A look at the weather map early this morning (a little past 2:00 a.m.) shows an ongoing but somewhat eroding mesoscale convective system moving east-to-southeast through the central Plains. Temperatures are ranging in the upper 60s to low 70s for most spots, the exception being areas where rain had just fallen. Dew points remain to be rather high, especially for this time of night, in the upper 60s to low 70s.
As convection continues to erode, a general northeastward turn will occur this morning alongside the aforementioned thunderstorms across the central Plains. Short-term model solutions have been suggesting precipitation will be cleared out by morning, allowing peak diurnal heating to thoroughly destabilize the environment.
Another mild and humid day is expected across the state. High temperatures will range from the low 80s north to the low 90s southeast. Apace with dew points peaking around the low-to-mid 70s this afternoon, heat indices nearing the upper 90s in parts of the state can be expected. Consequently, instability in the likes of 2500-3500 J/KG of CAPE will develop, creating a very unstable atmosphere as a whole. Increasing shear aloft throughout today will also be a supportive factor with the organized severe thunderstorm development later this afternoon.
Any storms that would end up forming in this type of environment will become supercellular quickly amidst organized updrafts, leading to a large hail threat (upwards of two inches in diameter) and damaging winds to 60-70 MPH initially. An isolated tornado also cannot be ruled out – this risk is highest near the warm front (see synoptics below). The Storm Prediction Center currently has an enhanced risk of severe weather over much of the state, centered right over the Des Moines metropolitan area.
The greatest threat will likely exist to the northern part of the enhanced risk area. It would not be of particular surprise to me to see this area moved farther north-to-northeast by the 8a or 12p SPC updates, where the environment is really the most “ripe” in a sense. The tornado threat will be greatest across north central Iowa, with decent instability and increasing shear. The tornado threat may also be localized enhanced by the mesoscale boundaries that are laid out by previous storms.
By afternoon, a low pressure center will setup shop across the northern Plains (somewhere in southern South Dakota). Extending east-to-southeast of this will be a warm front, draped through northern Iowa and southern Minnesota, into the Ohio Valley. These synoptic features, in combination with whatever boundaries linger from thunderstorms early this morning, will play a key role in the broad forecast for the remainder of the day. Bottom line: a few tweaks can be expected between now and noon as confidence in these features increases.
Storms should begin firing by mid-afternoon. As the night goes on, storms will grow upscale and band up into several different organized clusters or lines of storms. The threats by this point will shift to a growing damaging wind threat. However, with the sun going down, a lot of the energy that these storms typically use will diminish, therefore diminishing the severe threat as well. These types of setups are nothing uncommon to Iowa, especially during the summer months (an initial discrete, supercellular mode eventually evolving into a linear damaging wind event).
Another threat that will have to be monitored carefully will be flash flooding. National Weather Service offices have been considered and may end up issuing Flash Flood Watches for the northern end of the state where it won’t take much rain to cause flash floods with the ample precipitation as of late. With all of the moisture available, heavy rainfall will be a big concern with thunderstorms – especially if they’re widespread. If they end up being moreso on the small-scale, this threat will be somewhat mitigated. This is something that will also have to be monitored through later today.
Stay tuned to the Iowa Weather Network for further updates!