Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252008

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

308 PM CDT

Through Wednesday...

Most of the forecast concerns will be after the short-term period
by Wednesday evening, with the main focus on very warm and humid
conditions on Wednesday. In addition, can`t rule out a few
thunderstorms making it into portions of northern IL (mainly far
NW and north central IL portions of the CWA during the late
afternoon). A persistent cluster of showers and thunderstorms to
our northwest finally is dissipating, with only high cloud cover
spreading over the area.

The rest of the night will be quiet with increasing high clouds
from convection to our north and over the Plains. Overnight and
early morning temperatures will be milder than today`s lows and
mainly in the 60s. A warm front will lift north on Wednesday,
bringing a return to uncomfortable humidity levels, with dew
points rising to the upper 60s to lower-mid 70s. There should be
plenty of convective mid and high cloud debris overspreading the
area, which will limit temperatures somewhat. Progged 850 mb/925
mb temperatures would typically support around 90, but with the
cloud cover limitations, expecting mid to upper 80s (peak heat
index values in the mid to high 90s over IL corn belt areas).

Forecast soundings indicate fairly stout capping over the local
area into the afternoon, which should effectively prevent
thunderstorms until the mid to late afternoon. With the arrival of
main short-wave from the west toward evening, have indicated
slight to low chance category PoPs for the potential of lead
convection spreading into portions of northern IL as mentioned
above. Will need to watch for a slightly quicker onset of higher
coverage showers and thunderstorms just prior to 00z. The Long
Term discussion below contains full details on the risk for heavy
rain/flooding and strong to severe storms Wednesday evening and



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 18Z TAFs...

Primary concern through this afternoon is wind direction at
ORD/MDW. Winds at many sites have been southeast to south-
southeast. However, over the Chicago metro, particularly at ORD,
there has been variability in the southerly component between
south-southwest and south-southeast. Have held with a due
southerly direction in the ORD TAF to account for this, with a
potential to become more prevailing southeast this afternoon even
without a lake breeze shift. Regarding the lake breeze, the
primary boundary is visible on radar from near Calumet Harbor to
just northeast of PWK, and it`s only slowly pushing inland. Thus
confidence is low in a prevailing easterly wind shift at ORD/MDW.
MDW has had a more pronounced southeast direction thus far but
expect variability from south to southeast there as well at least
for the next few hours.

Otherwise, a slowly decaying cluster of thunderstorms is expected
to dissipate, with only air space impacts and high cloud cover
overspreading the region, and few-sct CU below. Light south-
southeast winds can be expected overnight and south-southwest near
10 kt on Thursday. VFR conditions are likely for the eastern
terminals through the afternoon, with some potential for a period
of MVFR CIGs at RFD in the morning. SHRA/TSRA potential will
arrive after the current TAF cycle.



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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