Late-Season Thunderstorm Risk Returns to the Red River Valley

A developing Colorado Low has brought very humid conditions to Manitoba’s doorstep, and a disturbance moving through the province today will tap into that moisture and spread showers and thunderstorms across the Red River Valley into the southeastern corner of the Province.

An upper-level disturbance moving through the province will bring a late-season thunderstorm risk to the Red River Valley with a small risk of severe storms. There’s a surprising amount of support for thunderstorms, largely due to the building humidity in Minnesota over the past few days. While the weather in Winnipeg is cool and dry, conditions have become positively balmy in Minnesota, where there are overnight lows in the 20s thanks to sticky dew points in the 20-22°C range.

A stationary front lies across northern Minnesota and far southeastern Manitoba this morning.

The dry air over the Red River Valley is separated from the muggy Minnesota conditions by a fairly strong frontal boundary running from Nebraska northeastwards through Minnesota and into northwestern Ontario. As an upper-level disturbance approaches today, that humid air will be lifted up over the front and into southern Manitoba. So while we won’t see particularly humid conditions here at ground-level, further up in the atmosphere will see significant moisture move in.

Before the weather gets busier this afternoon, temperatures will climb to a high near 18°C, but winds will be breezy, increasing out of the northeast to 30 gusting to 50 km/h.

By mid- to late-afternoon, things will likely begin firing up with widespread showers moving into the province from North Dakota. While showers and/or thunderstorms will be widespread over south-central and southeastern Manitoba, the severe thunderstorm threat will be confined to two regions. Within the Red River Valley, there will be a slight chance of an isolated severe thunderstorm or two, while southeastern Manitoba sees a higher chance of more widespread severe thunderstorm activity. The primary threats from today’s thunderstorms would be severe hail and/or wind.1

AWM Thunderstorm Outlook for September 22, 2017
AWM Thunderstorm Outlook for September 22, 2017

The rain and thunderstorms will move out of the region this evening. When all is said and done, most areas will have seen somewhere between 5-10 mm of rain, with amounts of 20-40 mm possible in areas that see more thunderstorm activity. Skies will remain fairly cloudy tonight as temperatures dip to a low near 10°C with winds diminishing to around 20 km/h.

Things calm down for Saturday as the region sees a lull between Friday’s disturbance and another system moving on for Sunday. Expect mostly cloudy skies, a high near 16°C, and winds out of the north at around 20 km/h. Skies will remain cloudy on Saturday night with a low near 10°C again.

GDPS 24hr. QPF valid 06Z Monday September 25, 2017
Some guidance suggests significant rainfall on Sunday

A Colorado Low will progress through the region on Sunday, bringing rainy conditions to much of Southern Manitoba. Unfortunately at this time, it’s not quite clear exactly how rainy things will be. Some guidance has this system a bit more progressive and tracking further eastwards, which would result in 5-15 mm of rain in the Red River Valley, but others — such as the GDPS shown above — are slower with the system and bring it further west. The slower solutions would result in higher rainfall amounts for the region, more likely in the 15-30 mm range with localized spots perhaps seeing 40 mm.

At this point, the lower rainfall solution seems most likely, but we’ll be keeping an eye on it. Otherwise, it will be a cool day with a high of just 12°C and more northerly winds around 30 km/h. Expect a cloudy low near 9°C on Sunday night.

Long Range

Cloudy and cool conditions are expected to persist into the beginning of next week, but Winnipeg will likely be done with any significant rainfall chances. By mid-week, it looks like things will clear out with temperatures returning to seasonal values. The more pleasant weather may continue through next weekend, which would be a nice change!

Winnipeg’s seasonal daytime high is currently 16°C while the seasonal overnight low is 4°C.

  1. EC³ considers hail with 20 mm diameter or larger and wind gusts in excess of 90 km/h to be severe.

August Brought Mild Days, Cool Nights, and Almost No Rain

August was an interesting month regarding temperatures, particularly because the overall trend was shaped largely by the distinct lack of humidity. The monthly mean temperature ended up slightly below seasonal, but that was driven by unseasonably cool overnight lows. In fact, the average overnight low for August 2017 was 2°C below the seasonal average. Contrast that with daytime highs that were actually warmer than normal; August 2017 ended with an average high slightly above the seasonal average.

Perhaps of note, humidity levels were quite low through the month of August. We don’t quite have the dew point climatology set up here at A Weather Moment, so I asked Winnipeg’s resident weather statistician how August 2017 ranked in terms of average dew point:

So August 2017 ended up the 16th least humid of the last 64 years. The reason cooler nights can be linked to the low levels of humidity come from a simple physics lesson many learn in high school: dry air heats up – and conversely cools down – more quickly than water. The drier conditions allowed temperatures to climb high, but also allowed it to cool off more significantly at night.1 As an aside, this is why many humid places have smaller temperature ranges from day to night, while the driest places on earth typically have huge temperature ranges between day and night.

Getting into the actual statistics, August 2017 ended with a mean temperatures of 17.7°C, -0.8°C below the seasonal average of 18.5°C. The average daytime high was 25.5°C, 0.5°C above the seasonal average of 25.0°C. The average overnight low ended up at 9.9°C, -2.0°C below the seasonal average of 11.9°C.

August wraps up Summer 20172, which ended up with a mean temperature of 17.8°C. That places it as the 87th warmest3 on record, and comes in -0.6°C cooler than the summer of 2016.

Rain, What Rain?

Another notable aspect of August 2017 was the distinct lack of rainfall. Typically Winnipeg will receive around 77 mm of rain through August, but this year the city received only 14.1 mm at the airport, a mere 18% of the normal monthly rainfall.

