these ingredients come together. Even after substantial precipitation events, fine fuels such as dead grass can quickly dry out to the point where it can easily ignite again in short order.

And that was the key to the fire behaviour we witnessed on October 17^th. The dry, dead grasses were already a potential powderkeg, but the exceedingly high winds greatly exacerbated the situation. When compared with other types of wildland fires that consume a wider range of fuel sources, the conditions required for this event are far more easily attained during the spring and fall. Large, destructive forest fires often result as a culmination of factors that have conspired for several months or even years in advance, such as in the recent, memorable spring of 2016. Previously dry winters, along with comparatively warm and dry springs, really help to dry out larger fuel sources as well as the surface duff layer itself, going down a few feet in some cases. This leads to deeper, hotter burning fires under the right conditions, which can take months or even years to extinguish (like the Fort McMurray fires). By contrast, grassfires and other types of surface fires that consume small, flashy fuels, derive their intensity by the degree to which they’re driven by the wind. They can spread much more rapidly in some cases, but are also extinguished far more easily.

Smaller diameter fuels such as dead grass, surface litter, leaves, and conifer needles, have a much shorter timelag constant than those of larger fuels, and those deeper within the ground. The timelag constant is specifically the amount of time it takes a given fuel size/type to lose 2/3 of its moisture; soaked fuels with diameters of greater than 7cm typically require as many as 7 weeks of dry conditions in the warm season to dry out sufficiently to support combustion, while fine fuels such as dead, standing grass can take less than a day. Smaller fuels are thus much more sensitive to the weather, including wind and relative humidity, while larger fuels tend to only be sensitive to appreciable amounts of precipitation as well as temperature. The Canadian Wildland Fire Information System (CWFIS) has developed a set of indices that take these factors into considering in anticipating fire behaviour, called the Fire Weather Index (FWI) system:

FWI Structure. For more information, check out this link.

Without getting too deeply into it, the FFMC value corresponds to finer fuels, while the DMC and DC correspond to progressively larger and deeper fuels respectively. Moreover, “ISI-driven” fires result from the dryness of finer fuels (and of course can also result when larger fuels are dry) as well as wind, and “BUI-driven” fires result from longer term drought conditions that dry out larger and deeper fuels. Thus, the fires of October 17^th were most certainly ISI-driven, due in large part to the fires burning predominantly over grasslands.

Forecast FFMC values for October 16th - a day prior to the fires (since a map for the 17th wasn't available). Note there are already extreme values in excess of 92 across the southern Alberta grassland region. This and the following map are courtesy of ACIS, which can be found here.

Forecast ISI values for October 16th. ISI values are made up of FFMC + wind. If we're already seeing extreme values across the southern grasslands the day prior, how much more serious would the situation be with warning criteria winds the following day!

The above factors are clearly contributing to the extreme fire danger forecast here, valid for October 17th. These factors, combined with the wind warning, were a clear sign that a potential grassfire outbreak was possible. Courtesy NRC; this information can be found here.

The forward rates of spread are the direct result of the wind over grass that is highly cured. Therefore, the types of burn paths, as well as the visual characteristics of the smoke columns that we saw tell us the grass was very cured, and the winds were “off the charts”. The latter is mentioned because warning criteria winds are much stronger than the highest input values we find in fire behaviour calculation tables. Burn areas were highly elongated, and were much more extensive on their greater axes than on their minor axes. Smoke columns were robust (signifying intensely flammable fuels), but highly tilted in the strong winds. The wind enabled forward rates of spread exceeding several kilometres an hour, which rapidly put communities in their paths in potentially grave danger. Even weather radars were picking up the upper portions of the tilted smoke plumes, revealing long and narrow columns driven by the extreme winds.

