Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261941

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
241 PM CDT Mon Jun 26 2017

203 PM CDT

Through Tuesday...

Showers and a few thunderstorms have increased across the I-80
corridor and southward ahead of a surface trough/cold front.
MLCAPE per RAP mesoanalysis is about 250 J/kg, thus the widely
scattered shower but generally isolated and brief thunder. This
activity will continue to slide southeast along and ahead of the
trough axis. A few showers are possible farther north, but the cu
field here is flatter and we need to await the upper low in WI for
a little better forcing later this afternoon. Instability is
weaker near this low, and with the later arrival expect some
isolated showers in far northeast IL late this afternoon.

Compact upper low will quickly dig southeast of the
area this evening, ending our lower precip chances fairly quickly
in its wake. Surface high pressure will slide just to the
southwest of the area tonight. With clearing skies in subsidence
behind this wave, it will be another fairly chilly night with lows
again in the upper 40s to mid 50s. We maintain a weak pressure
gradient over the area which will keep things from getting too
cool and limit any fog.

The surface high will move from central Illinois east through the
Ohio valley on Tuesday, while low pressure will develop across the
high plains. With lower level winds shifting southwesterly in this
pattern and plentiful sunshine, expect a nice rebound in
temperatures back to near 80. A lake breeze may also form, but at
this point it appears to remain confined to the near lakeshore



233 PM CDT

Tuesday night through Monday...

A busier pattern sets up with multiple chances of showers and storms
Wednesday through Friday, and then again early next week.

Surface high pressure shifts east Tuesday night, and a surface low
develops over the northern plains. The NAM and ECMWF still
suggest a dying convective line/MCS may move into northern IL
Wednesday morning while other models keep the convection over WI.
Continued with a dry forecast Wednesday morning, but if a
convective line or cloud shield move over the area, they could
limit storm development in the afternoon. Showers and storms are
expected ahead of the low Wednesday afternoon. The low`s warm
front lifts through the region leading to highs in the low 80s.
Expecting lift from an upper level shortwave, but forecast
soundings feature very little CAPE. CAPE increases to around 500
J/kg by 00Z Thursday/7PM Friday. Shear on the other hand is strong
at around 50 kt. Low level shear in particular looks strong and
LCL`s are low. CAPE values are much higher over Iowa so thinking
while we will see scattered thunderstorm development, the majority
of severe storms will occur over Iowa. Thinking storms to the
west will become linear and move through Wednesday evening/night.

The other issue for Wednesday will be wind gusts. As the previous
discussion mentioned, the potential exists for wind gusts to reach
or exceed 35 MPH. However, unsure at how well we will mix due to
cloud cover.

The low continues over the northern Great Lakes Thursday, and its
cold front stalls over north central IL. Heat and humidity build
with highs in the mid 80s Thursday. Showers and storms are expected
south of the boundary roughly south of I-88. Forecasted CAPE values
vary from 1000-2000 J/kg, but shear values are much lower around 20

Zonal flow aloft allows additional upper level disturbances to shift
east overhead through Friday.  In addition, a surface low is
forecast to form over Iowa/Missouri Friday morning and shift
northeast over the forecast area Friday.

Cooler air aloft arrives Saturday and it looks like precip will stay
to the north and east. High pressure moves over the region Saturday
night and Sunday. Guidance on how the next upper level system will
evolve early next week with the GFS featuring a stronger system than
the ECMWF. Therefore left PoPs as is.



For the 18Z TAFs...

Concerns center on isolated shower or storm development this
afternoon, along with a slight wind shift later this afternoon,
which could briefly shift ORD/MDW to possibly a more northerly
direction. Lake breeze development on Tuesday is also on the

Two focus areas for showers include the sheared out energy along
and south of I-80, which will lead to isolated to widely scattered
showers, possibly turning into a storm or two. But this is largely
south of the terminals. Next is the upper low across northern WI.
This will rotate through NE IL mid afternoon, though there is
some weak convergence along a surface trough in NE IL ahead of it.
Moisture is not great with either wave so anything would be very
short lived, but already seeing showers develop ahead of this
axis, so VCSH will be carried out of the gate with this set. CAPE
is very small and not forecasted to increase much, so held off on
thunder mention.

A weak surface trough will flip winds more northwesterly through
the afternoon. This pattern can sometimes lead to winds shifting
more northerly to even northeasterly at ORD/MDW. This trough
appears a bit weaker, hi-res guidance is still holding winds N-NW,
so have trended that way, though with medium confidence. Forecast
soundings depict quick drying this evening. There is a weak
gradient on the northeast side of high pressure that will maintain
a west wind component through the night.

Only concern for Wednesday is lake breeze development. Current
guidance keeps the lake breeze out of ORD and MDW and confined
just east as the lower level southwest winds will increase ahead
of developing low pressure across the plains.



233 PM CDT

Northwest winds have increased to 20-25 kt across the nearshore
waters. Winds will become even more northwest as a cold front moves
through this evening. Winds diminish to where a small craft advisory
will not be needed for the IL shore early this evening. However,
waves in the Indiana nearshore waters will remain hazardous past
midnight and slowly subside overnight.

West winds become southerly Tuesday as the high pressure ridge moves
over the lake. The next low forms over the northern plains Tuesday.
The pressure gradient tightens over the lake Wednesday between the
high over the central Atlantic Coast and the low over the northern
plains. South winds may increase to gales Wednesday. The low weakens
Thursday as it shifts east, and winds weaken and become west to

Another, weaker low may move over the southern end of the lake
Friday afternoon/evening.  West winds are expected behind the low
through Sunday.



IN...Beach Hazards Statement...INZ002 until 4 AM Tuesday.

     Beach Hazards Statement...INZ001 until 4 AM Tuesday.

LM...Small Craft Advisory...IL nearshore waters until 7 PM Monday.

     Small Craft Advisory...IN nearshore waters until 4 AM Tuesday.




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