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!

Massive water accumulations with these thunderstorm families, and multiple reports of funnel clouds in the general area!

Trav.

Just another (late) update to acknowledge that the smoke, at all altitudes of the display would move more towards right-hand to partially central sections of the La Ronde audience due to a faster shift in wind direction behind the cold front/shortwave trough. Also, convection would remain suppressed for this evening due to the rapid evacuation of the cold front and shortwave trough to the East, leaving partly cloudy skies in the form cirrus clusters. Wind speeds would also be a littler breezier from the SW (23-26 km/h), with occasional gusts nearing 40 km/h. This may prompt slight delays, given a slight change in direction that favors blowing somewhat more towards the on-site audience.

Trav.

The Belgian display, on July 20th, will face a return flow of moist air infiltrating this region, causing the humidity to achieve high status. As such, a maximum temperature of 27 C is favorable by mid-afternoon, accompanied by a maximum humidex of 32 C. Then by late-evening, this should decrease to 26 C (humidex of 31 C).

While lift would be limited enough to suppress particularly deep convection, there would, nevertheless, be some high-altitude cirrus interspersed with clusters of moderately-defined cumulus clusters, especially during the afternoon to evening, in the wake of an upcoming area of low pressure. This would create partly cloudy sky conditions during the late-evening. A shortwave trough would also eject East during the predawn period, making the thunderstorm risk more important on July 21st.

Winds would largely be in the form light SSW to even straight Southerlies but backing to SE tendencies by around sunset and going forward (8-11 km/h). Therefore, at all altitudes of the display, the smoke should be displacing slowly clear to the right of and slightly away from the La Ronde audience, though some periods of larger smoke accumulation will be favorable at the far right portions of the display along low- to mid-levels. Such extensive accumulations would, however, only be temporary and present mostly during active portions of the display. This wind regime would also continue to keep forest fire smoke concentrations farther North, some patchy hazy skies are favorable due to light winds and progressively high humidity.

Another update to follow by late tomorrow evening (July 19th) to early-predawn July 20th.

Trav.

Conditions described in the above post continue to be valid for the Belgian display, and July 20th collectively. As such, here are the expected weather conditions for Thursday, July 20th for the greater Montreal area.

Temperature/Humidity

A return flow of moister air would circulate into Southern/SW Quebec and would send humidity to high status by mid-afternoon. In response to this pattern, a maximum temperature of 27 C is favorable by mid-afternoon, which would be accompanied by a maximum humidex of 32 C. With respect to late-evening, including display time, this should decrease slightly to 26 C (humidex of 30 C).

Precipitation

Lift in the region continues to be limited. However, sufficient solar heating and progressive moistening would contribute to some moderate instability by late-afternoon into the evening. This would encourage some window of opportunity for slow-moving sparsely-distributed convective rain showers and isolated non-severe thunderstorms (collectively 30% probability during the mid-afternoon to end-evening period. Otherwise, modest overall lift would generally favor some high-altitude cirrus blended with defined cumulus clusters, especially during the afternoon to evening in response to the aforementioned upcoming area of low pressure that is more important for July 21st. Thus, partly cloudy skies are likely for the late-evening.

Wind

The overall synoptic-scale wind velocity would largely be in the form of light SSW to straight Southerlies. However, some backing would occur by around sunset to allow for SE winds by around sunset to ESE tendencies deeper into the evening (8-11 km/h throughout the day). As such, smoke accumulations should be displacing slowly clear to the right of and almost directly away from the La Ronde audience, though some periods of larger accumulation will be favorable at the far right portions of the display along low- to mid-portions during more energetic moments. Again, this overall wind regime would help to direct forest fire smoke concentrations farther North, but some haze is favorable as a result of light winds and high humidity by mid-afternoon. EDIT: For those on Notre-Dame Street, this more likely to be a hazy display, especially for those viewing farther away from the bridge. As such, it is best to be positioned closer to the bridge in order to avoid smoke interception and, thus, improve viewing clarity.

If necessary, another update will follow by this mid-afternoon (July 20th).

Trav.

The day of the Canadian display (July 27th) would be influenced by a cut-off area of low pressure. As this system ejects NE, it would sustain humidity levels into very high status as a result of the convective rainfall that would occupy principally the late-morning to mid-afternoon period. Thereafter, a gradual clearing would occur, leaving behind a few clusters of cumulus with large clear breaks between these, including during the evening and display time. Along with a display time temperature of 22 C in the greater Montreal area, the continued very high humidity would also support a humidex of 30 C at the time (32 C during the afternoon with a maximum temperature of 23 C).

