Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200758

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 AM CDT WED JUL 20 2016

859 PM CDT

For Evening Update...

Minor update to include some low chance pops for showers/isolated
thunder across the south/southwest IL counties of the cwa. Also
trimmed pops back during the night with a lull in activity
expected in the wake of current short wave.

Regional radar depicts an MCS propagating southeast across west-
central Illinois and far eastern Missouri this evening. While most
of this precipitation will not be a factor for the cwa, an MCV
which developed with strong convection over southern Iowa earlier
this afternoon has tracked eastward into parts of north central
IL, with some spotty showers/isolated thunderstorms continuing to
fester from around Sterling southeast toward Pontiac and
Champaign. Trends from the HRRRx and other high-res CAM`s suggest
not much will develop upstream for the next several hours, with
some subsidence in the wake of the short wave/MCV across central
IL. Based on this scenario, have added some low chance pops across
the south/southwest counties through midnight for continued
bubbling of weak convective cells. Farther west/north, have
removed pops at least until closer to morning. Various guidance
does develop elevated convection in the 10-12z time frame along
the Mississippi River as the low level jet (currently focused
across the mid-Missouri Valley and upper Midwest) veers more
west-southwesterly. Confidence is somewhat low in coverage farther
east across the forecast area into Wednesday morning based on
differences in CAM output, so have made no sig changes to pops
after 10z or so.

Updated grids available. ZFP shortly.



303 PM CDT

Through Wednesday...

Concern in the short term is on convective chances late tonight
through Wednesday night. Of note, it could be quite warm on
Wednesday too outside of convection, which will provide a
springboard into our upcoming stretch of hot and humid conditions.

Model guidance tonight is keying in on several features in water vapor
imagery. The first is the one driving the convection across Iowa
and far western Illinois. The instability gradient is positioned
just west of our area, possibly slightly grazing areas west of
I-39 and is evident in warming cloud tops on IR farther north. This
gradient is expected to remain in these locations through this
evening possibly sneaking a bit farther eastward. Convection to
our west should generally develop southeastward with some lighter
precipitation getting into our western and southern areas later
this afternoon and evening where showers and storms could fester a
little bit as the system as a bow echo with it. There is another
wave riding the ridge in the Northern/Central Plains. Ahead of
this wave the low level jet will re-intensify across Iowa and
southern Minnesota tonight. Some of this low level jet energy will
focus on the leading shortwave that will be sliding southeast
across western Illinois. Guidance is mixed on how much
redevelopment will occur with this wave, and if it will rob some
of the moisture from the more northern and later shortwave. This
later wave and its associated LLJ will be weakening as it veers
Wednesday morning, but some of the activity could survive across
Northeast Illinois in the morning. Confidence is low as several
other guidance bring the more focused area into Central Illinois
and would leave NE Illinois with some showers possible throughout
the day, favored in the morning. Confidence is medium-low with
this time frame.



303 PM CDT

Wednesday night through Tuesday...

The focus will be on increasingly hot and humid conditions late in
the week, with the complicating factors being possible periods of

We are still in a favorable position to experience our most
uncomfortable conditions of the summer season thus far, beginning
Thursday and continuing for many locations through the entire
weekend. Most guidance keeps any potential overnight convection
Wednesday night into Thursday along our northern and eastern
areas, and possibly not even in our area. Either way, we would
only be left with some debris clouds while 500 mb heights increase
to 594-596 dam and the west/southwest oriented 850 mb thermal
ridge builds in. Good agreement in the ensembles and operational
solutions for Thursday. With 850 temps of 23-24C and 925 temps of
26-29C, Thursday will be hot and humid with dangerous heat index

The last few runs of the GFS and ECMWF continue to paint a muddled
picture for Friday as Northeast Illinois and Northwest Indiana are
located on the northeastern fringe of the hot dome and have
trended downward some with the extreme temperatures. Hot and humid
conditions seem a very good bet regardless, but this is a
precarious position because the potential is there to be even more
dangerously hot and humid but also bring in convection and debris
cloudiness both Thursday night and later Friday. Even if the
convection remains to our north, outflows could bring winds off
the lake. This is most pronounced in the 18.12z and 19.00z ECMWF
runs which has much cooler MOS numbers (the 18.00z run at 98 for
ORD and the 19.00z has 89). In spite of that, the NAM keeps us
capped all day long and it quite plausible that these other models
are overdoing convection and breaking the cap when it may be hard
to do once the ridge builds in. Even if the cap were to break it
appears that it could wait until late in the day where the hot
conditions will be achieved easily anyways. Plus, we will likely
have a very warm starting point Friday with overnight lows only in
the 70s to possibly even near 80 in the city that it will not take
long to get into the 90s.

