Thunderstorm

Operations Duty Managers

Any size of a thunderstorm in proximity to an airport terminal along with duration can greatly impact operations depending on demand at that time.

This could necessitate TMI’s such as GS, GDP, MIT, holding at bedposts, halting ground operations due to nearby lightning and or wind shear/microbursts.

• Thunderstorm size, coverage, and intensity play a larger role EN-ROUTE operations. This may cause reroutes, SWAP, CAN Routes, and MIT. 

From a planning perspective, the following is most important to an operations duty manager at a major airport. It is two-tiered, with both the airport/terminal and the enroute taken into account:

• To note: Thunderstorms do not have to impact the field to lead to TMIs. If large swaths of enroute airspace are lost due to the presence of convective cells, this may require TMIs. This is where the TAF, forecaster notes, TCF panels, discussions with forecasters, Consolidated Storm Prediction for Aviation system (CoSPA), and Hailstop become critical.
• For both airport/terminal and enroute: confidence level on the level of organization of thunderstorms is critical. Are we expecting a well-organized line along a frontal boundary? Disorganized development not tied to any large-scale feature? This impacts TMI discussions depending on demand and impacts how precise we can be in planning any initiatives.
• We tend to be particularly concerned when our preferential routes are blocked by storms. This tends to greatly increase the workload on ATC.

Airport/Terminal:

• Timing of thunderstorms at the airport/in the terminal. From an airport capacity management perspective, knowing when the most likely time frame for loss of the field due to storms allows us to compare the timing to demand at that time to decide whether or not TMIs will be needed.
• Duration of thunderstorms impacting the airport/terminal. If TMIs are likely needed due to demand, knowing the potential duration will drive the conversation as to the type of TMI we may put in place. GS are usually favored over GDPs in short-duration events as it allows for optimal (and tactical) maintenance of flow. We want to avoid programs where we have fewer aircraft airborne than what we can handle.

Enroute:

Extent of coverage, timing, and trajectory of thunderstorms in the enroute

• The extent of coverage of thunderstorms across our enroute will impact our ability to hold aircraft, our ability to get aircraft to the terminal, and the workload increase felt by both crews and ATC when significant vectoring around storms is required. In situations of organized convection, the possibility of implementing SWAP routes is there, though this requires significant pre-planning and coordination with the NTMU and customers.
• The timing is also crucial as discussions are had with enroute ATC, the NTMU, and customers for the operational plan and potential up-staffing in the sub-unit.
• Understanding the trajectory of storms allows for planning within the sub-unit of which sectors are likely to be busiest and when, of potential offloads at bedposts, and of tactical non-formal TMIs that may be requested (MIT or APREQs for example). These are discussions that are had at Operations Duty Manager briefings or with supervisors on the floor prior to thunderstorm occurrence.

• Thunderstorms have one of the highest impacts on the 4 majors. Thunderstorms will greatly affect arrival rates, runway usage, loading and departing aircraft and additionally affect the routes in which the aircraft can get to the airports/bedposts. Thunderstorms have the highest impact on traffic movement and usually require slowing down the whole system.
• Thunderstorms will greatly affect routes when they extend into high level airspace. Affecting ability to get aircraft to bedpost for the four majors and additionally affect the decisions to accept offloads through Canada from the FAA.
• NTMU pays close attention to the forecast and CoSPA, the difference in thunderstorm type forecasted is important along with PROBs in the TAF etc, as this is a good indication of the persistence of the weather.
• Top questions are; how long will the airport be impacted, will the enroute be affected by thunderstorms, will there be breaks in the weather to recover?
• The NTMU closely monitors the forecast for thunderstorms at the four majors primarily using the TAF and CoSPA. The duration that the thunderstorms will affect the terminal/airport is important to understand whether storms are quick moving (allowing recovery) or longer in duration.
• A solid squall line is much easier to plan for than pop ups. Squalls are much easier to see on CoSPA tracking into the area, so it is much easier to predict timings for traffic management initiatives and predict rates and even time when the airport may be shut down. Pop ups are harder to predict timing and impact to traffic levels usually have to be handled on a much more tactical level.
• The sooner the event can be predicted the better, ideally the day before to allow for planning of staffing. For the traffic management portion, 0600-0700 am is ideal for an afternoon event (the time it will usually have the highest impact due higher traffic volumes).

