As we are now within a week of the first display of this year’s competition, I thought that it would be appropriate to post this.

Here, I will prepare weather forecasts prior to each fireworks day. This would include a preliminary report approximately 48 hours just before display time, and then a more finalized update about 18-24 hours before. These forecasts would include details of temperature, humidity, some discussion of rainfall probabilities, cloud coverage, as well as wind speed and direction. If necessary, more updates would follow in cases of rainfall risks (notably if/when there are thunderstorms possible in the area) and/or if winds are favored to be either too strong or too light. Feel free to also share any details!

Here is hoping that all fireworks evenings (and setup) meet with ideal weather conditions!

Trav.

This is a thread I was looking forward to! Thank you, Trav, to be back with this public service tailored for the event. I always read carefully your forecast to get prepared.

Fred

You're very welcome, Fred! Always a pleasure, and I am looking forward to the displays!

Trav.

Trav, with a such username, I suggest that you add air quality to your weather forecast!

Seriously, the current situation in Montréal (due to forest fires in northern areas) is impressive and disturbing. La Ronde is reportedly in discussion with the City and local public health authorities regarding the opening show. I hope the anticipated rain is enough to rule out the scenario of a cancellation.

Here is a report segment from La Presse:

OUI OU NON POUR LES FEUX D’ARTIFICE ?

Des discussions seraient par ailleurs en cours concernant l’International des Feux Loto-Québec, dans le contexte actuel. La Santé publique n’a pas souhaité se prononcer sur le sujet lundi. À la mi-juin, La Presse rapportait que ces grands spectacles pyrotechniques ont été responsables à eux seuls de 5 des 33 journées de mauvaise qualité de l’air en 2022 à Montréal, soit 15 % du total. La Ronde, responsable de l’évènement, a indiqué lundi être « très sensible aux préoccupations soulevées en regard des feux d’artifice, qu’il s’agisse d’impact environnemental ou du lien qui est établi entre les incendies de forêt actifs ». « Nous évaluons la situation avec la Ville de Montréal et attendons les directives de la Santé publique à savoir si notre premier spectacle pyromusical de ce jeudi pourra avoir lieu », a fait savoir la porte-parole de La Ronde, Carole Bricault.

https://www.lapresse.ca/actualites/grand-montreal/2023-06-26/qualite-d e-l-air-a-montreal/toujours-mauvaise-mais-en-voie-de-s-ameliorer.php

Following a brief but modest decrease in humidity, a return flow will re-introduce the high to progressively very humidity pattern that has reached this region since the afternoon of June 23rd. As such, this year’s opening display on June 29th (and more especially for the first in-competition display) will be affected by this tropical regime.

A maximum temperature of 26 C is favored for the day (June 29th), and by late-evening, this would decrease modestly to 23-24 C in the greater Montreal area. In light of the return flow that would restore high humidity, a maximum humidex value of 33 C by mid-afternoon, and then 29 C by display time would be favored.

A weak shortwave trough would also be advancing East during the predawn hours, which would encourage a few isolated light convective rainfall and a more continuous low cloud deck. Beyond the morning, more breaks in the cloud deck would occur, but with weak lift in the region, as well as some instability, this would favor patches of low-altitude cloud in the form of cumulus clusters (with large clear breaks), including into the evening. Rainfall development should be minimal following the morning period.

Wind speeds are predominantly in the form light WSW for the day. However, he latest thinking is that these wind fields would gradually undergo a transition to SW by late-afternoon to eventually SSW in time for display time, at speeds of 6-8 km/h. This would allow the smoke to displace gently to the right of the La Ronde audience (left for those viewing from Notre-Dame Street), This will be carefully reviewed, though, given that wind direction is more sensitive to light wind speeds.

