Heat Breaks with Soggy Saturday

The heat seen over southern Manitoba over the past week will be in place for just one more day before a low pressure system moves through the region on Saturday, bringing soggy weather and cooler temperatures.

Friday
28°C / 18°C
Mainly sunny
Saturday
25°C / 17°C
Mainly cloudy with showers. Risk of a thunderstorm.
Sunday
26°C / 16°C
Mixed skies with a chance of showers

Another hot day is on the way for Winnipeg, but humidity will continue to decrease slightly towards more comfortable levels. Winnipeg & area will see plenty of sunshine today with just a few puffs of cloud possible in the afternoon. Winds will be fairly light out of the west-northwest at 15-25 km/h. Temperatures will climb to a high near 28°C. Tonight will bring increasing cloud and light winds as temperatures dip to around 18°C.

A low pressure system will be pushing northeastwards out of Saskatchewan & Montana on Friday night.
A low pressure system will be pushing northeastwards out of Saskatchewan & Montana on Friday night.

Saturday will be an unsettled day as a low pressure system moves through the province. The daytime high will be in the mid-20’s with winds picking up out of the south to 20-30km/h in the morning.

There will be rain on Saturday, however at this point, there’s still some uncertainty on exactly where precipitation will fall. Indications are that amounts will generally be 5-15 mm, however convective elements may produce rainfall amounts up to 30-50 mm in thunderstorm activity.

One potential outcome of Saturday shows two main areas of convective rainfall with up to 50 mm of rain.
One potential outcome of Saturday shows two main areas of convective rainfall with up to 50 mm of rain.

Ultimately we’ll simply have to wait and see how things shape up and what the nature is of the precipitation that develops through Montana and Saskatchewan tonight. We’ll provide updates in the comments below.

Much of the rain will taper off on Saturday evening, leaving us with fairly cloudy skies and a low that falls to around 16°C by Sunday morning.

Sunday will be a bit of a mixed bag. Expect mixed skies and a high near 26°C, however winds will be breezy, peaking out of the northwest at around 40 km/h and we’ll see just a slight chance of some scattered showers. Skies should clear for Sunday night as temperatures head to a low near 16°C.

Long Range

The start of next week looks fairly pleasant with sunny to mixed skies and daytime highs in the mid- to upper-20s. It looks like it will be fairly dry with a ridge of high pressure deflecting things south of the Red River Valley, but that will be sensitive to the exact location that this feature sets up.

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

Heat Continues, Wednesday Thunderstorms Will Ease Humidity

The [extreme] heat will continue through the remainder of the week with daytime highs near or exceeding 30°C through Friday. A weak cold front passing through on Wednesday night will help flush some of the humidity out of the region, making for a slightly more comfortable end to the work week.

Wednesday
33°C / 19°C
Mixed skies with risk of thunderstorms
Thursday
31°C / 17°C
Mainly sunny
Friday
30°C / 17°C
Increasing cloud with risk of thunderstorms

Another hot and humid day is on tap for Winnipeg & the Red River Valley as the air mass that moved into the region yesterday remains in place. Daytime highs will soar towards to near 33°C which, when combined with oppressive dewpoint values in the 22-24°C range, will feel more like the low- to mid-40's. There will be little relief from the extreme heat as winds remain fairly light.

Dewpoint values are forecast to rise into the 22-24°C range today, creating oppressive humidity.
Dewpoint values are forecast to rise into the 22-24°C range today, creating oppressive humidity.

By late afternoon the risk for thunderstorms will ramp back up throughout much of southern Manitoba. There will be a moderate risk of severe thunderstorms along and south of the Trans-Canada Highway with a slight risk of thunderstorms extending northwards into Central Manitoba. Thunderstorms will likely initiate over southwestern Manitoba late in the afternoon, and then propagate eastwards across the Red River Valley overnight. The primary threats from these storms will be strong winds, torrential rain and damaging hail.1 When the storms first initiate in the late afternoon or early evening, they will also pose a tornado threat. It will be another all-hazards severe thunderstorm day for portions of Southern Manitoba.

