Tag: Rain

Cool Blast Across Manitoba

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A powerful low pressure system moving through the Interlake will bring significantly cooler weather to Manitoba over the next couple days. Read on to find out where the rain will be and how cool it’s going to get.

GFS Forecast for this evening

GFS 850*mb* temperature forecast for this evening. I’ve drawn on the strong high over the Western Prairies that is helping our low pressure system pull down cool Arctic air. Cold front and warm fronts have been drawn in blue and red, respectively.

As the low pressure system passes to our north today, it will drag a cold front through the Red River Valley over the course of the afternoon. Preceding this front will be an area of rain, and while a majority of the thunderstorm activity will be through the Interlake, closer to the low, there will likely be some isolated embedded thunderstorms through the Red River Valley.

Accumulations will generally be higher the further north from the international border you go; while up to 25-50mm will likely fall through the Interlake, only 2-5mm are expected over the southern RRV. Any location in the RRV that sees a thunderstorm could easily receive between 0.5 to 1” of rain. Here in Winnipeg, we’ll probably see between 5-10mm with some locally higher amounts if a thunderstorm rolls through by the end of the day.

On the backside of this system a strong northerly wind at all levels will drag down the most potent shot of cool, Arctic air we’ve seen in quite a while. While winds will be out of the south in the morning, gusty northerly winds will kick in quickly behind the cold front, with sustained speeds between 40-50km/h and gusts as high as 60-70km/h. 850mb temperatures approaching 0°C will push down into the Interlake region tonight. This will bring with it the chance for the first significant lake-effect shower event of the “fall”1 season. When looking for the generation of lake-effect showers, there’s a few things to look for:

  1. A temperature difference from the surface to 850mb of at least 13°C.
  2. Less than 60° of wind shear between the boundary layer and 700mb; preferably less than 30°.
  3. At least 100km of fetch which the air travels over open water.

Given that lake temperatures are still sitting at 22-24°C, we’ll certainly see plenty of clouds develop and move south off the lakes. Whether or not lake-effect showers develop will depend precisely on the winds, but it looks fairly favourable right now with winds forecast to be from the NW with minimal directional shear. We’ll likely see some streamers develop overnight, providing showers or drizzle to communities in lee of the lakes. Current forecasts put these narrow bands of precipitation between Portage & Winnipeg and just east of Winnipeg, but we’ll have to see what the actual wind direction ends up being as to where they’ll go. Right now it looks like there won’t be enough of a westerly component to the wind to bring them into Winnipeg.

If the surface winds end up too westerly, or the 850mb temperatures don’t cool off quite as much as forecast, then no streamers will form. It will be interesting to see what happens!

In the end, it could certainly be worse than it’s going to be. The coldest air will be over us during the overnight period, and we’ll likely have cloud cover as well which will help keep things a little warmer. By the time Thursday afternoon rolls around, the cold air will already be exiting the region, so we won’t end up dealing with temperatures quite as cool as many other areas across the Prairies will.

  1. It’s not fall yet! ↩

Change Is Coming

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This week will start out with more of the same normalish weather we’ve had for the past week or so…but it looks like a change in the pattern is coming…

A cold front will swing through Manitoba on Wednesday

A strong cold front (blue line) will swing through Manitoba on Wednesday ushering in cooler weather

Temperatures on Monday and Tuesday are expected to be around normal (or slightly below), with highs in the low to mid twenties expected. Our normal high for this time of year is 25C. We will be under a surface ridge of high pressure during this time period so rain appears unlikely.

A change in our weather is in store around midweek. It appears that a major cold front will swing through Southern Manitoba at some point on Wednsday. Models disagree with the timing of the cold front, but it looks most probably that the front will enter Western Manitoba sometime early Wednesday morning and be out of the province by Wednesday afternoon or evening. Regardless of the exact timing it looks like Wednesday will be a chillier day, with highs struggling to reach 20C and rain being likely. As we move closer to Wednesday the details of the frontal passage should be better known.

After the cold front passes through on Wednesday we’ll be in for a couple of cooler days with temperatures only in the upper teens or lower twenties for Thursday and Friday. Beyond this work-week there is some uncertainty in terms of what will happen next. Many models portray us sticking with more normal weather through the remainder of August, while others show us warming up again. As always, time will tell which forecast is correct!

