Tag: Humid

Warm But Stormy This Week

Scott 1 Comment

Warmer weather is on tap this week as we experience a pattern more characteristic of mid-July. However, this warmer weather will bring with it a risk of severe thunderstorms as higher humidity builds into southern Manitoba.

Today is expected to be nice as a surface high to our east brings a light southeasterly flow to southern Manitoba.
Today is expected to be nice as a surface high to our east brings a light southeasterly flow to southern Manitoba.

This Week

Today will be a nice day as temperatures rise into the twenties under mainly sunny skies. A surface high to our east will bring a light southeasterly flow to southern Manitoba while building heights aloft help to maintain dry and clear conditions. The only risk of significant weather will come overnight Monday as considerable theta-E advection1 brings a risk of elevated convection developing across portions of western Manitoba. Given the inherent uncertainty in predicting elevated convection, it is possible that a substantial elevated risk could develop, or none at all, depending on the eventual evolution of the system. Current models suggest fairly tame convection will develop in western Manitoba, with more substantial convection further west in Saskatchewan, but this remains uncertain as was previously stated.

Tuesday appears to be the most appreciable threat of severe weather this week as an approaching upper-level trough sets the stage for thunderstorms to develop. But before the thunderstorm threat is discussed, a brief word on the general weather for Tuesday. In most of southern Manitoba it appears temperatures will climb up around 30C by Tuesday afternoon, with high humidity pushing humidex values near 40. A gusty southeasterly surface wind is also expected as a result of the strong system developing to our west. Besides the possible elevated convection on Tuesday morning, as described in the previous paragraph, the main threat for new thunderstorm development is expected beginning Tuesday afternoon in western Manitoba as a surface trough pushes into the region. At this time all modes of severe weather appear to be possible, including large hail, damaging winds, and tornadoes. This summer has featured unusually large volatility in severe weather forecasts, so bear in mind that the forecast provided here is based on the current guidance and is entirely subject to change depending on the eventual evolution of this system.

Update: An updated discussion for Tuesday’s thunderstorm threat is available here.

A technical discussion based on the latest (Sunday afternoon/evening) guidance is provided using the MIST technique:

  • Moisture: Considerable moisture is expected to advect into eastern Saskatchewan and western Manitoba by late Tuesday. 100-mb mean mixing ratios near 18 g/kg are expected in the region, with surface dewpoints in the low twenties. ET enhancement will play some role in elevating near-surface dewpoints, but the depth of the moisture will largely result from the advection of deep moisture from the central US Plains.
  • Instability: Steep mid-level lapse rates of ~8 C/km are expected atop the boundary layer as a southwesterly mid-level flow advects an EML from the western US. These steep lapse rates in combination with rich boundary layer moisture are expected to result in MLCAPE values of 3000-4000 J/kg in the warm sector. MLCAPE could be even higher on a localized basis given the current model guidance showing mean mixing ratio values higher than those listed in the previous section – although such extreme mixing ratio values seem less probable given the current moisture values in the source region (central US Plains).
  • Shear: A 65 kt jet streak at 500 mb is expected to be edging into Saskatchewan by late Tuesday. In addition, a south/southeasterly LLJ of 20-30 kt is expected in the 925-850 mb layer. Surface winds will similarly be from the south/southeast in the warm sector, likely at 10-20 kt. This wind profile is expected to result in effective bulk wind differences (EBWD) of 25-35 kt along the international border increasing to 35-50 kt in east-central Saskatchewan by 00 UTC Wednesday. Considerable low-level veering will result in effective storm relative helicity values of 150-350 m2/s2 across the warm sector, with the highest values in southern Manitoba. Increases in effective helicity can be expected after 00 UTC due to the typical boundary layer decoupling.
  • Trigger: A surface trough is expected to extend southward through eastern Saskatchewan by Tuesday afternoon, extending from a strong low pressure system over the northern Prairies. A secondary, weaker low may develop near the MB/SK/ND border, depending on how the upper jet exit orients itself as it rounds the upper ridge. A warm front is expected to be located across Manitoba, although the exact location remains uncertain. It is possible that an outflow-warm front merged boundary may be present depending on the evolution of morning convection. The absence of outflow-reinforcement may result in a fairly diffuse warm front running somewhere through the interlake. Aloft, a strong shortwave is expected to cross the eastern Prairies on Tuesday morning, helping to maintain and/or trigger elevated convection. Later in the day, a weaker shortwave may push through eastern Saskatchewan and/or southern Manitoba, although the timing and placement of this feature are unclear. Weak height falls across southern Manitoba will maintain considerable capping, especially south of the Trans-Canada Highway, where 700-mb temperatures are expected to exceed 10C through the day, and may reach as high as 12-14C.
AWM Day 1 Convective Outlook for Tuesday July 19, 2016
AWM Day 1 Convective Outlook for Tuesday July 19, 2016 (Original Day 2 Outlook)

