Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202026

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

325 PM CDT

Through Thursday...

Main forecast concerns/challenges are with the hot/humid
conditions expected over the next couple of days, with an
Excessive Heat Warning and Heat Advisory issued across the CWA
this afternoon.

In the near term, small area of diminishing precip still slowly
pushing southeast through northern Illinois, primarily impacting
areas along the I-39 corridor. This area of precip will continue
to drift southeast over the next couple of hours while also
decreasing in coverage. Lightning has been limited over the last
hour, and although still possible over the next couple of hours,
think lightning will continue to be less with time. Then
anticipate dry conditions across the CWA for most of tonight.
Increasing LLJ could provide an isolated shower/thunderstorm late
tonight into early Thursday morning, but with the greater
forcing/focus expected to be well north of the area, am leaning
more towards a dry forecast. If any precip does develop late
tonight into Thursday morning, it will exit/diminish through mid

Had made a midday update to the heat headlines in place,
replacing the Excessive Heat Watch with a Warning and then issuing
a Heat Advisory across northwest Indiana. These updates were made
with increasing confidence for a long duration of hot and humid
conditions beginning late morning Thursday and persisting into
Friday. Guidance consistent with swinging much warmer air across
the CWA throughout the day on Thursday, with 850mb temps
approaching 24C. Lack of cloud cover and strong insolation in this
environment supportive of mid 90s and with very humid air in
place, expect dangerous heat conditions across the CWA with heat
index values ranging from 103 to as high as 115 degrees to be in
place. At this time, expect the highest heat indices to be across
north central Illinois where thermal ridge and higher dewpoint
axis will reside. However, do think there is a possibility that
temps could be slightly higher than currently forecast as well as
with slightly higher dewpoints. This is especially the case with
dewpoints, as dewpoints are around 80 degrees at this point across
Iowa. If this were to occur, the lower end of the heat index range
could be higher, even across the Advisory area in northwest



326 AM CDT

Thursday through Tuesday...

Thursday through Saturday: The main focus continues to revolve
around the potential heat wave in this timeframe; however,
considerable challenges have arisen with the duration and intensity
of the heat. Thursday looks to be the most probable day for the
heat, as the 500mb ridge will be pushing increased ridging
overhead coupled with the robust thermal ridge rising to 20-22 deg
c. The column continues to indicate minimal forcing/wind, and
expect only minimal cloud cover. This should allow sfc temps to
quickly warm into the lower 90s and possibly the middle 90s by
mid-afternoon. Channel of low- level moisture will begin to pivot
east/northeast and become oriented from Missouri to Northern
Illinois to Wisconsin with Td`s progged to approach the mid/upr
70s. This should push heat- indices to between 103-110 in the
afternoon. At this point based on confidence have opted to hold
onto the current excessive heat watch, and allow another cycle of
forecast guidance to determine if a warning or advisory will be

The warm/moist axis will remain overhead Thur ngt, keeping temps in
the mid/upr 70s. It is conceivable that with sfc winds strong enough
from the southwest temps near Chicago may struggle to fall below 80

Friday the focus will transition towards the 500mb ridge beginning
to relax, with a frontal boundary progged to be sliding south across
the Upper Midwest, which could approach Northern Illinois by midday
Friday and cause a shorter duration to the heat and humidity. If the
boundary arrives by early afternoon Friday winds will likely flip
northeast off of Lake Michigan for Northeast Illinois, keeping temps
several degrees lower from Thur with heat indices around 95-100
Friday. Elsewhere precip chances remain a challenge, along with heat
indices. Have held onto a slight chance POP Friday, but confidence
remains low on coverage and intensity of any storms that develop
given the lack of wind and forcing throughout the column; however,
the moist boundary will be overhead and sliding south and should
become a focal point for sct showers/storms.

Saturday: mid-lvl ridging begins to flatten slightly, allowing flow
along the northern periphery and systems to easily traverse the
northern CONUS from west to east. Expect another warm day with sfc
temps pushing back to the low 90s, and apparent temps nearing the
upr 90s to 110 in the south/southwest CWFA. The largest caveat for
Saturday will be precip chances as guidance continues to show a weak
lobe of vorticity sliding east across the upper midwest/western
great lakes region.

Sunday through Tuesday: Ensembles continue to show the 500mb ridge
lingering; however, the orientation will become focused across the
southern half of the CONUS, with a shortwave lifting northeast
across the upper midwest for the second half of the weekend and a
trailing frontal boundary that should clip Northern Illinois Mon.
Expect periodic chances for showers/thunderstorms through Sun/Mon,
then a trend towards a dry start to next week with surface ridging
returning to the Great Lakes region and temps relaxing to the
mid/upr 80s.



For the 18Z TAFs...

Vfr conditions and south/southwest winds in place this afternoon,
while showers and thunderstorms continue just west and northwest
of the Rockford area. It does appear that this activity may come
close to the Rockford area, and impact the terminal for a couple
of hours early this afternoon. However, these storms are
weakening and will continue to weaken and likely stay away from
the remaining sites this afternoon. Any further precip
development tonight into Thursday will likely stay to the north,
and have continued a dry taf. VFR conditions then expected through
the forecast period, while south southwest winds persist.



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 Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...NOON Thursday TO 7 PM

     Excessive Heat Watch...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039...7 PM Friday TO 3 PM Saturday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...NOON
     Thursday TO 7 PM Friday.




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