#upper low

Unsettled weather will mark the start of the last weekend of July, but conditions will improve for a rather pleasant end and persist into a fairly dry and seasonal start to next week.

Friday: Thunderstorms Possible

Friday

25°C / 15°C

Chance of showers or thunderstorms

Today is off to a cloudy start with a few showers likely through the Red River Valley[1] as the remnants of the overnight activity push off to our east. Skies will remain mostly cloudy until the cold front that brought copious amounts of severe weather to Saskatchewan yesterday pushes through the Red River Valley early this afternoon.

As that cold front moves eastwards, it’s likely that more thunderstorms will fire up along it. Dewpoints in the upper teens will combine with daytime highs in the mid–20’s to produce MUCAPE values as high as 2000J/kg ahead of the front. Shear will be marginal with only around 25–30kt of surface-to–500mb bulk shear and the vertical wind profile isn’t particularly favourable for severe weather. The cold front will be enough of a trigger, although it will be weaker than yesterday and not offer nearly as much forcing as it did then.

As a result, thunderstorms will likely fire up somewhere in the central or eastern Red River Valley and push into the Whiteshell and far SE Manitoba this afternoon. At this point it doesn’t look like there will be an organized severe weather threat, but we’ll be taking a look a little later this morning to assess whether or not the severe threat needs upgrading.

The HRRR forecast shows a line of showers and thunderstorms pushing eastwards along the cold front this morning.

Other than the storms, the day will be pleasant. Before the cold front passes the Red River Valley will be under a relatively light southerly or southeasterly wind. It will be fairly humid and temperatures in the low 20’s until the passage of the front, after which westerly winds at 30–40km/h bring in much drier air. The afternoon sunshine should help our daytime high climb to around 25°C.

Temperatures will drop to around 14°C under clear skies and light winds tonight.

Showery Saturday

Saturday

24°C / 13°C

Mainly cloudy with widespread showers

Tomorrow will be the least pleasant day of the next few. A strong upper-level low will slide across Southern Manitoba through the day, producing widespread showers underneath it. There may be a few isolated thunderstorms, but for the most part most areas in Southern Manitoba will be seeing a fairly wet day. Winds will generally be 20–30km/h through much of the day, shifting from southwesterly or westerly to northerly through the day and overnight.

Skies will begin to clear overnight as the temperature drops to around 13°C.

Sunny Sunday

Sunday

24°C / 13°C

Mainly sunny

Sunday will likely start off with some low cloud trapped in the boundary layer making for mixed to mostly cloudy skies. As the temperature rises through the morning, the clouds will gradually break up leading to a mainly sunny afternoon with some scattered clouds. Highs will be around the 24–25°C mark and temperatures will drop to around 13°C once again on Sunday night.

This pleasant weather will continue through the first half of next week. Another strong ridge is forecast to build into BC/Alberta, which will keep temperatures seasonal[2] and the weather dry.

Full discretion: I’m writing this on Thursday evening, and still unsure on how much of Thursday night’s convective activity will survive to the Red River Valley.  ↩

Wet weather will finally give way this weekend to sunshine and warm temperatures as an upper ridge building into the western Prairies induces a dryer, more stable northwesterly flow aloft over Manitoba.

Friday 25°C / 14°C A few morning showers giving way to afternoon clearning.

Saturday 27°C / 14°C Mainly sunny.

Sunday 28°C / 15°C Sunny.

We’ll see a few morning showers today as the dying remnants of an upper low cycle their way out of the province. Once the scattered showers end in the morning, skies will begin to gradually clear through the afternoon. We’ll see a high of 25 or 26°C with winds out of the north at 20–30km/h. We’ll see clear skies tonight as temperatures drop to around 14°C.

A high pressure system will begin to push eastwards across the Prairies this weekend which will bring some beautiful, dry weather to Southern Manitoba. Temperatures will climb into the high 20’s this weekend under sunny skies with no chance of rain in sight. Overnight lows will dip to around 15°C.

Cooler, more seasonable, weather is going to move into Southern Manitoba for the week ahead as a large upper trough swings into the region. [](https://img.skitch.com/20120903-bbgnmcen6334ycus6ah2ir2gif.png)

GEM-REG 500mb wind field valid Tuesday morning. A deep upper low is straddling the SK MB border with a trough extending to the SW denoted by the dashed black line.

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. 

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" (25-50*mm*) given precipitable water values of 40-50*mm* and large, damaging hail given the cool temperatures aloft. [](https://img.skitch.com/20120803-jryehbxtcxqa468j7sfysxyujj.png)

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

Warm weather will continue to dominate Southern Manitoba as we climb into the high 20's almost every day this week. We'll continue to see active weather as multiple systems move through the province over the second half of this week.

