Hot & Windy Saturday Brings Thunderstorm Threat

Today will be a scorcher over southern Manitoba as strong southerly wind pump hot and increasingly humid air northwards. With that heat, though, will come a fair amount of instability and one of the first widespread severe thunderstorm threats of the season.

To start the day, some elevated convection will move across the region, most likely through the Interlake. These thunderstorms may be strong to severe, but should remain north of the Red River Valley. Although the storms should pass north of Winnipeg, the city will still see some passing cloud as they move by. These storms and their cloud cover should clear out by midday. As the sun comes out, the wind will pick up. A tight pressure gradient over the region1 will produce southerly winds up to 50 gusting 70 km/h today, helping heat and moisture surge northwards out of the United States. With those strong winds in place, Winnipeg will see temperatures climb to a high near 33°C. It will also get a tad more humid as the day goes on as dew points rise into the mid-teens.

Strong southelry winds will be in place over southern Mantioba this afternoon. To the west, a trough of low pressure extending southwards into the United States may trigger a line of thunderstorms late in the day.
Strong southelry winds will be in place over southern Mantioba this afternoon. To the west, a trough of low pressure extending southwards into the United States may trigger a line of thunderstorms late in the day.

By late in the afternoon, the thunderstorm threat will begin to build. A trough of low pressure over southwestern Manitoba into the Interlake may trigger the development of thunderstorms. If they develop, they’ll likely be strong to severe with primary threats of large hail and strong winds. Any of these thunderstorms that develop will move eastwards towards the Red River Valley through the evening. The focus then turns southwards to the Dakotas. A shortwave lifting northeastwards through the state will trigger convection late in the day. These thunderstorms will grow upscale and expand in coverage overnight. As the evening progresses, this blossoming area of thunderstorms will gradually spread across southern Manitoba. The potential for strong to severe thunderstorms exists tonight, with large hail and damaging wind gusts the primary threats. The thunderstorms should be through the Red River Valley by Saturday morning.

Temperatures will remain mild on Friday night with a low near 19°C. Those strong southerly winds will ease into the 20 to 30 km/h range this evening and then diminish to light overnight.

If thunderstorms are able to develop along the trough, southern Manitoba may see a line of thunderstorms marching towards the Red River Valley early in the evening.
If thunderstorms are able to develop along the trough, southern Manitoba may see a line of thunderstorms marching towards the Red River Valley early in the evening.

Saturday will bring plenty of cloud to Winnipeg as the low pressure system responsible for all this heat and instability moves through. Temperatures will be much cooler with morning temperatures near 18°C dropping to 14°C through the day. Winds will gradually pick up out of west into the 20 to 30 km/h range. More showers with the risk of thunderstorms are likely to develop later in the morning and persist through much of the afternoon. Conditions will calm down on Saturday night with some clear breaks developing towards Sunday morning. Winnipeg should see a low temperature near 9°C.

Sunday will bring seasonably cool conditions to Winnipeg, but things will get a chance to dry out. The city should see a high near 19°C under partly cloudy skies. Westerly winds near 30 km/h will be in place for much of the afternoon before diminishing in the evening. Winnipeg will see a low near 8°C with just a few clouds on Sunday night.

Long Range Outlook

Next week will bring seasonably cool temperatures with daytime highs around 20°C and overnight lows close to 10°C. The city will see variable cloudiness through the week, and a couple of passing disturbances may bring a chance or two of showers through the week. Temperatures should begin to warm up near the end of the week, but that may bring a return to unsettled conditions.

Today’s seasonal daytime high in Winnipeg is 23°C while the seasonal overnight low is 10°C.

  1. The tight pressure gradient is a result of a 996 mb low over western Manitoba squishing against a ridge of high pressure extending through SE Minnesota into Iowa.

Thunderstorms Expected Across Southern Manitoba Overnight

A developing low pressure system in North Dakota will generate a widespread thunderstorm threat this evening and overnight that extends into southern Manitoba.

In North Dakota, there’s very high confidence that there will be some truly dangerous thunderstorms later today. A confluence of all severe weather parameters – extremely high amounts of energy, favourable wind profiles with high amounts of shear, a clear and potent focus for thunderstorm development – has led the SPC in the United States to issue a “moderate risk” thunderstorm hazard for the state today. Further north, things become a little more unclear and conditional in southern Manitoba.

The overall synoptic setup is summarized succinctly in the PASPC’s daily significant weather discussion:

[…] approaching long wave upper trough creating favourable synoptic setup tonight/overnight for a developing Colorado Low convective complex. This begins as a rapidly deepening baroclinic surface low in the Northern States with associated associated severe surface-based convection beginning this afternoon or evening.

