The heat and humidity will be building back into the state by the weekend, and so will the storms chances. Remnants of tropical storm Bill will continue to move off to the northeast across Missouri, which will continue to bring increased cloud cover to the state, especially across southern Iowa. On Friday, a warm front will begin to approach the state, which will allow our temperatures to warm into the mid to lower 80’s.
By Saturday, the heat will be on! Highs will be soaring into the mid 80’s to lower 90’s across the state. The warmest locations will be across southern Iowa. Dew points will be tropical as they will be in the lower 70’s, which will add a few degrees to the actual temperature. Heat indices across southern Iowa by Saturday afternoon will be near or above 100°F! A few of the models have a few locations topping out at a feel like temperature of 105°F-107°F. This heat will be a trigger mechanism for afternoon thunderstorms ahead of an approaching cold front, which will sweep through the state Saturday late afternoon into evening.
The atmosphere will be primed for strong thunderstorms by Saturday. There will be plenty of moisture to work with as dew points will be in the 70’s. Convective Available Potential Energy (CAPE) is off the charts currently on the models. Some CAPE values are approaching 6,000 J/kg of CAPE. For severe thunderstorms, all you need is 2,500+ J/kg of CAPE, and since the models are showing twice that amount, there is enough energy to work with in the atmosphere. This will help support the development of very large hail, and damaging winds. Initially, storms may pose a low tornado risk. However, storms will quickly line out along the cold front, so the tornado threat appears to be very low.
The only concern that we have is the potential for a strong CIN (convective inhibition) value which will limit thunderstorm development. CIN is a measure of the amount of energy needed in order to initiate convection, or in other terms, overcome a cap on the atmosphere. The strongest CIN will be across central and western Iowa. This will play a major role in thunderstorm development in these locations. However, if a storm does overcome this cap, and can develop in this region, then it will be explosive as the atmosphere is primed. But for now, the best area for thunderstorm development appears to be across eastern Iowa, in a region with lower CIN values. This why the Storm Prediction Center has outlined this area in a day three slight risk.
There is still much to be monitored as we are still 48 hours away. Continue to check back here for all of the very latest developments.