Change Is Coming

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

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.

Thunderstorm Risk Returns

The risk for thunderstorms returns to the Red River Valley today as a cold front with a history of producing severe weather pushes into the region.

850mb Analysis from the GFS

850mb Analysis from the GFS model for mid-morning today.

We’ll see some clouds/light showers clearing out early this morning across the Red River Valley, leaving us in sunshine for the latter half of the morning and early afternoon. By early afternoon, a cold front will be draped N-S over the western RRV. With temperatures climbing into mid-to-high 20’s by mid-afternoon, there should be enough heat and moisture to get storms going along the cold front. It’s likely, but not certain, that some thunderstorms will deveop on a north south line from near Winnipeg south towards the International Border. These storms will track eastwards across the RRV through the mid-to-late afternoon.

They will certainly have the potential to be severe. This cold front has a history of producing storms capable of damaging winds, large hail, and isolated tornadoes. For us, conditions don’t look quite as favorable as they did yesterday in SK or the day before in AB. We’ll be looking at:

  • 1000-2000 J/kg of CAPE
  • LI values near -7
  • Bulk shear values of only 20-35kt
  • 0-3km EHI values only 1-1.5

Wind shear will be the biggest limiter to storm development today. Storms may struggle to develop a distinguished structure, which could result in a relatively tame outcome. There’s just enough support to not be able to rule a severe storm out, however, and I expect we’ll likely see some watches issued for the RRV midday. The main threats for these storms will be heavy downpours with rapid rainfall accumulation and large hail. There’s a secondary threat of strong winds and, while tornadoes cannot be ruled out, the threat for the RRV is nowhere near the threat yesterday in Saskatchewan. Ultimately, though, the chance for seeing storms depends highly on the speed of the cold front; we’ll have to wait and see how quickly it moves into the RRV today to refine the thunderstorm forecast.

Things should clear out in the evening before a chance for showers returns late overnight and Thursday morning as the main upper low tracks over the RRV. We’ll see a mix of sun and cloud on Thursday with a high near 25°C. Friday heading into the weekend looks fairly nice. Sunshine should dominate and temperatures will climb back into the high 20’s or low 30’s. We’re certainly shaping up for another above-average month for temperatures. If July’s mean temperature is above normal, it will be the 13th consecutive month with above-normal temperatures in Winnipeg.

One Last Shot at Some Rain

Another frontal system tracking across Southern Manitoba today will bring one last shot at precipitation before the sub-tropical high begins to edge northwards and push us back into a hot, dry pattern.

15Z GEM-REG Analysis

Analysis of the GEM-REG model for this morning. The red and blue lines represent warm and cold fronts, respectively.

A cold front that brough severe thunderstorms and numerous tornadoes to Saskatchewan yesterday will push across the Red River Valley late this morning into the early afternoon. What sort of weather we’ll see from this depends highly on how much sunshine we’re able to see beforehand.

A rather large MCS will slowly decay as it moves through SW Manitoba this morning on approach to the Red River Valley. Some uncertainty exists as to what this system will do, however, and the result is dramtically different outcomes.

The NAM wants to keep this system quite progressive and push it eastwards fairly consistently. Under this solution, we’ll see a cloudy morning with some showers and thundershowers pushing through late morning/early afternoon. There’s a chance that rain could be fairly heavy under some of the thundershowers. Westerly winds would build in this afternoon flushing out the moisture in the Red River Valley and returning us to much more comfortable dewpoints in the low teens.

The growing trend in allh the other models, though, is for this system to stall out for a while over SW MB / extreme western RRV. If this is indeed what happens, things could be a little more interesting. Should the sun be able to poke through even a bit, that heating combined with the high surface dewpoints (should be in the 20-23°C range tomorrow morning) would build our CAPE values into the 2000 – 3000 J/kg range. That, combined with good deep shear and a SE wind at the surface veering to SW aloft, could provide an environment favorable for strong storms. The main threats would be heavy rainfall due to the slow-moving nature of the system (this threat would be centralized over the Western Red River Valley), strong winds and large hail. Stronger winds pushing in on the backside could favor the development of a squall-line type feature, but if they’re late to the party, a few isolated supercells certainly aren’t out of the question.

My take on all this? I think we’ll see some middle-ground solution play out. I think we’ll have a chance at seeing a bit of sun this morning, and by late morning some showers and thunderstorms will start pushing through the RRV. One can’t ignore the potential that exists for strong convection, and in that regard I would agree with the SPC’s “See Text” analysis for ND into Southern MB. The potential is there, but I wouldn’t say it’s likely. I hope that many places across the RRV see some rain as this front comes through, because it may be our last chance for a while…

8-14 Day Temperature Anomoly Outlook

NAEFS 8-14 Day Temperature Anomoly Outlook. The large area of red over the Canadian Prairies indicates a high probability of warmer than seasonal temperatures through the 8-14 day time period.

After this system, the sub-tropical high will begin to build northwards again, pushing plenty of heat into Southern Manitoba. Daytime highs will remain in the high 20’s to low 30’s through much of the next 7-10 days, with nary a drop of rain in sight for the Red River Valley. After many places have gone two weeks without any measureable precipitation, perhaps not the best of news.

We’ll keep a close eye on the storms this morning and post updates in the comments as the system develops!