That’s right, we again have a Marginal (1 out of 5) to Slight (2 out of 5) risk for Severe Weather with large hail and damaging winds being the main threat. The reason for this continued threat over the last fews has been due to a frontal boundary that has stalled out just to our south over Central Illinois. It is time for a quick lesson in meteorology and storm development. There are three ingredients that are needed for storm development, just your average storm:
- Moisture (dewpoint) [Generally want your dewpoint to be 55F or more, but can vary based on the type of convection]
- Instability (CAPE) [Dependent on the season, but generally 1000 J/kg is ideal]
- Source of Lift (Boundary, Front, etc)
For Severe Storm development there is one more ingredient that is needed, and this is the KEY between seeing severe storms and your average thunderstorm. That fourth ingredient is Shear. Shear is the change or increase in winds with height and direction. This can be broken down further into Directional Shear and Speed Shear. Directional shear is the change of wind direction with height. While Speed shear is the change in wind speed with height. Shear is important because it helps thunderstorms tilt with height allowing for the storm to sustain itself longer by keeping the updraft separate from the down draft. Which will help with the development of a mesocyclone (rotating updraft that defines a supercell), which can further lead to the development of a tornado if other factors align just right. For the record, this is really, really dumbed down, and much more goes into this.
If you haven’t fallen asleep yet or stopped reading you might be asking, why does this matter? Well, for today’s setup, all four of these ingredients need to come together in order for us to see a repeated threat for severe weather. Generally speaking, dewpoints across the area are around 55F or higher. The source of lifting, in simple terms, is going to be the boundary just to our south. The limiting factor right now is our CAPE, which according to the Mesoscale Analysis from the SPC is basically nothing. That is supposed to increase though this afternoon to between 2000-3000 J/kg, which is more than enough instability. When it comes to shear, thanks to that boundary, winds at the surface are out of the east. As we go up with height, between 925-850 mb, the winds shift to out of the southwest, which means we have great turning with height in our lower levels. Above that at 700 mb, our winds are out of the west and at 500 mb they are out of the west-northwest. Great shear profile, but because we are north of the boundary, all our storms will be what we like to call elevated and not surfaced based. This is why we are talking about a large hail threat initially, that could transition to a more straight line wind threat late tonight as winds start to become more parallel, or unidirectional with height (winds are all in one direction). If the storms were surface based, then we would have a better chance for tornado development, but the storms will be too high off the ground to favor that.
So to summarize, Marginal to Slight risk for severe weather late this afternoon and evening, into the late overnight hours. An initial round of showers and storms may move through around 4 pm this afternoon with a threat for large hail and wind. The main focus though is around Midnight as a cluster of showers and storms will enter the Rockford area. More recent model runs have been trending it around or just after Midnight, so that is something that we will keep an eye on.
I am sorry if this post was too long, I just felt it would be a good time to give a crash course in what it is meteorologist are looking at when it comes to forecasting thunderstorms. It was also a great excuse for me to talk about today’s severe weather threat while also incorporating a very small amount of Mesoscale Meteorology ahead of my final.