Tag: Severe Thunderstorms

Friday’s Surge of Heat & Humidity Leads to Unsettled Weekend

Brad

Heat & humidity will begin moving into the region today as a breezy southerly wind develops over the province, tapping into a much warmer air mass over the Northern Plains of the United States.

A potent low pressure complex developing over the western Prairies & Northern Plains will bring a thunderstorm threat back to Southern Manitoba today with the southwestern corner of the province under the threat for thunderstorms—potentially severe—this afternoon and this evening, while the Red River Valley and southeastern Manitoba seeing the threat move in this evening and into Saturday morning.

Before that, though, Winnipeg & the Red River Valley will have a fairly nice day ahead. Temperatures will soar through the morning, reaching within a degree or two of today’s high temperature of 28°C by lunch. Skies will be mixed today with cloudier conditions developing in the afternoon. Southerly winds to 30-40km/h will pick up through the day across much of Southern Manitoba, drawing moisture from the United States northwards into the province. By late afternoon, dew point values in the Red River Valley will climb to the 16-18°C range1 while in the southwest corner of the province, dew point values may reach the 20°C mark, making for very humid feeling conditions.

AWM Convective Outlook for June 24/25, 2016
AWM Convective Outlook for June 24/25, 2016
As the afternoon wears on, a leading shortwave lifting out of Montana will destabilize conditions over southern Saskatchewan and southwestern Manitoba, bringing a threat of thunderstorms to the region. There’s a slight chance that some of these thunderstorms may become severe. The threats from these storms will be:

  • Rainfall: Precipitable water values of 35-45 mm combined with storm motions of 20-30km/h will produce very intense rainfall, but the storms will be moving quickly enough that the overall rainfall accumulations will be limited.
  • Hail: With MLCAPE values in the 1500-2500 J/kg range and ample shear present, storms will likely take the form of discrete supercells early in their life cycle. These storms will be capable of producing large, damaging hail.
  • Wind: Overall, wind won’t be a widespread threat with these storms, but isolated damaging wind gusts are possible.
  • Tornadoes: The supercell thunderstorms in southeastern Saskatchewan and southwestern Manitoba will be in environments with very strong directional shear. A small chance of a tornado or two exists with these storms.

As we move into the evening, scattered showers or thunderstorms are possible over the Red River Valley while the thunderstorms closer to the SK/MB border continue onwards and grow into a larger complex of thunderstorms that will push eastwards overnight. Instability actually increases overnight as moisture continues to be pumped northwards along the low-level jet ahead of the incoming shortwave. As a result, a large area of rain and thunderstorms will likely progress eastwards overnight, reaching the Red River Valley between 3 and 6AM. These thunderstorms will present a slight risk of severe weather with primary threats of rain and large hail, with a secondary threat of damaging wind gusts.

Worth noting: if the western shortwave offering support to this system ends up slowing down, the overnight thunderstorm activity may diminish over southwestern Manitoba.

Temperatures will remain very warm on Friday night with overnight lows near 20°C in the Red River Valley.

Saturday: Muggy with Continued Thunderstorm Threat

Saturday will start with whatever convection moving through on Friday night exiting the region, and we’ll be left with very humid conditions with dew point values likely climbing to 20-21°C. Temperatures will climb towards the mid-20’s through the morning under fairly cloudy skies, but conditions will undergo a fairly significant change midday as a cold front sweeps through. The passage of this front will bring another threat of showers and thunderstorms to the region and with MLCAPE values near 1500-2000 J/kg and ample shear, these will also have bring the threat of severe weather with rainfall, winds and hail a concern.

4km NAM Dewpoint Forecast valid 15Z June 25, 2016
High dewpoint values over 20°C will be in place on Saturday morning in the Red River Valley.
Once the front passes, gusty westerly winds will usher in drier air as dew point values plummet into the single digits, bringing relief from the humidity. Skies will begin clearing as well, likely providing some afternoon/evening sunshine. Temperatures will likely top out at only 24 or 25°C, though, and a cooler night will be ahead with lows near 15°C.

Dreary End to the Weekend

Saturday night’s clear skies will be replaced by cloud on Sunday as the main upper-level low of this whole weather system moves through the region. It will bring showers to much of Southern Manitoba with most of the activity through the morning hours and then tapering off through the afternoon. Temperatures will be much cooler with a high near 17 or 18°C and strong northwesterly winds of 40 gusting 60 km/h picking up by the end of the morning.

The winds will taper off overnight as the valley heads to a low near 11°C.

Winnipeg’s seasonal daytime high is currently 24°C while the seasonal overnight low is 12°C.

