Thunderstorm Chances Continue into Weekend

The generally unsettled weather that has been in place over Southern Manitoba over the past week will continue for at least a couple more days as a large upper-level low pushes across the Prairies. For those tired of sweating the heat, though, a reprieve is on the way as drier air finally works its way into the Red River Valley.

Today will be a fairly pleasant day with a fairly sunny start and temperatures poised to climb up to around 26 or 27°C. As the temperatures warm up this afternoon, shower and thunderstorm activity will likely develop as a cold front pushes eastwards into the Red River Valley. Most areas through the valley will have a chance at seeing some shower or thunderstorm activity, however the chance will increase the closer you find yourself to the U.S. border. Any convection that does develop will push out the eastern RRV early in the evening, ushering in some cooler, notably drier air into the region behind the cold front. Friday night will mark the end of a very humid streak that has seen dew points in the upper teens or low 20’s for over a week. Expect a refreshingly cool overnight low of around 12°C.

GDPS Forecasted 500mb Heights & Vorticity for Mid-day Saturday
Saturday will bring a risk of thunderstorms and funnel clouds to the Red River Valley as a vorticity-rich upper-level low moves over the region.

Saturday will bring yet another chance for showers and thunderstorms as the main upper-level low moves over Southern Manitoba. Temperatures will climb to around 22°C, which may be enough to pose a marginal risk for some severe thunderstorms in the afternoon; the biggest impact to be aware of, however, is the potential for cold core funnel clouds. These funnel clouds are developed through entirely different mechanisms than their stronger, supercell thunderstorm counterparts, and are substantially weaker and rarely result in a touchdown. If a cold core funnel cloud does reach the ground, it’s considered a landspout tornado. These tornadoes are short lived and usually very weak. While they do not pose the level of threat that a supercell thunderstorm tornado would pose, strong winds of any type can be dangerous. The shower and thunderstorm activity will push out of the region in the evening and we’ll see clearing skies as the low heads back down to around 12°C.

Sunday’s forecast will be short. Beautiful weather is on tap with highs in the mid–20’s, comfortable humidity, light westerly winds and mostly sunny skies. No complaints! Expect a low on Sunday night in the mid-teens. Finally some good weather news for a change!

Unsettled Weather Continues To Week’s End

The unsettled weather that’s been in place for the last few days will continue through the end of the week as the general instability remains in place as more weather systems push through the region.

Today will be a fairly pleasant day that’s quite similar to yesterday other than a couple degrees warmer. Temperatures should climb to around 28°C this afternoon with more clouds popping up and developing into an afternoon/evening chance of some scattered thunderstorms. Thunderstorm activity should be fairly limited as the main story will be the low pressure system that will move into Southern Manitoba overnight and will set the stage for Thursday’s weather. Expect temperatures to dip to just 19°C as increasing cloud and the humidity continue to keep temperatures warm.

Thursday Brings Heavy Rain Potential

Tomorrow may actually end up being quite a wet day as multiple models are showing a low pressure system moving through tapping into the ample moisture in the region to produce a fairly large, and intense, area of rain.

NAM Precipitable Water Forecast for 15Z Thursday July 16, 2015
NAM Precipitable Water Forecast for 15Z Thursday July 16, 2015

The NAM (shown above) is forecasting precipitable water values as high as 1.8 to 1.9” (in the 45–50mm range) which is very high for our region. Precipitable water correlates strongly to potential rainfall amounts of storm systems[1], so it’s not unrealistic to say that rainfall totals of 1.5” to 2” (~ 35 – 50mm) are not out of the question albeit on the high end. If heavy convection were to set up, then even higher amounts would be possible. We’ll keep an eye on things and see how the storm progresses this evening; if the outlook changes much, we’ll provide an update at the bottom of this post.

So, the general forecast for Thursday would see the rain moving in overnight on Wednesday night and see the rain persisting at least until mid-day, but possibly into the late afternoon or early evening. Rainfall totals may end up being variable, but in general will likely be 20–40mm with the potential of seeing double that if system becomes vigorous enough. The high temperature will be in the 21 to 23°C range with an overnight low around 16°C. Winds will be fairly light much of the day.

Friday

The thunderstorm risk returns Friday, although it will be a fairly mild risk with just some isolated to scattered non-severe storms possible. The daytime high will sit near 26 or 27°C with light winds out of the southwest. The humidity continues to remain fairly high with dew points in the high teens. Skies will clear out Friday night as we head to a low near 15°C.


  1. Thunderstorm rainfall totals can greatly exceed PWAT values due to the convergence of the inflow wind that can “accumulate” PWAT from areas around the storm.  ↩

Heat Wave Hits Southern Manitoba

The first significant heat wave of the year starts today for Southern Manitoba as temperatures soar to the low 30’s and the humidity really starts to build into the region.

