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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 260524

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1224 AM CDT Wed Jun 26 2019

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 06Z TAFs...

1224 AM...Primary forecast concern this period is a lake breeze 
this afternoon.

Southwest winds under 10kts through this afternoon. There is a
weak outflow boundary moving through the southwest Chicago metro
area...currently near arr/jot. This may provide a brief period of
10-15kts in the next hour or so. As weak high pressure settles 
across the region today...winds will diminish enough to allow a 
lake breeze to form. Guidance does move this lake breeze through 
ord/mdw by late afternoon and maintained current timing for a 
shift to east/southeast. Timing may need some minor tweaking once 
the lake breeze emerges. Winds are then expected to turn light 
southerly later this evening.

Current showers across east central IL will remain south of the
terminals overnight. Isolated showers and thunderstorms are
possible in this same area this afternoon...south of the 
terminals. An isolated shower or thunderstorm is possible across 
northern IL this afternoon but coverage/confidence too low for 
any mention with this forecast. cms