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Heavy Rain, Severe Thunderstorms, Flooding and Significant Freezing Rain Possible Across Ontario


We are closely tracking a system expected to move into Ontario with heavy rain, possibly isolated severe thunderstorms, significant freezing rain, and snow. Currently (Tuesday evening), this system is over the United States and is spawning multiple severe thunderstorms and tornadoes. This system is pushing into Ontario.

Beginning in the late evening on Tuesday, moderate showers, at times scattered, will spread across Southwestern Ontario including the Extreme South. Local pockets of heavy rain are possible alongside non-severe thunderstorms. By the early morning hours on Wednesday (3-6am), light to moderate rain will begin to push across the GTA and into Central Ontario with scattered thunderstorms along the Lake Erie shorelines. Into the mid-morning (9-11am), moderate showers with pockets of heavy rain will be possible across portions of Southwestern Ontario, the GTA and into Central/Southern Eastern Ontario. Freezing rain and possible snow will develop across portions of Central and Eastern Ontario throughout Renfrew County and the Bancroft, Plevna region extending into Algonquin Park. The heaviest rain appears to fall from the shores of Lake Huron extending into the Muskoka region. In the Extreme South, the showers are expected to break leaving a period for clearing, possibly even clearing out the clouds to a sun and cloud mix. Into the afternoon (2-4pm), scattered showers (moderate to at times heavy) will be possible over portions of Central and Eastern Ontario with moderate to heavy freezing rain and light snow along the Ottawa River extending into Northeastern Ontario. Moderate rainfall is also possible along the southern shores of Lake Superior extending south over Manitoulin Island. At times, the rain could be heavy, especially around Georgian Bay. Across Southwestern Ontario, the clearing period throughout the late morning may lead to some pockets of convection or pop-up showers/thunderstorms. Any thunderstorms in this time period in this area will be non-severe. Into the evening hours (4-7pm), moderate to heavy freezing rain will be possible across Algonquin and Renfrew County alongside into the Bancroft region. Moderate to heavy snow will be possible north of North Bay including Gogama and Ruel. Showers, at times moderate to heavy, will be possible along the Bruce Peninsula extending into Muskoka and the Britt region.


There is some question about the risk of severe thunderstorms across the region. In this time period, there is some model disagreement on the location, intensity, and chances of severe thunderstorms. One model, for example, is displaying a line of thunderstorms, at times severe with strong wind, developing in Michigan and moving over Lake Huron to extend from Kincardine/Port Albert south to Windsor and Lake Erie as this moves east. Another model displays scattered thunderstorms, some severe, from Sarnia to London and then south towards Elgin County and Lake Erie, including Chatham and Windsor. We believe that the second object (isolated, scattered thunderstorms) appears more likely although we will wait to produce a final forecast.

Because of this difference in risks, we have created a severe thunderstorm outlook which can be found further down in this post. We will be creating a full severe thunderstorm forecast which will be attached to this post and as it's own post tomorrow (Wednesday) to display a more confident risk.

Following this into the late evening (9-11pm), scattered showers may be possible in Southwestern Ontario with moderate to heavy showers extending possibly into the GTA before moving out the region. Light to moderate, at times heavy, showers will push through Central and Eastern Ontario. At times, the rain may mix with freezing rain in the Renfrew County, Algonquin, Bancroft regions. Lingering snow is possible near North Bay and Temagami. By Thursday morning (3-6am), light rain may linger into Eastern Ontario while the remaining portion of Southern Ontario drys out.


Looking ahead, another round of showers and possibly thunderstorms may be possible later in the day on Thursday extending into Friday.

Rainfall

Rainfall Forecast, for Southern Ontario. Issued April 30th, 2019.

Blue or '1':

Areas in the blue or marked with a '1' can expect less than 10mm of rain. This includes portions of Northern Ontario from North Bay towards Espanola and areas further north.


