Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211533 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1033 AM CDT THU JUL 21 2016

1030 AM CDT

It remains a challenging forecast into the afternoon as we are
monitoring a mesoscale convective system (MCS) moving southeast
across east-central Wisconsin. Have continued the previous shift
thoughts that part of this complex, or at least redevelopment
from this, will clip northeast Illinois including parts of
Chicago metro area early this afternoon. Have upped thunderstorm
likelihood in the north metro for at least a brief focused period
in the early afternoon forecast. Also dropped temperatures a tad
over northeast Illinois, more so over the northern half of the
metro due to a combination of cloud cover and outflow influence.
Confidence remains low on duration of influence however. A key is
that no change to heat headlines will be made late this morning as
even with slightly cooler temperatures the very high dew point
values of the mid to upper 70s (even 81 at Pontiac) combined with
temperatures of the upper 80s to mid 90s across the area...with a
warm and humid night ahead and heat tomorrow...still certainly
warrant a heat headline areawide at this time.

The thunderstorm structure and behavior upstream on radar and
satellite in Wisconsin has had a gradual weakening trend and no
longer appears severe. However, pressure rises of 3mb/1 hr have
been observed at recent sites that had the storms pass indicate a
well-defined cold pool that should at least help outflow
propagate southward. In addition, there has been congestus and
light echoes across north central Illinois in what appears to be a
small warm advection wing ahead of the MCS. While the 12Z DVN
sounding indicated a capped profile in the low levels, aircraft
sounding data from northeast Illinois modified for expected T/Tds
at noon indicate little of a cap with over 2000 J/kg of MLCAPE.
Thickening cirrus likely will stunt the temperature climb over
northeast Illinois into the afternoon and that has already been
observed in southeast Wisconsin, so that would probably keep any
new growth isolated/scattered. But at this time favor that
possibility over northeast Illinois as it seems outflows moving
into such a high MUCAPE/PWAT atmosphere at this time of day rarely
completely "dry up". Overall convective allowing models are not
handling this well and have gone more with observational trends.

Southern and western parts of the CWA are still on tap for the
forecast highs and hourly temperatures into this evening and have
bumped dew points up a couple degrees leading to a tad higher
forecast heat index and wet bulb globe temperatures.

Redeveloping storms along what should be a slowing outflow
boundary are possible during the mid-late afternoon, though this
would likely be isolated as well given limited upper forcing and
stout upper level heights nearby/building in. Will continue the
chances later in the day as is. Any storms this afternoon will
have the likelihood of brief very heavy rainfall and temporary 30
to 40 mph winds. Anything stronger would require deeper growth and
organization and is reflected well in an SPC marginal risk for
most of the area.



335 AM CDT

Through tonight...

Today through tonight:
Main issues for today will be on a quickly advancing and bowing line
of storms, which early this morning was over Minnesota. Several
guidance members are struggling with initializing this system;
however, the national hi-res wrf arw/nmm members are doing well. The
theta-e advection would suggest this advancing line of storms would
persist to the southeast and arrive across Northern Illinois midday
and continue over the forecast area into the afternoon hours. The
challenges from this scenario will be on cloud cover, and remnant
boundaries that could result in additional spotty showers/storms
developing this afternoon/early evening.

Abundant low-level moisture continues to pool north/northeast, with
the axis of higher theta-e becoming oriented from Northeast IA to
Northeast Wisc later this morning, then sliding southeast. This
should allow the higher Td`s to rise to near the mid/upr 70s across
Northern Illinois and Northwest Indiana this afternoon, as sfc temps
warm into the mid 90s. This will likely result in heat indices
between 105 to 110. Now for the forecast wrinkle, how does the line
of convection work with the heat warning for today. It is
conceivable that with the clouds arriving midday/afternoon that
temps could be held down several degrees, inhibiting the excessive
heat indices. Will maintain no changes to the current headlines.

