Unsettled Weather On The Way

A broad trough of cold air aloft is set to park itself over Manitoba the next couple days, preventing any warmer air from spilling eastwards and keeping us cool, cloudy and slightly snowy.

24 Hour QPF Accumulation

24 Hour QPF Accumulation valid 12Z Thursday morning. This shows the total precipitation accumulation from 12Z Wednesday morning to 12Z Thursday morning.

A weak, broad upper trough will build over the province as a northerly at all levels provides cold air reinforcement. The general instability produced by the trough, combined with a slowly advancing cold front over the Interlake and a weak shortwave sliding across SW Manitoba from Saskatchewan, will produce plenty of cloud and occasional flurries. None of these features should generate significant snowfall, reflected by the generally meagre amounts produced by the model (<1mm of liquid equivalent over Southern Manitoba). Temperatures will remain relatively mild, moderated by the cloudy skies we’ll see for the rest of the week. Daytime highs should sit right around -5°C and overnight lows should be right around -10°C.

The cold front, after staying relatively stagnant for a few days, will push southwards this weekend as an upper trough swings southwards out of the Arctic. This should provide us with some sunny skies, but drop our temperatures down in the the -10°C to -15°C range for daytime highs with overnight lows closer to -20°C. By the end of the weekend, Southern Manitoba has another chance at snow, as a moderately strong low pressure system sweeps across the Northern Plains.

The general long-range forecast shows that after this slightly-above-average temperatures week is over, we’ll switch back into a slightly below-normal temperature pattern with daytime highs on the cold side of -10°C, with the possible return of overnight lows south of -25°C. Will March come in like a lion? We’ll have to wait and see…

Colorado Low Brings Most Significant Snowfall of Season

Many areas in Southern and South-Eastern Manitoba dealt with the heaviest snowfall of the Winter 2011-2012 season. The powerful Colorado Low that impacted our region dumped up to 20cm of snow in some localities, however due to an extremly sharp deformation zone, many residents may have been asking themselves where the snow was.

Snow pushed into Southern Manitoba late in the afternoon from North Dakota as the main area of lift was advected northward and intensified by a developing low pressure area in Northern North Dakota. Forecasting the event was very tricky, as model guidance had little consensus, with some models painting a swath of 10-20cm of snow over the entire RRV and others keeping all snow in Minnesota and Northwestern Ontario. This situation wasn’t unexpected, though; models often have difficulty dealing with the huge energy transfer that occurs in Colorado Lows.

Incoming Colorado Low

Satellite Image showing incoming weather system. Green shaded area represents main area of snow. Orange arrows show trajectory of the advection of the precipitation shield. Green arrows represent the deformation zone. Satellite image is from 2:15CST Monday Februrary 20th.

Heavy snow pushed into Southeastern Mantioba early in the evening with snowfall accumulation rates of 2-3cm per hour. As the system developed, it became more and more clear where exactly the snow was going to fall. What was surprising, though, was how exact one could be in their snowfall forecasts.

The areas that would recieve snowfall were completely dominated by what is known as the deformation zone. This is an upper-atomspheric feature that every low pressure system has; the clearest way to describe it is that it’s the feature created by a large area of air moving in one direction that is then split in two, with one stream heading 90° to the left and one stream heading 90° to the right.

Deformation Zone Diagram

Schematic diagram of a deformation zone. Given a hypothetical low pressure system with associated fronts (warm and cold), we can observe the general flow of air ascending over the warm front, wrapping back towards the low pressure system, and then encountering an area where the flow splits into two streams: one arcing back around the low-pressure center, and the other arcic anti-cyclonically away from the low. The area where the split occurs is called the deformation zone.

This feature often can be used to help forecast preicpitation because as air carries precipitation towards it, it is deflected in either direction and cannot push past that line. The strength of deformation zone varies, and how far past it the precipitation can penetrate is related to how strong the deformation zone is.

As the snow pushed into Southern Manitoba, it quickly became apparent that there was going to be a sharp line for where heavy snowfall occured. The following RADAR image shows how significant the deformation zone was:

Deformation Zone visible on RADAR

1.0km CAPPI image from the Woodlands RADAR demonstrating the strength of the deformation zone.

Typically precipitation will diminish over 50-100km across a deformation zone; on Monday night we were looking at distances of 10-20km dramatically altering how much snow any one location would recieve.

