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Severe weather outlook for Friday June 12, 2015

I did port early to the Twitter regarding the risk for a significant severe weather day for southern Ontario this coming Friday. Well I may have got a little ahead of myself after seeing the 06Z NAM from the morning of June 11 and I also only looked at one model. I'm not saying there isn't the risk now but the potential the potential that was being shown on the latest model runs is certainly not as high. The images shown are a 3 model comparison of the June 11 12Z (8am EDT) runs valid for 21Z (5pm EDT) for Friday June 12. Going clock wise from top left in the images are the NAM, GEM, and GFS.

Assessing the 500mb pattern all models show a broad trough swinging through southern Ontario. The main issue is going to be timing of the upper shortwave. Both the NAM and GEM agree on the timing and location of the shortawave. We can see that the GFS is a little more positively tilted with the upper trough. The tilt of the trough will be important as it will determine the strength of the large scale ascent within the warm sector and will have a direct influence on where the surface low will track. Associated with the upper trough is a 40-55 knot 500 mb jet maxima that will be pushing through southern Ontario as well. The bulk of the jet maxima will be off to the eastern parts but still ample flow over the area of interest. None the less this flow aloft is enough to provide about 30-50 knots of 0-6 km bulk shear across the region.

Moving down to 850mb a 35-55 knot LLJ is expected to be pushing through southwestern to eastern Ontario. Both the NAM and GEM agree with the orientation and location of the LLJ axis where the GFS is a little flatter an weaker with it. The GFS also keeps the LLJ axis mainly south of the U.S. border but brings it up into Lake Ontario. This is a result of the weaker and more positively tilted shortwave the GFS has in the 500 mb flow as shown above.

At the surface is where things between the GEM/NAM and GFS diverge a little more. We can see that both the NAM and GEM have low pushing into south central Ontario through the late afternoon. I've analyzed roughly where the frontal boundaries will be and notice that the NAM has the warm front a little farther north than the GEM. The GFS is really different with no real organized low into the province and maintains more of a stationary boundary with a few small scale vorticies along it. Again this is a direct result of the weaker and positively tilted 500 mb trough. The other thing to note on the GFS panel in the large cold pool. This is due to the fact that the GFS is maintaining the early morning convection / rain through the day. This is similar reasoning to why the GEM may have the warm front hanging back some in comparison to the NAM. Thus as the warm front and trough push in through the Friday morning hours a round of showers and elevated thunderstorms are expected. The evolution of this morning convection will determine how far north the warm front will be and thus how much moisture and instability will get into the province. The SREF maintains a lean toward the NAM/GEM solutions and I believe with the morning convection and cloud that the GEM may be onto the better solution with the warm front hanging up some.

Where the low tracks and where the warm front sets up will be important to storm mode and risk type as well. Warm fronts generate ample amounts of low level shear and helicity which forecast models may not always pick up. Below is a sounding from roughly the area just east of London where I did analyze the parcel ascent. The orange hatched area is the CAPE and the pink hatches is the CIN. From the get go I see that the CAPE is very skinny so storms may not be able to tap into 100%. There is also a pretty stout CAP as well but should be breakable with the shortwave and cold front coming through. The CAP may also help keep things a little more discrete as well only allowing the stronger updrafts to dominate but that is something we won't know until storms get going. The hodograph is a very broad looping hodograph through the 0-3 km range but is a little straight through the lowest levels. This fact with the deep layer shear available any storms that do form will likely be relatively well organized supercells. CAPE forecast from the the NAM and GEM suggest SBCAPE values near 750-2000 j/kg with the NAM staying toward the higher end of the range. How much instability builds will ultimately again be dependant on the timing of the morning convection and how much clearing there is through the day.

All factors assesed above and looking at some of the convective allowing models storms looks like there will be a slight chance for severe storms to develop across souther Ontario tomorrow afternoon with initation near 2-4 pm into Michigan and storms possibly coming through Ontario near the 3-5 pm range. The hodograph above is not very suggestive of it but taking more of a lean to the NAM/GEM low track and position of the warm front I think low level helicities will be a little bit more enhanced than what models are showing. So because of this I do believe supercell modes are possible with the small tornado threat. But shear vectors will lead to storm interactions and will become more linear relativelyquick. With the "skinny" CAPE the hail threat will be minimal so only small hail is likely pea to marbles maybe. In the risk map below the green area is the general T-storm risk area where some storms could become severe. The red area is where there is a higher severe risk with a very small tornado threat. Note that the tornado threat is CONDITIONAL on if the low tracks as the the GEM/NAM show and if there is sufficient instability.