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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 252121 AAA

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

421 PM CDT

A relatively concentrated area of scattered storms continues 
ahead of the long-lived but slowing and gradually weakening 
mesoscale convective vortex (MCV) scooting across northern 
Illinois. The storms have struggled to sustain themselves 
individually, likely owing to shallow boundary layer moisture 
and weak mid-level lapse rates. Recent Chicago area aircraft 
soundings indicate a well-mixed and deep boundary layer, up to
near 6,000 ft, and synoptic surface gusts have been 35 to 40 mph
at area sites apart from any storms. But surface observations also
indicate dew points having dropped to the lower 60s, and that is
barely enough to reach a convective temperature. Given marginal
low-level lapse rates (likely a little better than analyzed within
the heart of the MCV), the CAPE profiles are thin in the vertical 
for more robust storms despite the present of modest deep layer

If any storms can sustain and/or congeal into a cluster for more 
well-defined downdrafts, then a brief severe wind threat (60+ mph) 
could be realized before reaching the Illinois lakefront. Chicago
ORD recently had a convective gust to 48 mph, showing that 
potential. The storms should exit into southern Lake Michigan by 6
pm. There is a small chance for development into northern Lake 
and Porter Counties in northwest Indiana 5-7 p.m.

Otherwise, satellite and analysis trends in Iowa indicate just
south of I-80 appears to be the focus for the next round of
convection which could have isolated developing into our south of
I-80 area as early as near 10 p.m. but more so overnight.



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 18Z TAFs...

Forecast concerns include...

Gusty southwest winds this afternoon. 
Chance of isolated thunderstorms late afternoon and a chance for 
more organized thunderstorms overnight.

Current forecast still looks is on track for the afternoon. The
old MCV is tracking steadily east across the Mississippi River
into nwrn IL. Latest lightning data and satellite trends indicate
some initial thunderstorm development in far nwrn IL with some
towering cu development. Will maintain the vcts at the terminals
associated with this feature and monitor development closely for
any necessary updates should any direct terminal impacts become
more likely. Southwest winds with frequent gusts to arnd 25kt 
will persist through the afternoon. Some occasional higher gusts 
to arnd 30 kt are possible mid/late afternoon. Winds will 
diminish with sunset.

Convective chances are quite uncertain for the overnight hours are
trending farther south per the latest guidance which suggests the
development of a convective complex along the Iowa/Missouri border
late tonight and then tracking east into central Illinois. Given
this more southerly trend, have dropped the prob30 from all sites
with the exception of GYY which could be on the northern fringe of
the thunderstorm activity. 

Wednesday is looking to be relatively quiet dry/vfr conditions and
light winds to start the day. A weak pressure gradient and ample
sunshine and inland warming should allow a lake breeze to develop
and push inland in the afternoon. Expect lake breeze passage with
a wind shift to sely by late afternoon.


LM...Small Craft Advisory...nearshore waters 
     until 10 PM Tuesday.