WEATHER: Short Term Area Forecast Discussion



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Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
341 PM CDT Thu Jun 17 2021

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 

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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