Friday’s Surge of Heat & Humidity Leads to Unsettled Weekend

Heat & humidity will begin moving into the region today as a breezy southerly wind develops over the province, tapping into a much warmer air mass over the Northern Plains of the United States.

Friday
28°C / 20°C
Mixed skies with chances of showers or thunderstorms in the evening & overnight
Saturday
24°C / 15°C
Cloudy with a chance of thunderstorms or showers; clearing late in the day
Sunday
18°C / 11°C
Cloudy with showers

A potent low pressure complex developing over the western Prairies & Northern Plains will bring a thunderstorm threat back to Southern Manitoba today with the southwestern corner of the province under the threat for thunderstorms—potentially severe—this afternoon and this evening, while the Red River Valley and southeastern Manitoba seeing the threat move in this evening and into Saturday morning.

Before that, though, Winnipeg & the Red River Valley will have a fairly nice day ahead. Temperatures will soar through the morning, reaching within a degree or two of today’s high temperature of 28°C by lunch. Skies will be mixed today with cloudier conditions developing in the afternoon. Southerly winds to 30-40km/h will pick up through the day across much of Southern Manitoba, drawing moisture from the United States northwards into the province. By late afternoon, dew point values in the Red River Valley will climb to the 16-18°C range1 while in the southwest corner of the province, dew point values may reach the 20°C mark, making for very humid feeling conditions.

AWM Convective Outlook for June 24/25, 2016
AWM Convective Outlook for June 24/25, 2016
As the afternoon wears on, a leading shortwave lifting out of Montana will destabilize conditions over southern Saskatchewan and southwestern Manitoba, bringing a threat of thunderstorms to the region. There’s a slight chance that some of these thunderstorms may become severe. The threats from these storms will be:

  • Rainfall: Precipitable water values of 35-45 mm combined with storm motions of 20-30km/h will produce very intense rainfall, but the storms will be moving quickly enough that the overall rainfall accumulations will be limited.
  • Hail: With MLCAPE values in the 1500-2500 J/kg range and ample shear present, storms will likely take the form of discrete supercells early in their life cycle. These storms will be capable of producing large, damaging hail.
  • Wind: Overall, wind won’t be a widespread threat with these storms, but isolated damaging wind gusts are possible.
  • Tornadoes: The supercell thunderstorms in southeastern Saskatchewan and southwestern Manitoba will be in environments with very strong directional shear. A small chance of a tornado or two exists with these storms.

As we move into the evening, scattered showers or thunderstorms are possible over the Red River Valley while the thunderstorms closer to the SK/MB border continue onwards and grow into a larger complex of thunderstorms that will push eastwards overnight. Instability actually increases overnight as moisture continues to be pumped northwards along the low-level jet ahead of the incoming shortwave. As a result, a large area of rain and thunderstorms will likely progress eastwards overnight, reaching the Red River Valley between 3 and 6AM. These thunderstorms will present a slight risk of severe weather with primary threats of rain and large hail, with a secondary threat of damaging wind gusts.

Worth noting: if the western shortwave offering support to this system ends up slowing down, the overnight thunderstorm activity may diminish over southwestern Manitoba.

Temperatures will remain very warm on Friday night with overnight lows near 20°C in the Red River Valley.

Saturday: Muggy with Continued Thunderstorm Threat

Saturday will start with whatever convection moving through on Friday night exiting the region, and we’ll be left with very humid conditions with dew point values likely climbing to 20-21°C. Temperatures will climb towards the mid-20’s through the morning under fairly cloudy skies, but conditions will undergo a fairly significant change midday as a cold front sweeps through. The passage of this front will bring another threat of showers and thunderstorms to the region and with MLCAPE values near 1500-2000 J/kg and ample shear, these will also have bring the threat of severe weather with rainfall, winds and hail a concern.

4km NAM Dewpoint Forecast valid 15Z June 25, 2016
High dewpoint values over 20°C will be in place on Saturday morning in the Red River Valley.
Once the front passes, gusty westerly winds will usher in drier air as dew point values plummet into the single digits, bringing relief from the humidity. Skies will begin clearing as well, likely providing some afternoon/evening sunshine. Temperatures will likely top out at only 24 or 25°C, though, and a cooler night will be ahead with lows near 15°C.

Dreary End to the Weekend

Saturday night’s clear skies will be replaced by cloud on Sunday as the main upper-level low of this whole weather system moves through the region. It will bring showers to much of Southern Manitoba with most of the activity through the morning hours and then tapering off through the afternoon. Temperatures will be much cooler with a high near 17 or 18°C and strong northwesterly winds of 40 gusting 60 km/h picking up by the end of the morning.

