Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250841

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
341 AM CDT Tue Jul 25 2017

338 AM CDT

Today and tonight...

Dry, quiet weather conditions are expected to continue across the
area today and tonight.

08Z surface analysis depicts high pressure from northern IL into
lower MI, with calm or light easterly winds and mostly clear skies
in place early this morning. Small temp-dew point spreads and
moist ground conditions will likely support some shallow ground
fog early this morning away from the warmer urban core of the
Chicago area, though any fog should burn off quickly after sunrise
this morning.

Short wave upper ridging continues to strengthen aloft with
gradual warming of low levels as winds turn southerly. Surface
winds will remain more southeasterly however, as the surface ridge
drifts off to the east. Thus while temps moderate to around
80/low 80`s in most spots, northeast IL counties along the lake
will see some modest lake breeze cooling with temps in the mid-
upper 70`s. Patchy high clouds from convection across portions of
the Upper Midwest may spread into northwest parts of the cwa at

Low-level south flow becomes better established tonight, though
the main axis of moist advection will remain well to our north and
northwest overnight. Again, patchy high clouds can be expected
especially north overnight, with lows in the 60`s area-wide.



338 AM CDT

Wednesday through Monday...

Main focus during this period is on the brief return of warmer
and more humid weather Wednesday, and the potential for
strong/severe thunderstorms and locally heavy rainfall threat late
Wednesday into Thursday.

Surface high pressure will continue to pull away to the east of
the region Wednesday, as southerly low-level flow becomes
established across the Plains and Mississippi Valley. A broad
region of mid-level height falls will spread across the Upper
Midwest during the day, in response to a rather vigorous mid-level
short wave propagating across southern Canada, as well as a
smaller amplitude mid-level wave moving through a band of enhanced
mid-upper level westerly flow across the Plains. A cold front,
trailing from the main area of low pressure over Ontario, will
slowly sag south-southeast across the Upper Midwest into Wednesday
night, eventually pushing slowly southeast across the cwa through
Thursday. Low level south-southwest flow will result in an
increase in low-level moisture (70+ deg F surface dew point temps)
into the region ahead of the front, contributing to MUCAPEs of
1000-1500 J/kg by Wednesday evening. Forecast soundings do
indicate the potential for significant mid-high cloud cover across
the area during the day Wednesday, along with a warm layer around
800 mb which would both tend to slow development/initiation of
storms across the LOT cwa until late in the day, although the
approach of the mid-level short wave and associated mid-level
cooling through larger scale ascent (also aided by proximity of
right rear entrance to upper jet streak over northern Lakes)
should work to allow convection which is expected to develop west
of the Mississippi to spread into western parts of the cwa toward

As for the strong/severe/heavy rainfall threats, several
different models depict a weak surface low pressure wave
propagating northeastward along the cold front from KS-northern
MO-central/northern IL Wednesday night. This would support
slightly backed low level flow across northern IL/northwest IN
which would increase deep shear a bit, with the greatest threat
for wind/hail likely Wednesday evening when low-level lapse rates
are still somewhat diurnally favorable. Plan-view MLCAPE fields
from WRF, GFS and 3km NAM-NEST are highest from SE IA/western IL
into the LOT/ILX border region, suggesting perhaps a somewhat
higher threat across counties south of I-80. GFS forecast PW`s in
2-2.5+ inch range into this same area Wednesday night/Thursday,
with gradual veering of low level jet from SW to W and surface
wave potentially slowing MCS movement and presenting yet another
heavy rainfall threat. NAM/3km NAM-Nest and GFS all depict a
2.50-4.50+ inch swath of QPF south of I-80, though it is likely
too early to pinpoint mesoscale swath. Will highlight threat with
a hydrologic outlook (ESF), though could see the need for a flash
flood watch for a portion of the area as confidence in location of
heavy rainfall threat is refined.

Upper trough amplifies northeast of the Lakes on Thursday, with
the cold front progged to push southeast across the cwa during the
day. Forcing and instability are significantly weaker with low
level winds shifting west-northwest, and thunderstorm/precip
threat should sag south of the forecast area by late afternoon or
early evening. Heights begin to rise Friday as the upper trough
digs toward the mid-Atlantic states, and upper ridge builds from
the Northern Plains into the Upper Midwest into the weekend.
Expansive surface high pressure spreads across the region, with
north-northeast winds setting up dry and cooler weather through
the weekend and into early next week.



For the 06Z TAFs...

Quiet aviation weather through early Wednesday. A high pressure
ridge will move overhead this morning bringing calm, at times
light and variable winds. Cannot rule out shallow patchy fog near
daybreak at outlying sites, but do not foresee this being a big
deal at all. Winds turn southeast after daybreak, with more of an
easterly lake component by mid-afternoon possible at ORD and MDW.
Confidence in afternoon wind speeds being around 9 kt is high.



130 AM CDT

High pressure over the lake this morning will track east while a
deep low for summertime will scoot eastward across the northern
Canadian prairie provinces. The tightening of the pressure
gradient between these will bring strengthening southerly winds
this afternoon and especially tonight across the north. Stability
over the lake should keep gusts from getting too high, although
sustained speeds of 25+ knots are looking likely toward Beaver
Island and the U.P. shore.

The pressure pattern gets a bit murky midweek, as a surface
trough/cold front associated with the aforementioned northern low
eases southward over the lake Wednesday night. Low pressure is
expected to develop along this boundary, though there is high
uncertainty on where -- most favored at this time just south of
the lake by daybreak Thursday. Where this takes shape and its
magnitude will drive just how quickly strong northerly winds
envelop the lake on Thursday. These look to gust to 30 kt across
the central and south, including into the Illinois and Indiana
nearshores at some point (maybe not until Thursday evening). Waves
will certainly be high and persist into Friday before high
pressure prevails for the weekend.






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