Discussion

A significant severe weather threat exists from central Saskatchewan into southern Manitoba on Tuesday. Strong wind shear in combination with an extremely unstable environment will lead to numerous severe thunderstorms. There remains considerable uncertainty in terms of the timing and location of these storms, thus the following forecast is subject to change. Nevertheless, at the present time wind shear vectors crossing approximately normal to the cold front, along with considerable low-level veering in a moderately capped environment are expected to allow sufficient updraft seperation for numerous discrete cells to initially develop across eastern Saskatchewan by late Tuesday afternoon/early evening. Short bowing segments and supercell structures (HP) are probable, with the likelihood of a severe MCS developing as cold pool mergers occur late evening as cells move across west-central Manitoba. A few tornadoes may be likely in the late afternoon-evening whilst cells remain discrete or semi-discrete. A secondary threat of severe storms may develop in SW Manitoba should the aforementioned secondary surface low develop. However, given the strong capping in the region and uncertainty regarding this feature, convection initiation in this region is more uncertain. Should a MCS develop during the mid- or late-evening period, it would tend to initially propagate eastward, but would likely take on a southerly component as it begins to forward-propagate into the LLJ during the late evening-early overnight period. Such a course may result in the Red River Valley being clipped. Given that such a system would likely be surface-based as it passed through the Red River Valley, owing to slow nocturnal cooling in the early overnight period, damaging winds would be the main threat, with a secondary threat of large hail. Having said all that, an entirely different scenario may unfold should the morning elevated convection remain intense during the day. Some high-resolution guidance suggests the elevated storms will slowly move east during the day, eventually becoming surface-based as they encounter the extremely unstable southerly feed over eastern SK/southwestern MB. Under this second scenario new storms would be less probable, with the main convection focused around the cluster of storms from the morning. Regardless of which scenario plays out, the overall risk is moderate in east-central SK and west-central MB, with a surrounding slight risk region.

Wednesday

The weather for Wednesday will be dependent on how quickly Tuesday’s weather system moves to the east. Some guidance suggests the system may continue to linger on Wednesday, potentially bringing another round of severe weather to southern Manitoba. Conversely, the system may have exited the region by that point. Regardless, temperatures are expected to climb up around 30C with lingering humidity. Should Wednesday end up presenting another severe weather risk we will be sure to provide a new update.

Long Range

The long range forecasts this summer have been quite inaccurate, although that isn’t necessarily unusual. Forecasts have continuous suggested that we will see warmer than normal weather, although that has not often panned out. The current long range forecast continues to suggest we’re going to see generally hot weather through the end of July. This time it appears that this forecast may be a bit more certain than previous ones, although bear in mind that it certainly could end up being wrong again!

  1. Simply put, theta-E is a measure that combines moisture and temperature.↩

Thunderstorms & Showers to Start The Week

Brad Leave a comment

A severe thunderstorm threat returns to Southern Manitoba today as very warm and humid conditions persist in the region for one final day before a cold front pushes cooler and drier air into the region. The passage of the cold front will not be an end to the unsettled weather, though, as a couple more days of showers or thunderstorms will occur with a large upper-level low passing over the region.

Today brings a notable severe weather threat to southern Manitoba with a slight risk of severe thunderstorms developing over southwestern Manitoba and a moderate risk in the Red River Valley eastwards through the SE corner of the province.1 The thunderstorms will develop mid-day and push eastwards with the cold front through the remainder of the afternoon. It is most likely that the thunderstorms develop over southwest Manitoba first, and then push towards into the Red River Valley this evening. Stay aware of any watches or warnings issued by Environment Canada.

The HRRR develops a line of thunderstorms along the cold front and pushes it into the Red River Valley this evening.
The HRRR develops a line of thunderstorms along the cold front and pushes it into the Red River Valley this evening.

Otherwise, this mornings cloud cover will break up a bit to produce mixed skies and the relatively small amount of sunshine will push the daytime highs to around 26°C. With very humid air in place, it will be a fairly uncomfortable day, feeling more like the low- to mid-thirties. After the thunderstorms move through this evening, we’ll head to a low near 16°C with breezy southerly winds.

Tomorrow will likely bring some sun first thing in the morning, but more clouds will move in with a good chance of more shower or thunderstorm activity in the afternoon. At this point severe thunderstorms aren’t expected, but we’ll keep an eye on things to see if anything changes. Winds will be pick up to be gusty out of the north near 40km/h. Expect a high near 23°C and a low near 14°C with a continued chance for showers overnight.

