Severe Wx Analysis/Forecast for Saturday, August 4th
Over the past few days, forecast models such as the GFS, Euro, and CMC have been consistent in the potential for severe weather in Wisconsin and the Upper Peninsula of Michigan this upcoming weekend. Upon analysis of the latest GFS model output (18Z), the ingredients all align for a severe weather event.
There are four ingredients I look for in the forecasting of severe weather: (1) Instability, (2) Moisture, (3) Lifting, and (4) Wind shear. Let’s take a look at these parameters in the case of this upcoming event suggested by the latest GFS run.
Instability is the measure of an air parcel’s ability to reach buoyancy and rise to higher levels of the atmosphere freely. Conditions that cause instability are cold air overlying very warm air at the surface. Since warm air is less dense than cold air, the warm air at the surface can rise in the atmosphere until it becomes as cold as the surrounding environment.
A quick and convenient way to measure instability is CAPE, or convective available potential energy. Here’s a look at the GFS output of CAPE values for 1PM CDT for Saturday:
The yellow and red colors indicate CAPE values of 2000 J/kg or higher, indicating sufficient instability for air to rise. Throughout most of Wisconsin, the values are high enough to create strong storms.
Moisture at low levels of the atmosphere is the fuel for thunderstorm development. Rich moisture allows for the storms to feed themselves and become larger and stronger. In assessing low level moisture, the most obvious parameter to look at is surface dew points. Typically, dew points need to be 55 degrees Fahrenheit or higher to provide enough latent heat release for severe thunderstorms. Let’s look at the dew points for Saturday afternoon.
Dew points are expected to reach at least 70 degrees for much of Wisconsin and the Upper Peninsula. This is ample moisture to support severe thunderstorms.
Forced lifting of air is absolutely necessary for unstable air to reach buoyancy and rise on its own. There are a few lifting mechanisms that can allow the rising of surface air: frontal boundaries such as a cold or warm front, low level warm air advection, positive vorticity advection, and upper level divergence. Most of these mechanisms are quite involved, so I’ll skip the in-depth explanations for now.
I first look at the 500 mb chart to access shortwave troughs. Shortwave troughs indicate areas of positive vorticity advection (PVA), or vertical motion in the atmosphere.
Above, the 500 mb chart is shown. A very pronounced shortwave is present, with a trough axis indicated by the red dashed line. Downstream of the trough axis (to the right) is an area of PVA. This tells me that rapid vertical motion in the atmosphere is moving into the Wisconsin/Upper Peninsula region.
Another chart I assess is the surface chart. This gives a good indication of frontal boundaries, warm air advection, etc. Below is a surface analysis showing temperature, wind, and mean sea level pressure:
Indicated by the blue line, the cold front is a primary lifting mechanism for these storms. When the cold front moves into the area, it forces the warm, less dense air to rise above it, reaching its level of free convection and exploding into storms. The black arrows indicate that warm air is being transported into the region, creating greater instability. Conditions are looking favorable for severe weather so far, yeah?
Another chart I like to assess is the 200 mb chart. The 200 mb chart is used to located the jet stream depending on the season. The jet stream shows where air is divergent in the upper atmosphere. Divergence aloft is necessary for a storm to sustain itself. Jet streaks, or areas of stronger winds within the jet stream, are good indicators of strong divergence aloft.
The 200mb analysis for Saturday shows a jet streak of 80+ knots (92+ mph) located over northern Wisconsin and Michigan. This is an extremely strong jet streak given the time of the year. This tells me that there is plenty of divergence aloft. It also tells me that wind shear is likely present, which I will discuss next.
(4) Wind shear
Finally, wind shear is one of the final indicators I look for in severe weather. Wind shear is the change of direction and change of speed of wind with height. Wind shear can give us a good idea of the organization and type of storms that are likely and the hazards associated with the storms.
Above is the wind shear from the surface to mid-level in the atmosphere. As you can see, shear of up to 60 knots is present over the Wisconsin area. This is sufficient speed shear for strong severe storms. However, looking at the directional shear (which I will not show), winds appear to be mostly unidirectional. This is an indicator that the storms will not likely rotate.
Based upon the previous analysis, I believe Wisconsin and the Upper Peninsula of Michigan will be seeing some severe weather this Saturday. The storms will likely be multicellular, turning into bowing segments capable of strong, damaging winds and large hail. However, tornadoes cannot be ruled out based upon the amount of instability and wind shear. Most probable location for tornado formation is near the surface warm front, which as of right now is forecasted to be near the international border.
This weather event is quite far out yet to be accurately forecasted. Conditions can change drastically from now until then. I’ll be updating the forecast as required when we get into the 2-3 day time frame.
(GFS output charts courtesy of TwisterData.com)