Wednesday, June 26, 2019

Friday, June 28th. Severe Weather Threat Across Southern Ontario




First Look: Wednesday June 26, 2019.

Southern Ontario has been relatively quiet in terms of severe weather this year. We've had lots of rain and below seasonal temperatures, but a distinct lack of notable thunderstorms heading into the end of June. That severe weather drought could be looking at its demise coming this Friday.

The classic prerequisite statement for Ontario: there is a lot of uncertainty still to resolve, and that is to be expected several days away. Mesoscale parameters (lake breeze and other surface boundaries; potential ongoing convection pushing east into the province in the morning, amount of cloud clearing and therefore, CAPE available) likely won't be resolved until Friday, so that, and synoptic features (position of the surface low, magnitude of speed/directional shear) will determine what we get on Friday. Patience is a virtue (or so they say).

Depending on the model you look at, this could mean a severe weather outbreak
(the 6/26, 00z NAM), something less intense, but perhaps photogenic (6/26, 00z GFS), or little to nothing at all in terms of severe weather (6/26, 00z SREFs). Energy is the big question as of the 00zs.

In terms of general question marks, the strength and timing of speed/directional shear (higher amounts appear to arrive after storms are already in progress/dying), and the magnitude of CAPE (energy) available, are concerns with this system. It is a situation worth monitoring heading into the Canada Day weekend, as I'm sure plenty of people will be traveling around the province and/or enjoying the weather outdoors (whether sun or storms is good weather to you is relative).


Thursday 00Z Model Run Update


While the discrepancy between models seems to have become more focused (as expected), there are still questions on the timing, and strength, of the storms heading into southern Ontario on Friday. As of now, it looks like two waves of precipitation are likely to occur.

With the vort max (top left panel) nearing southern Ontario in the morning, ongoing convection due to the uplift from that vort max will enter the Lake Huron shoreline area in the late morning/early afternoon. The strength and duration of this precipitation will help shape the rest of the day. If it lingers, and the cloud cover with it prevents diurnal heating and instability to build, the second wave of precipitation likely won't be as strong. If the first wave doesn't last long and cloud cover moves east quickly (allowing for strong heating and instability to build), the second wave, which should form later in the afternoon, may be stronger.


In terms of threat, the first wave looks to be a strong straight line wind, torrential rain, and maybe some small localized hail in areas of greater instability, in the late morning/early afternoon. If instability builds (it looks to be modest- around 1200 J/kg SBCAPE by 18Z), this line may be into severe thresholds. The first wave should stay linear; more of a mesoscale convective system, as opposed to discrete cells, but it'll be come more clear in the morning runs if this isn't the case.

The second wave has more intense potential, but many conditions:

-How much (if at all) will the atmosphere recover after the first wave of storms passes through? Will CAPE rebound?
-Will storms that fire be discrete? Or will they quickly become linear; reducing the tornado risk?
-Will outflow boundaries/lake breeze convergences, a MCV from the west entering the area serve to provide surface backed winds; enhancing tornado potential for any potential discrete cells?
-The constant among nearly all models is that SW Ontario seems to continuously have the highest available energy, so it is the area to watch for any storm formation east of Lake Huron. Having said that, the further south in Ontario you go, the further away from the best speed/directional shear you go; meaning storms become disorganized and messy faster.
-Another constant is the 700 mb wind increase compared to lower levels. With well mixed soundings, some of those winds may get mixed down to the surface for more intense storms.

I'll look again in the morning and provide and update. I will likely be out trying to get some pictures as well.







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