Showers and Thunderstorms on Tap for Southern Mantioba

Brad Leave a comment

While temperatures will soar to the 30°C mark today, much of the Red River Valley should be prepared for showers and thunderstorms to develop by mid-to-late afternoon as a low pressure system moves into the area and taps the available heat and moisture.

850mb Theta-E Values

850mb Theta-E values at 00Z May 19th (this evening) from the NAM. A sharp warm front is evident jsut north of the US border in Southern Manitoba.

With a strong capping inversion in place over the Red River Valley today, heat and moisture being pumped northwards will be trapped at the surface, allowing our temperature to soar to the 30°C mark and pooling moisture that will push our dewpoints up to around 15°C. Much of the day will be a beautiful day, especially given that it’s only May 18th. Conditions will deteriorate somewhat by late afternoon, though, as a surface low lifts northwards into Southern Manitoba.

The surface low will travel along a pre-existing surface trough and be supported by a shortwave ejecting northeastwards from the main upper trough, still anchored through Montana and Wyoming. With the heat trapped near the surface, the Lifted Index is expected to drop to the -2 to -8 values, with the lowest values running along a line from Gretna, MB to Bisset, MB and increasing to the southeast of that line. The heat combined with ample moisture is also expected to produce high CAPE values generally from 2000 to 3000J/kg. Increasing mid-level lapse rates with the approach of the shortwave will provide enough destabilization to erode the capping inversion through the afternoon and allow showers and thunderstorms to develop along and ahead of the warm front, initially concentrated near the triple point of the system. The showers and thunderstorms will expand in coverage as the evening progresses and the low-level jet intensifies.

Severe Weather Threat

Today marks one of the first severe weather threats of the season. Things look promising, and have even caught the attention of some seasoned storm chasers from the United States:

On paper, things look fairly good that there’s a risk for severe weather. EHI values rise to between 1.5-2.0 by late afternoon, which when combined with the presence of 30-35kt of bulk shear certainly presents the chance for supercell storms to develop. A few things hamper my excitement, though:

  • Bulk shear isn’t really all that impressive; the really good shear moves through Western Manitoba through the day today, lifting northwards into the Interlake. The shear diminishes to the SE and isn’t particularly strong where the greatest CAPE is. When it comes down to it, it’s likely that we’ll end up with marginally strong shear on top of marginally high CAPE this afternoon, with the most favorable conditions missing each other.
  • CAPE values may be too high. The NAM & GFS have been forecasting dewpoints slightly higher than have materialized. The amount of energy for storms to work with is going to be heavily dependant on the dewpoints that develop this afternoon, and if they don’t make it as high as forecast then we’ll be seeing weaker storms.
  • Too many storms. With so many marginally severe features, we may see quite a few showers and thunderstorms develop this afternoon as the cap erodes. If too many fire off, then it will be difficult for any one storm to oragnize itself into anything that poses a severe weather threat.

All that being said, a potential for severe weather does exist today across south-central and southeast Manitoba. The main threats should severe weather develop will be hail and/or the possibility of a tornado.

Thunderstorm Outlook for May 18th

Thunderstorm Outlook for May 18th to May 19th. Please note: Analysis has only been done for Manitoba. Storms may develop elsewhere in the Canadian Prairies.

Residents in Southern Manitoba should stay up-to-date on the weather today to ensure that if severe storms do develop, the appropriate precautions can be taken.

The Big Warm-Up Begins; Moist, Gulf Air Mass Incoming

Brad Leave a comment

Today marks the beginning of skyrocketing temperatures over Southern Manitoba. Although daytime highs will be cooler again today, plenty of warming will be going on just off the surface, bringing us substantially warmer overnight lows and temperatures soaring into the upper twenties and low thirties by the end of the week.

Thursday Temperatures

Surface temperatures from the GEMREG model valid 00Z Friday (Early evening Thursday).

Winds will pick up out of the southeast today as we move onto the backside of the surface ridge that moved over our region behind the cold front that brought damaging winds to Southern Manitoba on Monday evening. This marks the beginning of a large-scale shift in the upper atmosphere that will rapidly push heat and moisture from the Gulf of Mexico northwards. Temperatures will only climb to around 20°C today, but our overnight low will show evidence of all the warm air moving in aloft by dropping down to only around 13°C, about 10°C warmer than our overnight low last night.

The heat and moisture will begin to build in on Thursday as the warm front lifts north of Winnipeg; our daytime high will push towards the upper twenties and settle around 28°C. This will feel quite different than our 29°C Monday afternoon because of one particular feature: moisture. While dewpoints will stay low today, they’ll begin to increase overnight and through the day on tomorrow, climbing into the low teens by Thursday evening. The heat and moisture will continue to build in for Friday, with daytime highs climbing to near 30°C with dewpoints climbing further into the mid-to-high teens. This will make things on Friday feel positively tropical, with Humidex values in the mid 30’s.