Behind the thunderstorms overnight, a weak cold front will be sweeping through the region. This will begin pushing out the oppressive humidity and returning dewpoint values to more reasonable values in the upper-teens. Winnipeg will see a low temperature on Wednesday night near 19°C.

Thursday will be a beautiful and warm day under mainly sunny skies (once any left-over nocturnal convection clears out) and a light west-northwest wind. The daytime high will be around 31°C, but the humidity will be much more comfortable, with the dewpoint dropping down to the mid-teens for the afternoon. Expect a low near 17°C on Thursday night.

Friday will bring another disturbance towards Southern Manitoba, spreading more cloud into the region through the afternoon. A thunderstorm threat will once again return to the region. With low dewpoints in place over the region, there will be much less energy to work with than Wednesday's thunderstorms, however the dynamics look relatively good, so there will likely be a slight risk of severe thunderstorms over the region with hail and wind the main concerns. Expect another warm day on Friday with a high near 30°C. The low will dip to around 17°C on Friday night with continuing unsettled weather.

Long Range

The weekend is a bit of a mixed bag with unsettled conditions persisting through Saturday and sunshine returning on Sunday. Temperatures will be cooler with highs in the mid-20's.

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

  1. It's worth noting that this unstable air mass was capable of producing baseball-sized hail in a major supercell thunderstorm yesterday.

Warm But Stormy This Week

Warmer weather is on tap this week as we experience a pattern more characteristic of mid-July. However, this warmer weather will bring with it a risk of severe thunderstorms as higher humidity builds into southern Manitoba.

Today is expected to be nice as a surface high to our east brings a light southeasterly flow to southern Manitoba.
Today is expected to be nice as a surface high to our east brings a light southeasterly flow to southern Manitoba.

This Week

Monday
24°C / 13°C
Mainly sunny
Tuesday
29°C / 19°C
Mix of sun and cloud with risk of a severe thunderstorm
Wednesday
30°C / 18°C
Mainly sunny

Today will be a nice day as temperatures rise into the twenties under mainly sunny skies. A surface high to our east will bring a light southeasterly flow to southern Manitoba while building heights aloft help to maintain dry and clear conditions. The only risk of significant weather will come overnight Monday as considerable theta-E advection1 brings a risk of elevated convection developing across portions of western Manitoba. Given the inherent uncertainty in predicting elevated convection, it is possible that a substantial elevated risk could develop, or none at all, depending on the eventual evolution of the system. Current models suggest fairly tame convection will develop in western Manitoba, with more substantial convection further west in Saskatchewan, but this remains uncertain as was previously stated.

Tuesday appears to be the most appreciable threat of severe weather this week as an approaching upper-level trough sets the stage for thunderstorms to develop. But before the thunderstorm threat is discussed, a brief word on the general weather for Tuesday. In most of southern Manitoba it appears temperatures will climb up around 30C by Tuesday afternoon, with high humidity pushing humidex values near 40. A gusty southeasterly surface wind is also expected as a result of the strong system developing to our west. Besides the possible elevated convection on Tuesday morning, as described in the previous paragraph, the main threat for new thunderstorm development is expected beginning Tuesday afternoon in western Manitoba as a surface trough pushes into the region. At this time all modes of severe weather appear to be possible, including large hail, damaging winds, and tornadoes. This summer has featured unusually large volatility in severe weather forecasts, so bear in mind that the forecast provided here is based on the current guidance and is entirely subject to change depending on the eventual evolution of this system.

Update: An updated discussion for Tuesday’s thunderstorm threat is available here.

A technical discussion based on the latest (Sunday afternoon/evening) guidance is provided using the MIST technique:

