Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200826

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

326 AM CDT

Through tonight...

Early this morning: Surface ridge centered near Southeast Ontario
continues to have an influence on Northern Illinois and Northwest
Indiana early this morning. With a decaying area of showers/storms
located well to the south across Southern Illinois, with IR imagery
indicating clouds clearing along and north of interstate 80 across
Illinois. Td`s remain in the low/mid 60s where the clouds have
departed, meanwhile further south clouds have held Td`s up in the
upper 60s to around 70. Expect prior to daybreak some areas could
see patchy fog develop.

Today through tonight: Main challenges for this morning through the
afternoon revolves around precip chances, with many guidance members
backing off considerably on development and coverage of
showers/storms today. It appears the focus for new development
around daybreak/late-morning will be tied to the better Theta-E axis
which will be displaced well west of the forecast area across
Central Iowa stretching south through Missouri; however, the
coverage looks to remain minimal and will likely be struggling to
for growth as the morning and afternoon progresses. The downstream
surface ridge will likely continue to be an influence with weak
diffluent flow over Northern Illinois, so have backed off on POPs
for this morning to slight chance trending to low chance given the
lack of a supporting signal. 850mb thermal ridge slowly arrives with
temps around 19 deg c, with p-cloudy skies should support max temps
in the mid/upr 80s. Low level moisture will be slowly increasing as
southerly flow allows low 70 Td`s to arrive this afternoon.

Through this evening and overnight, the challenge of POPs coverage
continues. Many solutions continue to point towards continued
diffluent flow in the mid-levels, with weak forcing through the
column to support any better growth/coverage to precip. Some of the
current operational solutions even support dry conds tonight, mainly
for the far west/southwest forecast areas. The very warm/muggy
conditions will be overhead tonight, with temps struggling to cool
below the lower to middle 70s.



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.

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 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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