Due to persistent cloud coverage during most of the early- to mid-sections of the day, the thunderstorm risk would be minimal, though periods of moderate to heavy rainfall are possible during the late-morning to mid-afternoon. This would keep the air saturated for the remainder of the day.

Winds would be light to calm throughout the day as a result of this system, and wind direction would appear in a variety. Early in the day through to the mid-afternoon, wind velocity would be in the form of light SSW to Southerlies, but at the back end of this system late-day, NW wind fields during the early evening followed by a fairly quick transition to straight light Westerlies (7-9 km/h) by display time appears favorable. This would allow the smoke, which could be extensive at times in response to the very high humidity and saturated air, to drift slowly towards the on-site audience. Smoke at the highest altitudes would be moving more quickly, and towards left-hand and partially central sections of the La Ronde audience overhead. Thus, a hazy display is possible, but this will continue to be monitored.

Another update to follow by later today (July 26th) to early-predawn July 27th.

Trav.

Weather conditions described in the above posting continue to be largely valid for the day today (July 27th). As such, presented here are the dominant patterns for the greater Montreal area.

Temperature/Humidity

As mentioned, July 27th would be under the influence of a cut-off area of low pressure. Within its warm sector would be a slight amplification of humidity and warming through the day. The latest thinking is that the sun would penetrate the low-cloud deck by early-afternoon, allowing for more time spent with solar heating. As such, a maximum temperature of 26-27 C is favorable by mid-afternoon. Very high humidity would further send the humidex value to 34-35 C during that time. By display time, a temperature of 24-25 C is favored under continued very high humidity (humidex of 32 C). As such, this would represent the most humid conditions during a fireworks display so far this year.

Precipitation

As this system ejects NE, convective rainfall would favorably occupy principally the morning to early-afternoon period (90% probability) downstream of the associated warm front. Thereafter, the risk for convective rainfall becomes sparse for the remainder of the afternoon as lift weakens (30% probability). A gradual clearing would generally follow the early-afternoon, but the near-saturated air and modest lift would leave behind clusters of cumulus with large clear breaks between them – this would persist during the evening and display time, though with larger clear breaks present. The thunderstorm risk continues to be limited for the day, including the morning, but periods of moderate to heavy rainfall locally remain very possible for the morning.

Wind

Winds would be light to occasionally calm throughout the day. During the morning, wind velocity would be in the form of light Easterlies to ESE (7-9 km/h), but at the back end of this system late-day, Northerlies to NW wind fields for the afternoon would transition swiftly to SW by late-afternoon to early-evening, followed by light WSW tendencies deeper into the evening towards display time (7-9 km/h). This would allow the often thick smoke accumulations (especially along low- to mid-altitudes of the display) to drift fairly gently towards right-hand and central sections of the on-site audience. Smoke at the highest altitudes would be moving somewhat more quickly, but more directly towards the La Ronde audience overhead, collectively creating a potentially hazy display at large. That said, it would help that the winds would be marginally strong enough to help displace the smoke.

Another update to follow by this mid- to late-afternoon to address wind direction/speed, if necessary.

Trav.

To provide an update as to wind direction, tendencies appear more so SW by display time (under the same speeds mentioned previously, at 7-9 km/h). This would allow the smoke to focus more towards right-hand sections of the La Ronde audience, though higher-altitude smoke would be moving towards there and partially central sections from overhead. Smoke accumulations would still appear thick along principally low- to mid-altitude in response to very high humidity, but most especially when the display is more energetic.

Trav.

For the American display on Sunday (July 30th), much cooler and less humid conditions would be favorable. To that end, the humidity would be borderline low to moderate, and so a humidex would be negligible under a 22 C display time temperature (25 C maximum for the day).

One aspect to monitor is a cold front advancing South during the day, having some influence in the area for the mid- to late-evening period. Some convective showers and isolated non-severe thunderstorms (30% probability) could initiate along this boundary at the time, so this will have to be examined.

Winds are generally following a pattern similar to the Canadian display, though extensive smoke accumulations would be limited during the American show as a result of much lower humidity. Wind speeds would be 9-12 km/h, from the SW, for the evening period, allowing the smoke to drift at a reasonable speed towards far-right hand sections of the La Ronde audience, though smoke at the highest altitudes would be advancing more quickly towards right-hand sections and partially central from overhead.

Another update to follow this late-evening (July 29th) to early-predawn July 30th.

Trav.

Conditions specified in the above posting continue to be valid for the American display. Adhering to these previously-presented details, here are the dominant patterns for the greater Montreal area for the day today (July 30th).

