The latest outlook updates from the Storm Prediction Center this morning continues the moderate risk of severe weather, albeit slightly farther south across the Quad Cities metro, and has it extending into much of Illinois, Indiana, and Ohio. The enhanced risk has been narrowed down to areas east of Interstate 380, north of Interstate 80, and south of Highway 3. These areas are the primary areas of concern for severe thunderstorm development later this afternoon and tonight.
A look at the satellite, radar, and current observations early this morning shows several distinct things. First, as we expected this morning, showers and thunderstorms with the associated cloud shield are moving away from the area. The leftover rain this morning was a good thing in many ways – it prevented temperatures from getting too out of hand heat-wise, and therefore prevented too strong of a cap from developing. But a moderate cap is still expected to develop this afternoon, which is also a good thing as that should hopefully prevent development from occurring too early.
Surface observations are showing temperatures are modest, ranging from the low-to-mid 70s in the northeast and east central portions, to over 90° in the southwest. Dew points are creating for a humid environment, in the upper 60s northeast to nearly the mid 70s southwest. This will lead to high instability in the likes of 4000+ j/kg MUCAPE. As we ride on through the day, the cap will begin to erode by mid-to-late afternoon courtesy of the aforementioned instability. This will be in addition to the extremely strong 60-70 MPH effective vertical shear, which is courtesy of a powerful W-to-NW mid-level jet. With all of the obvious parameters in place with plenty of moisture at play, a ripe environment will exist over the area.
Low pressure center currently sits north of Lincoln, Nebraska, or near the Omaha area. The positioning of this low will be crucial to the forecasting process as it will set the stage for frontal boundaries and such. The boundaries left by this morning’s rainfall should also be taken in consideration. A warm front exists to the east of this across southern Iowa into southern Illinois, with a cold front behind it, and a dryline to the north. This all should slide north this afternoon… where it exactly sets up is not 100% for-certain – this will need to be refined continuously in the near-term.
Our current expectations is that thunderstorms will begin developing in the mid-to-late afternoon, and initially be in a discrete mode. With the volatile environment, upscale supercellular growth with occur quickly, with all threats of severe weather likely, including tornadoes, potentially strong; large hail, to baseball size; and damaging winds, to 60-70+ MPH initially. Forecast soundings show significant veering with height, with many of the tornado parameters pointing toward a decent threat there. This is something that needs to be monitored carefully tonight.
By late evening, supercells will evolve into a linear-type system, potentially into the form of a derecho. This is the reason for the moderate risk of severe weather over much of the southern Great Lakes and Ohio Valley regions, where a 45% hatched probability of damaging winds exists. A straight-line wind event with winds to 80 MPH continuously are possible, with an associated tornado threat (spin-ups) along the leading edge. This issue will likely be east of the state, so it isn’t as much of a concern here, but those with interests farther east should keep an eye on things. The line will be speeding east by that point, likely causing a lot of damage in it’s path. The primary issue that we will see across Iowa will be the discrete mode threats, and the higher threat for tornadoes and large hail.
Stay tuned to the Iowa Weather Network for the latest information throughout this event. We will likely be live streaming on Facebook tonight, so check us out there.