  1. At a dew point of 16-18°C, it begins to feel mildly muggy. ↩

Special: Severe Thunderstorm Threat Returns for Father’s Day & The Manitoba Marathon

Brad 1 Comment

Sunday, June 19th marks both Father’s Day as well as the Manitoba Marathon, and unfortunately it appears that the unsettled weather we talked about in Friday’s post has the potential to bring a significant threat of severe thunderstorms to the region.

AWM Convective Outlook for June 18/19, 2016
AWM Convective Outlook for June 18/19, 2016

The Short Version

Severe thunderstorms are possible early Sunday morning, however that threat will be conditional on the expected development of thunderstorms in Saskatchewan on Saturday night.

The current expectation is that an area of thunderstorms will develop over Saskatchewan Saturday night, expanding in coverage and accelerating eastwards through the overnight period. This area of convection will then move through the Red River Valley with a primary threat of severe and damaging winds as well as large hail and torrential rain capable of producing flash flooding.

Thunderstorm Threat Outlook for Sunday June 19, 2016
Threat Type Threat Level Details
Rain Moderate Torrential rainfall possible with flash flooding.
Hail Moderate 2-4cm hail possible.
Wind Moderate Strong wind gusts in excess of 100 km/h likely
Tornado Low No tornado activity expected.

The thunderstorm activity is expected to occur between 4:00AM and 9:00AM, with the primary threat period between 5:00AM and 7:00AM. The timing may adjust depending on the exact speed of the system, but overall this is expected to be a morning event.

Forecast for the morning of June 19, 2016
6AM 8AM 10AM 12PM
17°C 19°C 21°C 23°C
Thunderstorms likely Thunderstorm activity diminishes Slight chance of showers Partly cloudy

Alongside the thunderstorms, conditions will get notably more humid as the morning progresses.[1] By the time any thunderstorm or rain showers taper off, things will feel fairly sticky. Temperatures will start around the 16-17°C mark in the morning, climbing to around 23°C by noon, and then on to a high near 27°C in the afternoon. Winds will be fairly light until mid-afternoon when they pick up from the west to around 20-30 km/h, ushering in drier air and providing some relief from humidity.

The Details

The potent thunderstorm potential for Saturday night and Sunday morning across the southern Prairies will be the result of a potent shortwave that ejects northeastwards out of Montana on Saturday and then rides eastwards along the international border. This will spawn a surface low with cold front draped southwards as cooler air builds in behind the shortwave and warm front extending eastwards across the northern portions of the Northern Great Plains. The surface low will then slide eastwards, gradually lifting the warm front northwards as it goes.

This shortwave will [likely] initiate an area of convection over southwestern Saskatchewan that will track eastwards along with the shortwave. The orientation of the warm front will provide plenty of support for the convection with shear profiles that indicate the likely development of a MCS[2] containing bow echoes.

Going over the MIST ingredients:

  • Moisture: Dewpoint values near 18°C will be ingested into the convection on Saturday night. This will provide substantial energy for the thunderstorms to work with. Additionally, the high moisture content of the air being ingested will increase PWAT[3] values to over 50 mm, making very intense rainfall likely with any thunderstorms that develop.
  • Instability: Steep mid-level lapse rates will combine with the warm and humid air of the nocturnal inflow to produce MUCAPE values in excess of 2000 J/kg.
  • Shear: 55-65 kt of bulk shear coupled with veering profiles within the convection will support the development of strong, organized thunderstorms.
  • Trigger: Shortwave-induced mid-level destabilization coupled with the development of a strong LLJ.

For these parameters, the primary concerns will be hail and rain, however should the storms be able to break through the inversion, wind will rapidly become a significant threat as gusts in excess of 100 km/h will be possible.

The steep mid-level lapse rates combined with high MUCAPE values will produce a moderate hail threat with hail between 2-4 cm in diameter possible. The heightened PWAT values make torrential rain likely with these storms, which can result in rapid accumulations and flash flooding. The strong winds & shear profiles of the storm environment make damaging wind gusts possible. Justification for the strong winds despite the nocturnal part of the convection will be the intense storm-relative winds that will be associated with this quickly-moving system. The SR-inflow will likely be strong enough lift the low-level air mass wholesale into the storm, resulting in a surface-based or near-surface-based thunderstorm.

Summary

Father’s Day of 2016, and the Manitoba Marathon, face the potential of being remembered for severe thunderstorms if current guidance continues painting the picture it does. Thunderstorm potential is conditional on nocturnal convection developing on Saturday night in Saskatchewan, but if it does, there is a significant threat of severe thunderstorms over Southern Manitoba on Sunday morning.