ATC (Major Tower)

• Thunderstorms have a major impact on the airport.
• Workload in the tower increases significantly in communications with terminal ATC especially when thunderstorms are along the runway axis.
• TAF is supplemented by weather radar and continuous communication with terminal ATC, the Operations Duty Manager, and the airport authority. Strategically planning operations is more difficult in situations where PROB30/40 are used for extended periods of time and often leads to tactical intervention.
• Use of weather radar is more tactical but a great source of information.
• Lightning detection systems will dictate when the airport goes into a “red alert” where personnel seek shelter for safety purposes (lightning strike within 5KM).
• Everything on the ground stops during the “red alert” and coming out of a long duration event can be difficult as there are aircraft that need to push from gates and aircraft that need to depart all while aircraft that have been holding need to land. Coordination with terminal ATC and the Operations Duty Manager becomes critical to avoid a gridlock on the ground. Can lead to a ground stop. 

ATC (Regional Tower)

Thunderstorms are relevant to our operation. If we notice them forecast on the TAF, or painting strikes on the RADAR we would inform VFR pilots who might be affected. Sudden changes in wind direction and speed also affect runway assignments. IFR Departures can also be affected if a cell(s) are located along the departure routes (SID), deviations may be requested and coordination is involved. TAF is the forecasting tool used, PROBs are taken into consideration from a planning or head’s up perspective but given demand controllers are more likely to respond to thunderstorms in the moment.

The type of Thunderstorm threat will be defined by the environmental factors which allow it to form, and these different situations demand slightly different approaches for Weather Briefers. An unstable airmass, such as found in a summer “Warm Sector”, may spawn individual thunderstorm cells. These are easy to see and avoid (unless already overhead). When conditions indicate that a cold front will pass through the warm sector, this demands a more pointed warning to aircrew, as it will be impossible to avoid the severe turbulence, precipitation and reduced visibility that is common to a solid line of thunderstorms. For Light Aircraft, this is a “showstopper”, whereas some Medium Aircraft can launch and climb prior to the arrival of the Cold Front. 

Should conditions favour the development of a really big system, like a Mesoscale Convective Complex, you’ll see Dispatchers repositioning aircraft out of the path of the anticipated MCC path, with inbound and outbound flights rescheduled or cancelled outright. 

So, one could say that the type and size of Thunderstorm elicits different proportional responses from FSS. 

Specialists are often at the ready during potential convective development as visibility is often reduced due to haze on hot summer days, and the approach of developing convective clouds isn’t always obvious (the CI or AC visible above the Haze may not just be CI or AC). Thunderstorm passage is often a busy time for Weather Observers as special criteria are met in rapid succession as a Thunderstorm transits.

AAS

Advisory specialists watch for the tell-tale wind shift as the outflow boundary passes, sudden rapid fluctuations in air pressure and temperature, and sudden increases in visibility: all signs of an approaching/developing area of convective activity. Ground crews at airports will often coordinate with Advisory/Observation specialists to stay up to date on approaching hazards for personnel on the apron and manoeuvring areas. We often amend the advisory and determined runway to keep aircraft clear of approaching convective weather, as well as solicit PIREPs from anyone in the area to build a better picture of the convective activity for other aircraft, briefers, and forecasters.

Thunderstorms have a massive impact on our work as terminal controllers. While some pre-planning takes place, it’s much more adapting and reacting to what’s currently happening. We tend not to differentiate between thunderstorm types. However, as soon as thunderstorms are forecast in a TAF(even 30 percent), a few planning steps must be taken.

Below are all specifics from Montreal’s terminal ATC:

• Corridor Weather Integrated System (CWIS) is used to visualize the progression of cells and assess potential impact at the terminal. Certain traffic flow control measures can be put in place depending on the expected duration of the event.
• Our entire airspace structure in Montreal Terminal changes when we have stormy weather. Normally, the traffic here is separated “by task”, rather than geographically.

Usually, we’ll have one or two controllers working departures and one or two working arrivals. ATC work both arrivals and departures together in a small airspace. This is done safely in a highly procedural environment and mostly without coordination between terminal ATC as aircraft are on prescribed routes with little deviation. During thunderstorms, aircraft are more likely to deviate off their prescribed routing due to convection, at which point division of ATC responsibilities by task is no longer viable.

