After a prolonged period of hot and humid weather across Southern Manitoba, a frontal wave pushing eastwards triggered numerous severe thunderstorms across southeastern Saskatchewan and southwestern Manitoba. None was worse than a thunderstorm that exploded over southwestern Manitoba and rapidly developed into a massive supercell thunderstorm. After a short while, this storm organized and produced its first tornado; this would end up being just one of numerous touchdowns over the course of the following 3 hours. The tornadoes this supercell thunderstorm produced were staggering in their intensity; perhaps most notably was the tornado it produced near Tilston, Manitoba whose size and power shocked even veteran storm chasers.

Pre-Storm Setup

Forecasters knew days ahead of time that Monday had the potential to be a significant day across southeastern Saskatchewan and southwestern Manitoba. A cold front that had pushed eastwards across the Prairies over the prior couple days stalled out over the Saskatchewan-Manitoba border on Sunday as a strong jet crashing in over the west coast dug into the upper-level trough, deepening and tilting it while the upper-level ridge built over eastern Manitoba in response. This allowed even more heat and moisture to build over the region at the end of a week that had already seen dew points regularly above 20°C with temperatures climbing into the upper 20’s or low 30’s. On Monday, a shortwave ejected out of the trough and lifted northeastwards, temporarily reversing the cold front and transforming the boundary into a nearly-stationary warm front. As the shortwave moved through the region, it triggered the thunderstorms and began to sweep the frontal wave off to the east-northeast.

On Sunday, it was already looking conditionally likely for tornadogenesis in any thunderstorms that developed over southwestern Manitoba. Pulled directly from the PASPC Significant Weather Discussion:

SIGNIFICANT WEATHER DISCUSSION ISSUED BY THE PRAIRIE AND ARCTIC STORM PREDICTION CENTRE OF ENVIRONMENT CANADA AT 2:00 PM CDT SUNDAY JULY 26 2015.

[…]

Eastern Prairies… The light southerlies are bringing even more heat and humidity. Most areas, especially the Red River Valley, look to be a degree or two warmer today and that will push humidex values into the low fourties today. A heat warning as thus been issued for the Red River Valley and Central Interlake. As of 18Z values are already between 36 and 38. The heat continues Monday but should be below 40 in most localities.

Meanwhile, the cold front from the low in Central Saskatchewan will lie along the Manitoba/Saskatchewan border by early afternoon today at which point its eastward progression begins to slow dramatically. This feature will be the focus of nearly all severe weather potential for the next 48 hours. Ahead of the front in Southern Manitoba, MLCAPE values increase to 2000 to 2500 J/kg this afternoon with 30-40 knots of deep layer shear. While the jet maxima remains well to the west, there are still 40 knot southwesterlies at 500mb and 40 knot southerlies at 850mb pushing in from the Dakotas. This could produce a few more supercells as it did Saturday over Alberta and Western Saskatchewan. However, continued warm air advection all the way up to 700mb makes most of the south capped for the day – particularly so in the extreme southeastern corner. Terrain and remnant outflow interactions closest to the front make the Westman and Interlake areas the most likely to see severe thunderstorms but overall the risk looks moderate this afternoon and early evening if the cap breaks.

A new low pressure system develops rapidly late Sunday in Eastern Montana bringing strong easterly winds in the low levels to the International Border areas in Southeast Saskatchewan and Southern Manitoba Monday afternoon. Hodographs have impressively veering profiles with up to 55 knots of deep layer shear accompanied by 2500 J/kg MLCAPE. This has potential to develop tornadic supercells, with forecast 0-3KM EHI values of 4+ and SIGTOR of up to 2 in the area with even higher values south of the border. However, this is all very conditional at the moment, as strong thunderstorm development today or tonight could shift boundaries somewhat or consume much of the instability over the area. Cloud cover in general could also be an issue for daytime heating, both from remnant overnight cells but also from the upper level jet closing in from the west. Finally, the same strong low level advections are building a very strong cap of -100 J/kg or stronger. Once again, regardless of daytime activity, this same setup is promising for severe elevated convection for Manitoba south of the Interlake and the Estevan area Monday night.

[…]

It was clear that there was going to be a heightened and dangerous severe weather potential on Monday. By Monday afternoon, things had become even more clear:

SIGNIFICANT WEATHER DISCUSSION ISSUED BY THE PRAIRIE AND ARCTIC STORM PREDICTION CENTRE OF ENVIRONMENT CANADA AT 2:00 PM CDT MONDAY JULY 27 2015.

Eastern Prairies… This afternoon looks like a repeat of yesterday for heat and humidity in the Red River Valley and East. Temperatures are already in the high twenties with dewpoints in the low twenties, making it feel like the mid thirties as of early this afternoon. The heat warning has thus been issued for the area for one more day.