Late this afternoon and overnight continues to be challenging, and
will hinge on the placement of a remnant boundary from the possible
midday/early afternoon line of convection over Northern IL. Some of
the solutions are trying to push the 500mb ridge axis southwest
slightly, which could allow a lobe of vorticity in the mid-levels to
slide southeast along the periphery of the ridge towards Northern IL
and help to develop spotty showers/thunderstorms given the
instability aloft and potential for a lingering boundary. Temps
tonight will likely not fall much below the mid/upr 70s, and expect
a few sites to not fall below 80.



335 AM CDT

Friday through Wednesday...

Friday through Saturday: Large dome of mid-level ridging will
continue to be parked over the Central CONUS, with only minor
fluctuations. This should hold the 22-24 deg c thermal ridge aloft
across the region, and support afternoon temps pushing back into the
lower 90s; however, as is the case with Thursday on possible
convection coverages could throw a curve at how hot surface temps
become. Have opted to only make minor adjustments given the
decreased confidence for Fri/Sat. Still appears heat indices will
once again push into the 105-110 range, and likely continue into
Sat. There is some indications that a boundary will be sagging
south/southwest Fri, which could flip winds to the northeast across
areas adjacent to Lake Michigan. But if this does occur temps are
not expected to cool much.

The stalled frontal boundary will become oriented from Central Iowa
stretching southeast through the southern CWFA. This boundary is
expected to begin lifting north/northeast later in the day Saturday.
Given the proximity to the boundary, will continue to maintain low
chance POPs. Winds will likely be northeasterly turning
southeasterly late Saturday, which could also keep heat indices
several degrees under excessive heat warning criteria. Confidence
does remain low, so have opted to hold the current headlines in

Sunday through Wednesday: Ensembles continue to feature a high
confidence in the ridging remaining over the Great Lakes Sunday,
with some minor amplification Sunday, then less amplified for early
next week. Operational solutions then begin to relax the northern
periphery of the mid-lvl ridge Mon/Tue, with surface ridging
arriving late Mon and continuing through Tue. This should provide
a couple periods of dry conditions and less heat/humidity. Sunday
may require a heat headline, with current progs indicating heat
indices around 102-110 degrees.



For the 12Z TAFs...

Unstable conditions will be in place across the region which will
result in thunderstorm chances throughout the day, though
confidence in the details is low to medium at best. Most likely
scenario for TSRA on station today will be for the line of
thunderstorms currently moving south across Wisconsin to continue
into northern Illinois by early this afternoon. The southern end
of the line has shown some signs of weakening, though, with
warming cloud tops noted on IR imagery and outflow outpacing the
main line of convection. This adds some uncertainty in how far
south the line will make it and how far south outflow will move to
serve the focus for renewed thunderstorm development this
afternoon. Based on extrapolation, the best timing for this line
to reach the terminals would be early this afternoon, around 17Z to

South winds will veer to the southwest mid to late this morning with
gusts increasing to near 20 kt. Outside of the influence of
thunderstorms, except south to southwest winds to continue through
the remainder of the day; however, cannot rule out more northerly
winds for period associated with thunderstorm outflow. Also of
note, this morning fog and low ceilings have developed, mainly
over portions of north central Illinois. Only anticipate it to
impact RFD and DPA, and it should burn off by around 13Z.



302 AM CDT

A strong south to southwest breeze is in place across Lake
Michigan early this morning and will continue through the day and
portions of the overnight hours. Late tonight into Friday, a cold
front will gradually move down the lake with showers and
thunderstorms expected along the front. High pressure will
briefly build across the region Friday night into early Saturday,
then low pressure will move across the Canadian Prairies with a
warm front lifting back north. The low will continue to Hudson Bay
late Sunday with a trailing cold front sweeping across the lake
Sunday Night. High pressure builds back across the Great Lakes
Monday and Tuesday.



IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-
     ILZ012-ILZ013-ILZ014-ILZ020-ILZ022 UNTIL 7 PM Friday.

     Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 7 PM




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