I am looking at a potential storm chase over the next few days. I have been gearing up, cleaning out my car and charging batteries. I set up my dash cam, washed some laundry and checked the fluids in the car.
It is still a little early, but the Storm Prediction Center (SPC) has issued an Enhanced Risk for Nebraska on Friday. The Day 4 SPC forecast has a 30% probability for Nebraska and Kansas. By Day 5, the system has moved farther east, and Day 6-8 are still listed as “Probability Too Low.”
Let’s look into this more closely. Driving to “Nebraska and Kansas, somewhere” isn’t really a plan, and I want to know whether to track east with the system or head back to New Mexico. Also, what “system,” what is happening?
I will start by examining the North American Model (NAM). I looped through each layer, but was especially interested in 0Z Saturday (Friday night) through 12Z Saturday (Saturday morning), as the model only extends 60 hours from now, and this is the last run available so far.
The 300 mb plot shows a shortwave trough approaching from the west. This trough is what is driving this entire severe weather event. At 0Z Saturday the threat area is at the exit region of the jetstreak.
As the trough moves east, it will increase the directional shear, and will increase the chances of severe weather for Saturday.
The 500 mb plot shows some Positive Vorticity Advection (PVA) extending into central Nebraska. Even though the vorticity is not as strong here as farther southwest, the vorticity gradient is strong, and the winds blow directly across it.
The PVA increases in strength and moves north into South Dakota by 12Z Saturday.
The 850 mb plot shows some Warm Air Advection (WAA) ahead of a warm front that is forming from a surface low pressure system due to east towards Wisconsin. The WAA is strongest in northwest Nebraska and southwestern South Dakota.
By 12Z Saturday, the low pressure system has deepened and become a closed low visible on the 850 mb chart. The warm front has moved slightly northward, leaving the strongest WAA over southern Minnesota and northeastern Iowa.
The surface plot shows a developing low pressure system over the Nebraska and Wyoming border. The surface winds are approaching from the southeast, advecting moisture from the Gulf of Mexico.
By 12Z Saturday, the low pressure system has moved north into South Dakota, with the surface winds still approaching this system from the southeast.
What does all this mean? It means that chances for severe weather will be higher on Saturday than Friday. However, the target area will be smaller on Friday; the focus will be where the WAA and PVA intersect, provided the skies are clear enough for diurnal heating. By Saturday, the threat area will be much broader, driven by a sharpening gradient along the trough edge. Storms will be more intense and more widespread on Saturday versus Friday, and so pinpointing an a target area will be a little more difficult until after convection has fired and fallen apart on Friday. The strongest storms may form at the intersections of outflow boundaries from Friday’s storms and synoptic scale boundaries.
I will likely target a little farther south on Friday. I don’t know if I could get to Nebraska in time for the storms there, so I may be behind the system. However, being a little farther south, I may be able to target anywhere in the 30% probability ring for Saturday, versus getting stuck trying to travel from South Dakota to the Oklahoma border.
Thank you for reading my prediction.
The Severe Weather Outlook is from the Storm Prediction Center website.
The satellite imagery is from the NOAA Satellite and Information Service.
The surface observation and upper level charts are from Unisys Weather.