Stormy Start to An Unsettled Weekend

Friday will bring a severe thunderstorm threat to Winnipeg & the Red River Valley as the first of several bouts of unsettled weather moves through, gradually flushing out the unseasonal heat and humidity and transitioning us towards almost fall-like weather.

Friday
29°C / 18°C
Mainly cloudy; showers and thunderstorms this afternoon

Saturday
24°C / 17°C
Mainly cloudy; slight chance of showers. Evening showers or thunderstorms possible

Sunday
24°C / 11°C
Mainly cloudy; slight chance of drizzle or light rain

Mainly cloudy skies will be over Winnipeg & the Red River Valley today while a low pressure system gather steam and begins to move into the region from North Dakota. Little in the way of weather is expected until the low approaches this afternoon and triggers a substantial amount of convective activity. Showers and thunderstorms are expected to fire up mid-afternoon and track to the northeast. There’s a decent chance that the stormy weather will persist well into the evening before it clears out and moves off to the northeast.

AWM Day 1 Convective Outlook
A slight risk of severe thunderstorms exists over south-central and southeastern Manitoba today.

A slight risk for severe thunderstorms will be in place over much of south-central and southeastern Manitoba today. Looking at the MIST basics for thunderstorms:

  • Moisture will be ample with deep boundary layer moisture evident through surface dewpoint values near 22°C thanks to a multi-day build-up of humidity in a southeasterly flow. Strong evidence of the substantial moisture in place is shown by the precipitable water values which are forecast to climb over 50mm through the afternoon.
  • Instability will moderate with MLCAPE values around 2000–2500 J/kg. Much of the instability will be brought through the substantial moisture present since mid-level lapse rates are only expected to be moderate and not excessively steep.
  • Shear will be ample with approximately 25–35kt of 0–6km bulk shear and broadly looping hodographs. This should support the development of organized thunderstorms.
  • Trigger: there will be several triggers converging on the Red River Valley this afternoon including a frontal boundary, approaching low, as well as shortwave aloft. There should be no troubles finding something to trigger thunderstorms.

The primary threats from today’s storms will be heavy rain and large hail. PWAT[1] values are expected to climb over 50mm, which means torrential downpours and flash flooding will be a notable threat with this afternoon’s thunderstorms. Additionally, with such high MLCAPEs and adequate storm structure through the shear profiles, large hail may be an issues, however that would likely be a more isolated threat than the rainfall.

The thunderstorms will gradually push off to the northeast. Total storm rainfalls may exceed 2–4” (50–100mm) in some places, while other areas may end up seeing very little. Be sure to stay alert for any watches or warnings issued by Environment Canada.

Less excitingly, daytime highs will sit around 29°C today (which will feel more like the upper 30’s thanks to the humidity) with winds out of the southeast at 20–30km/h. Expect a low near 18°C.

Cooler, Unsettled Weekend

Both Saturday and Sunday will be cooler days with highs in the mid–20’s. Some uncertainty exists on precisely how things will play out, but in general, a very powerful low pressure system is expected to develop and lift northwards through Saskatchewan through the weekend.

The biggest change, compared to Friday, will be that it seems like the timing for any precipitation will shift from daytime hours towards the evening and overnight hours. Saturday will be mainly cloudy with just a very slight chance of some showers through the daytime hours. As the evening approaches, though, it looks likely that thunderstorms will develop along the western escarpment of the Red River Valley and then progress eastwards across the valley overnight. At this point it looks like the storms will be non-severe, but we’ll keep an eye on them in case the threat increases as we get closer to the event.

The overnight low on Saturday night will be around 17 or 18°C.

Sunday will be another fairly cloudy day with a slight chance of some very light rain or drizzle as easterly winds keep the boundary layer socked in moisture while drier mid-level lift approaches. As the evening approaches, once again some showers or thunderstorms are possible. Expect a low in the low teens on Sunday night.

Next Week’s Weather?

At this point, next week looks much cooler and drier with more fall-like weather in place. Daytime highs through the first half of the week look to be in the 15–20°C range while overnight lows will drop towards the mid-single digits. Little precipitation is expected, and we’ll see more and more sun as the week progresses.