As mentioned above, today marks the start of a significant heat wave for Southern Manitoba that will see temperatures in the upper 20’s to lower 30’s alongside very humid conditions with dew point values climbing into the low 20’s. The humid conditions will result in humidex values[1] climbing into the upper 30’s or low 40’s:

Day Max Temp Humidex
Friday 31°C 39
Saturday 31°C 39
Sunday 30°C 40

These hot and humid conditions are the result of a significant upper-level ridge building into the Prairies will bring a prolonged south to southeasterly flow over Southern Manitoba as it spreads hot weather into the region. Significant humidity will build into the region as the southerly winds tap into Gulf of Mexico moisture, combining with local evapotranspiration to produce deep moisture with dew points near 20°C.

Median dew point values & associated spred from the Global Ensemble Foreacast System for Winnipeg, MB
Median dew point values & associated spred from the Global Ensemble Foreacast System for Winnipeg, MB

The big story here will be the duration the hot and humid weather is here to stay. The broad pattern will remain unchanged at least through the weekend. While the ECMWF & GDPS models have more progressive solutions that would result in the humidity being flushed out early next week, the NAM & GFS – which are generally slightly better at dealing with nocturnal convection – produce a blocked pattern that would keep the moisture in place at least until the middle of next week. Some of this uncertainty is reflected in the GEPS graph above with greater uncertainty (spread) showing up after Sunday. For the next few days, though, the heat will be on.

Severe Weather Potential

Alongside the heat and humidity, a severe weather threat will develop over the Red River Valley for both Saturday and Sunday. Unfortunately, it’s still too early to produce any sort of concrete outlook. As the main upper low pushes into Saskatchewan this weekend, multiple shortwaves will track over Southern Manitoba, providing instability for thunderstorm development. There are numerous question marks in place regarding the thunderstorm potential:

  • It’s still too early to try to pin down where exactly the shortwaves may track; southwestern Manitoba will likely see the greatest potential for thunderstorms, but that threat could easily shift eastwards into the Red River Valley or even southeastern Saskatchewan.
  • Instability will be quite high, primarily driven by the high dew point values. SBCAPE values will easily exceed 3,000 J/kg – which is very high – but the actual thermodynamics are marginal; 700mb temperatures are forecast to be near 9 or 10°C, which is quite warm in the mid-levels and makes it more challenging for thunderstorms to get going. If mixed-layer dew points are used instead of just surface-based dew points, the CAPE values drop fairly dramatically.
  • With a large portion of the instability provided through the excessive latent heat potential thanks to the high dew points while environmental lapse rates remain somewhat marginal, a trigger will be very important. At this point, any sorts of triggers remain fairly fuzzy. There are numerous small features that could produce thunderstorms, the big question of whether or not they’ll be strong enough to break the cap and get things going is crucial. At this point, the answer is very unclear.
  • If thunderstorms do develop this weekend, they’ll be struggling against very weak bulk shear values of only around 15–20kt. This would likely result in slow-moving thunderstorms whose main threat would be large hail and very heavy rain.[2]

At this point, it’s best to say that thunderstorms are possible throughout the weekend, and to keep track of updated forecasts. We’ll be keeping an eye on things and likely produce a convective outlook Friday evening for Saturday.

Update: Slight Risk of Severe Thunderstorms Saturday

There will be a slight risk of severe thunderstorms across southwestern Manitoba this afternoon, spreading eastwards this evening and through the overnight period into the Red River Valley.

AWM Day 1 Convective Outlook for Saturday July 11, 2015
A slight risk of severe thunderstorms exists today across much of southwestern Manitoba extending into the Southern Interlake and Red River Valley.

A very hot and humid air mass in place over the region will bring the threat of severe thunderstorms today. Scattered thunderstorms will develop through western portions of the slight risk area today and will have the potential to become severe due to the copious amounts of energy available to them. Their overall strength will be limited by relatively weak wind shear, however these thunderstorms may still become quite potent with the capability of producing large hail, torrential downpours and very strong wind gusts.

The hail threat is quite significant today; with MLCAPE values expected to be in the 2000-2500 J/kg range, there will be more than enough energy available to support strong updrafts capable of large hail. The growth may be limited by the lower shear values, but significant hail is definitely possible today.

Heavy rain will be one of the greatest threats today thanks to the humid air in place; PWAT values have climbed to around 35-40mm which, when combined with expected storm speeds of only around 10-25km/h, will result in rainfall accumulation potentials of upwards of 75-100mm.