Green or '2':

Areas in the green or marked with a '2' can expect between 10 and 30mm of rain. Rainfall totals between 10 and 20mm are likely in Sault Ste. Marie, Searchmont and Blind River. This region also includes Hawkesbury, Mattawa, and Stonecliffe. Rainfall totals between 10 and 20mm, locally 25mm are likely in Manitoulin Island, Britt and South River extending into Algonquin Park and along the Ottawa River (Pembroke, Cobden, Deep River). Rainfall totals between 15 and 25mm with local amounts near or very slightly over 30mm being possible in Cornwall, Maxville and Ottawa, as well as, Bayfield Inlet, Burk's Falls, and Sundridge. Throughout Algonquin Park extending towards Madawaska and Barry's Bay, rainfall totals between 10 to 15mm are likely with local amounts towards 20mm being possible, depending on the timing of the freezing rain to rain switch.


Green or '2' (Area of Interest):

Areas in the green or marked with a '2' inside the 'Area of Interest' can expect between 10 and 30mm of rain. Most areas in this region will see between 20 and 30mm of rain although local pockets between 10 and 20mm are possible. With thunderstorm enhancement, rainfall totals over 30mm are possible.


Yellow or '3':

Areas in the yellow or marked with a '3' can expect between 20 and 40mm of rain. In Eastern and Central Ontario including Brockville, Perth, Kingston, Belleville, Peterborough and Apsley, rainfall totals between 20 and 40mm are possible, including the local risk of 50mm. Further north, rainfall totals between 20 and 30mm are possible over Bancroft extending towards Calabogie due to a changeover to freezing rain which may limit rainfall totals. Depending on the timing of the switch, even less rainfall totals are possible. Further west, rainfall totals over Parry Sound, Huntsville and Gravenhurst can expect near 30mm of rain with locally higher amounts towards 40mm being possible. Over the Bruce Peninsula, rainfall totals between 25 and 35mm are possible with rainfall totals as high as 45mm. Towards the south over Sarnia, Grand Bend, Kitchener, and Vaughan, rainfall totals near 35mm are possible. Local totals towards 40 and even 50mm are possible. With thunderstorm enhancement, rainfall totals over 50mm are possible.


Yellow or '3' (Area of Interest):

Areas in the yellow or marked with a '3' inside the 'Area of Interest' can expect between 20 and 40mm of rain with local rainfall totals towards 60mm. Throughout the region, a widespread 30 to 40mm are possible. Local pockets towards 20mm may be possible. Local totals towards 50mm may be possible. There are some models displaying 'bands' across the region with rainfall totals over 50mm towards 60mm and in some cases even 70mm.


If the system tracks further north, which is possible, the heaviest rain will be shifted north over the Bruce Peninsula extending into Parry Sound and Algonquin Park. We have decided to not include this risk on our map.

Severe Thunderstorms

As stated in the timeline portion above, there is the risk of some isolated severe thunderstorms across Southwestern Ontario. This portion of this article will be updated to include a full severe thunderstorm forecast tomorrow (Wednesday) as soon as it's updated.


Forecast:

Severe Thunderstorm Forecast, for Southern Ontario. Issued May 1st, 2019.

Blue or '1':

Non-severe thunderstorms are possible for areas highlighted in the blue. Gusty winds and heavy rain are possible. Small hail can't be ruled out.


Green or '2':

Isolated severe thunderstorms may be possible in the region. The main threats are heavy rain and gusty winds. Small sized hail may be possible in the strongest storms. There has been a few storms showing minor rotation, therefore, a non-zero tornado risk does exist.


Outlook to Forecast Change:

We have shifted the risk further south (not including Sarnia, London, etc) and have extended the risk along the shores of Lake Erie to include Elgin County and the Niagara region. We feel the strongest risk will be along the shores of Lake Erie with a decreasing risk further inland.


Outlook:

Severe Thunderstorm Outlook, for Southern Ontario. Issued April 30th, 2019.

Blue or '1':

Non-severe thunderstorms are possible for areas highlighted in the blue. Gusty winds and heavy rain are possible.


Green or '2':

Isolated severe thunderstorms may be possible in the region. The main threats are heavy rain and gusty winds. Small sized hail may be possible in the strongest storms.


Reminder: An outlook is NOT a forecast.

Flooding

#TurnAroundDontDrown

Alongside this heavy rain, we are expecting the already record flooding to continue to worsen for portions of Muskoka and Eastern Ontario.


The Ministry of Natural Resources and Forestry have issued a FLOOD WATCH for Southern and Northeastern Ontario.