So! How much snow fell? It depends where you are. Snowfall totals:

Location Total
Winnipeg – St. Bonifice 6.25cm
Winnipeg – Airport 2.0cm
Winnipeg – Charleswood 3.8cm
Winnipeg – Downtown 3.0cm
Winnipeg – East 9.1cm
Winnipeg – River Park South 8.5cm
Beausejour 15.0cm
Landmark 14.0cm
Indian Bay 17.0cm
Oakbank 13.4cm
Pinawa 15.4cm
Steinbach 21.0cm
Woodridge 20.0cm

As the RADAR accumulations show, most areas within 15-20km of Highway 75 recieved 5-10cm of snow, and amounts dramatically decreased as you headed east, with essentially no snow in Morden. Even across the City of Winnipeg, snowfall amounts varied by almost 10cm.

Snowfall Accumulations

RADAR-based Snowfall Accumulations as of 5:00PM CST w/Actual Observations plotted on top.

The storm had far-reaching effects. Heavy snow on power lines knocked out power to much of Falcon Lake as well as parts of Sprague and St. Adolphe. 16 schools/divisions in SE Manitoba were closed today as a result of the snowfall. The storm left many roads in eastern Winnipeg as well as most highways south and east of the city slippery and snow-covered; icy conditions are suspected to be responsible for one fatal collision this morning on Lagimodier Blvd.

This snowfall was easily the largest one of the season, and was the largest single snowfall in Steinbach since the winter of 2009. If you were hoping for an early spring with a snow-free end to February…well, you may have to wait a little bit longer. The extensive snow cover will limit any typical warming we would get with southerly winds over the next while. The silver lining? The distinct lack of snowfall over the Western Red River Valley would allow warmer (above 0°C) temperatures to push into the Central and Eastern RRV if we were to be under a good westerly flow. We’ll have to shovel out and wait a bit for that, though…

Snow to Start the Week

A Colorado Low system will bring the potential for snow over parts of the Red River Valley and South-Eastern Manitoba on Monday and Monday night. This will not be a particularly potent Colorado Low, but it could cause problems nonetheless.

The Colorado Low (red L) at 6pm on Monday

The Colorado Low (red L) and its predicted track – Image valid on Monday, February 20, 2012 at 6pm

On Monday the Colorado Low system will be slowly moving north from the Central United States. The main low pressure centre will be located near the Nebraska/Kansas border around the noon hour and stall out over that region for the rest of the day. Meanwhile a secondary low pressure centre will be developing over Southern Manitoba or Northern North Dakota. This secondary low pressure centre will proceed to absorb the more Southern low centre on Monday night, creating one low near the Lake of the Woods by early Tuesday morning. Snow will begin in Southern Manitoba on Monday afternoon and the heaviest bands will exit the region on Monday night. Some light wrap-around snow may linger into Tuesday.

GEM-Regional's QPF outlook

Forecast snowfall accumulations by the GEM-Regional model. For simplicity assume the numbers along the side correspond to snowfall amounts in centimetres

All that information is nice, but that main question for most people is “how much snow will we get”. This is where things get tricky. Some models (such as the GEM-Regional above) are suggesting that South-Eastern Manitoba, including Steinbach, Sprague, and the Whiteshell will get upwards of 10cm of snow. This same model is predicting somewhere around 5 to 10cm for Winnipeg. As you can see in the graphic above, there will be a sharp cut-off in snowfall amounts to our west. This means that areas such as Portage, Morden and Carman will probably see no more than 2cm, if that. The fact that there will be a sharp cut-off in snowfall accumulations makes predicting amounts for given cities very difficult. If the cut-off line moves a little bit further west, then Steinbach and Winnipeg may see 10cm of snow. However, if the cut-off line moves a little further east, then Winnipeg may see very little if any snow, while Steinbach and the Eastern Red River Valley only get a few centimetres. Other models have completely different ideas for snowfall amounts with this system, so don’t assume that the GEM-Regional is correct (in fact we never assume that models are correct!).

Now that I’ve given you the background information on this snowfall event, I will provide my personal predictions. As I have alluded to above, there is still lots of uncertainty in snowfall predictions and as such the below forecasts are simply my best “guesses” given the information available.