No precipitation is expected today or tomrrow, but as the heat and moisture build in, there’s certainly a chance of showers or thundershowers on Friday and Saturday. The chance for precipitation is elevated for Saturday as there will be additional lift and triggers present in the RRV as a low pressure system pushes through and drags a cold front across S. Manitoba. More on that later in the week.

Get out there and enjoy the summer-like weather! Friday and Saturday will see more cloud, but the higher dewpoints will help keep it feeling nice and warm out.

Warm, Dry Weather To Continue; Fire Risk To Remain High

Brad Leave a comment

Warm weather will continue over Southern Manitoba this week which, while great news for those of us who enjoy summer-like weather in May, will continue to sustain dangerous fire risk conditions. This news is not welcome to firefighters who are already battling two substantial fires: a 1,300 hectare fire southeast of Steinbach and a 200 hectare fire near Piney, MB.

Bush fire burning near Whitemouth Lake

Wildfire burning near Whitemouth Lake. This is one of two large fires crews are battling to contain in southeast Manitoba. Photo credit: Anonomous CJOB Listener

Hot, dry weather is expcted to continue today as Southern Manitoba continues to be flooded with a mild, Pacific flow. We’ll see a daytime high today of 28°C with winds once again out of the west/southwest at 30-40km/h with gusts up to about 50km/h. Dewpoints will remain in the low single digits, which will continue to challenge crews as they try to contain the wildfires.

A cold front will sweep through this evening, however one would be challenged to notice. Winds will shift to the north, but we’ll see overnight lows similar to before as the temperature dips to only 8°C tonight. Very little cloud is expected with this moisture-starved cold front over the Red River Valley, let alone any chance of precipitation. There is a slight chance of a shower or thundershower over the Whiteshell, however if they occur they will be short-lived and produce little rainfall accumulation. It looks like the best chance for any showers is on the Ontario side of the border.

If you enjoy sunshine, the rest of the week will be right up your alley. Temperatures will hover in the low 20’s for Tues/Wed. before a warm front pushes over S. MB and pushes temperatures up into the mid-to-high 20’s for Thursday and Friday. It looks like there may be a chance of showers or thunderstorms as the warm front pushes in on Thursday, however it currently looks like the best support will remain in North Dakota, leaving us with some clouds, but no precipitation. The biggest change with Thursday’s system will be the switch from a Pacific flow to a Gulf flow, which in addition to warm temperatures will also bring moister air to the area, which should lessen the fire risk somewhat.

Pacific Flows vs. Gulf Flows

These two flows are large-scale weather patterns that dictate where the air is coming from and, especially here in Southern Manitoba, are two of the main large-scale flows we deal with in the summer months. Those two words carry with them significant information about the weather and one can make a quick assesment of what sort of weather to expect if you have a basic understanding of the differences.

Pacific Flow Diagram

Diagram of a Pacific flow. Image is from the GEMGLB model, valid this morning.

First, the Pacific Flow. This is what Southern Manitoba is under right now. This flow occurs when there’s a large blocking high over the western United States. Moist Pacific Ocean air flows NE up and over the high and associated ridge into the NW United States and British Columbia (represented by the green arrows in the diagram above). Once it reaches the mainland, it begins to rise up and over the mountains. As the air rises, it cools and water is forced out of it, falling as rain or snow on the upwind side of the mountains (represented by the shaded green area above). Once it reaches the other side of the mountains, the air descends downwards, warming as it goes. What’s left on the leeward side of the mountains is air that’s warmer than it was when it started it’s trek over the rocks, but substantially drier. This air then spreads eastwards over the Prairies (represented by the brown arrows above), bringing pleasantly warm weather, but very dry and often windy conditions as well, usually with dewpoints in the single digits.

Gulf Flow Diagram

Diagram of a Gulf flow. Image is from the GFS model, valid Thursday evening.

A Gulf Flow also brings very warm conditions to Southern Manitoba as well, however it does so not through trickery of thermodynamics, but rather through brute force. This flow develops when a longwave trough is present over the west coast of the United States (as opposed to a ridge). The flow in the lower atmosphere organizes itself to flow from south to north in response, and heat and moisture from the Gulf of Mexico begins its trek up through the Plains of the United States. The longer this flow can stay in place, the further north the heat and moisture can travel; Southern Manitoba usually ends up under the influence of this air mass at least a couple times each summer. What characterizes this air mass? Hot weather, usually in the upper twenties, though often in the low-to-mid 30’s, and high dewpoints, often at least 18°C, but commonly in the 20-24°C range.

