Snow Squall

Operations Duty Manager

Effects similar to snow.

Can be difficult to plan for due to variability and uncertainty as the TAF usually reflects a PROB30. Risk assessment for the operational plan and the critical factors are onset, accumulations (if any), duration and confidence. Forecaster notes, chats and discussions are critical during these events.

• Discussions will be had with airport authorities, tower, terminal, enroute ATC, and NTMU to put together an operational plan and recognize/highlight possible constraints.
• Comparison of snowsquall timing with demand through that timeframe is done, including the assessment of airborne holding capabilities and room for recovery as a function of airport capacity. These conditions are all brought forward in Collaborative Decision Making on National Operation Centre calls with customers, and also highlight the importance of forecaster uncertainty/confidence. 

Squalls and streamers can often be below the radar horizon so a supplemental weather watch with hi-resolution satellite imagery is favored. Additional tools include the METAR tool on ADDS to monitor reported precipitation and weather cameras depending on the location and geometry of the streamers.

Snow squall effects are most impactful on areas in close proximity to the Great Lakes. For airports outside of the Great Lakes, squalls can be more of an inconvenience than a hazard to aviation. Reduced visibility in squalls for VFR pilots caught unaware would be a concern.

These events are considered the same as a regular snowstorm, however the expectation is that it is more likely to be of a shorter duration with less predictable start and end times.

• NTMU heavily depends on details from the CMAC forecaster to determine accumulations, assess the potential variability of conditions, and gauge confidence. This will transfer to planning rates for arrivals and for how long the clean up will take, as usually when these events do occur there is significant accumulation in a short period of time.
• NTMU will usually plan on a more tactical response rather than a strategic plan as these events are not as predictable as large snowstorms. Once the events start occurring the process would be the same as a snowstorm.

ATC (Major and Regional Towers)

Occurrences vary across the country and are more frequently observed where/when conditions are favorable. Possible impacts:

• Required aircraft de-icing.
• When not forecast, this can lead to unplanned runway intervention by the Airport Authority which, depending on traffic demand and capacity, can lead to TMIs.
• Frontal squalls are also seen and will have similar impacts. This includes required runway sweeps, ATIS updates, increased communications with the Airport Authority and Terminal ATC, higher workload. 
• Understanding timing, duration, and accumulation are key.

In CYUL: occurrences tend to be short in duration but can lead to impacts.

• St. Lawrence River/Lake St. Louis can have local effects reducing visibility, ceilings and giving snow accumulation on the runways.

The term “Squall” has a very specific meaning in Aviation Meteorology, referring to a sudden increase in Wind speed of at least 15 KT over the two-minute mean wind velocity measured at an observing point. To qualify as a squall, this surge of wind must last at least 2 minutes in duration and must drop by at least 5 KT at the end of the squall. This phenomenon is almost always associated with an arriving Cumuliform Cloud (Cumulonimbus or Towering Cumulus) at the reporting point. A Squall is very bad news for any aircraft on approach or departure near an airport (or even taxiing, for that matter) as the sudden onset can easily overwhelm the established control settings on an aircraft in flight near the ground, with catastrophic results. For taxiing aircraft, especially “tail draggers”, a high risk exists for “ground looping” in Squall conditions. Unsecured aircraft parked in the open may sustain damage, by colliding with other objects on or adjacent to the ramp area. (MANOBS ReferenceOpen a new window)

For Advisory Specialists, an occurring snow squall can be treated very differently depending on where it is located in relation to an airport. A squall that is within visual range of an Advisory site, but not occurring in the vicinity (within 5SM) or at the airport proper will always be mentioned in an Advisory, and PIREPs (specifically for visibility remarks, turbulence and icing) will always be solicited. At a distance it is easier to approximate the lateral and vertical extent of a snow squall, but more difficult to determine the intensity of snow fall and associated drop in visibility. If the squall is in the vicinity or overhead it is always treated as snow would normally be, with the exception of cautions for turbulence from the inherently convective nature of a snow squall. Close up it is much harder to determine the lateral and vertical extent of a snow squall, but easier to determine the intensity and visibility within. Our workload always increases during any snow event with the coordination of Aircraft movements with Snow Clearing operations, accounting for de-icing and holdover times, increased taxi and runway occupancy times, and an increase in weather observation frequency.

Terminal controllers take a tactical approach in responding to snow squalls and use the airports around them to gauge impact of snow squalls and timing. 

Similar in impact to freezing rain/snow.

• RWY clearing crews may be slower to respond in unexpected events.
• If conditions deteriorate rapidly, there can be so much snow/contamination that the braking is too poor for the runways to be used. This requires immediate intervention and increases workload for controllers as they coordinate the following with traffic.
• Information must be passed to aircraft (sometimes in French and English).
• The length of the runway closure/necessary clearing may be difficult to estimate, particularly during intense weather events and not knowing how long runway clearing will take can increase workload across the system.
• Poor runway conditions trigger delays and increased workload (requiring vectors)and/or diversions at an alternate (new clearance).
• Increased coordination required with other adjoining sectors so that any other inbound aircraft can be delayed further away so that one controller/sector does not become overwhelmed. Other sectors will issue holding clearances, specifically away from the small terminal airspace.
• Once conditions improve and runways become available, workload remains high as aircraft in holds are then sequenced for arrival.
• Due to lack of on-site location, and often rapidly changing conditions, currently lack of ATIS access in the Terminal, there is a significant workload increase during these conditions coordinating via hotline/landline with the tower.

