The risk for severe weather will ramp up Wednesday through at least Friday across the portions of the Plains and Midwest as a new storm system comes in from the Rocky Mountains. This storm has formed due to a weakening upper-level low that has moved into the Southwest.
In terms of Wednesday’s forecast, a spinning low pressure over the northern Rockies will spawn the development of scattered and severe thunderstorms across the Great Plains. These storms will rapidly form during the mid to late-afternoon hours along a cold front. The best chance for severe storms will be in southwestern South Dakota, northwestern Nebraska, and southeastern Wyoming. The tornado risk is very low, but damaging winds and large hail will be possible in a few locations. Into the overnight hours, these scattered strong storms will move northeast. They will then weaken due to the loss of daytime heating early-Thursday morning.
Now on Thursday, the tornado risk will be greater due to increased moisture, instability, and wind shear. Wind shear is a big ingredient for severe storms, and is needed in order for tornadoes to form. This wind shear and higher instability levels will also keep the damaging wind and hail risk down. The below image from the NAM model shows the strong wind shear intersecting with moderate to high instability with CAPE (Convective Available Potential Energy) values up to 3500 J/kg.
Therefore, severe storms will be possible ahead of a cold front across parts of the Northern Plains and Upper Midwest. These storms will play out similarly as Wednesday, where much of the day will be dry but hot. Many locations will reach high temperatures in the 80s and 90s. Then by the late-afternoon, a line of storms will pop up. Even a few discrete supercells are expected to develop. It’s these storms that have the best chance at producing tornadoes. The majority of these storms will dissipate in the wee hours of Friday morning, but a line of strong storms with gusty winds is expected to push to the south down toward Iowa and Missouri.
On Friday, the risk for severe storms will decrease as low pressure begins to track into Canada. The strongest of the upper-level winds will be separated from the best environmental conditions at the surface to produce severe storms. Nonetheless, a few strong storms are still expected from the western Great Lakes back toward the Central Plains. The morning will be dry, then storms will pop up late in the day before weakening early in the morning on Saturday.
This May has been somewhat quiet in terms of severe weather. There is a big deficit of storm reports so far this year, and this latest severe weather event likely won’t get rid of the deficit.