Flood Alerts | April 30th, 2019 | Northern Ontario | Ministry of Natural Resources and Forestry

Multiple local conservation areas have issued FLOOD WATCHES (brown) and FLOOD WARNINGS (red).


The Ministry of Natural Resources and Forestry including the MOST up-to-date flooding information can be found on their website: here.

Flood Alerts | April 30th, 2019 | Southern Ontario | Ministry of Natural Resources and Forestry

Alongside these many flood watches and warnings, multiple municipalities and counties have declared states of emergency due to this flooding.


These municipalities include (as of 7pm, April 30th):

- Alfred and Plantagenet

- Bracebridge

- Clarence-Rockland

- Greater Madawaska

- Horton

- Huntsville

- Laurentian Valley

- McNab/Braeside

- Minden Hills

- Muskoka Lakes

- Ottawa

- Whitewater Region


These counties include (as of 7pm, April 30th):

- Renfrew County

Snowfall and Ice Pellets

Snowfall Forecast, for Southern Ontario. Issued April 30th, 2019.

Blue or '1':

Areas in the blue or marked with a '1' can expect less than 10cm of snow. This includes portions of Central and Eastern Ontario such as Kemptville, Bon Echo Park, Minden, MacTier and Manitoulin Island including everything east and north. This also includes all of Northern Ontario. Almost all areas should see less than 3cm of snow although local pockets of 10cm or possible 15cm may be possible over portions of Algonquin Park extending towards North Bay, Ruel, Temagami, and Gogama. There is some question about how heavy the snow will fall, therefore, we have gone with the lowest possible risk.

Snowfall Forecast, for Northern Ontario. Issued April 30th, 2019.

Freezing Rain

Freezing Rain Forecast, for Southern Ontario. Issued April 30th, 2019.

Blue or '1':

Areas in the blue or marked with a '1' can expect less than 2mm of ice accretion. This includes portions of Eastern Ontario such as Ottawa, Hawkesbury, and Alexandria where a trace of freezing drizzle may set over the area. This also includes portions of Northern Ontario such as Blind River, Thessalon, Sault Ste Marie, and Batchawana Bay. This also includes Wawa and Temagami.


Green or '2':

Areas in the green or marked with a '2' can expect between 2 and 5mm of ice accretion. This includes portions of Northeastern and Eastern Ontario. This includes Arnprior, Pembroke, Deep River, Plevna, and Calabogie. Most of these places will see amounts near 2mm of ice accretion. Due to the change over to rain, much of this ice accretion may not stick to the surface of many objects and amounts will appear less than forecasted. In Northeastern Ontario, places such as Sudbury, Espanola, Agawa and Ranger Lake can expect between 2 and 4mm of ice accretion.


Yellow or '3':

Areas in the yellow or marked with a '3' can expect between 5 and 10mm of ice accretion. This includes North Bay, Mattawa and South River. Most of these areas will see amounts closer towards 5mm although local pockets towards 10mm may be possible. Due to the change over to rain, much of this ice accretion may not stick to the surface of many objects and amounts will appear less than forecasted.


Orange or '4':

Areas in the orange or marked with a '4' can expect between 10 and 15mm of ice accretion. This includes portions of Algonquin Park including Bancroft and Denbigh. The exact location of this area may be pushed slightly north or south depending on the track of the low. Due to the change over to rain, much of this ice accretion may not stick to the surface of many objects and amounts will appear less than forecasted. Most places will see amounts between 2 to 5mm or even 5 to 10mm despite 10 to 15mm of ice falling.


Red or '5':

Areas in the red or marked with a '5' can expect between 15 and 25mm of ice accretion. This includes portions of Algonquin Park including Barry's Bay and Madawaska. Most places will see much fewer amounts towards 2 to 5mm or even 5 to 10mm due to the intensity of the ice limiting the amount to stick and the changeover to rain. Due to the change over to rain, much of this ice accretion may not stick to the surface of many objects and amounts will appear less than forecasted.


There is the risk of even more ice over Algonquin Park although, due to the change over to rain and the intensity of the ice, much of this ice accretion may not stick to the surface of many objects and amounts will appear less than forecasted.


Because of the intensity and change over to rain, the freezing rain WILL NOT cause an ice storm despite the totals shown in the map.

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