Temperature/Humidity

Under the influence of high atmospheric pressure, and as part of a cooler-than-normal pattern that would be persistent for potentially nearly two weeks, much cooler and less humid conditions would be favorable for this display. As such, these would represent the coolest and least humid conditions to appear during a fireworks evening so far this year. The humidity would be at the lower end of moderate, and so a modest humidex of 21-22 C is applicable during display time under a temperature of 20-21 C (a 24 C maximum during mid-afternoon – maximum humidex of 25-26 C). Note that if convective rainfall (40% probability – please see more details in the next section), this would cool the air temperature to 17-18 C (humidex of 20 C).

Precipitation

While the day itself is largely pleasant outside the chillier morning and evening, the aforementioned cold front (and shortwave trough) would continue to be prominent features for the evening and overnight period. Fortunately, the extent of lift downstream of these would not be particularly aggressive, though it would be sufficient for scattered convective coverage. As such, some convective showers (40% probability) would be possible in the area during the mid- to late-evening, and some modest instability partly gathered from earlier in the day would provide some window of opportunity for a few isolated slow-moving and briefly-sustained non-severe thunderstorms (30% probability). Following what would be a day spent with plentiful sunshine, extensive cloud coverage would gradually overspread the region by evening in response to the previously-mentioned features, with some small patches of clear breaks between this cumulus to cirrus cloud deck.

Wind


Winds are would remain light (9-12 km/h) for most of the day, interchanging between SW and WSW. During the afternoon, wind speeds, however, would adopt a breezier pattern (18-22 km/h from the same directions) before becoming light for the late-evening (9-12 km/h). By display time, however, wind fields would tend to be more in the form of WSW. This would allow the smoke to eject at a reasonable speed towards right-hand and partially central sections of the on-site audience from all altitudes of the display. Faster displacement of smoke at the highest altitudes of the show is also favored, though this smoke would be moving more directly towards the La Ronde audience overhead. No extensive smoke accumulations are expected, but the display would often appear hazy from principally the right-hand portion of the display (at mostly low- to mid-level) relative to the La Ronde audience.

If necessary, another update will appear here this mid- to late-afternoon (July 30th) to address wind direction and convective rainfall character.

Trav.

Fairly strong and slow-moving thunderstorm family over the West Island-Vaudreuil corridor this early-evening, along with convective rainfall farther East over greater Montreal and the fireworks site. Scattered convective rain showers and isolated non-severe thunderstorms remain possible for the evening, as described above, though improving conditions are favorable in time for the duration of the display.

Edit: Temperature, indeed, cooled to 17-18 C in response to the rainfall.

Trav.

though improving conditions are favorable in time for the duration of the display.

Yes, the conditions have improved shortly before the display, while we had a light rain for a couple of minutes at one moment during the show (just before Purple Rain).

(I even saw a very famous pyro without shoes in the grandstands!)

But what about the after-show! There was a thunderstorm and heavy rainfall when I had to leave the Salon des artificiers and have a walk across La Ronde to exit at the front, then doing the usual circumvolution on the right side (when we exit), before going toward the left side to get the bus, because the left path is always blocked by the countless fences! What a water ride!

Fred

An area of low pressure will advance East in time for the day tomorrow (August 3rd). As it does, a re-introduction of warm and very humid air will circulate into these regions. Discontinuities in the cloud deck during the afternoon would allow for a maximum of 24-25 C (humidex of 31 C in response to high humidity) in the greater Montreal area. By evening, the temperature would be 22 C under nearly saturated air. This would yield a humidex of 28-29 C.

Accompanying the aforementioned low pressure system would be a series of shortwave troughs ejecting SE throughout much of the day. The enhanced lift induced by these features does bring concern that several rounds of strong to severe thunderstorms could appear throughout much of the day, with the first of these appearing during the early- to mid-morning period. The primary hazards with these would be local flash-flooding, damaging winds and in some cases, intense lightning where instability can be maximized. Furthermore, prior to thunderstorm consolidations, some window of opportunity exists for an isolated weak tornado during the mid-afternoon. There are some indications, though, that the last of the shortwaves could affect the area just prior to display time, which would begin suppressing further developments thereafter. This would be by quite a narrow margin, but it will continue to be monitored.

Winds would be generally in the form of breezy SW (17-21 km/h, with occasional gusts of 29-33 km/h) during the morning to afternoon period, and undergoing a transition to light (SSW) tendencies by late-evening (8-11 km/h). This would allow the often rather thick smoke accumulations to slowly drift to the right of the on-site audience (much like during the American display, but with slightly slower displacement at low- to mid-altitude). This could sometimes create a hazy display at the right-hand portion of the display for La Ronde, enhanced by any smoke-rich products that may be used at low- to mid-level. If thunderstorms and/or convective rainfall are in the area, however, then the smoke would suddenly advance more towards the La Ronde audience.