  • Moisture: Considerable moisture is expected to advect into eastern Saskatchewan and western Manitoba by late Tuesday. 100-mb mean mixing ratios near 18 g/kg are expected in the region, with surface dewpoints in the low twenties. ET enhancement will play some role in elevating near-surface dewpoints, but the depth of the moisture will largely result from the advection of deep moisture from the central US Plains.
  • Instability: Steep mid-level lapse rates of ~8 C/km are expected atop the boundary layer as a southwesterly mid-level flow advects an EML from the western US. These steep lapse rates in combination with rich boundary layer moisture are expected to result in MLCAPE values of 3000-4000 J/kg in the warm sector. MLCAPE could be even higher on a localized basis given the current model guidance showing mean mixing ratio values higher than those listed in the previous section – although such extreme mixing ratio values seem less probable given the current moisture values in the source region (central US Plains).
  • Shear: A 65 kt jet streak at 500 mb is expected to be edging into Saskatchewan by late Tuesday. In addition, a south/southeasterly LLJ of 20-30 kt is expected in the 925-850 mb layer. Surface winds will similarly be from the south/southeast in the warm sector, likely at 10-20 kt. This wind profile is expected to result in effective bulk wind differences (EBWD) of 25-35 kt along the international border increasing to 35-50 kt in east-central Saskatchewan by 00 UTC Wednesday. Considerable low-level veering will result in effective storm relative helicity values of 150-350 m2/s2 across the warm sector, with the highest values in southern Manitoba. Increases in effective helicity can be expected after 00 UTC due to the typical boundary layer decoupling.
  • Trigger: A surface trough is expected to extend southward through eastern Saskatchewan by Tuesday afternoon, extending from a strong low pressure system over the northern Prairies. A secondary, weaker low may develop near the MB/SK/ND border, depending on how the upper jet exit orients itself as it rounds the upper ridge. A warm front is expected to be located across Manitoba, although the exact location remains uncertain. It is possible that an outflow-warm front merged boundary may be present depending on the evolution of morning convection. The absence of outflow-reinforcement may result in a fairly diffuse warm front running somewhere through the interlake. Aloft, a strong shortwave is expected to cross the eastern Prairies on Tuesday morning, helping to maintain and/or trigger elevated convection. Later in the day, a weaker shortwave may push through eastern Saskatchewan and/or southern Manitoba, although the timing and placement of this feature are unclear. Weak height falls across southern Manitoba will maintain considerable capping, especially south of the Trans-Canada Highway, where 700-mb temperatures are expected to exceed 10C through the day, and may reach as high as 12-14C.
AWM Day 1 Convective Outlook for Tuesday July 19, 2016
AWM Day 1 Convective Outlook for Tuesday July 19, 2016 (Original Day 2 Outlook)

Discussion

A significant severe weather threat exists from central Saskatchewan into southern Manitoba on Tuesday. Strong wind shear in combination with an extremely unstable environment will lead to numerous severe thunderstorms. There remains considerable uncertainty in terms of the timing and location of these storms, thus the following forecast is subject to change. Nevertheless, at the present time wind shear vectors crossing approximately normal to the cold front, along with considerable low-level veering in a moderately capped environment are expected to allow sufficient updraft seperation for numerous discrete cells to initially develop across eastern Saskatchewan by late Tuesday afternoon/early evening. Short bowing segments and supercell structures (HP) are probable, with the likelihood of a severe MCS developing as cold pool mergers occur late evening as cells move across west-central Manitoba. A few tornadoes may be likely in the late afternoon-evening whilst cells remain discrete or semi-discrete. A secondary threat of severe storms may develop in SW Manitoba should the aforementioned secondary surface low develop. However, given the strong capping in the region and uncertainty regarding this feature, convection initiation in this region is more uncertain. Should a MCS develop during the mid- or late-evening period, it would tend to initially propagate eastward, but would likely take on a southerly component as it begins to forward-propagate into the LLJ during the late evening-early overnight period. Such a course may result in the Red River Valley being clipped. Given that such a system would likely be surface-based as it passed through the Red River Valley, owing to slow nocturnal cooling in the early overnight period, damaging winds would be the main threat, with a secondary threat of large hail. Having said all that, an entirely different scenario may unfold should the morning elevated convection remain intense during the day. Some high-resolution guidance suggests the elevated storms will slowly move east during the day, eventually becoming surface-based as they encounter the extremely unstable southerly feed over eastern SK/southwestern MB. Under this second scenario new storms would be less probable, with the main convection focused around the cluster of storms from the morning. Regardless of which scenario plays out, the overall risk is moderate in east-central SK and west-central MB, with a surrounding slight risk region.