With respect to air quality concerns, as Fred outlined above, and being mentioned in Paul's interview, poorer air quality would progressively be favored today (June 28th). To that end, once the cold front and attendant shortwave trough from this morning to early-afternoon (June 28th) ejects East, fairly breezy winds from the NW would encourage forest fire smoke to once again overspread Southern Quebec and surrounding regions, reducing air quality potentially significantly by late-day. In turn, this may prompt local smog warnings to re-surface, as well as other forms of air quality advisories. There is some concern, too, that this smoke could linger into a good portion of June 29th as a result of the lighter wind fields featured by the day, so this will have to be monitored carefully. Air quality details will be incorporated into the Wind section of the more finalized report and subsequent updates as necessary. If the smoke remains concentrated enough on June 29th, the maximum temperature would also be a little lower than expected (perhaps closer to 24 C(, though the humidity would still remain high.

Another update will follow late this evening (June 28th) or early-predawn (June 29th).

Trav.

Trav, for the first time, I hope your forecast is wrong! If Sante Public is reading this, there's a chance the display could be cancelled. Other forecasts are more optimistic ... (such as this one https://aqicn.org/station/@373375) ... with Friday looking to be more of a peak than Thursday. Other outdoor events are also vulnerable to these forecasts - remember the cancellation of the final day of the Montreal Triathlon last Sunday.

Fingers crossed for tomorrow, indeed!

Paul.

Thank you for this forecast, Trav. And thank you for this source, Paul. Very interesting. I even learnt there is an air quality station in my district, next to the supermarket I visit every week!

Inasmuch as that forecast is accurate, I guess we will be OK for the fireworks show tonight. However, because I don't exactly know the rules applied (if any), I feel some uncertainties in the air...

Fred

The suspense continues...

À la veille du lancement de la 37e édition de l’International des feux de Loto-Québec, la mairesse de Montréal Valérie Plante rencontrera la directrice régionale de santé publique de Montréal, la Dre Mylène Drouin, cet après-midi pour parler ou non de la tenue de l’événement annuel.

«Si on doit annuler l’événement, on va l’annuler», a prévenu la mairesse ce mercredi en point de presse.

https://journalmetro.com/actualites/montreal/3112234/plante-pourrait-a nnuler-les-feux-de-loto-quebec/

Fred

Indeed, very unfortunately, the fairly strong synoptic-scale NW flow regime discussed above would cause abundant smoke concentrations to infiltrate Southern Quebec and surrounding regions into today (June 29th), rapidly deteriorating air quality. This eventually prompted smog warnings and/or air quality statements. Particulate matter 2.5 could reach as much as or slightly exceed 90 ug/m^3 locally today.

I was really hoping that this would not be the case for this opening display (and hopefully not for any other upcoming displays)!

Trav.

Outside of the air quality issue, weather conditions would have, indeed, been overall ideal for the opening show. The high humidity, though, would have encouraged some periods of extensive smoke accumulation during the more active sections of the display and/or during segments featuring smoke-rich products (if any). However, sufficient SE to South winds (~12 km/h) would have directed this (thickened) smoke clear to the right of the La Ronde audience (left for those on Notre-Dame Street) at all altitudes. The temperature on the Molson display might have been something like 22-23 C (humidex of 28-29 C). Sky conditions were mostly cloudy (consisting of large clusters of cumulus) during the evening but with large clear breaks in between.

Trav.

Expanding on my previous posting above from June 28th, the aforementioned tropical pattern will continue to be present for the first in-competition display (Ukraine). This could potentially contribute to a new all-time maximum temperature for the day (July 6th).

A maximum temperature of 33-34 C is favored for the greater Montreal area. With continued high to very humidity, this would send the humidex to as much as 42-43 C. The extent of warm air advection would hold strong into the evening, and so the temperature would not be liable to decrease much by display time (still 30 C by display time, along with a humidex of 38-39 C).

Cloud coverage would be fairly low during the evening due to limited lift, though some increase in cumulus clusters would begin overspreading the region during the evening because of an advancing shortwave trough, and an attendant cold front at the surface (both of which may yield scattered strong to severe thunderstorms on July 7th). During the afternoon itself, moderate instability would favorably encourage rather sparsely distributed non-severe and briefly-sustained thunderstorms and/or convective rain showers (collectively 20% probability) in this general area, but generally mostly sunny conditions are favored.

