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Southern Ontario Thunderstorm Potential - April 13

Looks like a real shot of mild air will push up into the region today thanks to a large amplitude upper air pattern. Below we have the Rap mesoanalysis of 500 mb geopotential heights and vorticity from 12Z (8am) this morning. The ridge that is sitting up through Québec is allowing for a good push of mild air to push up from the U.S. We can see there is a subtle trough pushing through this morning which is bringing the scattered cloud cover along with some isolated showers for the Bruce Peninsula and Manitoulin Island. There is a broad trough sitting through west central Canada down through the north central mid-west U.S. with and embedded shortwave trough sitting through Minnesota / Iowa. This shortwave will provide upper level dynamics and geopotential height falls across southern Ontario this afternoon which will aid in overall ascent across the region.

Looking up to 300 mb there is a strong 130-155 kt jet streak nosing into the Lake Superior region. This jet streak is expected to push eastward through today although expected to weaken some as it does so. This jet streak will push into southern Ontario which should help with upper level divergence as well.

At the surface from 12Z (8am) this morning, the cold front is currently sitting just along the western shores of Lake Michigan. I’ve also analyzed dew point temps in increments of 5C. We can see that the better moisture is still sitting south of the province, but a persistent 30-40 kt 850 mb southwesterly low level jet will help advect some of the better moisture into the province. Notice in western MI where the moisture is nosing in just ahead of the cold front where the LLJ is strongest. Thus we should be able to get near 10C dew points into southwestern Ontario. The Detroit 12Z sounding does show a good bit of dry air and has a convective temperature near 91F (~33C). So right away I have concerns as to how much instability we can get as moisture advection and due points being advected in will not help much. The hodograph and winds profile however is very interesting and I’ll touch more on that below. Current speed of the cold front has it pushing onto the Lake Huron Shores near 2-3pm and should begin to push into eastern Ontario this evening near 7-8pm.

The 9Z RAP forecast below shows surface T’s, winds, and pressure on the left with the 500 mb geopotential heights and winds on the right. The RAP timing of the front roughly agrees with the current speed. Thus I believe the timing for precipitation or possible thunderstorms development near 2-3pm for Windsor up the Lake Huron shore and through southcentral Ontario 4-6pm. Take note that we can see the 500 mb flow is slightly less than a 45 degree angle. So I think shear vectors could allow for some more discrete cells at first but as the 500 mb trough swings east catching up to the front the shear vectors will make for line along the front. This will be reinforced by the cold air form the precipitation itself.

Below are the NAM forecast soundings for London for 2pm and 6pm. We can see at 2pm that there is some capping in place and moisture is getting better. But still not enough to allow for any measurable surface based CAPE. But take note of the winds profile and hodograph. There is suitable veering winds with a not bad looping hodograph. As we move ahead to 6pm we can see that there is some surface base CAPE near 266 j/kg with elevated CAPE values near 370 j/kg thanks to the drastic increase in moisture with the precipitation immediately along the cold front. Thus if storms develop a little ahead of the front, we could see a few weakly rotating updrafts with some more organized storm structures initially before it lines out.  This will be farther reinforced with surface to 500 mb bulk shear values expected to be near 50-70 knots. Although some weak rotation is possible there is no substantial torando threat. Should the storms develop right along the front, than will line out almost immediately. Given the sounding along the front gusts could approach the severe limit of 90 km/h but I’m giving a low risk of that in line with the SPC’s marginal risk.