Wednesday

The weather for Wednesday will be dependent on how quickly Tuesday’s weather system moves to the east. Some guidance suggests the system may continue to linger on Wednesday, potentially bringing another round of severe weather to southern Manitoba. Conversely, the system may have exited the region by that point. Regardless, temperatures are expected to climb up around 30C with lingering humidity. Should Wednesday end up presenting another severe weather risk we will be sure to provide a new update.

Long Range

The long range forecasts this summer have been quite inaccurate, although that isn’t necessarily unusual. Forecasts have continuous suggested that we will see warmer than normal weather, although that has not often panned out. The current long range forecast continues to suggest we’re going to see generally hot weather through the end of July. This time it appears that this forecast may be a bit more certain than previous ones, although bear in mind that it certainly could end up being wrong again!


  1. Simply put, theta-E is a measure that combines moisture and temperature.

Near-Seasonal Temperatures This Weekend

Near-seasonal temperatures will be in place this weekend with just a chance of a shower or two across the Red River Valley as a few weak upper disturbances ripple through.

Friday
24°C / 13°C
A few clouds
Saturday
25°C / 14°C
Partly cloudy with a chance of showers
Sunday
25°C / 13°C
Mixed skies with scattered showers or thunderstorms

Winnipeg & the Red River Valley will see a beautiful day today with plenty of sunshine, light winds and a high near 24°C. There will be a very slight chance of a shower or two in the Red River Valley, but these will likely be confined to the extreme western portions of the valley, particularly close to the escarpment.

Skies will continue fairly clear tonight with a low near 13°C.

Saturday will bring more pleasant conditions to Winnipeg & the Red River Valley, although there will be slightly more cloud and an incrementally higher chance of some scattered showers throughout the valley, particularly through the afternoon hours. Temperatures will climb to a high near 25°C with light winds once again.

Expect a low near 14°C on Saturday night with partly cloudy skies.

A cold front located over central Manitoba on Sunday morning will sweep southwards through the day.
A cold front located over central Manitoba on Sunday morning will sweep southwards through the day.

A weak cold front will sweep into the Red River Valley on Sunday, almost certainly bringing some precipitation with it. Temperatures will still be mild with a high near 25°C, however skies will become mixed to mainly cloudy in the morning, with scattered showers developing by midday. There will be enough instability to produce a thunderstorm threat, but at this time no organized severe thunderstorm threat is expected.1 Winds will shift from 15-25km/h out of the southwest ahead of the cold front in the morning to about the same speed out of the northwest behind the cold front.

Temperatures will dip to a low near 13°C on Sunday night with clearing skies and diminishing winds.

Next Week Brings the Heat

Heading into next week it's beginning to look like a bit of a mixed bag: we'll definitely be seeing a prolonged stretch of summer-like heat as daytime highs climb into the upper 20's after one cool day on Monday, but with Southern Manitoba left on the edge of an upper-level ridge building through central North America, we likely won't see nothing but sunshine all week.

The CPC is forecasting a near-certain probability of above-normal temperatures extending into southern Manitoba next week.
The CPC is forecasting a near-certain probability of above-normal temperatures extending into southern Manitoba next week.

It will be impressive heat that Southern Manitoba will be on the fringes of, with daytime highs soaring into the upper 30's or low 40's through the Great Plains of the United States. Some of that heat may surge further north in the second half of the week and produce daytime highs in the 30-35°C range across the southern Prairies. These high temperatures combined with dewpoint values in the 18-20°C range could result in humidex values approaching the 40 mark across southern Manitoba in the second half of the week.

This warm weather will be hampered, however, by the potential for several rounds of thunderstorm activity.2 With the upper-level ridge not quite fully building into our region, we'll be under the threat for multiple "ridge riders," upper-level disturbances that move up and over the top of an upper ridge. These features are common producers of thunderstorms and can often result in severe weather as well, so although we'll be warm, it doesn't appear that we'll get a completely dry stretch either. Precipitation should primarily be convective, so you may be able to sneak through the week without getting anything, but the potential will be there for wet weather.

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

  1. As usual, we'll keep an eye on things and update if the thunderstorm potential looks worse as we get closer to the date.
  2. I've installed chicken wire for protection from the farmers.