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Another strong low-pressure system is on the horizon, and it looks like it may bring yet another round of winter weather across parts of northern Ontario -- the same regions currently experiencing every form of winter weather.
Canadian Weather | Ontario

Winter wallop (and a lot of rain) ahead for Ontario


Brad Rousseau
Meteorologist

Saturday, April 29, 2017, 7:09 AM - Despite what the calendar may say for seasons, the winter of 2017 is not over.

Another strong low-pressure system is on the horizon, and it looks like it may bring yet another round of winter weather across parts of northern Ontario -- the same regions currently experiencing every form of winter weather.


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Below shows a five-day average (from April 27 through May 1st) of what the upper air (500 millibar) pattern across North American is expected to look like. A broad trough is poised to, and currently is, setting up across the central portions of the continent while being flanked by moderately strong ridges on either coast.

The upper-air pattern setting up will have temperature anomalies across the continent looking as such.

How temperature is driving the bus

Remember, this doesn’t show actual temperatures but rather how they compare with respect to what the normal is.

With the trough setting up across the central part of the continent it comes as no surprise that the cooler air will setup across this region with the core of the anomalously cold air across northern Ontario and the central United States. Temperatures here are expected to be anywhere from 2 C to nearly 12 C below normal.

With the relatively strong ridge setting up to the east, above-normal temperatures will be expected here. Temperatures here will range anywhere from 2 C to nearly 5 C above normal through the end of the month.

Note, though, that this is a five-day average so larger swings or departures from normal are possible day to day. Also note that this product is deceivingly mild for southern Ontario. The very warm temperatures on Thursday are causing the overall five-day average to appear milder when in fact the region will likely be near seasonal through most of the week.


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The takeaway from this image is to show that the frontal boundary at the interface of these air masses will roughly mark where the storm track will setup. With the anomalous cold air in place across northern Ontario, lows tracking along the storm track will bring the potential for winter weather and this is exactly what looks to be on the horizon at the end of the weekend into early next week.

Over the Pacific, a bundle of upper level energy worked its way into the western half of the continent Thursday, and gradually trekked southeastward along the western flank of the upper trough through the day Friday.


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As it gets into the southwest U.S., it will induce a surface low to develop over northern Texas along the frontal boundary outlined earlier through the day Saturday as shown below.

As the upper energy rounds the base of the upper trough it will strengthen and start pushing northward on the eastern flank of the upper trough, roughly along the frontal boundary and or storm track pointed out earlier. As it does so the associated surface low will push northward and strengthen as well.

Impacts from this next low will push into southern, eastern, and central Ontario through Sunday with widespread rain, which could be heavy at times, and possibly even the odd thunderstorm.

Snowfall expected Monday

Late Sunday night into early Monday morning precipitation will begin to spread across northern Ontario as snow, potentially heavy at times.

The latest forecast is showing that through the day Monday, areas from Moosonee to about Kapuskasing eastward along the Highway 11 corridor and Trans-Canada Highway through to the Thunder Bay, Upsala, and Atikokan regions will be seeing snow, possibly moderate at times.

Along the transition area between rain and snow some mixing is likely and the potential for freezing rain will need to be monitored. For now, the pattern being advertised by the long-range models is not one that is looking like any pro-longed freezing rain is likely.

Across central and northeastern Ontario, steady rainfall -- which could be heavy at times -- will continue through the day Monday.

Across southern Ontario there is still some question as to whether it will be steadier rainfall or not. Generally with these systems, as the warm front approaches and clears there is a break in the precipitation, then a line of more scattered to isolated showers or even thunderstorms come in along the cold front. Based on what models are showing this could be the case for Monday.

The warm front should clear southern Ontario Sunday night through Monday morning. Monday could turn out to be a pleasant mild day to start. But once the cold front comes through expect to see a line of showers or even thunderstorms through the afternoon, should the current timing that forecast models are showing pan out.

Monday night through Tuesday as the low pushes off to the northeast the cooler air will start to funnel southward which will then bring the risk for some snow, or at least wet snow, across most of northeastern Ontario. Any snow and showers should push out of central and northeastern Ontario through the day Wednesday.

In terms of snowfall amounts, areas near Thunder Bay, Atikokan, and Upsala, through Kapuskasing (or just west of it) through to Moosonee will see the most regarding snow. Accumulations could be significant getting near 10 to 20 cm range potentially even more.

Rainfall is looking like it will be greatest through central, northeastern, and western portions of eastern Ontario, where current models are showing rainfall amounts near 25 to 50 mm are possible.

As with most long-range forecasts, adjustments to these amounts, location, and timing, will be required as the event draws closer, so be sure to check back over the coming days for the latest on how this system will impact your region.

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