The winds will taper off overnight as the valley heads to a low near 11°C.

Winnipeg’s seasonal daytime high is currently 24°C while the seasonal overnight low is 12°C.

  1. At a dew point of 16-18°C, it begins to feel mildly muggy.

Showers Give Way to Hot & Humid Weather

A weakening disturbance will push through the province today, bringing some rain to the region. As it clears out, the heat and humidity will build back in for week's end sending daytime highs back towards the 30°C mark.

Wednesday
21°C / 12°C
Cloudy with a showers likely
Thursday
24°C / 14°C
Friday
Friday
29°C / 20°C
Partly cloudy & muggy; risk of a thunderstorm

Today will see Winnipeg stuck under overcast skies as a disturbance rolls through the region, bringing some showers to the region. The best chance for precipitation will begin mid- to late-morning through the early afternoon, with diminishing chances later in the afternoon as the system moves out of the region. The cloud cover and shower activity will keep things a bit cooler with daytime highs near 21°C.

Skies will clear in the evening as the disturbance departs the region and temperatures will fall to a low near 12°C. Thursday will be a beautiful sunny day across the Red River Valley with a high near 24°C and light winds. Temperatures will dip to around 14°C on Saturday night with just a few clouds.

NAM Forecast 2m Dewpoint valid 00Z Saturday June 25, 2016
A plume of higher dewpoints, shown by the blue and purple colours, is forecast to move into southern Mantioba on Friday.
A weakening disturbance will push through the province today, bringing some rain to the region. As it clears out, the heat and humidity will build back in for week’s end sending daytime highs back towards the 30°C mark.

Friday will see a return of the heat and humidity as a southerly flow re-develops over the Red River Valley. Winds will strengthen to around 30 km/h out of the south ahead of a major developing weather system in the western Prairies. Temperatures will climb to the upper 20's in the afternoon with a few clouds moving into the image. Perhaps more notably will be the humidity, which will surface through the day after northwards transport on Thursday night. From a relatively comfortable dewpoint of 12-13°C in the morning, by the end of the day the dewpoint will climb into the 18-20°C range, making for quite a muggy afternoon.

Heading into Friday evening, more cloud will move into the Red River Valley with a risk of showers or thunderstorms. It's early at this point, but there may be a slight risk of severe thunderstorms throughout the Red River Valley, including Winnipeg, on Friday evening.

Long Range

Saturday will be a warm and very humid day that will likely bring mixed skies and the risk of thunderstorms. Sunday looks to be much cooler with more comfortable humidity with a continued chance of showers or thundershowers as a large upper low moves over the province.

Winnipeg’s seasonal daytime high is currently 24°C while the seasonal overnight low is 12°C.

Drier Weather to Start the Week

This week will start out on a drier note after the heavy rains of the weekend left many parts of southern Manitoba in a very soggy state.

Mild and breezy weather is expected in southern Manitoba on Monday
Mild and breezy weather is expected in southern Manitoba on Monday

This Week

Monday
22°C / 10°C
Mainly sunny
Tuesday
22°C / 10 °C
Mainly sunny
Wednesday
24°C / 12°C
Mainly sunny

Today will be mainly sunny with temperatures climbing into the low twenties. The sunshine will be welcomed by many in southern Manitoba as it will give a chance for many water logged areas to dry out. Winds will be northwesterly at 20-30 km/h.

Tuesday will feature much the same weather as today, with temperatures in the low twenties and mainly sunny skies. Winds will be a bit lighter however, making it an even better day.

Wednesday’s weather is the most uncertain of the early week period. Most models suggest southern Manitoba will see mainly sunny conditions with temperatures in the mid twenties. However, an area of precipitation is expected to pass to our south. Should the forecast shift that precipitation northward, we could see more rain. At this time that does not appear likely, but it is something to bear in mind.

Long Range

The long range forecast continues to show us in a generally above-seasonal pattern through the end of June. However, we are also forecast to remain near the storm track through month’s end, suggesting more wet weather may be on the way later this month. Forecasting weather in the long range can be quite difficult in summer, so unfortunately you’ll just have to take the conditions as they come to some extent.

Special: Severe Thunderstorm Threat Returns for Father’s Day & The Manitoba Marathon

Sunday, June 19th marks both Father’s Day as well as the Manitoba Marathon, and unfortunately it appears that the unsettled weather we talked about in Friday’s post has the potential to bring a significant threat of severe thunderstorms to the region.

AWM Convective Outlook for June 18/19, 2016
AWM Convective Outlook for June 18/19, 2016

The Short Version

Severe thunderstorms are possible early Sunday morning, however that threat will be conditional on the expected development of thunderstorms in Saskatchewan on Saturday night.

