A Cool Weekend Ahead

A cooler weekend lies ahead for Southern Manitoba as cooler air pushes southwards through the Prairies behind a strengthening low pressure system in Northern Ontario. Some of our coolest temperatures of the fall lie ahead although it does seem like warmer air will begin to nudge back in by the end of the weekend.

Today

Friday

14°C / 2°C
Mainly cloudy. Lake-effect showers or drizzle has a chance of moving into Winnipeg.

Today will be the most “active” weather-wise thanks to a surge of cold air at 850mb that will be diving southwards through the day today. As this air pushes over the lakes it seems quite likely that lake-effect showers or drizzle will develop and push inland in the lee of the lakes. It’s very difficult to attribute a chance of precipitation to these features as they will almost certainly form, but they produce narrow ribbons (often ≤ 50km wide) that extend along great lengths.

What are the ingredients for Lake-Effect Precipitation?

There are a few ingredients that work together to produce lake-effect precipitation. The basic ingredients are:

  • At least a 13°C temperature difference between the water surface and 850mb
  • At least 100km of fetch (the wind must travel at least 100km over the water surface).
  • Less than 60° of directional shear between the surface and 700mb; less than 40kt of speed shear between the surface and 700mb.

Which areas see precipitation will be extremely sensitive to the wind direction. Here in Winnipeg it seems that the wind will be a little too northerly for the lake effect precipitation from Lake Manitoba to move into the city, but should the winds have a slightly more westerly component we may see some light shower or drizzle activity move in. That being said, some areas in the lee of Lake Manitoba and Lake Winnipeg will likely see some showers or drizzle today. The temperature will struggle to a mere 13 or 14°C and we’ll see temperatures dip to around 2°C tonight. While there should be widespread clearing, some stratus cloud may still be advecting off the lakes. Areas that remain in cloud through the night may end up a couple degrees warmer.

Frost will certainly be a possibility through the Red River Valley tonight as skies clear and winds let up.

Saturday & Sunday

Saturday

16°C / 5°C
Sunny.
Sunday

21°C / 13°C
Sunny. Windy out of the south.

We’ll see a pleasant but cool day on Saturday with mainly sunny skies, light winds and a high near 21°C. Saturday night will bring clear skies and a low of around 4–5°C. Sunday will also be a mainly sunny day although winds will begin to pick up out of the south through the day as a low pressure system pushes towards the province. With cool air in the valley and warmer air struggling to push eastwards, it could actually become quite breezy. At this point it looks like winds will be around 30km/h with gusts to around 50, but if the cold air is a little more stubborn it could end up windier than that and closer to 40–50km/h.

Next week looks to start off with a disturbance pushing through on Monday or Monday night bringing a chance of some rain to the Red River Valley, but at this point it’s far to early to make any real judgements on what it will do. The rest of the week looks fairly quiet with seasonal temperatures.

Severe Thunderstorms Possible in Late-Season Setup

Severe thunderstorms will be possible across much of Southern Manitoba today as a rare late-season setup develops through the day today.

Severe thunderstorm outlook for Wednesday afternoon through Wednesday night.

Severe thunderstorm outlook valid Wednesday afternoon through Wednesday night.

Today

Wednesday

26°C / 15°C
Mostly cloudy; thunderstorms likely late in the afternoon into the evening.

A low pressure system will be pushing it’s way out of North Dakota into SW Manitoba through the day today, bringing with it a push of warm air that will lift a warm front northwards into the extreme southern portions of the province by late this afternoon. Plenty of cloud will stream into the province ahead of this system which will result in us having a mostly cloudy day with our high temperature climbing somewhere near 25°C.

This system will develop a fairly potent severe weather setup, especially for this time of year. A deep layer of moisture will build over the Southern Manitoba through the day with dew points climbing to around 18°C and extending through the boundary layer. Steep mid-level lapse rates will develop with falling 500mb heights, contributing to a large amount of instability ready to be realized should an appropriate trigger develop. 500mb winds are forecast to increase to nearly 50kt by late afternoon which when coupled with a developing low-level jet, contributes to anywhere from 40–60kt of bulk shear in place tomorrow evening/night. There’s plenty of moisture, instability and shear.

So how about that trigger? Things will remain capped through most of the province through the day thanks in no small part to the cloud cover that will inhibit much of our potential heating. Thunderstorms will likely develop in SE Saskatchewan or SW Manitoba mid-to-late afternoon as the cold front associated with this system pushes eastwards and is strengthen by an incoming upper-level disturbance. Over the SW portion of the province, the storms will likely be surface-based which will be a very important distinction. With the strong shear profiles in place, surface-based storms in SW Manitoba will have the potential to become tornadic due to a relatively strong southeasterly inflow wind. Any residents of SW Manitoba should stay alert for watches and warnings that may be issued with this storm system.

The storms that develop along the cold front will likely quickly grow upscale and develop into a squall line pushing eastwards. As it moves towards the Red River Valley the storms will become elevated as surface temperatures cool and a very strong low-level jet develops. This jet will sustain these storms as they push eastwards over the rest of the province through the evening.

Elevated thunderstorms present very little tornado risk. Thunderstorms today will have the potential of producing heavy rain, very large hail and damaging winds, all afternoon and evening.

After the storms push through (and I won’t rule out that we’ll see the umpteenth split around Winnipeg this year), we’ll see clouds break up as we drop to around 15°C.

Thursday and Friday

Thursday

20°C / 9°C
Mix of sun and cloud; chance of scattered showers.