Winds would be in the form of mostly light South-Southwesterlies (9-12 km/h) for most of the day, but undergoing a slight transition to light SSE by the evening period. The broader scale wind regime would ensure that forest fire smoke, as was the case this week, would remain well North of this region. The wind fields would further allow the (often thick) smoke accumulations to drift clear to the right of and slightly away from the La Ronde audience (left for those on Notre-Dame Street). Note, however, that due to very high humidity, periods of extensive smoke accumulation are possible during the more active segments of the display, especially at right-hand sections of the display. Overall, though, the wind should be strong enough to steer the smoke off to the right, especially at the highest altitudes of the display.

Another update to follow late this evening (July 5th) to predawn July 6th.

Trav.

A strong tropical air mass continues to circulate in this region and, thus, for this first in-competition display. As such, here are the overall conditions for the greater Montreal area for July 6th.

Temperature/Humidity

The maximum temperature should be 33 C to even 34 C for the greater Montreal area by mid-afternoon. As a result of very high humidity, the humidex would reach 42-43 C. By display time, an ambient temperature of 30 C is favorable, accompanied by a humidex of 38-39 C.

Precipitation

Instability would remain moderate for the afternoon to evening, encouraging isolated convective rainfall and non-severe thunderstorms. All of these would be fairly slow-moving and briefly sustained. In tandem with the aforementioned cold front and shortwave trough, this would provide some boost in lift into the evening, maintaining the isolated convective rainfall risk intact for the evening (30% probability). Cloud coverage, in the form of cumulus clusters and high-altitude cirrus, would generally increase as the day progresses, being more concentrated during the evening (with clear breaks). During the afternoon, the risk would collectively be at 20% probability in this general area.

Wind

The overall wind tendency would be light South-Southwesterlies at 9-12 km/h for most of the day. By evening, the directional component does take on a slight transition to light SSE. Such overall wind fields (and this synoptic-scale pattern altogether) would keep forest fire smoke closer to the source area(s), as has been the case over the last week. Also the (often thick) smoke accumulations during the display would displace clear to the right of and slightly away from the La Ronde audience (clear to the left for those on Notre-Dame Street). The relative light nature of the wind together with very high humidity would encourage periods of extensive smoke build-up, notably during any active segments featured in this show at low- to mid-level and especially hugging right-hand sections (La Ronde’s right) of the display. Overall, though, the wind should be marginally strong enough to mostly guide the smoke towards the right, most evident at the highest altitudes of the display.

Another update will follow to address the extent of convective rainfall for the evening period, if necessary.

Trav.

The tropical air mass will continue to circulate over this region, capturing the second in-competition display (July 13th).

The environment will be characterized by very high humidity, building on what has been an already record-long stretch of days featuring dewpoints reaching or exceeding 20.0 C. Although periods of sunshine would be limited, a maximum temperature of 26 C would be achievable for the greater Montreal area following the morning periods of convective rainfall. By late-evening this decreases to only 25 C due to very high humidity/near-saturated air. As such, a maximum humidex of 34-35 C is favorable by mid-afternoon, and then 32-33 C for late-evening.

The environment would also be moderately unstable despite the limited periods for sunshine. At the same time, a broad warm front will affect the region throughout the day, providing periods of light to moderate rainfall (heavy locally). This would maintain nearly saturated air. However, by mid-afternoon and going forward into the evening period, a rather vigorous shortwave trough will enter this region and would guide an increase in thunderstorm coverage. The thunderstorms could become at least scattered in coverage during the evening, with a few of these thunderstorms potentially becoming strong as they organize. As such, it is possible for Severe Thunderstorm Watches and/or Warnings to appear in this general area at some point during the afternoon of July 13th. In light of possible reduced lift at the time, there is some window of opportunity for a break during actual display time, however, so this will be monitored.

