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« Southern Ontario Thunderstorm Potential - April 13 | Main

Ontario storm potential April 9 to the morning of April 10, 2015

With the SPC issuing an enhanced risk for southern Ontario lets take a step back and look at what the potential actually entails. Lets take a step back and see what the setup looks like. Below shows the 8:00 pm EDT (00Z) 500 mb geopotential heights and vorticity (or upper level spin). What we have is a broad trough sitting over the western U.S. with several embedded short waves. These short wave troughs will become key a little later on.

At the surface we have out stationary/warm front draped across much of the mid-west, just south of the Great Lakes into the central plains with a lee cyclone (low) setting up in CO. This low will push east northeast tonight through tomorrow along with the associated cold front with the warm front gradually pushing northward  through the day.

Through the morning the surface front will still sit just south of the Great Lakes with the 925mb front pushing north. As that occurs there will be 500 mb pushing into southern Ontario which is circled in red on the image below. Forecast soundings for this time do show very subtle hint of some elevated instability thus some showers through the morning are expected with a few lightning strikes but nothing much else. This first wave will also introduce a good deal of mid-level cloud across the the province which will also be reinforces by the the second upper wave (circled in black) as it approaches the province as well. Thus the potential for day time heating is quite low.

Below we see the top two images show surface and 500 mb geopotential height and the bottom is 925mb Theta-e and with surface based CAPE. We will be well into the warm sector but notice how temperatures are still quite cool. Since out large scale synoptic flow will be from the east southeast coming off of Lake Erie and Lake Ontario. This will lead to the surface air being heavily modified by the lakes and thus hinder much warming along with the cloud cover as well. This is also reflected in the bottom right image which shows surface based CAPE. No surface based instability is forecast to make into southern Ontario. But looking at the bottom left which is 925 mb Theta-e we can see some decent values pushing into the province with the leading edge of the good theta-e values along the purple line. These values indicate that there is the presence of instability aloft and near the 21Z time frame forecast soundings around SW'rn Ontario do show elevated cape values near 200-450 J/kg. As the second upper wave interacts with the leading edge of the higher 925mb theta-e gradient some scattered weak T-storms are possible. Circled in purple is the 3rd upper wave that is expected to swing through in the early evening near 00Z or 8pm EDT. The forecast soundings below are for 00Z and we can see through south central to southwestern Ontario we will have near 400-800 j/kg of elevated cape again. CAPE values are within the red squares. Notice all soundings have an inversion around the 950 mb level. Thus with the 3rd wave some more robust or organized storms are possible late day through the evening but are NOT expected to be severe by any means as no storms are likely to be rooted in the boundary layer. Thus the main risk would be some heavy downpours and intermittent lightning.




Even assessing the 4km NAM for the 03Z time (11pm) it shows some more robust convection which will be associated with the 3rd wave. But again will likely all be elevated and non-severe.

Finally we have the cold front that comes through late Thursday night into early Friday morning. The bottom left image in the 4panel below shows again 925 mb theta-e. On it we can see a very sharp gradient associated with the cold front. Ahead of it we can see modest values which are indicative of moist unstable air. The top right shows surface based CAPE with the bottom right showing MUCAPE( Most Unstable CAPE). The MUCAPE field is higher here near 100-750 j/kg because again all the instability is elevated. The sounding below is from Wiarton which is closest to the temperature peak. At 06Z or 2am the sounding here peaks in instability with 1046 j/kg but again notice we have an inversion from 950 mb up to 850mb. But with the instability in place and a sharp cold font, some of the stronger winds aloft will likely be mixed down. Thus its not until the late overnight / early Friday morning hours where we will get into the more active period. It is during this time that with the cold front passage some wind gusts could approach the severe criteria of 90 km/h. 

Even then its difficult to know whether the winds will be associated with convection or with the front itself. The 4km NAM along the the cold front does not show much precipitation except for some scattered showers. But with the strong dynamical forcing and sharp cold front I do believe we will have a frontal squall along the cold front. With this squall will be heavy rain, some intermittent lightning, and the risk of some gusts possibly getting near 90 km/h. 

So the take away here is yes there will be the risk of scattered thunderstorms through the day tomorrow. But it is quite likely that any storms will be elevated in nature and will not pose a severe threat. Any severe threat would be late Thursday night into Friday morning along the cold front itself where there will be a high wind risk.