We’ll see things cloud back up early on Thursday as another cold front pushes through. There will be a chance of some scattered showers behind the cold front in some lingering instability as we climb to a high of around 20°C. Winds will be a little breezy out of the northwest to around 20–30km/h. Things will clear up for the most part on Thursday night, although there will likely be a narrow band of stratus cloud coming off of Lake Manitoba overnight. Where that cloud will end up depends entirely on the exact wind direction, but suffice to say it’ll likely end up somewhere in the Western Red River Valley.

Friday

17°C / 4°C
Mainly sunny.

Friday will be a cool and sunny day. Temperatures will only climb to around 16 or 17°C and colder Arctic air slumps southwards over the Prairies. We may see a few clouds in the afternoon, but not anything significant. Temperatures continue to cool aloft through the overnight period with 850mb temperatures dropping to nearly 2–3°C on Friday night. This should translate to an overnight low dipping into the low single digits, likely to around 3 or 4°C.

The weekend looks fairly quiet with sunny skies and temperatures rebounding back towards the mid–20’s by the start of next week.

Warming Up, Just to Get Knocked Back Down

Temperatures will rebound early this week, but a big weather-maker may interrupt that warm-up by midweek.

Warmer air will move into Southern Manitoba on Monday, but it will be windy.

Monday

Monday

Sunny
21°C / 9°C

Today will be bright and sunny with temperatures improving over Sunday’s cool values. Highs will be in the low twenties over Southern Manitoba, which is quite a recovery from this morning’s lows, which were around freezing. The only downside to today’s weather will be a stiff southerly wind that will be 30 to 40km/h gusting to 50 to 60km/h by late afternoon.

Tuesday

Tuesday

Mainly Sunny
27°C / 13°C

Tuesday will be one of the warmest, if not the warmest, days this week with high temperatures in the mid to upper twenties. The humidity may even increase a bit by late in the day over some portions of Southern Manitoba. It certainly won’t be an oppressive mid-summer type of humidity, but may be noticeable nonetheless. Tuesday will be slightly less windy compared to Monday, but the wind will still be fairly strong and from the south.

Wednesday

Wednesday

Mix of Sun and Cloud. Rain late. Risk of a thunderstorm.
25°C / 17°C

What has been a fairly boring weather pattern so far this September may begin to take an interesting turn on Wednesday. A strong low pressure system is forecast to develop in the lee of the Rockies on Tuesday and move eastward into North Dakota by Wednesday. This system will prompt seasonably high moisture values to stream northward into the Dakotas and Southern Manitoba. This moisture will help to destabilize the atmosphere over a large part of the North-Central US, perhaps extending up into Southern Manitoba. This system will also pump warm air up into the region, with models currently suggesting temperatures will once again reach the mid to upper twenties in Southern Manitoba on Wednesday.

As this strong low pressure system interacts with that unstable environment, numerous thunderstorms may develop on Wednesday through Wednesday night. These storms may affect parts of Southern Manitoba at some point on Wednesday or early Thursday. Given the nature of this system it looks like the main severe threat will be heavy rain. However, it is too early to rule out the possibility of other forms of severe weather. We’ll have more updates on this system as the week progresses.

Long Range

The long range forecast will largely be determined by what happens with Wednesday’s system. At this point it appears likely that we’ll receive rain from this system, but the timing of that rain is too early to predict. Unfortunately, once this system moves out of our area it will deliver a parting shot of cold air, which will probably set up a chilly weekend. Again, how cold it will get is still not certain, but getting above seasonal values next weekend may be a struggle.

Warm Weather Gives Way to Cool Weekend

Well above-seasonal temperatures will wash over Southern Manitoba today as a powerful low pressure system in the Arctic drags a swath of very warm air eastwards across the Prairies. It won’t be meant to last, though, as a strong ridge of high pressure builds in behind a cold front that pushes through on Saturday, bringing cool Arctic air with it for the weekend.

Friday

25°C / 15°C
Mainly sunny.
Saturday

22°C / 4°C
Cloudy with a good chance of showers or thundershowers, then clearing through the afternoon.
Sunday

19°C / 6°C
Mainly sunny and cool.

We’ll see a breezy wind out of the southwest today as temperatures climb into the mid–20’s thanks to a warm front that pushed through overnight. We might see a little bit of cloud, but for the most part skies should be mainly sunny. Still in the warm sector of this system, we’ll only drop to the mid-teens tonight with some increasing cloudiness towards Saturday morning as the cold front approaches.

Saturday morning will bring a fairly decent chance for some scattered shower activity as the cold front pushes into the Red River Valley from the north. There may be a few thundershowers as well but no significant or widespread thunderstorm activity is expected. Gusty northerly winds will move in behind the cold front – which should be through much of the Red River Valley by midday – with clouds scattering out to a mix or partly cloudy skies. The temperature should be able to climb a degree or two above the 20°C mark, however cold air advection will limit our high very close to that mark.

Skies will clear overnight as we drop down to a very chilly 4°C or so. Areas outside the city of Winnipeg may even see temperatures dip a little cooler than that. We may end up seeing the first frost of the year in some places on Saturday night but I don’t expect a widespread frost to occur.

Sunday will be a sunny but very cool day as the ridge of high pressure dominates the weather pattern over Southern Manitoba. We’ll see temperatures climb to only around 17 or 18°C by late afternoon but at least the winds should be relatively light, picking up out of the south later in the day to only around 15–20km/h. We’ll see an overnight low of around 7°C on Sunday night with clear skies.