So a rapidly deepening Colorado Low-type system will develop severe thunderstorms over North Dakota this afternoon. Continuing:

[As the convection] evolves and severe elevated convection becomes dominant through the night. Big question is where does [it] go? Details vary widely amongst model solutions, with some bringing severe convection clipping SE Saskatchewan then moving through southern Manitoba, and others confining the bulk of the severe weather to the United States.

And that is the big question. Where will the thunderstorms develop tonight? Here’s our thoughts, omitting much talk about what will happen in North Dakota.

The primary focus for thunderstorm development tonight in southern Manitoba will be split between two features. The first will be a warm front draped west to east along the Canada/US border for much of the night before lifting a bit further north over southeastern Manitoba late overnight. The second will be a mid-level shortwave that moves across southern Manitoba late overnight into Friday morning.

These two features will support independent areas of thunderstorm activity, then merge into a single area as the shortwave moves over the warm front.

There are two primary ways this could play out; we’ll cover the most likely way and then cover Plan B©.

Most Likely Situation

A few isolated showers or non-severe thunderstorms are possible along and north of the warm front late this afternoon across northern North Dakota and far southern Manitoba. A more organized area of thunderstorms will develop over western North Dakota this evening and become the focus for thunderstorm activity for the following 12 hours. The thunderstorms will develop into a QLCS – quasi-linear convective system – mid-evening and accelerate eastwards. The mid-level shortwave moving into the region will draw moisture northwards from the QLCS across the border into southern Manitoba, supporting an area of showers and thunderstorms into southern Manitoba.

This activity will develop over the southwestern corner of the province sometime between 9 and 11 PM, then spread into the Red River Valley overnight.

AWM Thunderstorm Outlook valid 6PM Thursday June 28 to 6AM Friday June 29, 2018

Much of the severe weather in this scenario will be confined to North Dakota. There would be a slight risk of a severe thunderstorm south of the Trans-Canada Highway in Manitoba with primary threats of overland flooding due to heavy rainfall and large hail.

The convection will clear out overnight or early Friday.

Plan B

The alternative solution is one where the QLCS developing in North Dakota ends up surging eastwards, breaking away from the mid-level shortwave and becoming its own entity as it races towards Minnesota. In this case, less thunderstorm activity would be seen around the mid-level shortwave as it pushes into Manitoba overnight, however the organized southerly flow riding over the warm front would be undisturbed, likely allowing elevated thunderstorms to fire up overnight across far southern Manitoba. These thunderstorms, were they to form, would be more scattered in nature, but would pose a marked hail threat alongside torrential rains.

Not As Bad As It Looked Just 12 Hours Ago

In the end, the situation doesn’t look quite as bad over Manitoba as it did earlier this morning. If our expected solution pans out, most areas should just see a decent soaking with 15 to 30 mm of rain. Localized higher amounts will be possible, particularly closer to the US border. If the hail stays away, then we could be getting up tomorrow in the wake of a much-needed rainfall.

That said, if Plan B is what happens, or a hybrid of our expected solution and Plan B, then residents will need to prepare for the potential for large hail and localized flooding. We’ll be keeping an eye on things and likely issue one last brief update on our social media accounts later this evening.

Showers on Friday, Long Range Shifts To A Nicer Outlook

A low pressure system and cold front moving through Manitoba today will bring morning showers or thunderstorms to the Red River Valley and a follow-up chance for rain this afternoon. Once this system clears out, there’s good news ahead as the long-range forecast has shifted to a more optimistic outlook!

To start Friday off Winnipeg & the Red River Valley will see cloudy skies with a good chance of showers as a line of precipitation moves into the region from the west. That line should be through by 8 or 9AM, and then Winnipeg will be left with cloudy skies and a breezy southerly wind near 30 gusting to 50 km/h as temperatures climb towards a high near 24°C. Then, around 2-3 PM, the threat for thunderstorms will redevelop — primarily for Winnipeg south to the US border and areas east — as a cold front moves across the region. Strong thunderstorms are likely with this front with the potential for isolated to scattered severe storms. The main threat with this afternoon’s thunderstorms will be large hail between the size of nickels and loonies alongside gusty winds. Things will calm down this evening as the wind swings around to the northwest and temperatures head to a low near 13°C.

PASPC Day 1 Thunderstorm Outlook valid September 1, 2017
The PASPC Thunderstorm Outlook highlights well where severe thunderstorms are possible today.

Late Saturday night into early Sunday morning, it appears a weak disturbance will roll through the region. It will mainly bring a bit of cloud to the region but there will be a small chance of some early morning showers on Saturday. Once it moves out, though, Winnipeg will be set for a beautiful day as weak ridging moves into the area, bringing mainly sunny skies, light winds, and temperatures in the mid-20’s.