• If it helps to explain, picture this: The arrival controller's airspace is a single tube, for much of the terminal. Aircraft on that route cannot deviate a single mile off track, or they will be in departure’s airspace. This is fine when nobody is deviating, but if we had to stay in this structure with deviations, we’d constantly be coordinating with each other and have traffic not under our responsibility in our airspace, so this would be difficult to manage.
• So therefore, the first thing we’ll do, usually within 1hr of the thunderstorm, is to reconfigure the airspace into what we call “geographic” mode or simply “severe weather configuration”. In this mode, the airspace is simply divided geographically, as the name implies. We’ll have one controller working the northwest side of the terminal, one other controller working the southeast side, and then one controller working specifically the low arrivals. This airspace structure greatly simplifies deviations, because now if an arrival wants to deviate, the arrival controller doesn’t have to “check” with the departure controller if it works or tell the departure controller to stop a departure’s climb, because the arrival controller IS the departure controller. Less coordination. However, this also increases the workload, because now for example, the arrival controller who used to just be sequencing arrivals for CYUL is also responsible for everything happening over CYMX Mirabel and is also handling all the north departures from CYUL. Also, it’s worth mentioning that we are working “by task” 98% of the time, so just the simple fact of being in “geographic mode” increases the complexity of the workload, and this is before any thunderstorm is added to the mix.
• This reconfiguring takes a few minutes and involves switching frequencies and a few system manipulations, so it’s way easier if it’s done ahead of time, before the thunderstorm hits. The terminal supervisor will decide when to reconfigure, and we will advise the adjacent sectors as we do it.
• Once that is done, most of what we’ll do is react to how thunderstorms evolve. We can plan a little, but thunderstorms can be so unpredictable that it’s really hard to just “follow a clear plan”. Some aircraft will go hundreds of miles of course to avoid a cell, and others will tell us “No, this is fine we can go through it”. It’s a case-by-case basis. It gets trickier when we are surrounded by many dense cells, as all aircraft (both arrivals and departures) want to go through the exact same opening (aiming for clear air), which obviously increases complexity as we’re used to having arrivals and departures on laterally separated tracks.
• For departures, if the storm is blocking their exit point they will want to exit the terminal somewhere else. This often means that the next sector who was expecting the aircraft will not be receiving the handoff, because the aircraft will leave the terminal towards another sector. We therefore have to coordinate (electronically or verbally) with the adjacent sector that will receive the handoff, because since they weren’t initially expecting said aircraft, they will have no info on him and no idea he’s going to enter their airspace unless we tell them. This additional coordination adds workload. We have procedures to simplify this but it is a constant readjustment, the weather is always changing.
• Also for departures, if the storm is near or on the airport, aircraft will often request to break noise abatement and fly a different heading than the SID calls for on departure. For us in Montreal all jets depart on runway heading, but in severe weather operations they can request any heading. For example, if we are on 24s and the cell is straight ahead by 6-7 miles, no aircraft will want to take off towards it and wait until we give them a turn- they will all request an initial heading with tower. It’s our responsibility in Terminal to approve the headings, and to make sure the airspace is clear. This is an additional workload because we aren’t used to dealing with jets taking off in any direction other than runway heading so we must be extra vigilant.
• For arrivals, it’s always interesting because it gets very busy right before the storm, then if the storm hits the airport, it gets very quiet as no one can land anymore, and then once the storm has cleared the airport, everyone wants to land immediately so it gets very busy again. We can break noise abatement just like with departures, which can be very handy. For example, if the cell is 9-10 miles on final from the airport (we’re usually aiming to intercept the final at around 10 miles out, for an IFR approach), we can descend aircraft to lower altitudes and then we can vector them in for a 6-7 mile final, if the pilots are okay with it. This often helps a lot, as the pilots on downwind can see the storm cell straight ahead and they’d really like to turn base as soon as possible to avoid getting too close to the cell.
• Once we’re confident the storm is over and that no one is deviating anymore, we will reconfigure the airspace to go back to our “divided by task” mode, and then everything goes back to normal.

Thunderstorms create the most difficult work environments for ATC. We do not differentiate between different types of thunderstorms. We receive a weather briefing and are made aware that there could be thunderstorms that could cause delays and closures, but thunderstorms are generally a tactical event to be handled.

When a thunderstorm is passing over an airport, it can disrupt the airport to the point that arrivals and departures must pause (also referred to as ‘shut down the airport’ internally).

• This can happen either when the airport is surrounded by storms or there is a lightning alert that causes ground staff to take shelter. When storms linger over a major airport, airport congestion can occur and this can lead to airborne holds in our airspace, ground stops, and ground-delay programs.
• Isolated thunderstorms can lead to airports pausing operations with little to no warning, while organized storms give better predictability.

Controllers have a much lower aircraft capacity on their frequency during storms as each aircraft needs more time and attention. We are constantly gathering and passing information to and from pilots, managers, supervisors, and other controllers.

• This is why thunderstorms do not have to impact airports in particular, especially major airports, to lead to ground-delay programs or ground stops. Understanding the timing and coverage of storms across each of our sectors helps supervisors plan the separation/merging of sectors and to maintain situational awareness.

Important in the high level especially during the summer months. This phenomenon has a real impact on the operational level to such an extent that we will change our staffing. Due to the unpredictability of storm formation, movement and height we treat storms with a tactical mindset.

• Additional staffing supports opening of additional sectors so that ATC on floor are able to work fewer aircraft through a smaller airspace. This is especially important on days that see significant thunderstorm activity during peak traffic periods as there is a strong likelihood the majority of aircraft will request deviations.