The main weather story is the rapid development of the low pressure system in Eastern Montana. While there was uncertainty yesterday about the placement and motion of the low, nocturnal thunderstorm activity in the warm sector was very quiet, and satellite has borne out the extremely sharp and digging trough over the west predicted by most models. Surface analyses early this morning shows the low placement as forecast and is bearing out the early stages of northwestward movement and deepening as well. Ongoing convection leftover from this morning has weakened substantially and will continue to move northward with the rapid lifting of the warm front and the tilting of the jet.

Thus, many of the conditions potentially preventing the development of more significant severe weather in southeast Saskatchewan and Southwest Manitoba have been removed. Over the Montana low itself, skies are partly cloudy so it does look like many areas will see some sun today. A plume of 20 degree dewpoints is expected to wrap around the low, intersecting steepest mid level lapse rates with good surface convergence at the warm front about the Carlyle area early this afternoon. Hodographs here now have even more impressively veering profiles compared to yesterday, still with up to 55 knots of deep layer shear accompanied by 2500 J/kg MLCAPE. Thus, the potential for tornadic supercells continues to rise for the afternoon hours. 0-3 KM EHI are currently 5+ with SIGTOR values still climbing towards 1 early this afternoon. Models are in excellent agreement, with values quickly falling off below 0.5 in about 150km radius on all sides. As a result, a tornado watch has been issued for the Carlyle region for this afternoon. The threat for tornadoes will likely persist in extreme southeastern regions until the early evening hours.

All that remains is the capping inversion, which is weaker but still present – again weakest around Carlyle with only -20 J/kg MLCIN by 21Z, and rapidly increasing to -100 J/kg to the northeast and northwest. A few models trigger afternoon storms about these upper features and surface outflow, but still in the same area. The HRRR and 4km NAM seem to delay the onset of the convection until 00Z or 01Z this evening by which time the upper ridging has pushed eastwards into Manitoba. At any rate, supercell development will be rapid where it occurs and quickly transition into a much larger linear system building north and east. All this warrants a moderate to high risk of severe thunderstorms with potential for tornadoes, significatn large hail, winds and rain in an area bounded by Coronach, Broadview and Pilot Mound. A slight risk is warranted northeast to the Regina area and north to Yorkton, where likely more elevated storms could still pose a wind, hail and rain threat into the evening. Severe thunderstorm watches may need to be expanded for these regions later today if such threats become slightly more probable.

[…]

By midday Monday, it was clear that most of the factors that would work against severe thunderstorm development did not materialize, and a significant threat for tornado-producing supercells existed in Southeastern Saskatchewan and Southwestern Manitoba.

Touchdown: Tilston, Manitoba

The primary supercell that became the main focus for Monday evening initiated near Carnduff, SK at around 7:00PM CDT. It was relatively weak initially, wrestling with a couple other storm cells trying to initiate nearby as it moved onto the Manitoba side. As it passed by Gainsborogh and Pierson, the cells merged and organized and rapidly intensified. The first funnel cloud developed near Edward, MB:

The funnel formed and dissipated a few times as the low-level inflow organized, and then reformed and developed into a significant tornado near Tilston:

The tornado continued to grow in size & intensity as it continued northeastwards. As the supercell continued to organize itself, extremely impressive velocities began to show up on doppler RADAR:

Despite the height of the RADAR beam in the storm at this time, this velocity couplet1 clearly indicated a major event underway. As it progressed to the northeast, the storm developed into a large, multi-vortex tornado:

Terrifying Intensity

There’s no question that this tornado was one of the more powerful tornadoes in Canadian history.2 While the tornado was on the ground near Tilston, it grew into a massive circulation that had multiple tornadoes embedded within it:

Shortly after this stage, the tornado became rain-wrapped.3 Tracking various reports alongside the velocity couplet visible on doppler RADAR, the tornado-producing portion of the storm appears to have tracked from just southeast of Tilston north-northeastwards to between Reston and Cromer. It then took a slight jog eastwards and tracked along a line from between Pipestone and Scarth to east of Virden to just south-southeast of Crandall. After this point, the circulation weakens and the storm loses it’s organized tornado-producing capacity.

[map autofit=”1″ disable_scrollwheel=”1″] [pin tooltip=”Carnduff, Saskatchewan” description=”The storms that eventually developed into the massive tornado-producing supercell initiated just east of Carnduff, SK.”]Carnduff, Saskatchewan[/pin] [pin tooltip=”Tilston, MB” description=”Just outside of Tilston, MB was where a massive multi-vortex tornado that shocked storm chasers touched down on Monday evening.”]Tilston, Manitoba[/pin] [pin tooltip=”Reston, MB” description=”The large, multi-vortex tornado passed just west of Reston, MB.”]Reston, Manitoba[/pin] [pin tooltip=”Cromer, MB” description=”The large, multi-vortex tornado was seen southeast of Cromer, MB.”]Cromer, Manitoba[/pin] [pin tooltip=”Pipestone, MB”]Pipestone, Manitoba[/pin] [pin tooltip=”Scarth, MB”]Scarth, Manitoba[/pin] [pin tooltip=”Virden, MB”]Virden, Manitoba[/pin] [pin tooltip=”Crandall, MB”]Crandall, Manitoba[/pin] [/map]