GDPS Surface Temperature Forecast valid 21Z Sunday September 3, 2017 with annotations
A low pressure system passing through Manitoba on Sunday will bring warm temperatures to the Red River Valley

Another low pressure system is then forecast to move across the Interlake on Sunday, drawing very warm air eastwards and pushing daytime highs up in into the low 30’s. Winds will be breezy out of the south at 30-40 km/h, and Winnipeg can expect partly cloudy skies. Temperatures will then dip back to around 13°C again on Sunday night with a breezy northwesterly wind.

Long Range

The long-range forecast is looking much better than it appeared it would be earlier this week! While a potent shot of cold air is expected to slide southwards behind Sunday’s system, dropping high temperatures on Monday and Tuesday into the upper teens, the cool weather is now expected to be short-lived. Instead of being stuck on the edge of a large upper-level trough as an upper-level ridge sits over BC, models have moved towards a more progressive pattern, allowing the ridge to push eastwards relatively quickly. This means a return to seasonal temperatures by mid-week instead of week’s end.

Otherwise, fairly quiet weather on tap for next week.

Winnipeg’s seasonal daytime high is currently 21°C while the seasonal overnight low is 9C.

Severe Thunderstorm Threat Leads Into Beautiful Weekend

The threat for severe thunderstorms exists today across the Red River Valley as a cold front pushing in from the northwest clashes with a hot and increasingly humid air mass. Once the cold front moves through, conditions will settle with a fantastic weekend ahead.

Today will see temperatures soaring across the Red River Valley as a southerly wind strengthening to 30 gusting 50 km/h brings in hot, humid conditions. Temperatures will climb to a high near 30 or 31°C in Winnipeg and the Red River Valley today with a comfortable morning quickly turning muggy as dew points climb up to the 19-20°C mark by late afternoon. The heat and humidity together will combine to make it feel more like the mid- to upper-thirties, so be sure to minimize prolonged exposure to the heat and stay hydrated.

A cold front will push southeastwards across the Red River Valley this afternoon. This is the estimated location of the frontal wave at 7PM tonight.

A cold front will slump through the Red River Valley late this afternoon, and as it does so there will be the threat for severe thunderstorms. With MLCAPE1 values climbing up to 2000 J/kg and 45-50 kt of deep shear2, the potential for explosive thunderstorm development exists. The biggest questions that need to be answered are:3

  1. Does nocturnal convection develop Thursday night, producing more cloud cover and delaying heating over the Red River Valley and/or eat up some of the instability over the region?
  2. What exactly is the expected strength of the capping inversion and will it be strong enough to prevent any thunderstorms from developing?

The first question we’ll know the answer to by the time you read this. If there is some thunderstorm activity overnight, it doesn’t necessarily rule out thunderstorms later today, just makes it more dependent on the clouds clearing out in time to get enough heating. The second question is much more difficult to answer, and we’ll just have to see how things go. If there happens to be an intermediate sounding available from Winnipeg later today, that should give a much better idea of how much of a thunderstorm threat there is.

As shown in yesterday’s Day 2 Thunderstorm Outlook, ECCC is forecasting a threat of severe thunderstorms over the region today.

That said, if thunderstorms do develop, which I think is likely, they will be bad. All hazards will be possible with these storms: large hail, damaging winds, torrential rainfall, and tornadoes. Rain may be the least of the concerns as the storms will be moving fairly quickly to the southeast at around 40 km/h. Tornadoes will be a threat with these storms, so make sure you stay aware of any severe thunderstorm or tornado watches and/or warnings issued by Environment Canada. The best chance for thunderstorms will be between 6PM and 11PM.

Rest of the Weekend

One the cold front passes, winds will switch around to the northwest and Winnipeg will be set for a great weekend. Saturday will see a high near 25°C with a bit of morning cloud clearing out for the afternoon. The humidity will flush out through the day, making for quite a comfortable afternoon. Temperatures will drop to a low near 12°C on Saturday night. Sunday will bring partly cloudy skies to the region with temperatures climbing back up to a high near 28°C with southerly winds increasing to 20-30 km/h. A disturbance looks set to move through on Sunday night which would bring the chance for some showers or thunderstorms and a return of the humid conditions for Monday. Expect a mild low near just 18°C on Sunday night.

Long Range

The beginning of next week looks hot and humid once again, but then temperatures return to near-seasonal values. It looks like there will be occasional chances for showers or thunderstorms through the week.

Winnipeg’s seasonal daytime high is currently 26°C while the seasonal overnight low is 13°C.

  1. MLCAPE stands for “Mixed Layer” CAPE and is an estimate of the amount of energy a thunderstorm has to work with. 
  2. Deep shear refers to how the winds change with height between the surface and 6km up. Under 20 kt would be considered very low, while in excess of 50 kt is extremely high. 
  3. And to note, I’m writing this late Thursday evening…