Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 221753
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1253 PM CDT FRI JUL 22 2016
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
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
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.
342 AM CDT
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
For the 18Z TAFs...
The concerns for the Chicago area airports are the potential for a
lake breeze shift to easterly winds late this afternoon, as well
as a chance for light haze/fog tonight. Finally, east winds may
reach 7-10 kt for a period Saturday afternoon.
Westerly winds have continued to be stout enough across northeast
Illinois to prevent a lake breeze development thus far. The
somewhat local pressure gradient in place is anticipated to weaken
and allow a lake breeze to develop, though it should be slow to
move inward and have backed up the timing of an easterly wind
shift. It is possible that this wind shift does not arrive at all
or that the speeds are below 7 kt when it does, and confidence is
medium in its arrival and low on the specifics.
Isolated storms are possible across eastern Iowa late this
afternoon though the expected organization in these if they
develop is low. Because of that, the potential to move over the
TAF sites tonight is even lower, especially the Chicago airports.
Saturday will see a high pressure ridge over the area allow for
easterly to southeasterly winds across the TAF sites. These could
see an increase to 7 to 10 kt during the afternoon especially for
Chicago area sites.
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
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...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.
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.
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