VCTS

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On a given day where forecasters have assessed that thunderstorm activity will occur, describing the spatial extent of the risk is a priority. While forecasters have many ways to describe thunderstorm occurrence in the TAF, the MSC adopted the use of VCTS in order to elaborate further on the confidence level of thunderstorm occurrence at the terminal.

There are often situations where forecasters are very confident that thunderstorms will occur but will not happen within the 5SM radius of the terminal. PROB30, PROB40 and TEMPO groups all speak directly to the probability of thunderstorms at the terminal directly, which occasionally forced forecasters to use PROB30s to describe thunderstorm activity that was likely to occur close but not at the terminal itself.

This GIF is an example of VCTS at Moosejaw, SK. Lightning strikes are recorded within 10NM of the airport itself, but not within 5NM. At this distance, the thunderstorm would not be recorded as “TSRA” at the airport, so putting TSRA in TEMPO is an overstatement of what the forecaster may believe will happen at the terminal. “VCTS” is useful in this case, as the forecaster can specify that thunderstorms will pass close by, but not impact the terminal directly.

Image source: Environment and Climate Change Canada

VCTS functions as an indicator that forecasters are confident in thunderstorm development close to but not at the terminal. Close thunderstorms still pose a risk to the airport themselves but can be regarded as a “PROB20” at the airport, with higher confidence that they will occur outside of the 5SM radius around the airport that the TAF describes. It is worth noting that VCTS covers any report of TS but does not cover a METAR/SPECI report of TSRA.

For most airports, VCTS describes thunderstorms happening between 5-10SM (4-9NM) from the terminal and is generally reserved for single cell, less organized thunderstorms.

In situations where the terminal area covers a large region, such as the four Canadian major airports (CYYZ, CYVR, CYUL and CYYC), VCTS can be used to describe organized thunderstorms occurring and/or expected to affect areas of importance (e.g. approaches, departures, terminal area entry fixes, etc.) within the terminal area (45SM/40NM radius) but not the aerodrome itself.

This GIF is an example of VCTS at a hub site. Since the terminal area covers a much larger area, traffic is affected by convection at much larger distances from the airport itself. VCTS covers the 5-45SM radius disk surrounding hub airports.

Image source: Environment and Climate Change Canada

There is always a challenge in communicating lower risk thunderstorm activity near a TAF site. When thunderstorms are disorganized and sporadic, a forecast of TEMPO or “PROBXX” may not adequately describe the risk at the terminal itself.

The area of the 5SM radius circle around the airport is approximately 78 SM², whereas the area of the 5-10SM ring is 236 SM². For small sites, the larger area is more likely to see TS activity during isolated thunderstorm days. However, these isolated thunderstorms outside the 5SM limit still have the potential to affect landing/take off or enroute aircraft, and VCTS can give forecasters another method of describing this risk. There are also cases where thunderstorm activity could be geographically limited and would not be expected to move within 5SM of an airport. This could be due to thunderstorms developing over higher terrain such as mountains that would undergo rapid dissipation if the storms moved off the terrain.

For high traffic airports, the 5-45 SM ring covers an area of approximately 6284 SM² around the terminal. While forecasting for a larger area is less challenging than for a very small area, risk communication remains challenging. Use of VCTS or organized features approaching and moving away from hub airports necessarily lengthens the TAF, as PROBXX TSRA cannot be used with TEMPO VCTS.

Image source: Environment and Climate Change Canada

The GFA valid June 24, 2023 at 1800Z shows a risk of thunderstorms across Ontario and Quebec, with isolated storms across southern Ontario and occasional storms along the cold front extending into central Quebec.

The CYYZ TAF has a VCTS between 1500 and 1800Z, followed by a PROB30 of thunderstorms between 1800Z-0000Z. In the early hours of the TAF, thunderstorms are forecast to occur within the bedposts surrounding CYYZ, but not at the terminal itself. At 1800Z, the forecaster determined through their analysis that these storms are 30% likely to strike within the terminal region itself. Forecasters will use VCTS rather than a PROB of TSRA in order to signal their level of confidence, used to express their belief that thunderstorms are likely, but most likely not to impact the terminal itself.