CIPS analogs based on the GFS 0z run still suggest 80-100% likelihood
of heat indices above 100 F for Friday and 50-70% likelihood of
heat indices above 105F on Friday. WPC maximum heat index values
top out in the 105-110 degree mark across the Chicago metro and
above 110 south and west in the higher dewpoint axis closer to the
ridge, with > 70% probability of heat indices greater than 105
away from the lake. The effects of any potential convection
scenario would be less farther south and west but do decrease
confidence some for Friday.

Given the confidence provided in ensembles at this point, and
given the fact that the convection could miss us or temps could
recover quickly thereafter in a muggy air mass, it is still
prudent to maintain the excessive heat watch and following the
ebbs and flows of individual model runs at this point is
fruitless. Also of note, in spite of these complicating factors
and convection potential even over the weekend, warm southwest
flow will maintain muggy conditions through the weekend as low
temperatures will not likely drop out of the 70s (with dewpoints
in the upper 60s to mid 70s) in most places until possibly Sunday
night. This is certainly an important factor to note that it is
the lack of cooling potential overnight that is equally dangerous
if not more so. 850-925 temps outside of precipitation chances
would certainly support highs in the 90s through the weekend. Will
maintain the chance pops for the weekend as a strong upper trough
currently off the PacNW coast flattens the ridge over the weekend
and brings a cold front Sunday afternoon or Sunday night.
Therefore the message remains, dangerous heat and humidity
Thursday and Friday, with uncomfortable to possibly continued
dangerous conditions in some areas this weekend. It will likely be
hazy as well with high dewpoint air mass coming in.

Headline wise...we will be extending the Excessive Heat Watch into
Saturday for our southern and western areas, given an expected
surge in low level warming ahead of the approaching cold front.
Areas closer to Chicago and farther north may be affected by
convection Friday and current heat indices in these areas are
forecast to drop below 100. If things do not pan out as such,
excessive heat could remain a concern for the entire area.



For the 06Z TAFs...

High pressure is centered just east of the terminals tonight
resulting in light southeast flow which will veer to the south-
southwest Wednesday morning. Expect winds to increase to 10-12kt
Wednesday afternoon, then should drop back below 10 kt after
sunset. Thunderstorm forecast is a bit tricky with a lot of
variability between the models. We will be under a moderately
unstable atmosphere; however, the better forcing still appears to
set up to our north on the nose of the mid-level warm advection
with rather benign forcing locally. Any thunderstorms that develop
to our north/northwest could track south along the instability
gradient over the terminals, but latest suite of high-res
convective allowing models generally paint very low coverage or
dry conditions over the local area. The GFS and ECMWF both
continue to bring thunderstorms into northern Illinois, but both
are also initializing poorly, so hard to put too much stock in
these at this time. Confidence in the thunderstorm details is low
and continue to feel that chances for TSRA on station are too low
to include any mention in the TAFs.



256 AM CDT

High pressure will shift east of Lake Michigan today allowing a
fresh to strong southerly breeze to develop, with the strongest
winds expected across the north half of the lake. South to
southwest flow will persist through Thursday, then a cold front is
expected to push down Lake Michigan starting on Friday. High
pressure is progged to briefly build over the lake Friday night,
then a warm front will spread back north on Saturday. Yet another
cold front is expected late in the weekend into early Monday.



IL...Excessive Heat Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-ILZ012-
     ILZ013-ILZ014-ILZ020-ILZ022...NOON Thursday TO 7 PM Friday.

     Excessive Heat Watch...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039...NOON Thursday TO 3 PM Saturday.




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