This excellent shot by Susan Durtnall from south of the Siksika area fires really reveals just how much these fires were wind driven. It also showed up nicely on XSM (Strathmore) radar. Having also observed this pyrocumulus plume from the Claresholm area, it was reminiscent of the tilted over columns we observed on the day part of Slave Lake was destroyed, in May 2011. The fires that day were also predominantly wind-driven, with an even larger number of starts occurring that day.

What may seem counterintuitive to many folks however, is the fact that the region hadn’t been in the grips of a severe, ongoing drought. Earlier in the month, a strong wintry storm dumped snow and rain across the region – much of which had only recently melted. So what gives? As we mentioned previously, the short timelag constant of fine fuels such as dead, standing grass means that the recovery time for it to become flammable again was brief – especially given at least a couple of warm, dry, breezy days in advance to allow for maximum evaporation of fuel moisture. Then, it’s just sitting there awaiting any strong winds that might come along, and a spark for ignition.

The 30 day Standardized Precipitation Index (SPI) on October 10 reveals that conditions were near normal, if not wetter than normal for the time of year(especially in the Empress area), owing in large part to the heavy precipitation event near the beginning of the month. This goes to show how quick fine fuels can recover from precipitation events, and given the right circumstances, how irrelevant long term precipitation amounts can be in such events. Courtesy ACIS.

Here is the historical weather for Leader, Saskatchewan leading up to the 17th (a town which was evacuated), to give a sense of the bigger picture. A heavy precipitation event near the beginning of the month occurred, with conditions being quite dry afterward (quite normal in this area at this time of year). Winds were never too extreme, and only a few days of milder, breezy weather occurred before the grassfire event. Courtesy ECCC; this imformation can be found here.

Of course there were also other factors at play in the grassfire outbreak. Despite the rapidly shortening day length of late, diurnal effects still played a role in fire behaviour. Typically, peak burning conditions occur in the late afternoon when temperature and wind speeds are highest, relative humidity is lowest. This is also the time when the boundary layer tends to be deepest and well-mixed, enabling ready transfer of momentum from stronger wind speeds aloft to the surface, making for gusty conditions and stronger sustained wind speeds overall. When relative humidity drops below 30%, dry fuels (especially fine) become more flammable, and anything below about 20% is considered critical. In wildland firefighting, the concept of crossover hints that fire behaviour may become volatile, when the air temperature becomes greater than the relative humidity value. The concept does not reflect any true thermodynamic relationship, but is a good reminder nonetheless. For the record, crossover most certainly existed on the 17th in the mild and dry downslope flow of the warm sector. Had the wind event occurred at night, there could have arguably been less grassfires, due possibly to higher RH values, weaker surface winds, and lesser human activity.

22Z surface plots/IR Sat reveal a warm, dry, and windy warm sector over the southwestern prairies. Take the temperature and dewpoint at Leader, SK for instance. 73/16F (temp/dewpoint) is 23/-9C, which yields a surface relative humidity of about 11%. Using the principle of "crossover", 23/11 reveals we have fairly significant crossover values both in Leader, and in the surrounding region. These conditions greatly increase fire danger. Courtesy COD.

18Z NAM3KM prog sounding valid at 22Z, which is representative of areas over the southern Alberta plain (away from the mountains). Even though winds are progged to be 30 knots sustained at the surface, winds near the top of the boundary layer (when it is deepest, due to the combination of daytime heating and mixing due to strong winds) are approaching 60 knots. The overall boundary layer average wind speed would be expected to be higher than 30 knots at the surface (which it was), with even higher gusts (which also occurred). Courtesy COD.

As for ignition sources, public speculation of cause is a contentious issue when human activity is involved, since the determination of responsibility often has huge ramifications. I personally have been reprimanded as a former wildland firefighter, when I merely gave examples of common causes of wind-driven wildfires during a radio interview in the wake of the devastating Slave Lake fire of 2011, that destroyed 30% of the town. These might be misinterpreted by the public, it was feared, when a potentially multi-million dollar suit was at stake. Nonetheless, in the absence of lightning, almost all wildfires are either directly or indirectly human caused. This includes industry, agriculture, powerlines touching trees or dry ground fuels, recreation; other careless actions such as not fully extinguishing campfires and discarding lit cigarette butts; and of course delinquency and/or arson – especially when you see a clustering of otherwise unexplained starts near communities or along a major roadway when fire hazard is high.