  1. PWAT stands for precipitable water, a measure of how much water is contained in a column of air.  ↩

Sweltering Heat Moves Into Manitoba

The coming days will be marked by a rare late-summer heat wave that will see hot and humid conditions moving into the Red River Valley, sending humidex values soaring towards the low 40’s. The hot weather will be relatively short-lived, though, as a cold front moving into the region on Friday will send temperatures back towards seasonal values alongside a returning chance for showers or thunderstorms.

Today and tomorrow will both be swelteringly hot days with daytime high temperatures in the low 30’s throughout Winnipeg & the Red River Valley. Humidity will increase dramatically this morning as temperatures begin to warm and the boundary layer deepens and taps into a significantly more humid air mass that’s moved in aloft overnight. By mid-to-late morning, dewpoint values will have climbed into the high teens or low 20’s across the Red River Valley. Combining the high dewpoints with the hot daytime high temperatures, humidex values in the upper 30’s to low 40’s are expected. Environment Canada has issued a heat warning for Winnipeg as well as many other areas across Southern Manitoba including the southwestern corner, Red River Valley, and southeastern Manitoba. Thursday will be slightly warmer and more humid than today is as both the temperature and dewpoint are expected to climb another degree or two higher.

RDPS Forecast Humidex valid the afternoon of Thursday September 3, 2015
The RDPS is forecasting a large swath of humidex values over 40°C (orange) in Manitoba on Thursday afternoon.

With such substantial humidity in place, overnight lows both tonight and tomorrow night will be quite warm, bottoming out near the 20°C mark. Additionally, expect just slightly breezy winds out of the south.

Friday will bring increasing cloud through the day as a cold front approaches from the west. Temperatures should manage to climb into the upper 20’s before the cloud cover cuts off our daytime heating. At this point, it looks quite likely that a large are of showers and thunderstorms will work their way across the Red River Valley through the afternoon. Severe thunderstorm potential looks minimal at this time, but we’ll keep an eye out and issue and outlook if it seems like it might pose more of a threat. Temperatures will cool down into the mid-teens on Friday night with clearing skies.

Summer Isn’t Over Yet!

We’ll see more summer-like weather this week in Winnipeg and across the Red River Valley as it continues to feel more like July than the beginning of September.

A shortwave trough (white dashed line) will pass through southern Manitoba on Monday, bringing clouds and a risk of thunderstorms.
A shortwave trough (white dashed line) will pass through southern Manitoba on Monday, bringing clouds and a risk of thunderstorms.

Monday

Today will start out mainly cloudy as a potent upper disturbance moves through southern Manitoba. This disturbance may trigger some thunderstorms early on Monday morning, some of which could impact Winnipeg and the Red River Valley. Once the disturbance moves through, skies should clear, likely at some point in the afternoon. Temperatures will be in the mid to upper twenties with the wind becoming westerly due to the passage of this system.

Tuesday

Tuesday will be a nice day, as temperatures climb into the upper twenties under mainly sunny skies. Winds will be relatively light and from the south. No precipitation is expected on Tuesday, although some storms may develop overnight into Wednesday.

Wednesday

Like Monday, we may see some morning thunderstorms on Wednesday. A strong push of moisture from the south may help to trigger these storms. Should storms develop, they could be severe, but this forecast is still a few days away, so it could change. Besides the risk of storms, Wednesday will be hot and humid, with temperatures near 30C and breezy south winds.

Long Range

The long range forecast shows us cooling down after this week. Once a strong system passes through on Thursday temperatures will drop down to more seasonal values near 20C. Enjoy this little blast of heat while it lasts, it could be one of our last warm spells this year!

Unsettled Friday Leads Into A Hot Weekend

Notably warm and humid weather in place over Southern Manitoba today will bring a threat for severe thunderstorm activity across the region as a shortwave trundles through. An upper-level ridge will then begin to rebound back into the Southern Prairies in response to a digging trough off the western coast of North America, bringing even warmer air into the region thanks to the resultant southwesterly flow aloft that will develop.