Wednesday will be a cool and cloudy day with a good chance of more rain. The daytime high will only be around 20°C and winds will continue gusty out of the northwest, although not quite as strong as Tuesday. The shower activity will taper off in the evening as temperatures head to a low near 14°C.

The latter half of the week is looking like a return to seasonal temperatures and a chance of actually drying out a little bit.

Winnipeg’s seasonal daytime high is currently 26°C while the seasonal overnight low is 13°C.

  1. Apologies for the lack of an outlook image; time constraints have prevented me from making one. ↩

Increasingly Muggy Weather on the Way for the Winnipeg Folk Festival

Brad 4 Comments

Unsettled weather is on its way back to the province as a trough of low pressure develops over the western Prairies and draws humid air northwards out of the United States. For people heading out to Bird's Hill for the Folk Fest, this means increasingly muggy weather through the weekend with the potential for thunderstorms both Saturday & Sunday.

Today will be a fairly pleasant day. The system that brought up to 20 mm of rain to the Red River Valley yesterday1 will be moving off the east and any remaining cloud from it should clear out through the morning leaving just a few clouds around for the afternoon. Temperatures will climb to a high near 25°C. Expect just a few clouds overnight and a low near 14°C.

Saturday

Saturday will see more humid air beginning to work into the region. Morning sun will be replaced by clouds as the day goes on while the temperature climbs to a high near 26°C. There will be a slight chance of thunderstorms in the evening, but the main threat will occur through the overnight period. There will be a very small chance of severe thunderstorms.

Update: Severe Thunderstorms Hit SW Manitoba on Saturday July 9, 2016

Severe thunderstorms developed over SW Manitoba on Saturday and focused into one primary strong supercell thunderstorm that organized near Virden, MB and then moved southeastwards through Minto & Ninette before continuing on across the US border. It produced a variety of severe weather, with numerous reports of heavy rain, large hail and severe winds:

A summary of the severe weather over southwestern Saskatchewan on July 9, 2016
A summary of the severe weather over southwestern Saskatchewan on July 9, 2016

Sunday

Sunday will see the humidity move into the region full force with dewpoint values climbing up into the low 20's. This will make the high near 27°C feel very warm, however conditions will be a bit more bearable due to the fair amount of cloud cover expected through the day. Any times the sun does poke out, though, will likely see things begin feeling very warm very quickly.

A organized line of thunderstorms sweeping across Southern Manitoba is possible Sunday night.
A organized line of thunderstorms sweeping across Southern Manitoba is possible Sunday night.

Rain-wise, we may see some remnant showers or thunderstorms in the morning from any activity on Saturday night, and then things should diminish for much of the day. By Sunday evening & overnight, the thunderstorm threat once again returns. Sunday night will see a slight to moderate threat of severe thunderstorms across the Red River Valley. Large MUCAPE values overnight coupled with 30-50 kt of bulk shear ahead of an incoming shortwave will support the development of organized thunderstorm activity across a wide area of Southern Manitoba. The primary threats will be large hail and strong winds, with rapid rainfall accumulations also a concern.2

It will be a very mild night with a low near 21°C.

Long Range

The start of next week continues warm and humid with a thunderstorm threat late in the day into the overnight period. Heading through Tuesday into mid-week, the main upper low associated with the weekend's weather pattern moves into the region, bringing relief from the humidity and cooler temperatures alongside more cloud and chances for showers.

  1. While Winnipeg saw nothing but a few light showers late in the evening, the rain managed to push as far north as St. Adolphe, which saw 2mm. ↩
  2. Although the thunderstorms will likely have very intense rainfall, they will be moving relatively quickly, making accumulations not as prominent a threat as the hail or wind potential. ↩

Stormy Start to the Week for Southern Manitoba

Scott Leave a comment

This week will start off on a stormy note in southern Manitoba, with a risk of thunderstorms in most areas from today until at least Wednesday. Severe storms will be possible on most days with this risk.

Severe storms will be possible along a dryline (brown) and cold front (blue) in southern Manitoba today.
Severe storms will be possible along a dryline (brown) and cold front (blue) in southern Manitoba today.