The thunderstorm’s circulation was not completely consistent throughout, though, and showed signs of cycling. This means that instead of one tornado being on the ground for a long time, it likely produced several tornadoes and had periods with no tornado present. There is no question, though, that when it was on the ground, it was a historic event. A simple sign of how intense it was? Reed Timmer, someone who has made a living off planting himself in the middle of some of the largest tornadoes the United States has seen, showed noticeable anxiety as his team got close to this tornado. As a caution, this video contains strong language.

Clean-Up Underway

Little in the way of damage was observed on Monday night; with the tornado occurring so late in the day. There were a few early indicators that this tornado was very strong, including asphalt ripped right off a bridge:

#MBstorm pic from this evening N of Pierson, MB Tornado was so violent it ripped the asphalt from the hiway @680CJOB pic.twitter.com/RhqgzBYzBw

— Greg Johnson (@TornadoGreg) July 28, 2015

Come Tuesday morning, though, more signs began to roll in of the damage:

For now, the clean-up continues for the few people that were affected by it and the rest of us can be thankful that, somehow, this long-tracked tornadic thunderstorm managed to miss almost all population centers in the region.

Environment Canada Rates Tilston Tornado High-End EF2

At 5:38PM CDT on Wednesday, July 29th 2015, Environment Canada issued a preliminary rating of a high-end EF2 to the Tilston tornado:

AWCN11 CWWG 292238
WEATHER SUMMARY FOR SOUTHERN MANITOBA ISSUED BY ENVIRONMENT CANADA AT 5:38 P.M. CDT WEDNESDAY 29 JULY 2015.

ON THE EVENING OF JULY 27, 2015, A TORNADIC SUPERCELL DEVELOPED NEAR PIERSON TRACKING NORTHEASTWARDS TO VIRDEN BY 11:00 PM. THE STORM EXHIBITING STRONG ROTATION ON RADAR FOR OVER TWO HOURS AND PRODUCED A LARGE VIOLENT TORNADO NEAR TILSTON, MANITOBA.

THE FOLLOWING IS A PRELIMINARY SUMMARY OF THE TILSTON TORNADO. FURTHER INFORMATION WILL FOLLOW FROM THE ON-SITE ASSESSMENT BY ENVIRONMENT CANADA METEOROLOGISTS.

PRELIMINARY STORM ASSESSMENT:
EVENT TYPE: TORNADO
START TIME: JULY 27, 2015 8:30 PM CDT
PRELIMINARY RATING: HIGH END EF2
LOCATION: NEAR ROUTE 254 BETWEEN PIERSON AND TILSTON

ENVIRONMENT CANADA METEOROLOGISTS CONTINUE TO SURVEY THE AREA AND ARE ACTIVELY SEEKING PICTURES OF THE TORNADO OR DAMAGE IT MAY HAVE CAUSED. SHOULD YOU HAVE ANY INFORMATION REGARDING THIS EVENT, OR TO REPORT SEVERE WEATHER AT ANY TIME, PLEASE CALL 1-800-239-0484 OR SEND AN EMAIL TO STORM(AT)EC.GC.CA.

PLEASE NOTE THAT THIS SUMMARY MAY CONTAIN PRELIMINARY OR UNOFFICIAL INFORMATION AND DOES NOT CONSTITUTE A COMPLETE OR FINAL REPORT.

END

This may be changed in the future as more information becomes available to Environment Canada. As we said earlier, with the EF scale determined by damage, and with the Tilston tornado moving over such a sparse area, there may be insufficient evidence to rank how strong it actually was.

Chase Diary #1: Matthieu Desorcy

Part of the A Weather Moment team went out to storm chase on Monday. Here’s how the day unfolded in his own words.

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Storm Chase Log – July 27, 2015
by Matthieu Desorcy

Well, it was quite the chase day in southeast Saskatchewan and southwestern Manitoba. It all started off leading into the weekend when models began showing some tornado potential and there was fairly good consistency between runs in the region of concern. A few of us, including another AWM member – Scott Kehler, decided to take the day off make the drive out west and go chasing.

