Tag: Thunderstorms

Scorching Summer Weather Sizzles Southern Manitoba

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Temperatures will soar into the low 30’s over the coming days as a broad upper-level ridge continues to build across the Canadian Prairies, bringing with it some of the warmest temperatures of the year. Alongside the heat will come several bouts of humidity; at times over the coming days, humidex values – a “feels like” temperature that combines the effects of heat and humidity – will approach or exceed 40, making for exceptionally sweltering weather. In addition to the heat and humidity, today will bring a risk of severe thunderstorms…if they’re able to develop this afternoon.

Wednesday: Hot, Humid & Significant Thunderstorm Risk

Today will be a scorching hot day that will be made oppressively hot by the increasing humidity through the day. Temperatures will soar quite quickly today with the mercury reaching around 30°C by lunch time and then climbing a few more degrees above that this afternoon. All the while, the dew point will climb to the 19–20°C mark, resulting in humidex values in the 37–41 range for much of the day.

The biggest weather story for today, though, is the thunderstorm potential. First, here’s our outlook for today, but the discussion is important, so don’t skip over the rest!

AWM Day 1 Convective Outlook for August 12, 2015
AWM Day 1 Convective Outlook for August 12, 2015

A slight risk of severe thunderstorms exist across the Red River Valley, the Interlake region and eastwards to the Ontario border. Any storms that manage to develop today will have the potential to become very potent storms capable of all types of severe weather, including tornadoes, however there remains a single big question: will there be any storms?

As always, lets take a look at the basic MIST principles of thunderstorm forecasting:

  • Moisture: Ample moisture will be in place as surface dew point values climb to 20°C. 30mb mixed layer dew points are also expected to be in the high teens, which will make for ample fuel availability in convection.
  • Instability: Instability is strong but conditional. Given the high moisture values, MLCAPEs will sit in the 2000–2500 J/kg range while SBCAPE values may exceed 3000 J/kg. The crux is, however, the capping inversion. Strong insolation will chip away at the cap through the day, however 30–50 J/kg of inhibition will likely remain.[1] The big question is, will the combination of surface trough and lake breeze interactions provide enough lift to break the cap? If any storms do manage to initiate, it’s all clear for explosive growth in a strongly unstable environment.
  • Shear: Shear looks fantastic for the development of strong, sustained supercell thunderstorms. 0–6km bulk shear values are expected to be in the 30–35 kt range while hodographs show excellent curvature. No questions exist about how favourable the shear is for supercell thunderstorm development.
  • Trigger: As mentioned above, two triggers will be in place today. The first is a trough line pushing through the Red River Valley & Interlake this afternoon. The second will be various surface boundaries developed through differential heating on escarpments (RRV, Gunton Bedrock) or lake breezes. It’s only slightly likely than any one of these features would be able to provide enough lift to trigger a thunderstorm, however if two or more of these features interact, it could trigger thunderstorm development. The trigger is the biggest uncertainty with today’s thunderstorm potential.

All these factors together combine to give a slight risk of severe thunderstorms across a wide region of Southern Manitoba. Despite the “lower” threat classification, all types of severe weather – flooding rains, large and damaging hail, severe wind gusts, tornadoes – are possible in thunderstorms in the Red River Valley today. The slight risk is given not for thunderstorm intensity – any thunderstorms that develop today could be very, very strong – but rather for the uncertainty associated with if they’ll even occur and expected isolated nature of the storms.[2]

On tornado potential: Today brings with it a non-zero tornado threat, particularly for areas in the northern half of the Red River Valley and southern sections of the Interlake region. Hodograph curvatures are very impressive, and when storm-relative values are taken into account, helicities will be quite high in any thunderstorms that manage to develop. Cloud bases will be fairly high, but high dew points should help diminish significant evaporative cooling below the cloud base. Numerous parameters show favourable environments for thunderstorms capable of producing tornadoes as well. It’s impossible to forecast a tornado this far in advance, but if you live in the slight risk area, it would probably be wise to keep up to date on any watches/warnings issued by Environment Canada.

Temperatures will dip to around 19°C tonight with slightly less humid conditions.

Thursday: A Brief “Cool Down”

Marginally cooler air works into Southern Manitoba behind Wednesday’s trough line which will be reflected in daytime highs a whopping 1–3°C cooler, but still likely at 29–30°C or a touch warmer. Perhaps the bigger difference will be more tolerable humidity levels as dew point values drop into the low teens by the end of the day. Skies will be mainly sunny with relatively light northwesterlies as a ridge of high pressure builds in.

Winds shift southerly in the evening as the Red River Valley moves onto the back-side of the surface ridge and warmer air begins to push in again. Expect a low near 16°C.

Friday: Don’t Worry, It’s A Dry Heat

The heat is back on Friday with daytime highs climbing back to around 33–34°C. It won’t feel as hot as Wednesday, however, thanks to significantly lower dewpoints in the low- to mid-teens. While we’re not talking Arizona desert heat, it’ll be far more comfortable than the 20°C dew points earlier in the week.

