A wild rollercoaster of weather conditions begins today as a fairly potent weather system develops today, rocketing temperatures close to the 30°C mark before things take a turn for the worse as a stormy transition sends daytime highs struggling into the low teens by the end of the weekend.

Friday: Another Beautiful Summer Day, Stormy End?

Today will be a gorgeous day as warm air surging northwards ahead of a developing low pressure system in the Northern Plains of the United States.  Temperatures will climb into the upper 20’s this afternoon under fairly sunny skies. Some cloud cover will start working into the region in the afternoon alongside a slight chance of showers as a weak mid-level disturbance rolls through. In the evening, a low-level jet (LLJ) begins to intensify as the low pressure system deepens and brings more lift into the region, spreading more cloud into the region.

A round of thunderstorms is possible tonight along the nose of the LLJ.  The risk is somewhat conditional, dependent exactly how how much moisture is advected northwards within the LLJ. Assuming enough moisture is present to realize the instability, we head to MIST:

• Moisture: This will be the most uncertain aspect of tonight’s thunderstorm potential.  Dewpoints at 850mb are forecast to be around 10°C, which should be enough to realize the elevated instability, however it’s a farily narrow ribbon, and if amounts are lacking a bit, convective initiation will struggle.
• Instability: Fairly strong instability aloft will be present.  Models suggest mid-level (700 – 500mb) lapse rates of 7–8°C/km this evening, which would be considered quite strong.  If the expected moisture builds into the region, MUCAPE values of around 1000 J/kg should be realized.
• Shear: The overall shear looks great, with widespread bulk shear values of 40–50kt expected over the Red River Valley this evening. The high bulk shear values are produced through a combination of strong velocity shear through the vertical coupled with large, looping hodographs indicative of favourable directional shear.
• Trigger: Isentropic ascent of the LLJ as it overruns a warm front draped west-to-east along the international border coupled with falling heights ahead of the incoming upper-level low should provide a strong enough trigger to initiate convective activity.

All this combined means that there’s a fairly good chance of some thunderstorm activity through the Red River Valley this evening.  It likely won’t be severe; I think the higher threat for severe storms activity will be more in southwestern Manitoba where things will initiate earlier and rooted closer to the warm front.  That said, it certainly can’t be ruled out that there may be a rouge severe thunderstorm cell through the evening into the first half of the overnight period. Expect a low temperature near 17°C.

Mixed Bag On Saturday

There remains some uncertainty as to how Saturday will play out, but it seems quite probable that we’ll see some shower or thunderstorm activity through the day. There may be a few sunny breaks, but it should be cloudier than not through much of the day. If any rain manages to hold off until later in day, marginally severe thunderstorms could be possible in the Red River Valley, however it would be dependent on how much heating occurs (read: how sunny it manages to be). Even with decent heating shear looks marginal which will provide a barrier to significant thunderstorm development.

This comes with a caveat, however, as the forecast will be dependant on the timing of the cold frontal passage. With current best indications, there will be a slight risk of severe thunderstorms to the east of the Red River Valley on Saturday afternoon, however if the front slows down a little, that threat could shift westwards into the Red River Valley. We’ll take a closer look at it in an update tonight or tomorrow morning.

That said, temperatures will top out around 25°C with fairly light winds.

Saturday night will see the Red River Valley on the back side of the low and behind the cold front; colder air will begin pushing in alongside strengthening northerly winds. Expect a low near 13°C.

Miserable Sunday

Sunday will be a miserable day by any standard. Very cold air over the region—850mb temperatures are forecast to be a chilly 0–2°C—will keep our daytime highs to just the low teens. Very strong northerly winds will be in place over the region; sustained winds will likely reach 50–60km/h with gusts reaching as high as 80km/h or so. On top of all that, wrap-around precipitation will work it’s way across the Red River Valley, bringing more rain and/or showers to the region.

To top it off, with lake temperatures near 21°C and 850mb temperatures forecast to be near 0°C, lake-effect or lake-enhanced rain/drizzle is probable in the lee of the lakes, persisting possibly into Monday morning.

The low temperature on Sunday night will be around 6 or 7°C.

Hopeful in the Long Range

The good news is that the long-range forecast looks good. Rather quickly behind the passage of this low, a broad upper-level ridge is forecast to begin building back into the Prairies. This will bring significantly warmer weather back to the region. Considering it’s still August and we’re talking about daytime highs in the low teens, a quick bounce back into the mid- to upper–20’s for daytime highs will be much appreciated.