Tag: Humid

Humid Weather Brings Thunderstorm Threat to Southern Manitoba

Brad Leave a comment

The remainder of this week will be marked by a notable increase in the humidity as a deep southerly allows the moisture to build into Southern Manitoba. Alongside the sweltering conditions, a low pressure system pushing across the Prairies over the coming days will bring a threat for strong to severe thunderstorms across Southern Saskatchewan & Southern Manitoba.

Today will be a hot day with the humidity gradually building in through the day. Winds will strengthen through the day out of the south to southeast to around 30km/h or so as temperatures climb into the upper 20’s. Skies will remain mainly sunny with just a few clouds expected. While the humidity will be comfortable to start the day with dew point values around 14°C or so, the weather will get considerably stickier as the day progresses with dew point values climbing into the upper teens or perhaps even hitting the 20°C mark in the evening. This will make for very humid conditions with humidex values in the mid–30’s.

RDPS Surface Dewpoint valid 21Z July 23, 2015
The RDPS shows dew point values climbing over the 20°C mark Thursday afternoon resulting in very humid conditions.

Thursday will be the significant weather day for the second half of the week as a trough of low pressure passes across Southern Manitoba. Temperatures will soar towards the 30°C mark with significant humidity as dew point values once again push towards the 20°C mark, making the afternoon feel more like the upper 30’s. Winds will be lighter on Thursday than they will be on Wednesday, but there may be some brief breeziness late in the morning or early in the afternoon.

AWM Day 2 Convective Outlook for Thursday July 23, 2015
A slight risk of severe thunderstorms exists today across much of southwestern Manitoba extending into the Interlake and Red River Valley.

The heat and humidity will combine to pose a risk for some potent thunderstorm activity tomorrow afternoon. A trough of low pressure moving across the Red River Valley will serve as a trigger for convective activity sometime mid-to-late afternoon.[1] Winnipeg & the Red River Valley will see the risk for strong to severe thunderstorms – given a few conditions work out, more on that later – with large hail, strong winds and torrential downpours the primary threats. If an organized supercell manages to develop, the tornado threat with it (or them) will be slightly elevated. Looking through the guiding MIST principle for assessing convection:

  • Moisture: Abundant. Moisture transport coupled with vigorous evapotranspiration will combine to produce elevated surface dew point values near or exceeding 20°C by the end of the day.
  • Instability: Moderate instability with marked height falls ahead of an approaching shortwave coupled with the elevated moisture available through the boundary layer will produce MLCAPE values in the 2500 – 3000 J/kg range. The instability will carry into the evening hours with MUCAPE values remaining in the 2000+ J/kg range.
  • Shear: Pretty good. Bulk shear values of 20–30kt will increase towards 30–40kt in the evening hours. Additionally, low-level profiles show moderate veering. The wind profiles support supercell or multi-cell storm development with upscale growth likely in the evening hours.
  • Trigger: As mentioned, the trough pushing through the region associated with a low pressure system moving through the Dakotas will provide sufficient convergence to trigger thunderstorm activity.

The end result of all this is that thunderstorms are likely through Southwest Manitoba in the afternoon, with the threat pushing eastwards into the Red River Valley late afternoon into the evening. Storms will likely originate as supercells or multi-cell storms. Primary threats will be large hail and torrential rain, shifting towards hail/rain/wind as the storms organize & expand in coverage heading eastwards. Any supercell storms that manage to develop will likely pose a slight tornado risk.

Showers or thunderstorms are very likely Thursday evening/night as the system moves through. Stay alert of any watches or warnings issued by Environment Canada through the day into the evening on Thursday.

Friday will be a fairly quiet day. Things will stabilize behind the departing trough of low pressure, bringing clearing skies and a high in the upper 20’s with decreasing humidity. Expect a low in the upper teens on Friday night.

Summer Heat Returns for Weekend

The reprieve from the heat and humidity will be short-lived, however, as temperatures will climb back up to the 30°C mark with humid conditions returning as dew point values of 20°C or greater building back into the region. No precipitation is expected through the weekend.

  1. Although a wildcard remains of how long nocturnal convection from Wednesday night will persist into the daytime hours on Thursday.  ↩

Heat Wave Hits Southern Manitoba

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The first significant heat wave of the year starts today for Southern Manitoba as temperatures soar to the low 30’s and the humidity really starts to build into the region.