The timing for the primary severe thunderstorm threat will be in the early morning, buffering the starting time for the marathon, with much of the thunderstorm activity expected to be out of the region by 10-11AM. In the wake of this system will be a warm day that will feel fairly humid until westerly winds kick in mid-afternoon, pushing out the humidity and drawing in drier, slightly cooler air for the start of next week.

  1. Dewpoint temperatures are expected to rise from 12-13°C early in the morning to around 19°C by mid-morning.  ↩
  2. Mesoscale convective system.  ↩
  3. Precipitable water.  ↩

Stormy Start to An Unsettled Weekend

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Friday will bring a severe thunderstorm threat to Winnipeg & the Red River Valley as the first of several bouts of unsettled weather moves through, gradually flushing out the unseasonal heat and humidity and transitioning us towards almost fall-like weather.

Friday
29°C / 18°C
Mainly cloudy; showers and thunderstorms this afternoon

Saturday
24°C / 17°C
Mainly cloudy; slight chance of showers. Evening showers or thunderstorms possible

Sunday
24°C / 11°C
Mainly cloudy; slight chance of drizzle or light rain

Mainly cloudy skies will be over Winnipeg & the Red River Valley today while a low pressure system gather steam and begins to move into the region from North Dakota. Little in the way of weather is expected until the low approaches this afternoon and triggers a substantial amount of convective activity. Showers and thunderstorms are expected to fire up mid-afternoon and track to the northeast. There’s a decent chance that the stormy weather will persist well into the evening before it clears out and moves off to the northeast.

AWM Day 1 Convective Outlook
A slight risk of severe thunderstorms exists over south-central and southeastern Manitoba today.

A slight risk for severe thunderstorms will be in place over much of south-central and southeastern Manitoba today. Looking at the MIST basics for thunderstorms:

  • Moisture will be ample with deep boundary layer moisture evident through surface dewpoint values near 22°C thanks to a multi-day build-up of humidity in a southeasterly flow. Strong evidence of the substantial moisture in place is shown by the precipitable water values which are forecast to climb over 50mm through the afternoon.
  • Instability will moderate with MLCAPE values around 2000–2500 J/kg. Much of the instability will be brought through the substantial moisture present since mid-level lapse rates are only expected to be moderate and not excessively steep.
  • Shear will be ample with approximately 25–35kt of 0–6km bulk shear and broadly looping hodographs. This should support the development of organized thunderstorms.
  • Trigger: there will be several triggers converging on the Red River Valley this afternoon including a frontal boundary, approaching low, as well as shortwave aloft. There should be no troubles finding something to trigger thunderstorms.

The primary threats from today’s storms will be heavy rain and large hail. PWAT[1] values are expected to climb over 50mm, which means torrential downpours and flash flooding will be a notable threat with this afternoon’s thunderstorms. Additionally, with such high MLCAPEs and adequate storm structure through the shear profiles, large hail may be an issues, however that would likely be a more isolated threat than the rainfall.

The thunderstorms will gradually push off to the northeast. Total storm rainfalls may exceed 2–4” (50–100mm) in some places, while other areas may end up seeing very little. Be sure to stay alert for any watches or warnings issued by Environment Canada.

Less excitingly, daytime highs will sit around 29°C today (which will feel more like the upper 30’s thanks to the humidity) with winds out of the southeast at 20–30km/h. Expect a low near 18°C.

Cooler, Unsettled Weekend

Both Saturday and Sunday will be cooler days with highs in the mid–20’s. Some uncertainty exists on precisely how things will play out, but in general, a very powerful low pressure system is expected to develop and lift northwards through Saskatchewan through the weekend.

The biggest change, compared to Friday, will be that it seems like the timing for any precipitation will shift from daytime hours towards the evening and overnight hours. Saturday will be mainly cloudy with just a very slight chance of some showers through the daytime hours. As the evening approaches, though, it looks likely that thunderstorms will develop along the western escarpment of the Red River Valley and then progress eastwards across the valley overnight. At this point it looks like the storms will be non-severe, but we’ll keep an eye on them in case the threat increases as we get closer to the event.

The overnight low on Saturday night will be around 17 or 18°C.

Sunday will be another fairly cloudy day with a slight chance of some very light rain or drizzle as easterly winds keep the boundary layer socked in moisture while drier mid-level lift approaches. As the evening approaches, once again some showers or thunderstorms are possible. Expect a low in the low teens on Sunday night.

Next Week’s Weather?

