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Another Significant Snowfall on the Way

February 24, 2012April 3, 2015 Brad Leave a comment

After Manitobans dug out from as much as 21cm of snowfall, it looks like some portions of Southern Manitoba may need to keep the snow shovel ready as another significant weather system tracks across the Northern Plains, bringing significant snowfall to North Dakota. The question is, just how much snow will make it into Southern Manitoba? Read on for our best guess…

Winters Around – Photo by Jennifer Molnar

First: for today we’ll see a mix of sun and cloud and light flurries through much of the day as the shortwave that brought around 3cm of snow to Winnipeg yesterday slumps southwards into Minnesota. Temperatures will be cooler today than the past few days now that we’re on the back side of the cold front, with daytime highs around -10°C in Winnipeg and the Red River Valley. Overnight, skies will clear and temperatures will drop considerably; in Winnipeg the temperature should get down to around -22°C, while some regions in the Central RRV south of Winnipeg will see temperatures dipping just below -25°C. Saturday will see temperatures rebound back to around -10°C with increasing clouds late in the day.

Another system will bear down on Southern Manitoba by the end of the weekend, and this time we’ll be looking at a very strong snowfall gradient from north to south instead of east to west. A powerful low pressure system will make landfall on the British Columbia coast by this evening and then accelerate across the Rockies and slice through the Northern Plains, spreading an intensifying area of snow across North Dakota and Southern Manitoba.

The exact question to be asked is “How much snow?” In this case, that’s not quite so easy a question to answer. In the heart of the heaviest snowfall, this system will pack quite a wallop, dumping up to 20cm (~ 9”) of snow in North Dakota. The situation in Southern Manitoba will be complicated by our old friend, the deformation zone.

While with the last system the deformation zone set up in a north-south orientation, dramatically reducing snowfall amounts across an area as small as Winnipeg, this system’s deformation zone will have a more typical west-east(ish) orientation and will not be quite as strong. This will again produce a relatively sharp drop-off in snowfall amounts from the south side of the deformation zone to the north side. Currently, indications are that it will set up somewhere slightly north of the Trans-Canada highway. This will allow snow to push across Southern Manitoba and give areas along the Trans-Canada highway, such as Brandon and Portage La Prairie, roughly 5cm of snow. Areas north of the Trans-Canada would see a couple cm, but nothing significant. Current indications are that if things pan out as currently forecast, areas south of the Trans-Canada would receive 5-10cm of the white stuff, with the potential for isolated areas of 10-15cm within 50km or so of the International Border.

Any forecast for this system will be extremely sensitive to the track the system takes, which is essentially impossible right now, given the fact the low pressure system is still off the Pacific seaboard. The best way to approach a system like this is through the use of ensemble forecasting.

Ensemble forecasting is a technique where instead of outputting one “answer,” a forecast model produces many “answers,” each a result of slightly altered initial conditions. We then combine all that information to be able to produce probabilities of some weather condition at any given location. This technique produces *probabilistic forecasts compared to the traditional single-“answer” deterministic forecasts.*

We’ll use the NAEFS (North American Ensemble Forecast System) to look at this problem. It consists of output from two models, a Canadian one and an American one, each supplying 21 “answers” to the weather. This will help us find the most likely solution given the conditions we know right now. So, moving forward…

NAEFS Ensemble Snowfall - Probability of > 5cm

Ensemble probability of greater than 5cm of snowfall accumulation, valid for 06Z to 18Z Monday February 27.

This shows us the probability of any one location getting more than 5cm of snowfall. For various reasons, although it may not seem like it would be, anything above 50% is a pretty good chance of more than 5cm of snow. We can see that Winnipeg sits right on the border of the 30% probability, which means we’ll likely see at least 3 or 4cm for the time period covered by this image.

My best guess for storm-total snowfalls from Sunday through Monday evening, given the current information would be:

Winnipeg: 7-10cm
Brandon: 8-12cm
Portage la Prairie: 8-12cm
Steinbach: 8-12cm
Morden/Winkler: 10-13cm
Emerson: 12-15cm

With the very sharp drop-off visible immediately north of Winnipeg, it becomes quickly apparent how important the track will be for this system. A shift of even 50km could dramatically increase or decrease the amount of snow that ends up on the ground. We’ll have updates in the comments through the weekend as this system develops. I get a feeling that once we can actually see the low and where it’s heading I’ll be updating those numbers above, but hopefully not too much!

Unsettled Weather On The Way

February 22, 2012April 3, 2015 Brad Leave a comment

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

February 22, 2012April 3, 2015 Brad Leave a comment

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 Snowfall
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

February 20, 2012April 3, 2015 Brad Leave a comment

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 Weather Moment

Author: Brad