Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 341 PM CDT Thu Jun 17 2021 .SHORT TERM... 244 PM CDT Through Friday night... A very complex thunderstorm forecast is unfolding across the area through Friday night, with the potential for multiple rounds of severe convection amid at least two very different forecast solutions. Steeper lapse rates aloft have advected over the area today, with isolated very high-based (>10kft) convection recently developing over Cook County along the lapse rate gradient and mid-level isentropic ascent. A mid-level speed max between a longwave trough over southern Canada and a flattening ridge over the southern Great Plains is already firing some convection across southern Minnesota early this afternoon, with assistance from a surface cold front. Steering flow as well as the general E or NE push of the unstable airmass associated with this convection should keep much of the earlier activity well north in Wisconsin. The main focus will be the potential for upscale growth of convection west of the area this evening as a 50kt+ LLJ interacts with the cold front as it sags SSE. As has been a concern, and remains a concern per the 18Z DVN RAOB, is the substantial lack of moisture advecting into the region this afternoon. A theta-e axis is currently edging northward into southwest Iowa. Given this trajectory and overall model trends to shift the focus of the LLJ farther north than previously anticipated, convective initiation and subsequent potential upscale growth tonight will also favor a more northern solution. However, the forecast beyond early evening will likely evolve in one of two ways: 1) Convection in Iowa congeals into a well-defined MCS. In this case, the self-sustaining properties of the MCS in a highly unstable/sheared environment will allow for convection with an inherent damaging wind risk to track ESE into northern Illinois late this evening into the overnight hours. However, the MCS will be encountering an increasingly hostile environment with eastward extent toward the Chicago metro and northwest Indiana as it outruns the better moisture feed. 2) Convection fails to congeal into a well-defined MCS. In this case, most of the convection will likely remain north of the forecast area with some southward propagation into far northern Illinois. The slow southward motion with continued regeneration along any low-level outflow will result in a backbuilding storm threat with the potential for locally very heavy rainfall rates of 1- 2"/hr and a narrow swath of 2"+. The strength of the regenerating elevated cores also pose a damaging hail risk. Either way convection evolves, some form of outflow will be shifting south across northern Illinois by daybreak Friday. In the first solution, the more robust MCS will produce a cold pool that should clear a large portion of the CWA. This scenario is becoming less likely based on observational and model trends shifting the initial focus farther north. With the second scenario, the effective boundary will either slow or wash-out somewhere across the central CWA. Air-mass recovery on continued seasonably strong flow through the mid-levels will result in a exceptional, but conditionally, unstable airmass generally south of I-88 by early afternoon. 925 hPa temps of 26-28C will place a notable cap over the low-levels, so convective initiation will hinge heavily on low-level moisture and any local forcing mechanisms. However, clearing may be sufficient to push temps into the mid 90s south of I-88 corridor and erode some of the cap. Initially, there is increasing concern that a developing early afternoon lake breeze may be an impetus for the rapid development of severe convection across the southern Chicago metro into northwest Indiana, especially with 50kt of deep layer shear still in place. Additional scattered explosive severe convection will likely develop on any additional boundaries through the afternoon before an approaching mid-level wave allows convection to blossom by early evening across the far southern CWA. Convection will likely continue in an E/W fashion over the far southern CWA or more likely just south of the CWA Friday night. But with so many factors in place before then, confidence is quite low on exact placement.
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