Today

Today will feature a risk of severe thunderstorms in many parts of southern Manitoba due to the warm and humid air mass over the region. It will be a fairly unconventional severe weather setup with a complicated frontal regime in place. It appears that a dryline-type feature will be present through the Red River Valley on Monday afternoon, with a cold front cutting through the southern Interlake. The most unstable air is expected to reside near the cold front and east of the dryline, putting the Red River Valley and south-eastern Manitoba at the highest risk for severe weather. However, a secondary severe risk may develop along the cold front, especially through the interlake. It appears storms could also form further west along the cold front in parts of western Manitoba. The instability regime further west is less certain, therefore the thunderstorm risk in that region is also less clear. Prior to the initiation of thunderstorms, most of southern Manitoba will be quite warm, with temperatures near 30C in many areas. Winds will range from southerly/easterly ahead of the cold front/dryline to westerly/northerly behind these features. A technical discussion using the “MIST” technique is provided below for those wanting more specific details.

Technical Discussion

Moisture: Copious low-level moisture is expected ahead of the triggers today. Surface dewpoints of 20-22C are expected (ET enhancement should be in full swing), with 100-mb mean mixing ratio values of 14-16 g/kg.

Instability: Steep low- to mid-level lapse rates are expected today due to the presence of an elevated mixed-layer (EML) between 925 and 700 mb. These large lapse rates combined with the aforementioned mean mixing-ratio values should result in MLCAPE of 2500-4000 J/kg by late afternoon. MLCIN should be mostly eliminated with surface temperatures near 30C, assuming the 14-16 g/kg mean mixing ratios are realized.

Shear: Deep layer wind shear values will be variable and uncertain today. The 500-mb flow is expected to be 30-40 kt from the west, which would normally offer respectable wind shear on such a convective day. However, surface winds are also forecast to have a strong westerly component, even ahead of the triggers. Since southwesterly winds are expected ahead of the dryline, the effective bulk wind difference (EBWD) is expected to be less than the 500-mb flow. Conversely, some models have suggested easterly winds could be present ahead of the triggers, which would enhance the EBWD. A conservative approach would give an EBWD of 20-35 kt ahead of the triggers tomorrow, with the caveat that an easterly low-level flow could push said values to 40-45 kt. Given the expected westerly-component surface flow, it follows that low-level veering is also expected to be poor. Helicity values in the 0-3 km layer are only expected to be between 100-200 m2/s2, again with the caveat that an easterly surface flow would enhance these values.

Trigger: As mentioned previously, a cold front slicing in from the north is expected to provide a focus for storm develop along and north of the Trans-Canada highway through most of southern Manitoba. A north-south oriented dryline is expected to be in place in the Red River Valley during the day, providing a secondary trigger. Height falls during the afternoon-evening are unimpressive, and indeed could actually end up being height rises. The absence of any strong mid- and upper-level forcing for ascent should help keep storm coverage on the more isolated side, especially given that there will be a respectable, but breakable, cap to surface-based convection.

Discussion: A severe weather setup with impressive thermodynamics, but unimpressive kinematics/dynamics, is expected in southern Manitoba today. A respectable cap will be in place to surface-based convection until late afternoon, at which time strong low-level convergence along the dryline/cold front should be sufficient to trigger isolated severe storms. However, temperatures must reach near 30C for this to happen, therefore if strong surface heating does not occur, storms may fail to develop.

AWM Convective Outlook for July 4, 2016
AWM Convective Outlook for July 4, 2016

The CAPE-shear space today is sufficient for marginal supercells structures, but stronger supercells are possible should the stronger shear develop (previously discussed). The main threats today appear to be large hail and damaging wind gusts. A few very large hail reports are possible given the extreme instability and steep lapse rates, especially if the stronger wind shear is realized. The tornado threat is marginal given the weak helicity values. However, low-level CAPE and LCL heights are in the tornadic range, suggesting that a right-deviating supercell could produce a brief tornado. The overall risk of severe weather is slight.

Tuesday

Tuesday could see another risk of severe storms develop across southern Manitoba. Models suggest a front may be stalled just north of the international border in the afternoon, providing a focus for storms to develop. Any storms that develop could become severe, with damaging winds and large hail being the main threats. Areas away from the storms should see temperatures in the mid to upper twenties with skies being a mixture of sun and cloud.

Wednesday

A third risk of storms may evolve on Wednesday, though forecasting thunderstorms three days into the future is far from an exact science. Current models show a general non-severe storm risk across southern Manitoba. This means gusty winds, small hail, and heavy rains are the main threats from any storms that develop. Like Tuesday, conditions outside of storms will be nice, with temperatures in the upper twenties and breezy easterly winds.

Long Range

The long range forecast is looking unsettled for the remainder of the week in southern Manitoba. At this time there don’t appear to be any additional threats of severe weather late in the week, but more thunderstorms can’t be ruled out. Looking out further in the long range, models hint at a more sustained period of hot weather as we move into mid-July, although that would certainly not be unusual for this time of year!