  • Winnipeg: 2 to 4cm
  • Steinbach: 4 to 8cm
  • Portage: 0-2cm
  • Morden: 1-3cm
  • Whiteshell: 5 to 10cm, locally higher amounts near the Ontario border

Once we get through Monday, the weather will turn nice again. Tuesday and Wednesday will be warm with highs just below the zero mark in most parts of Southern Manitoba. Another low pressure system will be passing through our neck of the woods later in the week…but we’ll have more details on that later. Models are hinting at colder weather (i.e. below normal weather) for next weekend. Winter may not be done with us yet!

Elsewhere in Weather News

Giovanna – The Aftermath

Since reported last week, Giovanna, a cyclone in the western Indian Ocean with category 4 intensity at its peak, reached the central coast of Madagascar and caused considerable damage. The dangerous cyclone left 15 people dead and about 40,000 people homeless, most of them on the eastern coast where the cyclone made landfall and brought with it a significant storm surge and intense rainfall. Thankfully the capital city of Antananarivo (population 1.4 million) managed to avoid the center of the cyclone but the city still experienced widespread blackouts.

The track of Cyclone Giovanna

Super cyclone Giovanna’s complete track, added with its forecast track where it will eventually dissipate. (Source: HEWS)

Had the cyclone’s track had been slightly shifted to the north by about 40km, the damage would certainly have been greater in the capital. At its peak (over the western Indian Ocean before making landfall) the storm reached an astonishing 1,000 kilometres in diameter with sustained wind speeds estimated at about 230km/h.

Since heading back out into the Indian Ocean (Mozambique Channel), Giovanna’s track was more favourable to those in the region as it had taken a southerly track then turned easterly, not only avoiding a second landfall in Madagascar, but also completely avoiding Mozambique –the country that was ravaged by disastrous floods this year.

Elsewhere in the world, weather has been relatively quiet compared the last couple weeks. Some other weather news that could possibly make headlines this week is: another winter storm arriving in the UK this past Sunday, strong thunderstorms affecting the southern United States this past weekend, flooding in Peru and Brazil’s recent heat wave.

A Beautiful Weekend Ahead

Our left-over moisture will slowly clear out today and tonight ushering in a beautiful weekend with plenty of sunshine and warm temperatures. With Winnipeg’s average daytime high sitting at around -8°C right now, we can expect to see tempeatures nearly 10°C above normal by the end of the weekend!

Sunny Skies

Many areas in Southern Manitoba that are typically covered in snow at this time of year are completely bare. PHOTO BY ERIN GARDIN.

We’ll see a mix of sun and cloud here in Winnipeg today, with the ever-present chance of a few snowflakes, but any snow that does develop will be less significant than the few light flurries we saw yesterday in the city. The daytime high today will be cooler than yesterday, around -4°C, as a trough of cool air aloft swings over Southern Manitoba. The snow much of the Red River Valley recieved between 2 and 5cm of on Wednesday as a shortwave embedded in an upper trough swung across the province will not be sticking around for very long. An upper ridge has begun to build into the Prairies and will continue it’s trek into Southern Manitoba over the next two days. Here in Winnipeg, we’ll see winds switch around to southerly overnight tonight, which will usher in milder air and push our temperatures up for the weekend.

Upper Ridging

GEM-REG 500mb Winds valid 12Z Friday Feb. 17 (6AM CST)

We’ll see temperatures soar this weekend with a light south-southwest flow and mild air pushing in aloft. 850mb temperatures are expected to increase to slightly above 0°C by Sunday night, which will ensure that through the weekend we have daytime highs of 0°C – 3°C in Winnipeg.

Temperatures are expected to reach as high as +2°C this weekend; that’s 10°C above normal for Winnipeg at this time of year.

Any areas that manage to return to snow-free quickly will be able to add a couple degrees to those temperatures. Overnight lows will still be cool, with temperatures dropping down to around -11°C tonight and tomorrow night.

By Sunday, a low-pressure system will begin ambling it’s way out of Alberta into Saskatchewan. This will increase the southerly upper-level flow and provide the needed kick to really push those 850mb temperatures up. With the dramatically warmer 850mb temperatures in place, our overnight low on Sunday night will essentially be just a degree or two cooler whatever the daytime high makes it to.

The low will then track eastwards across Manitoba Monday into Tuesday. Models disagree significantly on what will happen happen for Winnipeg, with some giving us 5 to 10cm of snow and others keeping the precipitation 250 miles away from us. We’ll certainly keep our eyes on this system as it develops!