Conclusion

Though a strong wind event was anticipated, the grassfire outbreak was a consideration that escaped many. Weather forecasters and emergency planners should be prepared whenever future cases like October 17 come together, to help anticipate them and communicate to the public to be cautious about certain activities. Some wildfires seem inevitable, but many are not. It should be realized that, due to the nature of fine fuels and their rapid recovery time, extreme fire hazard can quickly arise after a few mild, dry, and breezy days – even after significant precipitation. This is not uncommon in Alberta, especially during spring and fall. Windstorms like the event we saw are more uncommon, so when there are signs that an intense weather system may bring widespread severe winds, combined with dry fuels – whether grass, or otherwise – we should expect that an outbreak of grassfires is possible. Once again, these conditions typically come together in fall and spring, when greater baroclinity leads to potentially stronger weather systems and their attendant winds at the same time that potential fire fuels are driest.

Personal Observations from October 17th:

I just wanted to give a brief account of what I personally observed that day. Being unable to work suspended by rope from tall buildings in such high winds, I drove to the southwest corner of the province to observe the weather and take wind measurements. Having a growing personal interest in the mechanics of mountain waves and downslope windstorms, I was in search of a steep lee slope to place myself beneath. I found an excellent spot a little ways north of Highway 3 on Cowboy Trail (Hwy 22), where a near-perfect north to south front range ridge parallels the road several kilometres to the west. Under the right conditions (which I won’t get into now), severe winds can (and do) reach the highway in this area, evidenced by the multiple signs warning of potentially severe crosswinds. My Kestrel 3000 measured a peak gust of 45.3 knots, which was sufficiently strong enough to compromise balance. I did expect much stronger winds however, and as the day progressed, the downslope component to the wind lessened in this area. I would speculate that this occurred due to daytime heating, which would act to weaken the mountaintop inversion responsible for the strong downward deflections of energy that drive many downslope winds in the first place. Stronger winds would be observed well to the east over the Alberta plain where deeper boundary layers would permit the transfer of higher momentum air aloft to the surface.

12Z NAM Oct 17 valid for 15Z along the front range in an area of SW Alberta. Note the strong inversion several hundred metres above ground, near mountain-top level, with ~75knot cross-barrier flow through the inversion layer. Areas on immediate, steep lee slopes would expect to see the highest winds - due both to the deflection of high momentum flow and gravity. Courtesy COD.

The whole day went from 0 to 100 in no time, with the rapid unfolding of events making it quite an interesting, if not scary day weatherwise. But the second thing I observed was very interesting. After the cold front passed, a post frontal precipitation band (with some convective elements) swept its way southeast across the province (which would help with fire suppression efforts). The highly tilted, in-situ smoke column from the Gleichen area fires set up a more or less west to east axis (the winds were veering from southwesterly to westerly, and then northwesterly as the front passed) of mesoscale updrafts comprising the horizontally extensive, vertically limited smoke plume. As the precipitation band crashed into and began interacting with the updrafts, we saw a momentary intensification of precipitation (due also possibly to the injection of new CCN into the cloud matter), with several lightning strikes detected. Once the precipitation band finished crossing through this area, it weakened again. Fascinating!

XSM (Strathmore) radar. First and most obvious are the Siksika area fires (with one main plume) originating from a point, with the upper parts of the long, narrow plume being sampled by radar. A change in wind direction from southwesterly to westerly is observed while post-cold frontal precipitation charges down from the northwest. There seem to be some convective elements along the leading edge, however, note the rapid intensification and subsequent weakening of echoes as the line passes through the area of the smoke plume. Loop courtesy RadarScope.