A Weather Moment

Southwestern Manitoba will be under the gun again for the potential of strong to severe thunderstorms.

In what has suddenly become a somewhat busy start to the season, more thunderstorms are expected through the remainder of the week, with a threat for severe thunderstorms tomorrow in the southwest corner of the province.

Today & Tomorrow

Wednesday

25°C / 13°C

Cloudy periods

Thursday

28°C

Chance of morning showers or thundershowers. Risk of a thunderstorm late in the day.

Thursday Night

15°C

Showers or thunderstorms likely.

In Winnipeg today we’ll see some cloudy periods but overall a very nice day with light winds and a high near 25°C. The main focus for weather will begin in Southern Saskatchewan where conditions are very favourable for severe thunderstorms. Storms will likely initiate mid-afternoon in south-central Saskatchewan and begin tracking eastwards.

The storms will quickly grow into supercells thanks to ample available energy and strong veering wind profiles. The storms across Southern Saskatchewan will carry with them the risk of all severe modes of weather: damaging winds, flash flooding due to torrential downpours, large hail and tornadoes. It will be a potentially dangerous afternoon in south-central Saskatchewan.

The storms will continue northeastwards through the overnight period, sustained by a strong low-level jet. The storms will enter Manitoba later in the overnight period, with the main focus through the Parkland region, but with the potential for storms extending southwards into SW Manitoba to the US border. At this point, it doesn’t appear that the storms would carry a significant severe threat, but there does seem to be a slight chance that they would be able to produce marginally severe hail if they do develop.

We’ll see the remainders of that convection move through the Red River Valley Thursday morning. It may still be thunderstorms at that time, it may just be some scattered showers and cloud; it will depend highly on exactly what develops in the overnight period.

Moving into Thursday afternoon, the thunderstorm risk returns to Southern Manitoba with a slight risk of severe thunderstorms in southwest Manitoba and possibly creeping into the southwestern Red River Valley. A low pressure system lifting northeastwards through the province will drag a cold front slowly eastwards through the afternoon. While a few unknowns still hinder the forecast – primarily what effect morning convection may have on the environment and how much cloud will linger, preventing things from warming up too much – the setup looks favourable for the development of strong to severe thunderstorms.

A favourable storm environment will be in place by the afternoon hours in southwest Manitoba, with CAPE values exceeding 1500J/kg, decent convergence along the cold front, very unstable mid-levels and a fairly favourable vertical wind profile. It seems quite plausible that supercell thunderstorms could develop along the cold front in the mid-to-late afternoon and then slowly track eastwards. Should severe storms develop, all modes of severe weather would be possible, including localized flash flooding, large hail, and strong winds. Tornadoes cannot be ruled out, however the low-level wind structure may not be quite conducive enough to generate the required environment.

Some uncertainty plagues the forecast, however, and it’s mainly focused on the “cap” in place. A “cap”, or capping inversion, is a layer of warm air usually somewhere in the first 2-5km of the atmosphere that inhibits air being lifted from the surface from continuing to higher altitudes. This feature can be overcome with enough forcing, which then allows the air parcel to rise since it’s warmer than the air around it – depicted with red shading in the image.

If the cap stays strong, there will be little hope for storms until late evening into the overnight period, and those storms would only be marginally severe.

It seems likely, though, that the cold front will erode the cap and provide enough forcing to generate thunderstorms, so our current outlook calls for a slight risk of severe thunderstorms over southwestern Manitoba into the Red River Valley. There is a chance that we may push the slight risk area further north and northwest, including more regions along the Trans-Canada Highway as well as some areas in Parkland Manitoba, but we’ll wait for further evidence that supports that move before we jump on that.

The storms will slowly track eastwards and showers and thunderstorms are likely over much of the Red River Valley overnight Thursday into Friday morning.

Friday & Beyond

23°C / 15°C

Showers likely, then clearing in the afternoon.

Friday will bring the remnants of any precipitation along the cold front through the Red River Valley in the first half of the day, then we’ll see clearing skies as this system pushes off to the east. The high will be near 23°C and the temperature dip into the mid-teens on Friday night.

The weekend is looking drier for the most part. A system tracking through North Dakota may spread some rain into the southern Red River Valley, but at this point its looking like things should remain south of the border. Sunday looks like quite a pleasant day with sunny skies and temperatures in the mid-20’s.

AWM Chase Day

Much of the AWM team will be out chasing storms in SW Manitoba today. You can follow along with them through their tweets here:

Tweets from https://twitter.com/aweathermoment/lists/the-awm-team
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