Forecast 500mb wind field with heights depicting powerful upper low entering Southern Manitoba on Friday evening.

July ended up being a very hot month for Winnipeg, with 14 days of the month with daytime highs over 30°C. The month ended with an average temperature (highs and lows both considered) just over 22°C, which is about 2.5°C above the normal average temperature of 19°C. July marked the 13th consecutive month that Winnipeg has seen above-normal temperatures.

July marked the 13th consecutive month that Winnipeg has seen above-normal temperatures.

We'll see a chance of afternoon thunderstorms across most areas of Southern Manitoba as a weak disturbance rounds the cold trough left behind yesterday's system that brought a thunderstorm threat to Southern Manitoba. Chances for thunderstorms will be significantly higher through SW MB northwards through the Parkland region while the Red River Valley will see just a chance of thundershowers in the evening as the upper feature passes by. Daytime highs will be near 28°C through the Red River Valley with an overnight low tonight near a comfortable 15°C.

We'll see a reprieve on Thursday, with no precipitation expected and a high near 28°C again. A powerful upper low will begin pushing across the Western Prairies bringing rain and cooler temperatures tommorow and will be the main weather-maker on Friday for us. Showers and thunderstorms will push into Western Manitoba on Thursday night and slide eastwards across the province through the day on Friday. It's likey that we'll see little precipitation on Friday as most of it pushes through the Interlake, however there's a good chance of showers overnight on the backside of this system. We'll see a daytime high of 24°C on Friday and a considerably cooler high near 20°C on Saturday.

While the temperatures may not be completely summer-like, today will be a day that indicates a clear shift towards summer weather.  An incoming disturbance will produce widespread convection today with showers and thundershowers covering much of Southern Manitoba.  Pleasant weather should make an appearance for tomorrow's U2 concert, but Winnipeg will quickly return to rainy weather on Monday as a system that will bring significant challenges to those with housing near the lake moves into the province.

A substantial stacked low pressure system currently positioned just east of Regina is set to move across Southern Manitoba today.  A few factors will combine to produce what could be a fairly interesting afternoon today.  

First, a southerly wind, combined with yesterday's rain has pushed dewpoints to nearly 10°C throughout the Red River Valley with a relatively deep layer of evidenced by the stratus cloud that has managed to form overnight despite winds of 15-20km/h.  This moisture will provide the fuel needed for thundershowers. 

Secondly, rather cool mid-level temperatures will be in place.  As the low pushes in, temperatures in the mid-levels should drop to -7 to -8°C, which when combined with daytime heating will produce ample instability to get convection initiating.  It is also indicative of fairly low freezing level, which will help in the generation of small hail.

And last but by far not least, the fact that this upper disturbance is stacked on top of a surface low means that there will be significant rotation through the mid and upper levels which will help storms develop rotation.

When these things combine, it's considered a pretty classic case for cold core funnels.  These funnel clouds do not develop in the classic sense; instead of developing in the rapidly rotating updraft of supercell thunderstorms, they develop through the descent of cold air through a storm that has developed rotation due to significant mid-to-high level spin.  Think of it as a funnel cloud that develops because of how air is coming out of the cloud rather than how air is being ingested into the cloud.

These funnel clouds do have the potential to touch down and become tornadoes.  If they do, they are often extremely short lived (< 1-2 minutes) and are much weaker than the tornadoes that form from supercell thunderstorms.  Any of the storms that could produce a cold core funnel cloud are also extremely likely to be capable of producing hail.

I think that most areas across Southern Manitoba could see hail today, although the threat is biggest in the Red River Valley and east.  Winnipeg and areas south should most likely see pea-sized hail should it develop.  Further east in the Whiteshell and Sprague regions, the potential exists for marginally severe hail (about dime to quarter-sized).

The biggest hindrance to the development of all this is the lack of focus for convection.  With no strong front or trough, convection may simply blow up all over the place instead of in any organized fashion, which will leave all the storms fighting each other for resources.  If this happens, it's likely that it would quickly blossom into a big area of showers with only the odd lightning strike.

My bet for the Red River Valley is this: low stratus this morning will burn off quickly with the morning sun.  By mid-morning, most areas should be seeing a mix of sun and cloud.  Daytime heating, combined with a weak cap, will initiate the convection earlier than typical, resulting in showers spreading into the Red River Valley around 12-1PM or shortly thereafter.  Any showers that develop in the RRV are just as likely to become thundershowers, and I think there will be numerous funnel clouds today.

As for Sunday and the big U2 concert, it should be quite a pleasant day.  We'll see a sunny morning give way to a mix of sun and clouds in the afternoon with a daytime highs around 15-17°C.  It will cool off quick in the evening, so make sure to bring a sweater if you're headed to the concert.

12hr. QPF valid 06Z May 31 from the 06Z May 28 GFS