Friday: Thunderstorms Possible Midday

A weak mid-level shortwave trundling across the region will bring a risk of thunderstorms to Winnipeg & the Red River Valley today[1] as warm and humid weather meets the colder air associated with the shortwave. As always, using our MIST principles:

  • Moisture: Adequate. Moderate moisture will be in place both at the surface and through the lower levels of the atmosphere as deep-layer moisture continues to build into the region.
  • Instability: Favourable. With both thunderstorm possibilities, both elevated and surface-based, enough instability will be in place. Surface-based convection would have greater potential to be severe, however it will rely on strong surface heating with mainly sunny skies until the shortwave arrives, which could be in question depending on what sort of nocturnal convection develops.[2] If it does end up cloudy, strong mid-level lapse rates associated with the shortwave should be enough to sustain showers or thunderstorms as it heads eastwards. All said, storms should have around 1000–2000J/kg of CAPE to work with, depending on their exact timing and where they’re based.
  • Shear looks quite good with around 35kt of 0–6km bulk shear and gently looping hodographs. Low-level winds are weak enough that tornado activity doesn’t look like a particular threat, but the directional shear is favourable for the organization of supercell thunderstorms.
  • Trigger: The shortwave moving through coupled with a weak surface trough will provide the convergence and trigger for convective initiation.
AWM Day 1 Convective Outlook
A slight risk of severe thunderstorms exists in the Red River Valley and southeastern Manitoba today.

All things considered, it’s likely that we’ll see some activity at some point today. The primary threats with today’s thunderstorms will be large hail and strong winds. Storm motions will generally be towards the southeast. Later in the day, a more organized heavy rainfall threat may develop with any storms still ongoing.

Those things aside, it will be quite a nice day. Temperatures will climb to around 28 or 29°C with a few clouds through the day. The humidity will begin to be more noticeable as dewpoints climb into the high teens by the afternoon ahead of the weak surface trough/low moving through. Winds will remain light.

Things clear out tonight with the humidity remaining in place and temperatures dipping down to about 17°C.

Hot & Humid Weekend Ahead

Temperatures will soar this weekend as warmer air moves into the region and sends daytime highs to 30°C or higher. Saturday will be a sunny day with a high around 30°C and dewpoint values in the high teens or low twenties making it feel more like the upper 30’s. Winds will be light. Expect a low near the 20°C mark on Saturday night.

Manitoba Health – Heat Alert Response System in Relation to the Heat Dial
This weekend could see provincial (pictured above) or federal responses to the heat and humidity over the region.

Sunday will see the daytime high climb even higher, most likely towards the 32 or 33°C mark as breezy southerly winds develop ahead of a developing low pressure trough. The humidity will remain in place with dewpoints in the high teens or low 20’s, making it feel more like the upper 30’s. There’s a slight chance we may see humidex values hit 40 on Sunday, which is Environment Canada’s criteria for a heat warning. That said, both Saturday and Sunday will likely qualify for the Province of Manitoba’s Heat Advisory and Response Pre-Alert which requires a humidex of 37 or higher.

There appears to be a slight chance late Sunday night or early Monday morning for some showers or thunderstorms to move through the area, but it looks very conditional at this point and it’s far to early to say much about it. We’ll put an update here early Sunday afternoon regarding the precipitation potential for Sunday night. Temperatures will likely only drop to around 19°C on Sunday night, however.

Long Term: Slight Cool-Down to Start Next Week

Looking ahead, it seems like we’ll see a slight cool-down next week as a frontal boundary stalls out over the region and brings a chance for multiple bouts of unsettled weather through the first half of the week. We’ll have more details about that bright and early Monday morning!


  1. Exactly when depends on the timing which will be affected by how much and what type of elevated convection developed with this feature overnight in Saskatchewan.  ↩
  2. If the nocturnal convection is in the right spot or strong enough, our sunshine could be replaced by cloud cover instead, dramatically limiting surface heating.  ↩