Under the influence of a stifling gulf flow, Carman, MB set the record for the highest Humidex value (a ‘feels-like’ for heat) ever recorded in Canada in 2007. On July 25, 2007, the temperature climbed to 34°C with an unbelievable dewpoint of 30°C, which produced a Humidex value of 53, beating out the old record of 52.1 set in Windsor, ON in 1953.

It’s always a good idea to keep an eye on the forecasts when under the influence of gulf flows; given the transport of so much heat and moisture, it is under the influence of these flows that we often see the risk for powerful thunderstorms.

Dewpoint vs. Relative Humidity?

You may notice that we often refer to the dewpoint on our blog, and not to the more commonly used term, Relative Humidity (RH). What’s the difference? There are several:

  • The dewpoint is an absolute measurement. Relative humidity is, obviously, relative.
  • The dewpoint is reported in °C, and is a measurement of how much water is contained in the air. Relative humidity is reported in % and is a measurement of how much of the water-bearing capacity of the air is being used. As air becomes warmer, it can hold more water.
  • The easiest way to understand the dewpoint is simple: how cold would something have to be so that when you put it out in the open air, it would “sweat”.
  • When dewpoints are low, the air has little moisture in it, and when they are high, it has more water in it. People often notice the moisture in the air once dewpoints reach around 12-13°C. Once they exceed 16-18°C, many people would actively notice that it was humid, and once the dewpoint exceeds 20°C, many begin to find it uncomfortably humid.
  • Conversely, one cannot make any absolute claim with a relative humidity measurement alone, nor does the RH actually let you know if it’s humid or not. A RH reading of 100% only means that the air is holding as much water as it can. There are two major problems with this. The first is that the RH changes through the day, even if the amount of water in the air (the dewpoint), does not. If the day starts off at 8°C with a dewpoint of 8°C, the RH is 100%. If the dewpoint stays at 8°C but it warms up to 21°C, the RH drops to 43%, despite the fact that there’s no more or less moisture in the air. The RH would then climb back towards 100% in the evening as the air cools. The second problem is that a RH of 100% can be extremely deceptive. If the temperature is 30°C and the RH is 100%, one could barely spend any time outside it would be so unbearably humid. Fortunately, this situation is extremely rare. If the temperature is -25°C and the RH is 100%…well it’s certainly not humid out. In fact, RH values are often quite high in winter since as the air cools, it can hold less water, so it uses more of it’s water-carrying capacity with smaller amounts of water.

All that to say that RH is a pretty messy, inconsistent measurement that when presented by itself doesn’t actually tell you much about anything. Dewpoint, on the other hand, is a consistent measurement that quantifies how much water is actually in the air and can tell you something about the weather when presented by itself. And that’s why we prefer to use dewpoint here on AWM.

Elsewhere in Weather News: May 12th, 2012

Matthieu Leave a comment

China Gets Battered by Torrential Rains

On the afternoon of Thursday May 10th, the north-west part of China (province of Gansu) saw severe thunderstorms roll through its higher elevations and bring with them dangerous hail and downpours that wreaked havoc across the region. With heavy rains being the most threatening hazard in this part of the country because of the mountains and easily-triggered mudslides, around 3,000 residents were evacuated for precautionary measures in the areas most prone to flooding.

Residents evacuating

After heavy rains fell many residents had to be evacuated or rescued as roads were blocked off. (Source: Shanghai Daily)

As the large hail and heavy rain fell, the Minxian County in the province of Gansu was hit the hardest when 70mm of rain fell in about an hour. Mudslides and large hail impacted the county significantly as they destroyed 7,000 acres of farm fields, closed roads leading in and out of the county including a large interstate, and damaged more than 30,000 houses. The mudslides also sent 40 people to hospital, killed 37 others, and the toll may rise as over 20 other residents are still missing. In total, 17 of the 18 townships in the region were affected – this comprises of one fifth of the county’s entire population. China’s flood control headquarters issued more warnings in the same regions on Friday in an effort to raise awareness about the dangers of flash floods and the need to prepare for another possible bout of heavy rains again this weekend. The Red Cross chipped in by donating quilts and coats to the now homeless, and Chinese government pledged $320,000 to fix damaged schools, hospitals and water treatment plants.

Torrential rains

Roads become small creeks as torrential rains fall in the province of Gansu, China. (Source: China.org.cn)

About 3,000 first response workers, still battling with Mother Nature’s wrath, worked fervently around the clock on Friday evening, one day after the storms struck, to find those still missing.