Potentially Severe Weather Nears Stateline

A strong midlatitude cyclone pushing across the Plains at the time of publication is expected to have significant impacts on the Stateline late Saturday afternoon and evening.

As of 2:00 PM on Saturday, the storm in centered over northeastern Nebraska with its cold front extending eastward across northwestern Missouri, southeastern Iowa, and central Illinois. This warm front is expected to be the initiator of strong to severe weather across north and northcentral Illinois. As the system propagates northeastward through central Iowa, the advancing warm front will lift warm, moist air from the surface into a rapidly destabilizing atmosphere. Ample vertical wind shear and strong vorticity streams closer to the storm’s triple point promote long-lived, rotating updrafts and, thus, supercell development.

Diabatic heating will play a big role in the severity of the thunderstorms. Significant warm air advection will be present with the approaching warm front which should pull temperatures in the Stateline into the lower to middle 60’s by late in the afternoon. Although additional sunlight could bring temperatures closer to, and possibly exceeding, the 70’s. This slight increase in temperature could help destabilize the atmosphere even further thus raising the probability for supercells including tornadoes and hail. Skies will remain mostly cloudy for the remainder of the afternoon, but notable clearing is possible at times.

 

All criteria for severe weather are possible including strong winds, sizable hail, and tornadoes. With significant low-level wind shear expected, organized vorticity streams promote the development of lengthy and strong low-level helicity swaths. This increases the probability of long-lived tornadoes. With an environment able to sustain strong updrafts, large hail is also a big concern with the approaching storm system.

The primary time frame for severe weather in the Stateline is between the hours of 4:00 PM and 8:00 PM, though the greatest chance for severe weather looks to arrive closer to 5:00 PM and lasting through around 7:30 PM. The Storm Prediction Center has issued a moderate risk, category 4 of 5, for severe weather for much of central and northcentral Illinois. The moderate risk encompasses Whiteside and Lee Counties, reaches into far southern Carroll and Ogle Counties, and includes the southern half of DeKalb County. This includes the cities of DeKalb, Sterling, and Amboy. An enhanced risk stretches nearly to the Illinois-Wisconsin stateline and includes the cities of Rockford, Freeport, and Galena. The Storm Prediction Center has also issued a 10 to 15% hatched probability for tornadoes as well as a 30% hatched probability for hail for majority of the Stateline. A “hatched” a probability is a 10% or greater probability for significant impacts. For hail, significant would mean two inches in diameter or larger and a significant tornado is an EF-2 tornado or stronger.

Have a plan in place for when severe weather, particularly a tornado, threatens your immediate area. Have a predetermined storm shelter that is easily and quickly accessible to you. Have multiple ways of receiving the most up-to-date weather information, one of these means should still be available to you should you lose power (weather radio, cell phone, etc.).