Wind is the final significant threat today and comes from two possible places. The first is during storm collapse; the low shear values may make it difficult for storms to persist for a long time and we may see some storms cut off their own energy source and collapse; if this were to happen, strong winds would be possible as outflows from those storms. The other possibility comes from the significant energy available to the storms; if a north-south line of storms manages to develop, there’s a chance that they may develop bowing segments or into a bow echo; if that were the case then we would see a wider swath of significant winds. The former possibility is more likely during afternoon and early evening while the latter case would be more likely as we head later into the evening.

The threat for severe thunderstorms across Southwestern Manitoba is already underway; it will gradually spread eastwards through the day and into the Red River Valley through the evening.

A slight risk of severe thunderstorms will likely exist across Southern Manitoba tomorrow as well, but shifted further east. We’ll have an outlook for tomorrow available later tonight.

Update: Hot & Humid Sunday Brings Severe Thunderstorm Risk

A heat warning is in place for a Sunday that will see the heat and humidity cranked up over the Red River Valley bringing humidex values in the low 40’s. All that heat and humidity will bring a slight risk for severe thunderstorms in the Red River Valley alongside portions of Saskatchewan and much of Southern Manitoba.

AWM Day 1 Convective Outlook for Sunday July 12, 2015
A slight risk of severe thunderstorms exists today across much of southwestern Manitoba extending into the Southern Interlake and Red River Valley.

In Manitoba, huge CAPE values are expected to develop today with SBCAPE values exceeding 4500 J/kg and MLCAPE values exceeding 3500 J/kg. Bulk shear will be on the lower side at around 20kt and is likely the major limiting factor in today’s storm potential. After a bit of elevated convection clears out this morning, a capping inversion of around 25-50 J/kg should hold storms back until later in the afternoon. With weak shear, the tornado threat should be minimal today, although if any storm realizing 3000+ J/kg of CAPE manages to get organized, I’d go with “all bets are off.” Storm development has the chance to be truly explosive today, so be sure to stay alert for any watches and warnings issued by Environment Canada as the day progresses.

The main threats will be large hail and torrential downpours. Storms will likely be moving faster than they did on Saturday, but will likely also be more intense. Strong winds will be a threat with any sufficiently organized thunderstorm.

The thunderstorm threat will persist into the evening and overnight, spreading eastwards into SE Manitoba & Lake of the Woods.


  1. The Humidex is a calculated value that attempts to create a “feels like” temperature that takes into account the reduced evaporation occurring on the skin surface that results in a diminished ability for the body to shed heat.  ↩
  2. With dew point values near 20°C or even creeping above that value, perceptible water values are expected to climb towards 45mm, it’s not impossible that a slow-moving storm would be able to produce 3–4” of rainfall (roughly 75–100mm) in a relatively short time.  ↩

A Smoky Transition Back to Summer Heat

Today will be a bit of a mixed bag here in Winnipeg as a dying cold front completes its slump into Southern Manitoba bringing a chance of showers alongside another batch of forest fire smoke gradually working into the Red River Valley. Fortunately, winds will become more southwesterly as the week progresses, pushing the smoke to our north and bringing back hot summer weather to the area.

Today will start off with a slight chance of showers or thunderstorms until around midday after which the clouds will start pushing off to our southeast and see increasingly smoky skies build in. Temperatures will only climb to around 21°C today and then dip down to the low teens tonight.

NOAA 1hr. Vertical Smoke Forecast valid July 08, 2015 7PM EDT
The “total smoke” product from NOAA is forecasting a thick layer of smoke across Southern Manitoba by this evening.

Thursday and Friday will herald the return of summer. Winds will shift more westerly to southerly and begin tapping into warmer air to our south. There should be a deep enough layer of southerly winds to push the smoke north of Winnipeg through the day. Temperatures will be soaring into the upper 20’s on both days under mainly sunny skies.

The biggest difference between Thursday and Friday will be the humidity; while dew points in the mid-teens on Thursday will make it feel slightly humid, it’s expected that dew points will climb up to the 20°C – or even a bit higher – for Friday, making it feel downright tropical out there.

Overnight lows on Thursday & Friday will be in the upper teens.

Unsettled Weekend?

Weather models are pointing towards a potentially unsettled weekend as a large upper-level low lifts northwards into Alberta with a deep trough extending southeastwards across the Prairies.

GDPS 24hr. QPF valid 00Z July 13, 2015
Total precipitation forecast from Saturday evening to Sunday evening from the GPDS.

It’s not worth getting caught up in details at this point when dealing with a large, highly convective system such as this. The main thing to take away is that with a large low lifting into Alberta, it’s likely our weekend will continue humid, warm and potentially bring some wet weather.