This model derived atmospheric profile, valid at 1500Z on June 24th shows the atmospheric instability and high dewpoints (indicating ample available moisture) present in CYYZ, giving two of the three ingredients required for surface-based thunderstorms. Forecast lake breeze convergence in the vicinity is an example of a trigger, the third required ingredient that sets the stage for thunderstorm development. Forecasters that day concluded in their analysis that forecast conditions at CYYZ, along with this trigger expected to initiate far enough from CYYZ were better described using VCTS rather than a PROB30/PROB40 in the TAF.

Image Source: Pivotal Weather

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One of the ingredients supporting thunderstorm development on June 24th was daytime heating. GIF of forecast 2-meter temperatures between 1200Z June 24th until 0300Z June 25th shows the increase in temperatures expected that day. Daytime heating like this, coupled with high dewpoints and the presence of a trigger, provides a great setup for thunderstorm development as it destabilizes the lowest levels of the atmosphere (see atmospheric instability for more information).

Image Source: College of DuPage

The presence of atmospheric instability, quantified graphically here as CAPE or Convective Available Potential Energy, provides a first glance of where the risk of thunderstorms may exist. In this GIF, valid between 1200Z on June 24th and 0300Z on June 25th shows positive CAPE values across southern Ontario between 1200Z and 0000Z. Combined with high surface dewpoints, two of the three triggers needed for thunderstorm development are present. Forecasters then assess where triggers, high instability and moisture are all likely to be located at the same time in order to time thunderstorm risks at airports. Though CAPE and high dewpoints were present at CYYZ for much of the day, the VCTS represents the forecaster’s determination that initial thunderstorm development is most likely to be outside of the terminal area and airport. There could be a variety of reasons for this, such as: the forecaster finding that the model did not capture the real-time situation very well at CYYZ, seeing stronger convective ingredients further away, climatology suggesting later development, etc.

Image Source: College of DuPage

Forecast reflectivity from the HRRR, valid 1200Z June 24th until 0000Z June 25th shows the initialization of thunderstorms across southern Ontario throughout the day. Arrows in the map indicate the forecast magnitude and direction of surface winds.

Image Source: College of DuPage

This is a snapshot of the forecast reflectivity at 1600Z. The HRRR is indeed picking up on the lake breeze, with the axis of thunderstorm development aligning with convergence zones where the lake breezes off the various Great Lakes meet. These zones are defined by two red circles, with the red star indicating the location of CYYZ. Light gray arrows pointing towards each other indicate likely areas of forecast surface convergence. For example on the northeastern shore of Lake Erie arrows have a northeasterly component, while further north winds are southeasterly off Lake Superior. This small area is also showing possible thunderstorm development, pointing towards this lake breeze convergence zone being a likely trigger. The forecaster’s analysis of multiple variables such as these, models, and current observations then translates into their choice to use a VCTS or a PROB30. This is only one example of the many tools available to forecasters to assess convective development and timing of impacts.

Image Source: College of DuPage

This multispectral GeoColor satellite imageryOpen a new window from CIRA from 1415Z to 1945Z June 24th captures many features related to the lake breeze convergence zone and associated thunderstorm development. Though high-level clouds are present, after 1700Z these clouds clear out and tell-tale signatures of a lake breeze become evident (such as the small line of clouds remaining along the length of lake Erie). Evidence of convection starting to fire can be seen in a line from about London, ON (CYXU) to Peterborough, ON (CYPQ) at the start of the GIF by evidence of the puffy cumuliform clouds. Consistent with the VCTS forecast, these storms are not over CYYZ throughout the period shown but are likely impacting bedposts and within 45NM of the airport.

Image Source: CIRA

Radar imagery shows the development of convective clouds and thunderstorms across southern Ontario throughout the 1230Z to 1800Z period on June 24th. The VCTS in the TAF captures the storms that developed outside of 5NM but within 45NM of CYYZ. Differentiating between this type of situation and one where thunderstorms are likely at the airport itself is done to provide enhanced detail in support of operational decision-making, including whether ground stops or re-routing of enroute traffic is necessary.

METARs are included only to show that thunderstorms did not occur at the airport, though they did verify in the vicinity. VCTS does not equate to a zero percent probability of occurrence at the airport, but rather communicates that the likelihood is far higher outside of the terminal area. It indicates high confidence of thunderstorms occurring elsewhere.

Image Source: OGIMET

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