As mentioned above, today marks the start of a significant heat wave for Southern Manitoba that will see temperatures in the upper 20’s to lower 30’s alongside very humid conditions with dew point values climbing into the low 20’s. The humid conditions will result in humidex values[1] climbing into the upper 30’s or low 40’s:

Day Max Temp Humidex
Friday 31°C 39
Saturday 31°C 39
Sunday 30°C 40

These hot and humid conditions are the result of a significant upper-level ridge building into the Prairies will bring a prolonged south to southeasterly flow over Southern Manitoba as it spreads hot weather into the region. Significant humidity will build into the region as the southerly winds tap into Gulf of Mexico moisture, combining with local evapotranspiration to produce deep moisture with dew points near 20°C.

Median dew point values & associated spred from the Global Ensemble Foreacast System for Winnipeg, MB
Median dew point values & associated spred from the Global Ensemble Foreacast System for Winnipeg, MB

The big story here will be the duration the hot and humid weather is here to stay. The broad pattern will remain unchanged at least through the weekend. While the ECMWF & GDPS models have more progressive solutions that would result in the humidity being flushed out early next week, the NAM & GFS – which are generally slightly better at dealing with nocturnal convection – produce a blocked pattern that would keep the moisture in place at least until the middle of next week. Some of this uncertainty is reflected in the GEPS graph above with greater uncertainty (spread) showing up after Sunday. For the next few days, though, the heat will be on.

Severe Weather Potential

Alongside the heat and humidity, a severe weather threat will develop over the Red River Valley for both Saturday and Sunday. Unfortunately, it’s still too early to produce any sort of concrete outlook. As the main upper low pushes into Saskatchewan this weekend, multiple shortwaves will track over Southern Manitoba, providing instability for thunderstorm development. There are numerous question marks in place regarding the thunderstorm potential:

  • It’s still too early to try to pin down where exactly the shortwaves may track; southwestern Manitoba will likely see the greatest potential for thunderstorms, but that threat could easily shift eastwards into the Red River Valley or even southeastern Saskatchewan.
  • Instability will be quite high, primarily driven by the high dew point values. SBCAPE values will easily exceed 3,000 J/kg – which is very high – but the actual thermodynamics are marginal; 700mb temperatures are forecast to be near 9 or 10°C, which is quite warm in the mid-levels and makes it more challenging for thunderstorms to get going. If mixed-layer dew points are used instead of just surface-based dew points, the CAPE values drop fairly dramatically.
  • With a large portion of the instability provided through the excessive latent heat potential thanks to the high dew points while environmental lapse rates remain somewhat marginal, a trigger will be very important. At this point, any sorts of triggers remain fairly fuzzy. There are numerous small features that could produce thunderstorms, the big question of whether or not they’ll be strong enough to break the cap and get things going is crucial. At this point, the answer is very unclear.
  • If thunderstorms do develop this weekend, they’ll be struggling against very weak bulk shear values of only around 15–20kt. This would likely result in slow-moving thunderstorms whose main threat would be large hail and very heavy rain.[2]

At this point, it’s best to say that thunderstorms are possible throughout the weekend, and to keep track of updated forecasts. We’ll be keeping an eye on things and likely produce a convective outlook Friday evening for Saturday.

Update: Slight Risk of Severe Thunderstorms Saturday

There will be a slight risk of severe thunderstorms across southwestern Manitoba this afternoon, spreading eastwards this evening and through the overnight period into the Red River Valley.

AWM Day 1 Convective Outlook for Saturday July 11, 2015
A slight risk of severe thunderstorms exists today across much of southwestern Manitoba extending into the Southern Interlake and Red River Valley.

A very hot and humid air mass in place over the region will bring the threat of severe thunderstorms today. Scattered thunderstorms will develop through western portions of the slight risk area today and will have the potential to become severe due to the copious amounts of energy available to them. Their overall strength will be limited by relatively weak wind shear, however these thunderstorms may still become quite potent with the capability of producing large hail, torrential downpours and very strong wind gusts.

The hail threat is quite significant today; with MLCAPE values expected to be in the 2000-2500 J/kg range, there will be more than enough energy available to support strong updrafts capable of large hail. The growth may be limited by the lower shear values, but significant hail is definitely possible today.