On the morning of July 27th, our group left Winnipeg around 10:30AM make our way to southeast Saskatchewan, where we expected storms to initiate. The setup looked promising from the get-go early in the morning as most of the MCS activity stayed well north of our target area and little cloud prohibited surface heating from happening, allowing the warm front to lift into southeast Saskatchewan. Not knowing exactly which town we were targeting, we headed down along highway 2 all the way to Souris to start off where we had our usual chase day lunch – Subway. Knowing that the cap would hold strong until early evening, we weren’t in a huge rush to get to southeast Saskatchewan. We made a quick analysis which revealed by early afternoon the warm front was already across the border and was visible as a fine line on Foxwarren radar as well as Glasgow radar, stretching from southwest to northeast. We then decided to head to Redvers, SK, which was located just south of the warm front.

Inflow winds were gusting into the storm at a speed of an estimated 70-80km/h – the strongest inflow I’ve ever felt.

When we arrived in Redvers it was obvious the warm front was nearby – the wind came from the east southeast and the air felt “chillier” than just to the south in Carievale, SK. After observing several attempts of towering cumulus trying to break the cap, then moving north of the warm front and dying off near Redvers the setup suddenly wasn’t looking too promising. The cap was holding strong. Resisting the urge to relocate to another area further southwest we held strong in Redvers while getting a nice suntan and living from radar scan to radar scan.
It suddenly started to get exciting in the mid-evening. A few radar echoes began to show up south of Carnduff, SK near the international border which gave us hope the disturbed area would be able to get its act together and break the cap. In addition to the new echoes on radar, we observed some high level cirrus which implied that ascent was beginning to take place, associated with a shortwave we had identified earlier, moving into the region. This would give storms the extra push they needed to break the cap.

Echoes steadily strengthened from scan to scan and finally it was obvious that the storm had broken the cap – it had tops exceeding 50,000 feet. We watched the storm as it grew and slowly moved northwards, organizing itself and finally producing the first wall cloud of the day – the chase was on. Around 7:30PM the wall cloud really beefed up in the same area – a bowl formed and one side of the wall cloud was rising rapidly into the updraft. After watching it for a good 20 minutes the storm had appeared like it gusted itself out, the wall cloud disappeared and the base was higher. Taking advantage of the opportunity and knowing that nothing was imminent, we relocated to a location just north of Gainsborough, SK. Here we watched it for another 20 minutes as the supercell cycled wall clouds and produced what appeared to be a funnel.

As the storm slowly moved north northeast it moved away from us – we once again repositioned, this time on the Manitoba side, to the southeast of the storm not too far from Pierson, MB. This is when we noticed that things were really getting serious and potentially dangerous for surrounding communities. Minot radar showed that the storm had really tightened up its rotation. Once we were able to stop and take a close look at the storm. We noticed that a tornado was already in progress – and a large one that that, around 8:30pm northwest of Melita. We watched it for a good 20 to 30 minutes in the middle of nowhere in southwestern Manitoba northwest of Melita, snapping pictures amazed at what we were seeing a wedge tornado, here in the province of Manitoba. Inflow winds were gusting into the storm at a speed of an estimated 70-80km/h – the strongest inflow I’ve ever felt. It was obvious that the storm was moving away from us so we decided to move once again, this time to the north of Melita. By this time (9:20pm) we could not see the tornado anymore but the tornado could have been rain-wrapped. We cautiously watched the storm from the comfort of our car as cloud to ground lightning intensified and positive strikes coming out of the anvil peppered the area just to our north. By this time it was getting dark and the circulation would have been rain-wrapped – we watched the storm for a while longer and decided to call it a day as it was getting too dangerous to chase in the dark.

This was one of the stronger storms any of us had ever seen in person and will be a chase to remember for a long time to come!

Chase Diary #2: Justin Hobson

In addition to Matt, friend of the site Justin Hobson was also out chasing that day. Here’s what he experienced:

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Tilston, MB Chase Log
by Justin Hobson

We started the day in Oxbow, Saskatchewan where we dealt with tropical conditions as temperatures shot up near 30 degrees with dew points in the low 20’s. A deepening low pressure system was located to the southwest of us over northeast Montana and winds increased out of the east throughout the afternoon. A warm front was anchored from the southwest to the northeast across extreme southeast Saskatchewan and storms started to fire around 6 pm where they then rapidly intensified. There was some cell interaction going on at first due to the forcing that came in but one supercell was able to survive and latch on to the warm front around 8 pm (Manitoba time) where it became tornadic over the next 2-3 hours.

We caught a glimpse of a brief rope tornado while driving around 8:25 pm. We kept heading north where we then witnessed a violent stovepipe tornado at about 8:30 pm. This stovepipe then transitioned into a multi-vortex tornado which then got wrapped in rain where it appeared to have dissipated. We then repositioned ourselves for the new occlusion just to the northeast of the old circulation. Shortly after, we saw another tornado develop which started off as a large cone. This cone tornado quickly wedged out and grew to about 1/2 mile in size. This wedge lasted for a few minutes before becoming rain-wrapped as the sun set. It then continued to move to the northeast very slowly for another hour or so towards the Virden/Oak Lake area. What a day!