The current expectation is that an area of thunderstorms will develop over Saskatchewan Saturday night, expanding in coverage and accelerating eastwards through the overnight period. This area of convection will then move through the Red River Valley with a primary threat of severe and damaging winds as well as large hail and torrential rain capable of producing flash flooding.

Thunderstorm Threat Outlook for Sunday June 19, 2016
Threat Type Threat Level Details
Rain Moderate Torrential rainfall possible with flash flooding.
Hail Moderate 2-4cm hail possible.
Wind Moderate Strong wind gusts in excess of 100 km/h likely
Tornado Low No tornado activity expected.

The thunderstorm activity is expected to occur between 4:00AM and 9:00AM, with the primary threat period between 5:00AM and 7:00AM. The timing may adjust depending on the exact speed of the system, but overall this is expected to be a morning event.

Forecast for the morning of June 19, 2016
6AM 8AM 10AM 12PM
17°C 19°C 21°C 23°C
Thunderstorms likely Thunderstorm activity diminishes Slight chance of showers Partly cloudy

Alongside the thunderstorms, conditions will get notably more humid as the morning progresses.[1] By the time any thunderstorm or rain showers taper off, things will feel fairly sticky. Temperatures will start around the 16-17°C mark in the morning, climbing to around 23°C by noon, and then on to a high near 27°C in the afternoon. Winds will be fairly light until mid-afternoon when they pick up from the west to around 20-30 km/h, ushering in drier air and providing some relief from humidity.

The Details

The potent thunderstorm potential for Saturday night and Sunday morning across the southern Prairies will be the result of a potent shortwave that ejects northeastwards out of Montana on Saturday and then rides eastwards along the international border. This will spawn a surface low with cold front draped southwards as cooler air builds in behind the shortwave and warm front extending eastwards across the northern portions of the Northern Great Plains. The surface low will then slide eastwards, gradually lifting the warm front northwards as it goes.

This shortwave will [likely] initiate an area of convection over southwestern Saskatchewan that will track eastwards along with the shortwave. The orientation of the warm front will provide plenty of support for the convection with shear profiles that indicate the likely development of a MCS[2] containing bow echoes.

Going over the MIST ingredients:

  • Moisture: Dewpoint values near 18°C will be ingested into the convection on Saturday night. This will provide substantial energy for the thunderstorms to work with. Additionally, the high moisture content of the air being ingested will increase PWAT[3] values to over 50 mm, making very intense rainfall likely with any thunderstorms that develop.
  • Instability: Steep mid-level lapse rates will combine with the warm and humid air of the nocturnal inflow to produce MUCAPE values in excess of 2000 J/kg.
  • Shear: 55-65 kt of bulk shear coupled with veering profiles within the convection will support the development of strong, organized thunderstorms.
  • Trigger: Shortwave-induced mid-level destabilization coupled with the development of a strong LLJ.

For these parameters, the primary concerns will be hail and rain, however should the storms be able to break through the inversion, wind will rapidly become a significant threat as gusts in excess of 100 km/h will be possible.

The steep mid-level lapse rates combined with high MUCAPE values will produce a moderate hail threat with hail between 2-4 cm in diameter possible. The heightened PWAT values make torrential rain likely with these storms, which can result in rapid accumulations and flash flooding. The strong winds & shear profiles of the storm environment make damaging wind gusts possible. Justification for the strong winds despite the nocturnal part of the convection will be the intense storm-relative winds that will be associated with this quickly-moving system. The SR-inflow will likely be strong enough lift the low-level air mass wholesale into the storm, resulting in a surface-based or near-surface-based thunderstorm.

Summary

Father’s Day of 2016, and the Manitoba Marathon, face the potential of being remembered for severe thunderstorms if current guidance continues painting the picture it does. Thunderstorm potential is conditional on nocturnal convection developing on Saturday night in Saskatchewan, but if it does, there is a significant threat of severe thunderstorms over Southern Manitoba on Sunday morning.

The timing for the primary severe thunderstorm threat will be in the early morning, buffering the starting time for the marathon, with much of the thunderstorm activity expected to be out of the region by 10-11AM. In the wake of this system will be a warm day that will feel fairly humid until westerly winds kick in mid-afternoon, pushing out the humidity and drawing in drier, slightly cooler air for the start of next week.


  1. Dewpoint temperatures are expected to rise from 12-13°C early in the morning to around 19°C by mid-morning.  ↩
  2. Mesoscale convective system.  ↩
  3. Precipitable water.  ↩