Snowsquall impacts are highest in regions that tend to have prolonged events such as around the Great Lakes and the Maritimes. They do not usually have a huge impact on the operations in CYUL or Quebec City (CYQB) and are known to impact the Great Lakes region more.

Squalls tend to move through airports quickly and may create some missed approaches and closures, causing a brief backlog of arriving and departing traffic.

• Depending on the airport, geographical location, snow squalls tend not to last very long, so any runway closures for cleaning are less likely to occur for extended periods of time.

En-route controllers may be forced to hold inbound aircraft if there is a snow squall at the major airport. Generally, the holds do not last for too long and the controller and airspace does not need too much time to recover and get back on schedule.

• If, for example, a snow squall and its fallout (runway treatment) last for thirty minutes, ATC can hold some aircraft at the terminal entry point and slow down other inbound traffic to avoid them holding. It creates a brief, intense period of work but is not nearly as consequential as a long-term event (freezing rain, blizzard, etc.).
• If the snow squall arrives during peak traffic, the fallout can last much longer as back-up from arrivals in airborne holds can cascade into subsequent periods and impact later arrivals. Longer-lasting events, short staffing (inability to hold), or events that occur at peak traffic times, may lead to cascading delays and the need for TMIs.

Regional airports that get hit with snow squalls will have missed approaches and sometimes NOTAM closures.

Most regional airports only provide weather updates each hour or via an automated station, which can lead to a controller not having current information to provide a pilot. If there is a snow squall that the controller is not aware of and they clear a pilot for an approach in a regional airport, the pilot may return to the frequency to inform the controller that they would like to hold or may be forced to overshoot and head to an alternate destination. Collaboration is important between the controller and pilot during these rapid events.

If there are snow squalls in the weather forecast, high-level ATC could perhaps expect to have to hold airplanes in our sectors and also maybe have to reroute planes to their alternate. This requires a certain amount of coordination between sectors and increases workload to clear aircraft to their alternates, change the routes in the system, etc.

• Actively monitor ATIS for rapidly changing weather conditions. Plan alternate airports and adjust flight plan fuel for contingencies. 
• Ground delays due to snow squalls may lead to exceeding Holdover times. Plan fuel accordingly.
• Possible low visibilities resulting in arrival delays and possible CAT II/III operations. Increased risk of go-around especially in highly variable conditions.
• Most pilots will be familiar with both phenomena. Both are local, one static and one in motion. We will rely on TAF to alert us in those two conditions. Given that both phenomena are local, typically either our destination or alternate airport will be affected, and not both. If the TAF shows a persistent lake-effect snow condition at the airport, the forecast ceiling and visibility will be close to alternate limits, and from the planning stage, dispatch will most likely choose another airport. If the forecast changes after departure including now a lake-effect snow occurrence, we can consult with dispatch to change the alternate, if warranted (see note below on the need to change the alternate airport in flight). Changing the alternate planning airborne in this case is easy due to the limited reach of the squall.
• We can expect delays from snow squalls but rarely does it lead to a cancellation on its own. It could lead to a diversion also, but many factors have to line up: bad timing with the passage of the squall, worse conditions than forecasted, peak traffic at the destination airport, and flight planned without sufficient contingency fuel.
• We will rely entirely on the TAF to warn us about the passage of a snow squall and the presence of a lake-effect snow squall. We will take into account the forecast condition and its timing as well. The other product we can use to supplement our awareness of the timing of the passage is the ground radar imagery, if our onboard wifi is working. The latter is not necessarily a step every pilot will do. They might just assume the worst of the TAF will be the condition on arrival. Either way, we are always planning for an IFR approach to minima.
• The onboard weather radar is of limited use for this kind of forecast. Above FL300, it will hardly pick up any return as the precipitation is typically low for lake-effect snow squalls, and our radar operating on millimeter wavelength is not suited to pick up frozen precipitation. It might show a bit of green but nothing to raise flags to pilots.

Note on changing an alternate airport in flight

It is not required to change the chosen alternate airport after the flight has taken off. Alternate airport selection is a flight planning requirement. Both companies I have worked at had policies regarding changing the alternate airport in flight. If the forecast or actual condition drops below alternate minima, part of the flight watch responsibility of our dispatch is to pass this information along to us. The policy does not require a change if the forecast and observed weather holds above landing minima. If the alternate forecast or observed weather drops below landing minima, the policy recommends designating a new alternate, if fuel onboard permits. Depending on the crew and the dispatcher, that discussion will usually be started when the weather drops below the alternate minima. Dispatch will suggest a new alternate and calculate a fuel burn. We can hold more than one destination alternate as well, if we have the fuel on board to fly to either airport. Dispatch quite often plans for two alternates in bad weather, one which is more desirable operationally but where the forecast is closer to the alternate limit, and another where the forecast condition will be CAVOK but is less desirable from a standpoint of the operations. The captain can then make the best decision when the time is up at the destination airport.

General aviation (GA) VFR pilots will not be flying if snow squalls are predicted in any aviation weather products. This will usually be easily identifiable due to significant lack of visibility associated with the snowfall.