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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 260226

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
926 PM CDT Tue Jun 25 2019

925 PM CDT

Closely watching the small MCS over west central Illinois that has
persisted for quite a while from back to south central Illinois.
Recently, this has contained a small bowing segment with severe
wind in the ILX CWA. The trajectory of the RIJ and wind signature
with this would most likely take it just south of southern
Livingston and Ford counties. In addition, recent scans from KILX
indicate that outflow has surged out ahead of the leading edge

However, of note, a pair of supercells approaching the Peoria 
metro may threaten the Livingston, Ford, and possibly Iroquois
counties with hail/wind if they maintain intensity. Radar 
extrapolation of these cells would place them near far southwest 
Livingston County near 1040 PM CDT. With increasing convective 
inhibition, it's possible but not certain that these storms will 
weaken in time to preclude a severe threat, but will continue to 
monitor them. A pocket of enhanced deep layer shear may help them
sustain to close to counties mentioned above despite the
increasing CINh.

Otherwise, based off the overall trajectory of the lead MCS over 
west central Illinois, showers and thunderstorms will be favored
primarily south of the Illinois and Kankakee Rivers, with likely
little/no threat of rain to the north, particularly north of I-80.
Another cluster of thunderstorms over southern Iowa is tracking
east-southeast and it to would favor the far southern CWA
overnight if it holds together, though this is of low confidence.



140 PM CDT

Through tonight...

For the remainder of the afternoon, main concern will be convective 
trends associated with an old MCV moving along the IL/WI border. 
As of 115PM CDT, convective activity is on an increasing trend as 
the forcing from the MCV is beginning to tap an environment with 
increasing instability, temperatures in the lower to middle 80s, 
dewpoints in the upper 50s to lower 60s and ML CAPE of 1000 j/kg, 
and more favorable wind shear as winds turn from sly 15-25 kt at 
the sfc to wly 35-50 kt in the 2-4km AGL layer. The strongest 
storms are currently along the northern Winnebago County border, 
with the current trajectory taking the storm into srn WI. Latest 
satellite imagery indicates increasing coverage of enhanced cu 
developing to the south of the main upper level circulation center
with the potential for upscale development of thunderstorm 
activity into a more organized cluster. Based on the current path 
of the MCV, locations along and north of the I-88 corridor will be
the most likely to see strong to, possibly, severe thunderstorm 
development, but strong thunderstorms as far south as the I-80 
corridor cannot be ruled out. Based on the current speed of the 
the MCV, thunderstorms will be possibly until 23z to 00z this 
evening. Guidance has also been suggesting a second round of 
thunderstorms late tonight, after 06z. The latest guidance is 
suggesting that convective initiation with this second round of 
thunder will occur along the Iowa/Missouri border and then spread 
to the east. This would suggest that the most favored location for
this second round of thunder will be south of the I-80 corridor.


250 PM CDT

Wednesday through Tuesday...

The primary forecast concerns remain the thunderstorm chances and
frequency especially into the weekend.

Any thunderstorms tonight will likely have shifted south of the 
CWA and/or weakened by Wednesday morning. The low-level boundary 
should be extremely diffuse, and probably more of a weak outflow
composite boundary draped somewhere just south of the area. The
mid-level belt of west-northwesterlies will continue to be modest
on Wednesday atop a gradually developing warm advection regime.
All of this together points toward a somewhat nebulous focus for
convection during the afternoon. If an impulse, such as a remnant
MCV like today were to become more evident in the mid-level flow,
then its possible something could be keyed in on more so for
scattered storms. By later in the evening into overnight, the
low-level jet focus into the Upper Mississippi River Valley will
likely support scattered storms there that could become semi-
organized and evolve into northern Illinois.

With expected sun, Wednesday should again see widespread mid 80s.
A lake breeze is expected that will cool immediate lakeside 
locations into the 60s during the mid afternoon or so. 

The end of the week through early next week is looking very warm 
and humid with highs each day in the mid/upper 80s with dewpoints 
from the mid 60s to lower 70s. The 00z gfs/ecmwf build the ridge 
into the western Great Lakes and possibly across the entire area 
by Saturday. Convection that forms to the northwest will have the 
potential to move southeast into the cwa along the edge of the 
ridge. Timing is difficult but there does appear to be some 
consensus for Thursday night into Friday morning. If this were to 
occur it may not have a large affect on high temps on Friday 
unless additional afternoon thunderstorms redeveloped Friday 
afternoon. As the trends stand now...Saturday may end up being dry
with temps possibly into the lower 90s but confidence is low and 
maintained blend temps in the upper 80s. In the absence of any 
outflow boundaries...forecast soundings would suggest surface 
winds would be strong enough to prevent a lake breeze Friday and 

Going into Sunday/Monday, the 12Z ecmwf continued to build high 
pressure southeast across the Lower Great Lakes region. This 
would result in a cold front across the area in the Sunday 
timeframe which would probably lead to a period of thunderstorms 
and then drier/cooler easterly flow off the lake. This solution is
currently not depicted in the forecast/blend and its possible 
that the upper ridge does build farther east as previous model 
runs had shown. Thus confidence is very low. 



For the 00Z TAFs...

Overall, while the chances of additional thunderstorms late 
in the period will not be zero, but the chances are too low to 
include in the TAFs at this time. Any additional thunderstorm 
activity tonight is likely to remain southwest of the terminals. 
So, it is looking to be a quiet period of weather tonight into 
Wednesday. If any additional storms were to develop near any of
the terminals it would not be until later Wednesday afternoon or 

The gusty west-southwesterly winds will abate over the next couple
hours, with light west-southwesterly flow likely for much of the
day Wednesday. A lake breeze could make a run on ORD, MDW and GYY
later in the afternoon. If it makes it to these terminals, expect
a wind shift out of the east for a few hours into early Wednesday