Saturday, August 23, 2014

Interesting near-end-of-season setup

We've both been pretty quiet here recently; most of this is due to personal circumstances but also it's been due to a lack of good storms and setups to chase.

Tomorrow may be different.  Finally, something to talk about.

Moisture: good dewpoints are already in place, in the mid-teens, but 20s are lurking in ND.  They should advect north by tomorrow.

Instability: this is conditionally problematic, but with cooling aloft, should the dry slot of the upcoming system be relatively cloud-free (even for an hour) this will cease to be a problem, with 2000+ j/kg MLCAPE.

Shear: this is interesting; the deep-layer shear will be adequate but the profiles themselves may lead to weird storm modes.

Trigger: not a problem with a sub-1000 mb low coming our way, accompanied by all the requisite fronts.

Here's a prog sounding for Carman, MB, for 7 PM.  Note the implied convection.

So here's what I think will happen: a bunch of overnight and morning storms will drop tons of moisture.  This moisture as well as that advected in will make for potential instability.  The dry slot of the low-pressure system will move over the Red River Valley by mid afternoon, setting the stage.  It'll just be a question of whether the low clouds in the dry slot will mix out.  I think they will, as it'll be a rather windy day (turbulent mixing) and the sun is relatively high in the sky (convective mixing).  Then, around 5 or 6 PM, a few storms will go up and quickly become severe.  I think supercells with heavy rain, wind and hail will occur.  (Justin disagrees with me on the hail because of the warm atmosphere; I think the steep lapse rates and supercell structures will compensate for that and make for loonies or bigger; we have a bet on it.)  The vertical wind profile looks a little bit goofy for tornadic supercells, because it exhibits a veer-back pattern, which tends to make the storms rain into the updraft, and as a result they can't live as long.  As well, the resultant storm motion, aside from strongly deviant motion, will be into lower and lower instability.  So it's not ideal for tornadoes, but it's not horrible.  Low-level turning could provide for a few tornadoes,  but this doesn't scream long-track violent tornadoes, either.  I think tornadoes will be relatively early in the event, if any.  Then it'll blow up into an area of rain and thunderstorms that will move into northwestern Ontario.

Consecutive runs of various models have been pulling the low farther and farther west; a few days ago this setup looked like it would be primarily a northwest Ontario one, and perhaps in the jungles of eastern Manitoba.  Now, it's changing in favour of chasers.

Good luck and stay safe!