Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220843

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
343 AM CDT FRI JUL 22 2016

342 AM CDT

Through Saturday...

Heat and humidity continue through Saturday as does an unstable
atmosphere that could result in more storms.  The only changes I
made to the heat headlines was adding a heat advisory Saturday for
Kendall and Will counties in IL and Jasper, Benton, and Newton
counties in Indiana.  Otherwise the going heat headlines seem
accurate with no need to adjust them.

Today: A weak cold front is across southern MN and central WI this
morning and it will drop south into northern IL today.  At the same
time a weak upper level disturbance currently over SD will shift
east and weaken as it does so.  Not expecting much if any forcing
from these features today.  Also expecting cloud cover to linger
through at least the morning.  The impressive convective cloud
shield overhead is inhibiting fog formation and will likely take
awhile to clear today.  Given the lack of forcing and lingering
cloud cover, have low confidence in convection today.  Forecast
soundings feature a stout cap that I don`t think there will be
enough forcing nor mixing to overcome it. However, with a little
forcing in place, did not feel comfortable going dry either.  Liking
the CR-HRR and NMM-East solutions that feature mainly dry conditions
with perhaps a few isolated showers and storms.  As such kept PoPs
in the slight chance to low chance range.

The cloud cover may impact temps slightly, and I did lower high
temps by a couple of degrees today.  Also lowered dewpoints a bit
given current conditions.  This resulted in max heat indices ranging
from 110-115 east of I-39 and south of I-80 but all areas will see
heat indices of 100+. Decided to keep the heat headlines as is
because it will feel icky outside regardless of the exact heat
index.  The message of taking precautions to avoid heat related
illnesses is still valid.

The front hangs around Saturday and could see additional convection
fire along it. Guidance once again features a cap in the morning,
but this time it looks like the cap will be eroded leading to
convection especially Saturday night. Confidence increases through
Saturday afternoon into Saturday night as the cap weakens and an
upper level shortwave swings through.

Kept high temps in the 90s except along the lake where a lake breeze
should keep temps in the 80s.  Added a heat advisory for a few
counties where heat indices will be around 105.



342 AM CDT

Sunday through Thursday...

A cold front swings through Sunday with a chance of showers and
storms ahead of it.  Thinking Sunday will be the final day of heat
and humidity. A heat advisory may be needed for most of the warning
area. Heat indices vary from 100 to 110.

Slightly cooler and drier conditions move in Monday and Tuesday with
highs in the 80s.  A more active pattern returns mid and late in the
week with periodic chances of storms.  High temps look to remain in
the 80s.



For the 06Z TAFs...


- Gusty south winds for the next few hours
- Lower visibilities late tonight into early Friday
- Thunderstorm chances today
- Wind shift to east and timing
- Fog Friday night into early Saturday

A wake low behind earlier thunderstorms will create a temporarily
strong pressure gradient over our eastern terminals such that
gusts to 25-30 kt are possible for a few hours. The rain shield
will be shifting south in the next few hours. The next concern
shifts to visibilities later tonight. The back edge of the thicker cirrus
shield is moving southeast through Central Wisconsin, and
visibilities are tanking. The trailing end of the cirrus still
has several more hours before getting close to the Illinois-
Wisconsin border. Have indicated some mvfr vsby for now as as the
clearing may take place just past the diurnal minimum, but it is
possible that things drop quickly to ifr or lifr at or around
sunrise and this will bear watching given the light winds that
will arrive in a few hours once the wake low eases.

Next concerns center around convective chances. Have hung onto the
Prob30 timing for now, but confidence is low. Forcing mechanisms
center around a south ward moving frontal boundary or possible
lake breeze, and any subtle west-southwest moving shortwaves. The
RUC/HRRR dampen the waves in the plains as they approach, and
convergence on the front/lake breeze appears weak, such that a cap
in forecast soundings may not be broken. Will re-evaluate in the
9z and 12z AMDs to see if thunder chances can be pulled.

If no convection occurs, light winds and moist conditions will
likely result in fog Friday night.



259 AM CDT

Winds this morning should ease as high pressure gradually shifts
east over the lake later today into tonight. A weak cold front
will also move south across the lake today but winds should remain
relatively weak. The high will shift east Saturday, and this
coupled with low pressure that will move to south of Hudson Bay
Sunday will allow a warm front to lift north across the lake
Sunday. Winds may gust to 25 possibly 30 kt Sunday. A weak cold
front will return Sunday night followed by high pressure through
mid week.

Waves are still a bit disturbed this morning following
thunderstorms last night, but expect that they will continue to
ease this morning.



IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-
     ILZ012-ILZ013-ILZ014-ILZ020-ILZ022 UNTIL 7 PM Friday.

     Heat Advisory...ILZ020-ILZ022...7 PM Friday TO 7 PM Saturday.

     Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ001-INZ002 UNTIL 7 PM Friday.

     Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.




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