Plaincast

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Periods of thunderstorms will be possible this afternoon and eveing. Any storms that occur could be severe with damaging winds and potentially some large hail.

Dangerous heat and humidity will be found around outlying areas through this afternoon.

Dangerous swimming conditions due to large waves and strong currents are expected late tonight through early Wednesday morning.

Discussion

Early this afternoon, a broken line of severe convection with embedded supercells is tracking east across the interior Chicago metro area. Since the start of the afternoon, these storms have likely been responsible for at least one tornado, if not a few, large hail, and some road flooding. Latest SPC mesoanalysis suggests up to 5,000 J/kg of MLCAPE resides over NE IL which have been allowing for convective cloud tops up -80C reaching heights over 60,000 ft. This cluster had developed along a tight instability gradient on the leading edge of a low level shortwave disturbance pushing across northern IL. The system as a whole is being carried toward the east with the synoptic wave while individual cells are tracking more southeastward with a NW shear profile. With activity beginning to cross into northwestern IN as of 1PM, would imagine these storms should have little to no trouble maintaining themselves over the next couple of hours as they spread SE over our IN counties. All hazards will remain possible with these storms into late afternoon before moving away to our east including damaging to destructive winds, large hail, flooding, and possibly a tornado.

Depending on how long these current storms and isolated trailing activity want to fester in our eastern forecast area, we may see a brief relative lull in activity during the mid-late afternoon. Thereafter, additional thunderstorms may target our area along and ahead of a cold front during the late afternoon and evening. Severe storms associated with this feature are already ongoing over east-central WI. Guidance has been of little help today going back and forth with solutions for later today. This morning, camps were backing off on additional convection this evening, but latest CAM guidance is starting to get excited again. It's not entirely clear to what extent the ongoing convection will influence the low level thermodynamic environment for later. Outflow from ongoing storms is pushing away from the metro into the outlying areas. Wouldn't imagine at this point in the day that the unstable airmass will have time to recover much where its been removed. Nonetheless, the environment aloft will remain plenty unstable and supportive of additional severe storms later if parcels can be sufficiently forced. However, forcing along the front appears rather meager with not a ton of help from features aloft. Additionally, lots of deep layer dry advection, including through the low levels, will take place on the back edge of the shortwave between now and this evening. This will work against deep convective development down into northern IL this evening along the front. With this sort of instability though and with storms already ongoing, imagine it's rather likely we see at least some activity during the evening hours; whether it's severe remains unclear. The highest coverage during the evening is heavily favored across the eastern half of the forecast area near and east of the track of the disorganized surface low. If storms can mature along the front later today, damaging winds will be the biggest concern with hail and flooding also a possibility. Little to no low level instability and unfavorable hodographs don't appear to present much of a tornado threat this evening, even with surface-based storms.

Storms should move away from the area by the end of the evening. Gusty northerly winds will develop in the wake of the front late tonight into tomorrow, especially over the lake. This will quickly generate high waves and dangerous currents on the lake and a High Swim Risk is forecast at area Lake Michigan beaches from late tonight into Wednesday morning. Some patches of fog may also attempt to develop in the wake of the front late tonight, but not seeing much of a signal for any widespread dense fog. Rather, we may be stuck beneath a blanket of gloomy low stratus during the morning tomorrow.

The cold frontal passage will bring relatively cooler (albeit still warm) conditions to the middle of the week with lower and middle 80s forecast Tuesday and Wednesday afternoons. Regional high pressure will bring quiet and largely sunny conditions to the middle part of the week before another synoptic system looks to bring more rain and storm chances to the region by Friday.