What’s important for us is to know what the tops of the cumulonimbus will be. Depending on the altitude the clouds reach, it could influence the deviations planes decide to take. Products such as CoSPA and TCF are used to determine echo tops and storm motion.

Thunderstorms increase workload as flights require updated information on thunderstorms and active SIGMETS. A risk of thunderstorms will drive conversations around pre-planning routes (SWAP) that avoid active areas of thunderstorms. 

• Thunderstorms that also occur at the same time as clear air turbulence add another layer of complexity to an ATC workload.
• If a major airport is significantly impacted by thunderstorms, and operations are forced to stop for extended periods of time, airborne holding can extend into our airspace or miles-in-trail requested from the low-level enroute sectors.

General Aviation (GA) pilots are taught to be very cautious about TS, both directly and indirectly. We all know not to fly into them, but also not near them, or under them, especially in smaller GA planes, due to the surrounding potential effects – turbulence, precipitation, updrafts, and downdrafts. 

Thunderstorms are to be avoided at all costs. The combination of low cloud, heavy winds and rain can severely damage a GA float plane and potentially leave a crew stranded. GFAs and FIC weather briefings are most useful to pilots who are not flying to/from aerodromes providing TAFs. Examples include float plane pilots and pilots flying to private airfields.

When planning a flight, thunderstorm possibility will usually be easy to identify, but the degree to which they will be present, as well as the various types of thunderstorms, can make for a challenging decision-making process.

When relying on forecast products, the GFA is useful to see the bigger picture of what regions are likely to contain storm activity, but they usually refer to the conditions of an air mass over a very broad area. Unless associated with a specific phenomenon such as a cold front, it can be hard to predict exactly where they occur, so a good pilot will read the GFA and create a plan of action with contingencies assuming that there is a possibility of isolated or scattered storms in the region they plan to fly.

If a TAF is available, the more predictable the occurrence, the easier it is to plan accordingly.

• In other words, if there is a good chance of storm activity (ie VCTS or TEMPO), most pilots will proceed with caution and will try not to plan any flying during the active times with an extra margin before and after.
• When listed as a PROB30/40, in conjunction with the information on the GFA, many pilots will assess the risk and often decide to fly with the knowledge that they may encounter storm cells that they can deviate around.
• Proper planning is essential in these cases and one must always have a backup plan, and ideally a backup to the backup, which includes at a minimum alternate airports and extra fuel reserves.
• Staying local is always a good idea to help mitigate these risks. A pilot shouldn’t plan a longer trip with these risks unless there are many potential diversion options.

When available, radar observations are a very important and often underused tool that can show the size, speed and severity of a larger storm system.

• For isolated storms it can be useful as well, but extra caution must be taken when using radar before flight because these storms tend to be more erratic and can appear and disappear more quickly from radar.
• On a clearer day with isolated storms, it will be far easier to visually identify cells, but on a scattered or overcast day, the cell formation and/or dissipation may be harder to identify as they “hide” among the clouds.

In summer when there are chances of less predictable thunderstorms not related to a cold front or squall, pop up storms tend to keep everyone on higher alert in terms of go/no go decisions to fly. During all flight phases, good pilots who have assessed the risks and decided to fly will be constantly looking around to monitor changes in the weather activity in all quadrants surrounding them.

• In flight, special attention must be paid during the presence of thunderstorms. Commonly, pilots are taught to give a wide safety margin (10 to 20 miles distance from the storms) which can vary depending on their severity and who you ask – there are different schools of thought on this subject, but common sense would suggest that farther is better. However, it can be hard to judge your distance from a storm, even if the aircraft is equipped with a satellite weather service (given the inaccuracy due to the movement of the storm or the aircraft) and therefore the use of ground based visual references can be useful to help assess your distance.

Flying directly beneath or in close proximity to a thunderstorm should never be attempted. This might be obvious when there is precipitation below the thunderstorm due to reduced visibility and it would be automatically avoided by a VFR pilot. However, during developing or dissipating stages, precipitation may not be visible and extra caution must be taken due to updraft or downdrafts that may be present and creating extra risk. Updrafts may be hard to counter and poor corrective action may cause pilots to enter a dangerous or excessive dive. A failure to react to an updraft could push the aircraft closer to cloud and potentially cause the pilot to enter IMC, which could have severe consequences for an untrained and/or panicking pilot. A downdraft could be difficult or impossible to overcome if the aircraft’s vertical climb speed is less than the speed of the downdraft, causing it to approach the ground. Incorrectly adjusting for the downdraft could result in a dangerously low airspeed and/or aerodynamic stall.