I have a terribly busy day tomorrow, including a presentation I'm still preparing the slides for (I shouldn't even be on LJ but I'm taking a short break) and an afternoon drive up to Chicago. This just got mailed by a respected forecaster to a stormchasing list I'm on. The area he outlines is only a couple hours' drive from here; in fact we'll drive right through the eastern portion of it late tomorrow afternoon. I'm thinking: bring the camera, and hope for something interesting to take a picture of. We won't have time to chase after anything, but there's always the possibility of a chance meeting. Stranger things have happened.
Subject: Forecast: Friday, northern/central IL
I expect a few tornadoes, maybe strong, in an area bounded approximately by I-74 on the west, I-80 on the north, State Route 1 on the east, and U.S. route 24 on the south. Best chance may be in the northwest quadrant of this forecast area, with major complications coming to this forecast tonight.
Latest NAM model is showing a low pressure over Galesburg, IL tomorrow at 7 PM, with a quasistationary front along I-80, and a cold front to St. Louis. The GFS is a bit further west and weaker with the low, but it breaks out storms late tomorrow afternoon along I-74. They all lock onto a shortwave which will be moving through after 4 PM in the afternoon in the warm sector. However...forecast soundings show that the atmosphere will become uncapped by 2 PM...this may be an early show. A deep, neutral-tilt upper trough will increase mid and upper level winds to 50 knots or higher (60+ MPH) in at least the northwest quadrant of the target area. The NAM has 30 knots at 850 MB, 56 knots at 700 MB and 50 knots plus at 500 at Lacon at 0Z tomorrow. The upper disturbance helps spin up the surface low over western/northwestern IL by late afternoon. This should back the winds to south/maybe southeasterly ahead of it right at the warm front. Warm, moist air, with temperatures in the lower 80s and dewpoints in the middle 60s will be lifted and produce thunderstorms by early-mid afternoon. Given the increasing speed shear and low-level directional wind shear in the afternoon, supercell thunderstorms are likely before it evolves into a squall line.