Heading into Friday night, deep-layer moisture transport ramps up and will begin bringing significant amounts of moisture into the region aloft. This, combined with warmer air moving in, will lead to a fairly balmy night with lows near the 20°C mark.

Long Range: Severe Storm Threat Returns on Saturday

It looks like a threat of severe thunderstorms returns to Winnipeg & the Red River Valley on Saturday. Very humid conditions with highs in the upper 20’s will clash with a cold front moving in from the west. Showers and thunderstorms are probable with this front, and with significant energy and shear in the region, it’s entirely possible for severe thunderstorms to develop. It will all depend on the exact strength & timing of the cold front, so we’ll take a closer look at that on Friday when the event is closer.

Sunday will be a comparatively cold day with partly cloudy skies, a bit of a breeze and highs in the low 20’s.

  1. Many studies show that some of the strongest supercell thunderstorms form in environments with between 25–50 J/kg of inhibition.  ↩
  2. At this point, we’re not expecting a huge line of thunderstorms to roll across the Red River Valley; rather it seems probable that there would be just one or two very strong storms.  ↩

Severe Thunderstorm Threat Marks Transition to Summer Heat Wave

Brad 1 Comment

A threat of severe thunderstorms will come to Southern Manitoba today as a trough of low pressure pushes across the region, tapping into very muggy air near the surface. The passage of this system will mark the beginning of a transition towards a summer heat wave as a potent upper ridge begins building over North America.

Friday: Severe Thunderstorm Threat

This morning’s cloud and drizzle will break up through the morning as our winds finally begin swinging around to the west/southwest, finally allowing some sun to begin poking through and allowing us to start climbing towards our daytime high near 25 or 26°C. As we move towards the afternoon, however, a trough of low pressure will begin pushing into the Red River Valley from the west, bringing with it the threat for severe thunderstorms.

Taking a look at the basic MIST principle for today’s convective potential:

  • Moisture: Moderate moisture will be in place today with surface dewpoints near 18°C and mixed layer dewpoints around 16°C.
  • Instability: As the trough swings into the region, mid-level lapse rates will steepen significantly alongside the erosion of the capping inversion thanks to daytime heating. By mid-afternoon, MLCAPE values near 2000 J/kg will develop with little to no capping inversion along the trough line.
  • Shear: Slightly backed surface winds ahead of the trough line coupled with approximately 25kt of 0–6km bulk shear will be sufficient to provide organization to any thunderstorms that develop.
  • Trigger: This is the most questionable aspect. Models differ on the strength of the trough line as it swings through; while some show little strength to it resulting in a lack of convergence and no convection, others show a stronger trough which would be sufficient to focus the energy and produce more substantial thunderstorm activity.
AWM Convective Outlook valid 12PM Friday August 7, 2015 to 7AM Saturday August 8, 2015
AWM Convective Outlook valid 12PM Friday August 7, 2015 to 7AM Saturday August 8, 2015

As a result, a slight risk of severe thunderstorms is warranted for much of Southern Manitoba. Any storms that develop will likely initiate as single-cell or supercell thunderstorms before growing upscale fairly quickly into linear convective elements. The primary threat with any storms that develop today will be strong winds and large hail, however rainfall rates may pose significant impact in storms despite accumulations likely not hitting any severe criteria.

Thunderstorms will move off towards the east through the evening, leaving behind clearing skies and a pleasant overnight low near 15°C.

Beautiful Weekend Ahead

There isn’t much to say about the coming weekend other than it will be quite nice. Daytime highs on both Saturday & Sunday will be near the mid–20’s with overnight lows in the mid-teens. Winds will be relatively light out of the northwest on Saturday and fairly calm on Sunday. There will be a bit of cloud cover left over on Saturday while Sunday will be mainly sunny.

Heat Wave Builds In Next Week

Next week will see very warm weather return to Southern Manitoba as a potent upper-level ridge builds across much of North America.

GFS 500mb Height & Wind Forecast valid 00Z Thursday August 13, 2015
The GFS 500mb Height & Wind Forecast valid 00Z Thursday August 13, 2015 shows a significant upper-level ridge over much of North America.

As the upper ridge builds, a deep southerly flow will develop over the eastern Prairies, bringing significantly warmer air into the region. With 850mb temperatures forecast to reach 20°C or warmer, it seems likely we would see daytime highs in the low 30’s several days next week. One of the big questions is exactly how humid it will get, but we’ll have to wait until we’re closer to better assess that question. At this point, though, it seems quite likely that we’ll see a stretch of dry and hot weather next week.

Unsettled Wednesday Marks Return to Warmer, Humid Weather

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The pleasant spell of cooler, drier weather is coming to an end today as a warm front pushing eastwards across the Prairies brings unsettled weather today and then a deeper southerly flow which will help the heat and humidity slowly build up through the region into the end of the week.

