February Thundersnow in Calgary

 On Monday evening, February 22, thousands of Calgarians were treated to a rare display of wintertime thunder and lightning - all while heavy snow, graupel, and soft hail fell across parts of the city. At least two flashes were detected, with the first occurring over Broadcast Hill in the city's southwest just after 9PM MST, and a second flash at about 920PM over the downtown core. Reports and photo evidence from an area resident confirms the first flash likely hit a TV tower, with the orientation of the lightning bolt indicating an upward flash. In many ways, this was a textbook incidence of self-initiated upward lightning, given the background meteorological environment and presence of tall towers. Let's take a closer look.

Look at Calgary there with its own lightning for miles around. In fact, the next closest lightning was off the Florida coast at about 3900km away!

In terms of the bigger weather picture, a potent shortwave trough moving over the Pacific Northwest would result in the development of a lee cyclone in southern Alberta during the afternoon and evening hours. Significant cooling aloft would accompany the approaching trough and result in destabilization - which, when combined with strong forcing for ascent in the mid-levels of the troposphere, would further serve to steepen mid-level lapse rates and facilitate the ascent of parcels originating in relatively mild and moist Pacific air to their levels of free convection - permitting the development of moist convection. 

500mb RAP analysis, valid at 04Z Feb 23 (9PM local). 500mb temperatures near -32C over Calgary are indicated, with the shortwave trough axis over the Pacific Northwest.

Wind barbs on the station plots indicate the cyclonic wind field associated with the lee low just east of Calgary at 04Z Feb 23. 

Radar trends indicated that snow flurries developing during the mid-evening west of Calgary were becoming more convective in nature, evidenced by intensifying echoes and reflectivity gradients, as a strong vort max that had a history of facilitating the development of lightning in the BC interior a few hours previous moved into southern Alberta. Following the occurrences of lightning in Calgary (which likely occurred as the storm was still in the process of maturing), radar echoes intensified as soft hail 5-10mm in diameter fell across southern sections of the city. 

Radar screen grab from 941PM, a few minutes after the second lightning flash. 

Springbank Airport (CYBW) was more directly impacted by the convective cell than Calgary International Airport (CYYC), so I have selected this site for more representative observations of the storm environment. At 8PM, the temperature was -2C, with a dewpoint temperature of -5C and a slight northwesterly breeze - hardly an environment one would expect to soon be preceding lightning nearby. By 9PM, winds were northwest at 22 gusting to 27 knots, with heavy snow and 1/4 mile visibility as the convective flurries were pushing through. 

According to a study done on the climatology of thundersnow events in the CONUS, mean thundersnow environments were characterized by a stable boundary layer, and a nearly saturated neutral thermal profile above the low level inversion. Thundersnow events in their study did not favour any given time of day, suggesting that dynamic forcing plays a large role in its occurrence, irrespective of diurnal effects. 

Forecast soundings reveal such an environment, with an elevated inflow base, and steep mid level lapse rates. They can also give us a sense of where charge centres within the cloud may occur as a result of the charge separation that results from strong convective updrafts - with the negatively-charged region likely developing more or less centred around parts of the cloud that were about -20C, where mixed phase precipitation including the presence of graupel exist. 

Cloud temperatures were around -20C at about 3km AGL

Now that we've got ample charge separation ongoing associated with these convective, snow-bearing clouds, how do we get lightning? It is an interesting thought to ponder that had this activity not passed over the TV antennae in SW Calgary that top out around 5000 feet ASL, or the towering high-rises of downtown, that this lightning may not have occurred at all. 

It turns out that in these kinds of environments, tall towers such as radio antennae can initiate their own lightning. The tip of the tower is closer to the negatively-charged region (when one exists) within convective clouds than areas on the ground below, which can result in a positively-charged upward leader that brings about an upward lightning flash as negative charge is brought down from the cloud. Therefore, despite the relatively common occurrence of positive lightning in the winter months, it's likely this mechanism resulted in the Calgary lightning flashes having a negative polarity. 

Another very interesting fact is that self-initiated upward lightning becomes more likely when it is windy, since the wind acts to reduce the "corona-produced screening layer" as charge builds up near the tips of tall objects. As can be seen from the observations at CYBW, strong winds were associated with the passage of the convective flurries, which would also result in blowing snow as they passed to the east of Calgary later on. This might also explain how similar conditions led to lightning strikes earlier the same morning in a line of convective flurries that passed over the Wintering Hills wind farm to the east of Calgary, as the churning turbine tips potentially initiated upward lightning when charged convective clouds passed overhead.

Plain Language Summary

- The unstable air necessary for the formation of thunderclouds resulted from a combination of cooling temperatures and rising air in the middle levels of the atmosphere, atop the relatively warmer and more moist Pacific air at the surface. That same rising air also served as the "trigger" for the convective clouds that brought heavy snow, graupel, and soft hail

- Strong updrafts within the convective clouds resulted in the separation of electric charge that is necessary to get lightning discharges. A negative charge likely developed in the clouds some distance above the surface, while the Earth below possessed a positive charge

- Since the tall structures that were hit (TV antennae and high-rise buildings) are closer to the negatively-charged area of the cloud than surrounding areas on the ground, they likely initiated their own lightning. The presence of strong winds atop the towers also made this more possible. 

- Had these convective clouds passed over an area without tall towers, it is possible that no lightning would have occurred at all. Thus, it was a special convergence of factors that led to the thundersnow event in Calgary!