Squall Line

Operations Duty Managers

• Forecaster Notes in HubWx normally provide the first indication of a squall line and in most cases this is identified during the previous day. Operations Duty Managers consult a variety of weather sources including model data and prioritize discussions with the meteorologist to share aviation impact information that will play an important role in the decision-making process.
• As the event approaches (<12 hrs) the forecaster notes remain a primary source of additional information supplementing the TAF, and variability and confidence levels take on higher importance. CoSPA and NASA hi-res satellite imagery allow monitoring the location, movement and development of the squall line and TCF helps provide guidance on geometry and density. Organization of the storms is a concern, will it be a solid line, broken or scattered. The impacts to airspace, especially the bedposts, are a primary concern. Forecasters have become very adept at recognizing bedpost impacts and typically embed this info in the forecaster notes.
• Significant planning and discussions also occur with speciality supervisors to help determine the structure for optimal break times and the opening and closing of sectors that are most in-line with the forecast timing of the squall line passage. This allows for the delivery of the best possible service to our customers and stakeholders.
• There are 4 questions that can be used to define a weather impact: 1- onset timing, 2- duration, 3- severity, 4- extent of coverage. These 4 questions are applicable to both bedpost and airport impact. The 8hr FCST on CoSPA helps validate forecaster timing and if there are differences helps an Operations Duty Manager asks for clarification on forecast uncertainty. Forecast arrival of the squall line and its impact is compared to expected throughput at each arrival route entry point. This helps to strategically move flights when possible and balance volume across each arrival entry point into the terminal. CoSPA, when performing at 75% or better can be a helpful resource for decision makers. Fix balancing is normally done tactically in conjunction with the NTMU. Once a fix is impacted the next question becomes “when can I use it again?”. The same tools are used to determine when the weather will move off the fix. If balancing is not an option due to volume or other impacts then airborne holding occurs.

Airport impact with a squall line is often predictable, especially when it is well defined. For example: a line approaching YYZ from the NW will impact BOXUM first, then NUBER and IMEBA followed by the airport, then RAGID and LINNG. 

• The TAF is used for reference. However, even with a well-defined front will typically contain PROBs or VCTS, making supplementation with forecaster notes and discussion with the forecaster critical. The TAF, when PROBs and VCTS are long-lasting, is less useful as a planning tool when used alone. Operations Duty Managers are trained to rely on additional information and the insight forecasters can provide on timing, confidence, and trend.

• Usually squall lines are forecast well in advance and monitored in the Traffic Situation Display, CoSPA and RADAR.
• These are the easiest thunderstorms to plan for as usually timings and effects are known well in advance. Often NTMU will contact other units that are currently experiencing the phenomenon and get an idea of what to expect.
• Solid lines are the easiest to plan for whereas lines moving with holes that may miss certain airports require more tactical and flexible updates to the plan. Solid Squall lines are much easier to track, predict and plan for, whereas single cells or pop ups are the most difficult. A solid line will usually be planned out in advance, where single cells or pop ups will require a more of a wait and see approach and handled tactically as opposed to strategically.

ATC (Major Tower)

Thunderstorms that are reflected as PROBs in the TAF can be challenging to create a plan for. On these days, there is an increased need from Operations Duty Managers to forecasters to give us the bigger picture if there is a need to make adjustments from a tower perspective.

• Impacts are similar to regular thunderstorm events.
• Benefits of a squall line is the better organized the line, the easier it is to plan for from a timing perspective. The line is usually easy to see coming from a long way away.

ATC (Regional Tower)

• Squall Lines wouldn’t trigger special operations at regional airports.
• Squall lines are treated tactically by many regional towers like CYAM.
• Controllers monitor weather and look for lightning strikes and inform pilots accordingly as well as coordinate deviations for IFR aircraft on departure routes (SIDs) as with thunderstorms.

If conditions exist for very large, intense thunderstorms, there is a chance of squall line formation. This can mean either a line of thunderstorms associated with a fast-moving cold front, a Mesoscale Convective System, or a Mesoscale Convective Complex (in ascending order of magnitude). 

These circumstances will have the undivided attention of every dispatcher in the region and nationally, as the tops of the thunderstorms are likely to be above FL500, meaning that enroute “High Flyers” will have to reroute to avoid severe turbulence and even potential hail, ejected through the top of cloud and thrown downstream. Traffic flow managers across Canada and the USA will be keeping an eye on those kinds of conditions. 