At this point, next week looks much cooler and drier with more fall-like weather in place. Daytime highs through the first half of the week look to be in the 15–20°C range while overnight lows will drop towards the mid-single digits. Little precipitation is expected, and we’ll see more and more sun as the week progresses.

  1. PWAT stands for precipitable water, a measure of how much water is contained in a column of air.  ↩

Unsettled Friday Leads Into A Hot Weekend

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Notably warm and humid weather in place over Southern Manitoba today will bring a threat for severe thunderstorm activity across the region as a shortwave trundles through. An upper-level ridge will then begin to rebound back into the Southern Prairies in response to a digging trough off the western coast of North America, bringing even warmer air into the region thanks to the resultant southwesterly flow aloft that will develop.

Friday: Thunderstorms Possible Midday

A weak mid-level shortwave trundling across the region will bring a risk of thunderstorms to Winnipeg & the Red River Valley today[1] as warm and humid weather meets the colder air associated with the shortwave. As always, using our MIST principles:

  • Moisture: Adequate. Moderate moisture will be in place both at the surface and through the lower levels of the atmosphere as deep-layer moisture continues to build into the region.
  • Instability: Favourable. With both thunderstorm possibilities, both elevated and surface-based, enough instability will be in place. Surface-based convection would have greater potential to be severe, however it will rely on strong surface heating with mainly sunny skies until the shortwave arrives, which could be in question depending on what sort of nocturnal convection develops.[2] If it does end up cloudy, strong mid-level lapse rates associated with the shortwave should be enough to sustain showers or thunderstorms as it heads eastwards. All said, storms should have around 1000–2000J/kg of CAPE to work with, depending on their exact timing and where they’re based.
  • Shear looks quite good with around 35kt of 0–6km bulk shear and gently looping hodographs. Low-level winds are weak enough that tornado activity doesn’t look like a particular threat, but the directional shear is favourable for the organization of supercell thunderstorms.
  • Trigger: The shortwave moving through coupled with a weak surface trough will provide the convergence and trigger for convective initiation.
AWM Day 1 Convective Outlook
A slight risk of severe thunderstorms exists in the Red River Valley and southeastern Manitoba today.

All things considered, it’s likely that we’ll see some activity at some point today. The primary threats with today’s thunderstorms will be large hail and strong winds. Storm motions will generally be towards the southeast. Later in the day, a more organized heavy rainfall threat may develop with any storms still ongoing.

Those things aside, it will be quite a nice day. Temperatures will climb to around 28 or 29°C with a few clouds through the day. The humidity will begin to be more noticeable as dewpoints climb into the high teens by the afternoon ahead of the weak surface trough/low moving through. Winds will remain light.

Things clear out tonight with the humidity remaining in place and temperatures dipping down to about 17°C.

Hot & Humid Weekend Ahead

Temperatures will soar this weekend as warmer air moves into the region and sends daytime highs to 30°C or higher. Saturday will be a sunny day with a high around 30°C and dewpoint values in the high teens or low twenties making it feel more like the upper 30’s. Winds will be light. Expect a low near the 20°C mark on Saturday night.

Manitoba Health – Heat Alert Response System in Relation to the Heat Dial
This weekend could see provincial (pictured above) or federal responses to the heat and humidity over the region.

Sunday will see the daytime high climb even higher, most likely towards the 32 or 33°C mark as breezy southerly winds develop ahead of a developing low pressure trough. The humidity will remain in place with dewpoints in the high teens or low 20’s, making it feel more like the upper 30’s. There’s a slight chance we may see humidex values hit 40 on Sunday, which is Environment Canada’s criteria for a heat warning. That said, both Saturday and Sunday will likely qualify for the Province of Manitoba’s Heat Advisory and Response Pre-Alert which requires a humidex of 37 or higher.

There appears to be a slight chance late Sunday night or early Monday morning for some showers or thunderstorms to move through the area, but it looks very conditional at this point and it’s far to early to say much about it. We’ll put an update here early Sunday afternoon regarding the precipitation potential for Sunday night. Temperatures will likely only drop to around 19°C on Sunday night, however.

Long Term: Slight Cool-Down to Start Next Week

Looking ahead, it seems like we’ll see a slight cool-down next week as a frontal boundary stalls out over the region and brings a chance for multiple bouts of unsettled weather through the first half of the week. We’ll have more details about that bright and early Monday morning!

  1. Exactly when depends on the timing which will be affected by how much and what type of elevated convection developed with this feature overnight in Saskatchewan.  ↩
  2. If the nocturnal convection is in the right spot or strong enough, our sunshine could be replaced by cloud cover instead, dramatically limiting surface heating.  ↩