Spetember 9-11th Severe Weather Potential

Hello everyone and welcome back to the Severe Weather Centre. I've been keeping a close eye on the models lately and have been quite impressed with some of the severe weather parameters for this upcoming Wednesday across MI,IN, IL, OH and S On. Subtropical moisture will get drawn from Norbert's circulation off the West coast of Mexico in addition to an upper level low which is expected to spin off the B.C coast for several days. Note the two features on the latest run of the GFS(18z initialization).

By Monday evening this upper level low will come inland, crossing the Northern California coast and taking a track through the rockies and up towards Lake Superior. This will try to partially phase with an area of vorticity currently located across Nunavut, which will cause anomalous deepening of the low pressure area for this early in the fall. The euro goes as deep as a 982mb low centre near Sault St Marie Wednesday night while the GFS/CMC are a little faster and weaker with the SLP centre, albeit still very impressive.

18z GFS

12z Euro

 This deep low pressure area creates a very impressive kinematic environment with a 45-55kts LLJ across parts of MI, IN and NW OH. This jet strengthens even further after sundown with a 60-65kts jet max across S On late wednesday evening.

GfS model 850mb wind field valid 6z Thursday 

Really impressive low level speed/directional wind shear in a sounding near guelph, On. Notice the strong clockwise turning, small T/td spreads and extreme water loading/saturation.

 The sounding lacks instability and steep mid level lapse rates, however.

 These winds will be accompanied by a 90kts upper level jet streak nosing SE with strongly backed sfc flow out ahead of the front. PWAT values are in access of 2" across a widespread area.

GRR had this to say in this afternoon's discussion. Very interesting.

"THE RETURN INTERVAL FOR THE MEAN SEA LEVEL PRESSURE AS THE
SYSTEM MOVES THOUGHT THE GREAT LAKES IS ABOUT 1 YEAR IN 10. THE
PRECIPITABLE WATER RETURN INTERVAL IS GREATER THAN 1 YEAR IN 30 FOR
THE 10TH-11TH OF SEPTEMBER. SIMILARLY THE 850 TEMPERATURE ANOMALIES
ON THE COLD SIDE OF THE SYSTEM (NEAR ZERO) ARE AROUND ONE YEAR IN 5
FOR THAT TIME IN SEPTEMBER."

CAPE's do appear to be lacking on most of the models, with a general 500-1000j/kg on most modelling across IN/IL with little to no CAPE further NE. This, along with timing are going to be huge players for S On as a faster vort ejection will increase the chances of a more significant outbreak although super cells will likely continue well after sundown as the low level wind fields continue to intensify. Keep in mind, the models due tend to underestimate WAA at this range so a trend upwards in the instability department is likely as we edge closer to the event. Right now, I'm in agreement with the SPC outlook in terms of the areas likely to be effected. If modelling continues showing the dynamic's it has been showing, a dangerous situation is possible for S On from Windsor through the GTA, including much of SW Ontario. On the other side of the border Michigan, NW Ohio and Indiana are at risk of severe weather. Primary threats appear to be high precip super cells with strong winds, flooding potential and possible tornadoes. I'll keep you posted on twitter in the days ahead!