Area Forecast Discussion
Issued by NWS Chicago, IL

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550
FXUS63 KLOT 221655 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1155 AM CDT FRI JUL 22 2016

.UPDATE...
1153 AM CDT

Having monitored morning trends and looking at the overall message
of the forecast, have made some shifts to heat headlines. Ended
up replacing the warning in northeast and far north central
Illinois with an advisory to better reflect temperatures and heat
index readings this afternoon, as well as taking into account last
nights lows around 70 (thanks to evening rain) and the lack of
deep heat persistence.

Will further assess heat headlines this afternoon for Saturday
and if an advisory will be needed for the Chicago metro and
Rockford areas.

The large footprint of the nocturnal MCS that slowly dropped
southward across Illinois last night has continued into this
morning, with cloud cover now finally thinning. This had hindered
morning warming but still should see decent afternoon recovery.
Dew points are running 4 to 8 degrees lower than yesterday at
this time thanks to the MCS cooling. Expect gradual recovery in
these values with some uncertainty over western and southern
parts of the CWA where mature crop evapotranspiration can play a
greater role. Have accounted for this with continuing the
Excessive Heat Warning. Also climatology for area 925mb
temperatures indicate mid 90s still possible in the west and south
if we can attain full sun, which is gradually looking less
likely.

Warm advection off the deck is expected to gradually increase
across eastern/northeastern Iowa late this afternoon where there
already is some agitated mid-level clouds. Isolated to scattered
storms appear possible late this afternoon into the evening. In
the absence of a strong low-level jet and any well-defined short
wave disturbance, do not anticipate this convection wanting to
move easterly and would move more so southeast within the
850-300mb flow. Have trimmed mentionable chances some, including
out of Chicago.

MTF

&&

.SHORT TERM...
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.

JEE

&&

.LONG TERM...
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.

JEE

&&

.AVIATION...
For the 12Z TAFs...

Concerns:

- Lower visibilities or MVFR cigs for a brief time around sunrise
- Thunderstorm chances today
- Wind shift to east, though winds should be on the lighter side
- Fog tonight into early Saturday

The back edge of the thicker cirrus shield is moving close to
Illinois, and visibilities have dropped upstream, but there is additional
convective debris from storms over Iowa that look to keep cigs and
vsbys from impacting the terminals this morning.

Next concerns center around convective chances. Have removed the
Prob30 for thunder for today. Even though the chance is not zero,
at this point it is below my threshold to include at this time.
Forcing mechanisms center around a southward 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.

If no convection occurs, light winds and moist conditions will
likely result in fog tonight. Left the TAF fairly simple tonight
given uncertainties on any convective debris, but it is plausible
given the recent rain and light winds that lower visibility is
possible.

KMD

&&

.MARINE...
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.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.

     Heat Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-ILZ012-ILZ013-
     ILZ014 UNTIL 7 PM Friday.

     Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.

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

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

LM...NONE.
&&

$$

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