August was just another month in the story of summer 2017: it was dry. The summer season accumulated 136.7 mm of rain at the airport, which was just 56% of the seasonal normal of 244 mm.

2017 Annual Precipitation Statistics – Updated September 20, 2017
Winnipeg is currently experiencing one of the driest years since 1981.

The drier pattern began in May, and then has not relented. While other areas in the Red River Valley are not quite in as dry a spell as Winnipeg, a general trend of dryness has persisted across much of the region.

August 2017 had measurable precipitation on just 6 of 30 days, with the single largest one-day rainfall total of 6.7 mm on Wednesday August 9th.

August 2017 was actually quite a pleasant month in many ways: most days brought a pleasant and dry summer warmth while evenings were cool. The month brought plenty of sunshine with rain being an elusive sight, continuing the dry trend that began in May of this year.

September has been quite a different month so far, with significant warmth in place this month. Winnipeg hit the warmest day of 2017 on September 12th when temperatures climbed to a record-setting high of 34.8°C. Daytime highs and overnight lows have largely been above-normal much of the month. While rain was sparse at the beginning of the month, the latter half has so far shifted into a more unsettled pattern, bringing relatively frequent rainfalls to the region, although Winnipeg has managed to elude much of the activity.

  1. I also asked Julien to calculate the correlation between humidity levels and highs and lows. Daily low temperatures had an r value of 0.61, showing a much stronger correlation than daytime highs which had an r value of 0.28.
  2. Meteorological summer runs through June, July, and August.
  3. …or 59th coldest, depending how you prefer to look at it.

Generally Unsettled Weather Ahead

Winnipeg and area will continue to see a variety of generally unsettled weather ahead thanks to a persistent as a south-to-southwesterly flow aloft continues throughout the remainder of the week.

Today will be the nicest day of the next few as a few remaining showers clear out from last night’s activity early this morning, leaving the region with sunny skies for the rest of the day. Winds will be out of the southwest at around 30 gusting 50 km/h all day with temperatures climbing to a high near 20°C. Tonight, temperatures will dip to a low near 11°C with winds out of the south at around 20 km/h.

Thursday will see the winds taper off through the morning as skies cloud over. A chance of showers will persist much of the day1 and then taper off in the evening. Temperatures will climb to a high near 18°C. Skies will clear out overnight with temperatures dipping to a low near 10°C with light winds shifting to the north.

Winnipeg may be clipped by a band of showers that moves northeast through the day on Thursday

Friday will once again see cloud move in early in the day with winds picking up out of the north to northeast to around 15-25 km/h. There will be another chance for light showers throughout the day, but it looks like they will be light and scattered. Temperatures will climb to a high near 18°C. Skies will remain mostly cloudy on Friday night with temperatures dipping to a low near 9°C.

Long Range

The long range forecast continues to provide more of the same, with guidance suggesting cloudy skies throughout the weekend and into the start of next week. Another system will pass through on Saturday night, bringing the potential for another round of organized rainfall, followed by another system Sunday night into Monday. Temperatures will be below normal through this entire period with winds out of the north to northeast.

So not the nicest fall weather ahead, but potentially some more needed rain coming to the region.

Winnipeg’s seasonal daytime high is currently 17°C while the seasonal overnight low is 5°C.

  1. Being honest, it’s about a 50% chance, but people don’t really like hearing that in their weather forecast.

Warmer Weather Returns, But More Chances For Rain Ahead

Temperatures will rebound back above seasonal values today as southwesterly winds aloft bring warmer air back to the region, but another major low pressure system is heading into the Prairies that will spread rain across southern Manitoba on Tuesday.

Today will be a beautiful day in Winnipeg after a bit of a dreary weekend with plenty of sunshine and a high near 19°C. Winds will be light out of the south to southeast. Through the afternoon hours, a bit of cloud will move through the region, bringing mixed skies to Winnipeg and cloudier conditions to the southeast. Some areas may even see some light showers, but that looks like it will be more confined towards the Whiteshell and Sprague regions. Temperatures will then head to a low near 10°C tonight under a few clouds.

RDPS 2m Temperature Forecast valid 21Z Monday September 18, 2017
Daytime highs will sit in the upper teens across much of Southern Manitoba on Monday.

Tuesday will bring the arrival of the next significant low pressure system in Manitoba. Temperatures will continue to be relatively mild with highs near 19°C again, but winds will be breezy picking up out of the southeast to 30-40 km/h. Cloud will spread into the region early in the day, with rain blossoming1 mid-day over southwestern Manitoba and mid- to late-afternoon over the Red River Valley. The winds will then taper off in the evening, with the bulk of the rain pushing off east of the Red River around midnight. Temperatures will dip to a low near 11°C.

GDPS 24hr QPF Forecast valid 18Z Wednesday September 20, 2017
The GDPS suggests very high rainfall totals possible west of Winnipeg on Tuesday afternoon and evening, but the overall rainfall totals will likely fall into the 10-25mm range over the Red River Valley.

Wednesday will start off with a slight chance of a few lingering showers, but will then transition towards drying out and clearing as the day progresses. Temperatures will be mild with highs near 20°C again with a bit of a breeze out of the west to southwest at around 20-30 km/h. Expect a low near 11°C on Wednesday night under mainly clear skies.

Long Range

Thursday looks to be another nice day with above-seasonal highs near the 20°C mark, but then another major low pressure system will track into the Northern Plains Thursday night through Friday, bringing another chance for rain alongside brisk northeasterly winds and cooler temperatures. Once it passes by Sunday, it appears that Winnipeg will then see a more extended period of near- to below-seasonal temperatures.

Winnipeg’s seasonal daytime high is currently 17°C while the seasonal overnight low is 5°C.

  1. There may even be a chance of seeing some thunderstorms!