Stormy Start to the Long Weekend

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The August long weekend will be off to a stormy start today as the threat of severe thunderstorms bears down on the Interlake, Red River Valley and Whiteshell this afternoon.

Tornado near Lake Diefenbaker

One of a couple of tornadoes that the incoming system spawned in Saskatchewan yesterday. Photo by @TheMrsCogs.

A very powerful low pressure, responsible for at least two tornadoes in Saskatchewan yesterday, will move into Southern Manitoba today and push a cold front across the province through the day. Very strong dynamics will accompany this system, a stark contrast from the last few systems which have had comparatively strong thermodynamics instead. Despite the lack of strong thermodynamic parameters that are often looked at when diagnosing severe thunderstorm potential, such as CAPE, EHI or LIs, the strong dynamics will present a significant severe weather threat over south-central and southeast Manitoba.

Dynamics are physical, kinematic properties of the atmosphere, such as jets, shear and lift. Thermodynamics are thermal/energetic properties of the atmosphere such as temperatures, humidity and instability.

A band of showers will push into the western Red River Valley early in the afternoon ahead of the upper low. As the afternoon progresses, the upper low will begin to deepen and enhance the destabilization over the Red River Valley. By mid-to-late afternoon, despite the cool temperatures and cloudy skies, there should be rapid development of storms along the cold front. Tornadoes are unlikely, although not impossible, here in Southern Manitoba; that threat should be relegated to the Dakotas where even greater dynamics and substantially better thermodynamics are present. If any tornadoes do form tomorrow, it’s likely that they will be of the short-lived variety and relatively weak. The greatest threat from today’s storm will be extremely heavy rain giving rapid accumulations of 1-2” (25-50mm) given precipitable water values of 40-50mm and large, damaging hail given the cool temperatures aloft.

Thunderstorm Outlook

Day 1 thunderstorm outlook valid 18Z today through 12Z tomorrow morning.

The storms will exit the province overnight, and we’ll have a brief reprieve from precipitation. On Saturday, some showers will wrap down into the RRV on the back side of this system. Amounts will vary across the region, but in general less than 5mm is expected. Temperatures will struggle to climb to even 20°C; daytime highs of only 18 or 19°C are expected. Winds will be breezy from the NW.

This system will clear out Saturday night before nicer weather builds back in. The second half of the long weekend will see increasingly sunny skies and daytime highs near 25°C.

More Showers Than Thought?

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A few systems are expected to bring chances for showers and thunderstorms to Southern Manitoba over the next few days as the upper ridge takes a few days longer to build in than previously thought.

30-Day Rolling % of Normal Precipitation for the Canadian Prairies

30-day % of normal precipitation for the Canadian Prairies. This map depicts the increasing drought conditions over much of the Red River Valley, with most areas only seeing only about 60% of the normal amount of precipitation over the last 30 days.

Offering a slight bit of relief to the dry conditions over the Red River Valley, an area of rain blossomed in North Dakota overnight and has pushed into Southern Manitoba this morning. Amounts are generally expected to be between 5-10mm along the International Border with amounts dropping off quickly to the north. This system will push into NW Ontario by late morning and skies will clear out behind it. Under sunshine our temperatures will soar to nearly 30°C in the afternoon.

The subtropical ridge that was previously progged to push into the Southern Prairies has instead decided to stay more or less where it was for a few extra days, which will leave us with a more zonal flow through this weekend. Fortunately, this means that we’ll have more chances for precipitation than previously thought.

The next chance for rain will come on Saturday afternoon/evening as a shortwave that will move across Saskatchewan today slumps southeastwards across the Interlake and Red River Valley. A couple showers and thunderstorms are likely to fire up underneath the shortwave as it crosses the RRV in the late afternoon. Currently all the convective parameters look to be fairly middling, so I don’t expect any severe weather to occur. Saturday will have a daytime high of 28°C before clouds move in in the afternoon. The low on Saturday night will be around 15°C.

Sunday should be a sunny day with a high near 28°C.

The next chance for rain will be Monday night as a shortwave slumps down the upper ridge that will build into the Western Prairies over the weekend. After that, it looks like the ridge will continue building into the Eastern Prairies, bringing us sweltering hot temperatures ove 30°C by the end of next week.