Isolated thunderstorms, sometime referred to as “airmass” thunderstorms, usually form in the warm sector behind a warm front, prior to the associated cold front sweeping through the area. The hot, humid conditions found in this sector will be very convectively unstable, and all that is needed to initiate an isolated thunderstorm is some microscale feature, such as a small hill or bend in a river valley. This minor orographic influence can have truly spectacular effect, and this type of thunderstorm should not be underestimated for its disruptive effects on flight operations. Although it will be shorter in duration and smaller in geographical extent than other, more organized mesoscale convective events, an airmass thunderstorm can still pack a very heavy punch in generating severe weather. 

Upstream observations are indicators that are used to assess incoming weather, including squall lines and the associated changes in wind patterns ahead and behind them. The factor that can differentiate this from thunderstorms is the length of time the terminal airports are likely to be affected depending on the line path. Otherwise the effects are the same.

Another very useful clue for identifying when a squall line is getting close is the ground speed/airspeed difference of aircraft on final. For example on runways 06L/R in CYUL:

• The weather there often arrives from the west, so “something happening on the final of 06s” is going to hit the airport eventually. We often have a tailwind on 06s, for example at 4000ft the winds will be 240/40, then as the aircraft descend on the localizer it transitions to 060/10 on the ground. During the day, we’ll see that the “transition” from tailwind to headwind is getting lower and lower (closer and closer to CYUL). We can get this info by asking pilots to give us the current winds (known as a spot wind check), but also by simply looking at the groundspeed on the radar. If aircraft are assigned 160kts and are ground 200kts, there’s a significant tailwind, and we will see the groundspeed reduce as the wind transitions into a headwind. At some point, the transition will be very low, at 1000ft, then even lower, and then suddenly the winds on the ground shift to 240/30 too.

Squall lines are very apparent on ATC radar screens. When controllers see a line of storms (almost always moving from (north)west to (south)east in Montreal), they will perform some analysis and ask the following questions:

• How far does the line extend?
• How fast is it moving?
• At that pace, when will the squall line pass over the major airports?

Once this analysis has been completed and a plan developed, controllers will provide the best possible service to aircraft given the potential impacts of a thunderstorm situation. Aircraft will be re-routed to avoid squall lines, if possible. Re-routing aircraft well clear of squall lines avoids deviations and allows controllers to better plan their traffic.

The difference between a regular thunderstorm day and a squall line day is that traffic can be re-routed to avoid squall lines with more precision than compared to thunderstorm build-ups that are more unpredictable. When storms are directly over major airports, there is no difference. Delays and holds will happen.

• Squall lines can greatly affect the high-level sectors. If the line is passing through a busy flight corridor, most if not all flights will need to deviate, and a lot of aircraft can end up in a small portion of an airspace.
• Turbulence often is associated with squall lines.
• Supervisors may request more information from CMAC forecasters regarding the TAF, its variability and forecaster confidence, to know the extent of the line coverage.
• All of this greatly increases our workload, airspace complexity, frequency congestion and fatigue. Our planning might change in the way we might require more staff, we might request other sectors to reroute aircraft around the squall line, and it can add delays for aircraft.

• While single cells are easier to navigate around, squall lines with embedded cumulonimbus become more difficult to navigate and plan around.
• Usage of moving maps and graphical forecasts are most useful.
• Planned routes are assessed before takeoff with weather radar overlaid on navigational charts.

For General Aviation (GA) pilots, awareness of squall lines usually comes from looking at a GFA for activity on the front, combined with radar images when available to determine if storms will be discrete (single cell) or a solid line. When it is a solid line, it means no flight will take place near or in direction of the line. If it is discrete and there is enough spacing between storm cells, it may be possible to pass through them, with proper awareness of the risks involved (see section on thunderstorms for more detail). This is where calling an FIC for an interpretive weather briefing becomes critical as they can provide this information. When storms are related to a cold front and the conditions are predicted to be similar along the entire frontal line, especially in an unstable air mass, it should be presumed that there will be a squall line along the front, because not assuming this would pose a greater danger. In VFR GA, we must always consider the risk and assume a worse situation than what is predicted. Perhaps a convenient fact is that frontal storms travel and appear at a more consistent rate and thus timing for a flight can be more accurately planned. For other storms not related to a squall line, this is often made harder because they may appear and disappear with less predictability (see thunderstorm section for details).