Wednesday

Today will be the first active day of the week as a warm front approaches from the west and pushes through the Red River Valley later this afternoon & into the evening hours. This system will spread showers and thunderstorms across southwestern Manitoba this morning and gradually push them towards the valley through the day. The rain will likely hold off until late in the afternoon or early this evening here in Winnipeg. Temperatures will climb into the low 20’s across the valley with winds picking up out of the south-southeast to 20–30km/h with some gustiness on top of that by mid-to-late afternoon.

NAM Simulated Reflectivity valid at 00Z Thursday August 6, 2015
The NAM simulated RADAR refliectivities show a large swath of showers moving through Southern Manitoba this evening as a warm front pushes eastwards.

With the showers there will be a slight chance of an embedded thunderstorm…if we can manage some heating through the day with a bit of sunshine. If it ends up being overcast for a large area ahead of the front, the thunderstorm activity will likely be fairly mild with little-to-no chance of severe weather. On the other hand, if some sunshine manages to heat things up a bit, there will be a very borderline risk of some marginally severe wind or hail with some of the storms. That said, it doesn’t look particularly likely that we’ll see too much thunderstorm activity and, rather, we’ll see more moderate shower activity. The exception to this rule will be over southwestern Manitoba where there will be a slight risk of severe thunderstorms with main threats of large hail and damaging winds.

By the time things clear out late in the evening, most places will likely have seen around 5–10mm, but localized amounts of 10–20mm are possible. Temperatures will drop to lows in the mid-teens tonight.

Thursday

Thursday will be a muggier day with temperatures climbing into the mid–20’s. There will be some sunny breaks, but skies look to be trending towards the cloudier side of things. Another disturbance begins pushing into our region later in the day, which will bring another chance for showers or thunderstorms mid-afternoon into the evening hours. Temperatures will dip to the mid-teens again on Thursday night with a continued chance of shower activity.

Friday

Friday will see a more organized thunderstorm threat return to Winnipeg & the Red River Valley as the main upper-level disturbance swings through the province. Temperatures will climb to around 26°C or so with any leftover humidity getting flushed out by drier westerlies at around 30km/h. We may see some pop-up thunderstorms through the day, but at this point they don’t look like they’ll bring any severe weather. Friday night will see temperatures dip back to the mid-teens with a chance of some wrap-around shower activity pushing into the Red River Valley form the north on the back-side of the departing upper low.

Weekend Looks Pleasant

Looking ahead, the weekend looks great with high temperatures in the upper 20’s, relatively comfortable humidity, and little in the way of precipitation expected thanks to a ridge of high pressure working its way across the Prairies. Finally, rainy weather during the work week turning beautiful for the weekend!

Unsettled Conditions Bring Relief From the Humidity

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Winnipeg is finally seeing a break from the relentless heat and humidity of the past week thanks to the passage of a cold front yesterday evening that pushed the humid air mass that had been so persistent over our region off to the east and ushered in significantly drier air from the western Prairies. Conditions will remain fairly unsettled over the next few days, however, as multiple shortwaves slide down over the province in the northwest flow left in the wake of a departing low pressure system.

Wednesday

Today will be a fairly cloudy day as the upper low associated with the system that brought a terrifying long-tracked, rain-wrapped, nocturnal, multi-vortex tornado to southwestern Manitoba moves over the region.

NAM 500mb Vorticity valid 12Z Wednesday July 29, 2015
This morning’s forecast 500mb vorticity from the NAM shows the intense upper-level low departing Southern Manitoba.

With little heat or humidity left in the region, scattered showers are expected with minimal accumulations. The bigger story than the precipitation today will be that the wind will be quite prominent this morning out of the northwest at around 40km/h with gusts to 60–70km/h. The wind will taper off as this system pushes to the northeast, and by evening we should see it sitting at a manageable 20km/h or so. Temperatures will climb into the mid–20’s today with an overnight low in the mid-teens.

Thursday

Thursday will be a fairly nice day with mixed skies and a high in the mid-to-upper 20’s. Humidity will remain comfortable as well, making for great summer weather.

RDPS 3hr. QPF valid 00Z Friday July 31, 2015
The RDPS shows a line of thunderstorms late Thursday cutting across Southern Manitoba.

Most indications are that a shortwave trough – evidenced above by high vorticity values at 500mb – will move through midday into the afternoon. As it pushes across Manitoba, it will bring with it the risk of a few thunderstorms; at this point no severe weather is expected should any thunderstorms get going.

Skies will clear out Thursday evening as the temperature drops into the mid-teens.

Friday

Not much to say for Friday other than it will be another beautiful day. Just a few clouds are expected as temperatures climb into the mid- to upper–20’s once again. There are some hints of some thunderstorm activity, but at this point it looks like it should remain well north of the Red River Valley. Temperatures will dip down into the mid-teens on Friday night.