Heavy rain will be one of the greatest threats today thanks to the humid air in place; PWAT values have climbed to around 35-40mm which, when combined with expected storm speeds of only around 10-25km/h, will result in rainfall accumulation potentials of upwards of 75-100mm.

Wind is the final significant threat today and comes from two possible places. The first is during storm collapse; the low shear values may make it difficult for storms to persist for a long time and we may see some storms cut off their own energy source and collapse; if this were to happen, strong winds would be possible as outflows from those storms. The other possibility comes from the significant energy available to the storms; if a north-south line of storms manages to develop, there’s a chance that they may develop bowing segments or into a bow echo; if that were the case then we would see a wider swath of significant winds. The former possibility is more likely during afternoon and early evening while the latter case would be more likely as we head later into the evening.

The threat for severe thunderstorms across Southwestern Manitoba is already underway; it will gradually spread eastwards through the day and into the Red River Valley through the evening.

A slight risk of severe thunderstorms will likely exist across Southern Manitoba tomorrow as well, but shifted further east. We’ll have an outlook for tomorrow available later tonight.

Update: Hot & Humid Sunday Brings Severe Thunderstorm Risk

A heat warning is in place for a Sunday that will see the heat and humidity cranked up over the Red River Valley bringing humidex values in the low 40’s. All that heat and humidity will bring a slight risk for severe thunderstorms in the Red River Valley alongside portions of Saskatchewan and much of Southern Manitoba.

AWM Day 1 Convective Outlook for Sunday July 12, 2015
A slight risk of severe thunderstorms exists today across much of southwestern Manitoba extending into the Southern Interlake and Red River Valley.

In Manitoba, huge CAPE values are expected to develop today with SBCAPE values exceeding 4500 J/kg and MLCAPE values exceeding 3500 J/kg. Bulk shear will be on the lower side at around 20kt and is likely the major limiting factor in today’s storm potential. After a bit of elevated convection clears out this morning, a capping inversion of around 25-50 J/kg should hold storms back until later in the afternoon. With weak shear, the tornado threat should be minimal today, although if any storm realizing 3000+ J/kg of CAPE manages to get organized, I’d go with “all bets are off.” Storm development has the chance to be truly explosive today, so be sure to stay alert for any watches and warnings issued by Environment Canada as the day progresses.

The main threats will be large hail and torrential downpours. Storms will likely be moving faster than they did on Saturday, but will likely also be more intense. Strong winds will be a threat with any sufficiently organized thunderstorm.

The thunderstorm threat will persist into the evening and overnight, spreading eastwards into SE Manitoba & Lake of the Woods.

  1. The Humidex is a calculated value that attempts to create a “feels like” temperature that takes into account the reduced evaporation occurring on the skin surface that results in a diminished ability for the body to shed heat.  ↩
  2. With dew point values near 20°C or even creeping above that value, perceptible water values are expected to climb towards 45mm, it’s not impossible that a slow-moving storm would be able to produce 3–4” of rainfall (roughly 75–100mm) in a relatively short time.  ↩

A Smoky Transition Back to Summer Heat

Brad Leave a comment

Today will be a bit of a mixed bag here in Winnipeg as a dying cold front completes its slump into Southern Manitoba bringing a chance of showers alongside another batch of forest fire smoke gradually working into the Red River Valley. Fortunately, winds will become more southwesterly as the week progresses, pushing the smoke to our north and bringing back hot summer weather to the area.

Today will start off with a slight chance of showers or thunderstorms until around midday after which the clouds will start pushing off to our southeast and see increasingly smoky skies build in. Temperatures will only climb to around 21°C today and then dip down to the low teens tonight.

NOAA 1hr. Vertical Smoke Forecast valid July 08, 2015 7PM EDT
The “total smoke” product from NOAA is forecasting a thick layer of smoke across Southern Manitoba by this evening.

Thursday and Friday will herald the return of summer. Winds will shift more westerly to southerly and begin tapping into warmer air to our south. There should be a deep enough layer of southerly winds to push the smoke north of Winnipeg through the day. Temperatures will be soaring into the upper 20’s on both days under mainly sunny skies.

The biggest difference between Thursday and Friday will be the humidity; while dew points in the mid-teens on Thursday will make it feel slightly humid, it’s expected that dew points will climb up to the 20°C – or even a bit higher – for Friday, making it feel downright tropical out there.