Another update to follow late this evening (August 2nd) into early-predawn August 3rd.

Fred: Yes, that was quite a deluge of rainfall tied to that isolated thunderstorm family after the display! The one over the West Island-Vaudreuil corridor during the early-evening also led to large amounts of water accumulation there!

Trav.

Weather conditions specified in the above-posting for today (August 3rd) remain largely applicable. As such, here are the dominant conditions for the greater Montreal area for August 3rd.

Temperature/Humidity

The aforementioned area of low pressure will advance East and re-introduce a (very) humid air mass into this general area. With continued broken skies for the afternoon, sufficient solar heating would help to guide a maximum temperature of 24-25 C by mid-afternoon (humidex of 31 C as a result of high humidity) in the greater Montreal area. By evening, the temperature would be 21 C under nearly saturated air, correspondingly generating a humidex of 27-28 C.

Precipitation

A series of vigorous shortwave troughs would advance SE throughout much of the day ahead/downstream of the low pressure system’s cold front. The enhanced lift would favor widespread convective rainfall and scattered thunderstorms, some of which would continue to prove to reach strong to severe status during the afternoon period. As such, a Severe Thunderstorm Watch could appear early today. The risk for rainfall collectively would be 90% for the morning to afternoon period, and dropping to 60% for the duration of the evening as a result of slightly reduced lift that would otherwise prompt lesser coverage of convective rainfall, especially deeper into the evening. The sun should make some appearance during the afternoon, however, which would partly provoke a few organized multi-cell thunderstorm formations locally. The primary hazards with these continue to be local flash-flooding, small hail, damaging straight-line winds and potentially intense lightning where instability is at a maximum. The previously-mentioned isolated weak tornado risk, while still existent, should remain minimal within this environment. Sky conditions would be overcast with a low cumulus cloud deck for the evening. There remains some opportunity for lift to reduce just enough around display time, however, to allow for a brief dry slot for the display to be fired safely, so this will continue to be checked. Nevertheless, there is potential for the rounds of heavy rainfall to cause some technical issues for the display.

Wind

Winds remain in the form of breezy SW for much of the day, reaching speeds of up to 23-27 km/h, with occasional gusts of 37-41 km/h during the early- to mid-afternoon. Directional tendencies still show a transition to light Southerlies by late-evening (9-12 km/h). As such, the often thick smoke accumulations would fairly slowly drift clear to the right of the on-site audience, almost similar to what was observed during the American display but with slightly slower displacement at low- to mid-altitude. This would, thus, create a hazy display at its right-hand portions relative to La Ronde, enhanced by any smoke-rich products that may be used at low- to mid-levels of the display space. Note, however, that if thunderstorms and/or convective rainfall are in the area directly, then the smoke would suddenly turn towards the La Ronde audience before the synoptic-scale flow can be restored, but the latest thinking that the thunderstorms would be most prominent up to the early-evening.

Updates to follow this early- to late-afternoon (August 3rd) to address wind direction and notably convective rainfall.

Trav.

Lift associated with upcoming shortwave troughs continues to be rather aggressive for not only this early-evening but throughout the evening (August 3rd), and so the latest thinking is that the thunderstorm risk, including for strong/severe multi-cell thunderstorms, will favorably persist. Caution should, thus, be exercised in case of severe weather.

Synoptic-scale wind fields would still be in the form of light SSW this mid- to late-evening (easing from their breezy/gusty nature this afternoon), which would displace the smoke clear to the right of the on-site audience. Speeds would be a little stronger than what was previously specified, however, at 12-16 km/h, allowing for quicker displacement at all altitudes. Again, if thunderstorms do interfere, this would cause the smoke to rapidly turn towards the audience (notably left and centralized sections), especially since strong downbursts and very torrential rainfall remain primary hazards with these thunderstorms.

The sun has been out in the West Island-Vaudreuil area and closely surrounding areas for the last 83 minutes, which has partly caused a few briefly-sustained isolated thunderstorms/convective showers to recently appear, around Lachute and Chambly, as well as one this morning over Eastern greater Montreal and the East End previously this morning. At the very least, earlier cloud cover limited thunderstorm potential for earlier in the afternoon, keeping conditions dry and overcast with local light rain beyond this morning's steadier rainfall.

Trav.

Glued to the radar again this week Trav! Let's hope that, whatever precipitation there is, we have wind to move the smoke!