Wind fields are generally in the form of light SSE (8-11 km/h) throughout the day, persisting into the evening. This would favorably allow the likely often extensive smoke accumulations, due to fairly light winds and near-saturated air, to drift clear to the right of the on-site audience (clear to the left for those on Notre-Dame Street). Smoke at the highest altitudes would evacuate at faster speeds, so most of the smoke accumulations would be directed at low- to mid-altitudes at La Ronde’s right-hand side of the display area. Extensive smoke accumulations are notably expected whenever the display becomes more active and/or during times with any smoke-rich products present at low- to mid-level. Note, however, that if thunderstorms are in the area during display time, or slightly before, the smoke would more likely turn towards the La Ronde audience before the broader-scale wind flow can be restored.

Air quality, in general, would remain good, since this continued tropical pattern would keep forest fire smoke closer to its source and downstream of there.

Another update to follow late this evening (July 12th) or predawn July 13th, with follow-ups likely throughout the day July 13th to address the thunderstorm situation during the late-afternoon to evening.

Trav.

Forecast details presented in the above posting remain generally valid. However, some revisions to synoptic-scale wind velocity and thunderstorm environment are made below. As such, here the dominant conditions for the greater Montreal area for Thursday, July 13th.

Temperature/Humidity

As mentioned previously, tropical air will continue to circulate in this region. Late-morning to mid-afternoon sunny breaks and stronger warm air advection would encourage temperatures to reach as much as 27-28 C before thunderstorms/convective rainfall stunt further warming. The very high humidity would send the humidex to 36-37 C by mid-afternoon. By display time, a value of 23 C is favorable. By this point, humidity will have decreased from very high to high, creating a humidex of 27-28 C. If convective rain showers are in the area, this rain-cooled air would lead to 21 C (humidex of 25-26 C).

Precipitation

The environment described in the above post also remains. Large destabilization would occur by the late-morning to early-afternoon once the cloud deck undergoes enough discontinuity to allow for periods of extensive sunlight to get through. At this time, a strong shortwave trough and surface cold front will migrate East/SE and will likely increase thunderstorm coverage rapidly, forming what would be large and strong thunderstorm families along a broader line of deep convection with Eastward progression. The primary hazards with these thunderstorms would be damaging straight-line winds (inducing possible widespread power failures in Eastern Ontario to Southern Quebec), as well as locally small hail and significant sub-hourly rainfall rates. Embedded supercells would also support a narrow window of opportunity for a possible isolated weak tornado. Lightning frequency would also be high. As such, Severe Thunderstorm Watches to Severe Thunderstorm Warnings are favorable to appear in this general area at some point during the late-morning to early-afternoon of today (July 13th). During the morning itself, a warm front will provide periods extensive cloud cover, as well as moderate to heavy rainfall locally. Fortunately, as also suspected above, lift associated with both the shortwave and cold front would be much less significant by around display time, and so the thunderstorm risk would become less important. However, some residual scattered convective showers (40% probability) would remain present for the evening, followed by further declines in humidity after midnight. Mostly cloudy conditions, with some clear breaks, would be present during display time.

Wind

Wind direction during the morning would be in the form of somewhat breezy ESE to SE at 14-18 km/h. Due to the more rapidly movement of the humid, saturated air mass, relative to what was described in the above posting, a shift in wind direction to breezy SSW (16-19 km/h) is instead favorable by late-afternoon to early-evening. This would allow smoke accumulations to drift clear to the right of the on-site audience (clear to the left for those on Notre-Dame Street). Smoke at the highest altitudes would displace even more rapidly. Another revision made here is that smoke build-up should correspondingly be lesser, overall, due to both stronger wind speeds and somewhat less humid air by late-evening. Convective rain showers in the area could, however, still disrupt the local wind fields during display time, and so such convective showers would temporarily direct the smoke towards the on-site audience before the broader-scale flow could be restored in the area.

Air quality, in general, would remain good and forest fire smoke remains well North, closer to the source area.

If necessary, another update will follow late this mid-afternoon (July 13th) to address any changes to evening wind speed/direction and the ongoing convective rainfall risk.

Trav.

Well that's interesting - a tornado watch for the Montreal area for late this afternoon ... I guess that would help clear any smoke!