Overnight lows on Thursday & Friday will be in the upper teens.

Unsettled Weekend?

Weather models are pointing towards a potentially unsettled weekend as a large upper-level low lifts northwards into Alberta with a deep trough extending southeastwards across the Prairies.

GDPS 24hr. QPF valid 00Z July 13, 2015
Total precipitation forecast from Saturday evening to Sunday evening from the GPDS.

It’s not worth getting caught up in details at this point when dealing with a large, highly convective system such as this. The main thing to take away is that with a large low lifting into Alberta, it’s likely our weekend will continue humid, warm and potentially bring some wet weather.

Warm Weekend Brings Severe Thunderstorm Threat

Brad 1 Comment

True summer warmth is on the way for Winnipeg with temperatures soaring into the upper 20’s and overnight lows in the mid- to upper-teens. However, alongside the warmer weather, multiple threats for severe thunderstorms will return to the region.

Friday

Friday
29°C / 17°C
Partly cloudy, chance of a thunderstorm

Today will be a very warm day with temperatures climbing into the upper 20’s and light winds. Skies will start off sunny, and then we’ll see some afternoon cloudiness develop over the valley alongside some scattered thunderstorms. There’s little organizational feature, so it will be rather hit and miss as to whether you see a thunderstorm where you are or not.

Further north and west, there will be a slight risk of severe thunderstorms. MUCAPE (a measure of instability) values of 1000–1500 J/kg coupled with 25 kt of bulk shear will be substantial enough to support the organization of severe thunderstorms. A mid-level disturbance will slump to the southeast into Parkland Manitoba and the Interlake by early- to mid-afternoon and provide a focus and trigger for thunderstorm initiation.

AWM Day 1 Convective Outlook valid Friday June 26, 2015
A slight risk of severe thunderstorms exists today over southwest Manitoba into the Interlake region.

Severe thunderstorms are not expected widespread, but isolated severe cells are possible. Over the western sections of the slight risk area, large hail and heavy rain will be the primary threats. Those threats also exist further east in the slight risk area – mainly along the Red River Valley escarpment into the Interlake – there will also be a slight chance for funnel clouds or weak landspout tornadoes due to the additional “spin” provided by a shortwave moving through.

Temperatures will remain quite warm overnight with lows dipping to just around 16 or 17°C.

Saturday

Saturday
27°C / 14°C
Mixed skies; risk of a severe thunderstorm

Saturday will see a severe thunderstorm threat return to Winnipeg as more humidity pushes northwards ahead of a significant shortwave and associated trough line approaching from Saskatchewan. Dew point values are expected to climb into the upper teens producing MLCAPE values over 2000 J/kg. Strongly veering wind profiles coupled with over 40 kt of bulk shear will result in a moderate severe thunderstorm threat. While some models show surefire convection, there may be issues with a capping inversion depending on exactly how warm it’s able to get.

AWM Day 2 Convective Outlook valid Saturday June 27, 2015
A slight risk of severe thunderstorms exists on Saturday through the Red River Valley, Interlake and eastwards to the Ontario border.
An updated convective outlook graphic is available in the comments below.

Given the uncertainty, a slight risk for severe thunderstorms exists on Saturday through the Interlake into the Red River Valley. As the issues with the capping inversion become clearer, the threat may be upgraded to a moderate risk.

The primary threat with storms on Saturday would be damaging hail, strong winds and torrential downpours. Shear profiles are supportive of supercells, and while it’s not considered a significant likelihood, it’s important to remember that supercell thunderstorms can produce tornadoes.

Regarding non-thunderstorm weather on Saturday, temperatures will climb into the upper 20’s which will feel more like the low- to mid–30’s with the increased humidity. The winds will be out of the south at around 20km/h. Skies should clear on Saturday night as temperatures drop to the mid-teens.

Sunday

Sunday
28°C / 15°C
Mainly sunny

Sunday will finally see both warm temperatures and quiet weather. Temperatures will climb back into the upper 20’s with winds out of the north at around 15–25km/h. With those northerly winds, the humidity will be quite a bit more comfortable than on Saturday.

Some thunderstorm activity will occur in Manitoba on Sunday, but it will most likely be constrained east of Lake Winnipeg & down into the northern Whiteshell. The thunderstorm activity does not look like it has an organized severe threat.

Expect temperatures to dip back to the mid-teens on Sunday night.