Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 AM CDT FRI JUL 22 2016

340 PM CDT

Through Tonight...

Will continue the heat headlines as is for the area with an
Excessive Heat Warning for northern Illinois and a Heat Advisory
for northwest Indiana.

Immediate subsidence in the wake of the now re-strengthened MCS
across northern Indiana should keep much of the CWA dry through
sunset outside of Jasper and Benton Counties Indiana which will
have a localized strong/severe threat for a couple more hours.
Residual outflow boundary is draped northwestward from the west
Chicago metro through Janesville, Wisconsin and northwest from
there to scattered convection in west central Wisconsin. There
has been some agitation of cumulus along this boundary but with
the warm thermal nose near 850mb atop northwest/north central IL
and southwest Wisconsin, the expectation is for this area to
remain slightly capped. As for the storms in western Wisconsin, a
short wave helped to instigate these and that looks to move more
east of south into eastern Wisconsin. However, steering vectors
for anything organized would bend propagation more southerly. The
next couple hours should tell if storms will become more organized
there and pose a larger potential to work their way into northern
Illinois this evening, or if in fact just continue to fester in a
scattered state along the surface boundary. Again being at the
edge of a strong heat ridge with high CAPE/PWAT air is a
precarious place to be and one of lower confidence for
thunderstorm specifics. There would be a gusty wind and heavy
rainfall threat with any storms tonight.

While outflow has cooled northeast Illinois, already quick
recovery of the low-level air mass is occurring with almost 90
degrees already observed again in Lake County, Illinois as of 330
pm. Dew points CWA-wide outside of the rain-impacted area are in
the upper 70s with a few lower 80s, and even in the rain-cooled
area remain primarily in the 70s. So certainly a very muggy and
warm evening is in store. It is somewhat uncertain how much
downtown Chicago will recover late this afternoon with still winds
off the lake, but temperatures may actually warm this evening.
Below will note some of the uncertainty regarding tomorrow (which
will be similar to today), but feel the best message is to
continue the heat headlines as is.



415 PM CDT

Friday through Thursday...For Friday and through the weekend,
forecast concerns will remain the potential for excessive heat, at
least for portions of the CWA, and chances for thunderstorms.  For
Friday, confidence is relatively high that much of the CWA will once
again see heat index values reach warning criteria of heat index
greater than or equal to 110 F. There is some lingering concern
about early day convection which could, once again, significantly
impact temperatures, particularly over nern IL. Have opted to
maintain the going excessive heat warning for IL and the heat
advisory for nwrn IN, though there is some concern that either far
nern IL might not reach warning criteria or nwrn IN may rise above
advisory criteria,105-109 F and reach warning criteria.  Another fly-
in-the-ointment is that even with early day convection lowering
temperatures, there is the potential that temperatures could quickly
rebound high enough to reach Excessive Heat Watch or Warning
criteria since convection would likely have little impact on
prevailing dewpoints and temperatures could quickly rise is sky
cover clears out quickly.  A weak cold front is expected to push
down the length of Lake Michigan by Friday evening and then stall
out somewhere over nrn IL/IN.  This would bring some cooler, drier
air into the nrn portions of the CWA, particularly locations
experiencing onshore flow from Lake Michigan.  This should limit
temperatures and dewpoints over the nrn locations, though the
greater impact should be on dewpoints.  So, with upper ridging still
impacting the upper midwest, max temps for the weekend should still
be arnd 90 F, the upper 70s dewpoints should be limited to the
southern portions of the CWA while dewpoints over the northern
location should be a bit lower, perhaps only the lower 70s.  So,
heat index values may only exceed 100 F for locations south of the I-
80 corridor.  Convective potential will also largely focus on the
location of the weak frontal boundary.  The latest guidance suggests
that the boundary should hang up over the Upper Mississippi Valley
through central IL.  With pwats expected to remain well in excess of
2 inches through this period, locally heavy rainfall will be
possible and focused the highest PoPs and QPF generally along a
corridor from Rockford to Watseka.  The best model continuity is in
a relatively short period of Friday afternoon through Friday night,
so have highlighted this period with the highest QPF and likely
PoPs, with greater uncertainty before and after this period.

By early next week, the general model consensus is that a weak cold
front should push across the region as a series of northern stream
shortwaves suppress the upper ridge to the south, and the main
vorticity track shifts across the Upper Mississippi Valley and Great
Lakes regions.  The general trend for early next week should be for
lower temperatures, closer to seasonal normal levels, highs in the
middle 80s with periodic slight chance PoPs.


For the 06Z TAFs...


- Gusty south winds for the next few hours
- Lower visibilities late tonight into early Friday
- Thunderstorm chances today
- Wind shift to east and timing
- Fog Friday night into early Saturday

A wake low behind earlier thunderstorms will create a temporarily
strong pressure gradient over our eastern terminals such that
gusts to 25-30 kt are possible for a few hours. The rain shield
will be shifting south in the next few hours. The next concern
shifts to visibilities later tonight. The back edge of the thicker cirrus
shield is moving southeast through Central Wisconsin, and
visibilities are tanking. The trailing end of the cirrus still
has several more hours before getting close to the Illinois-
Wisconsin border. Have indicated some mvfr vsby for now as as the
clearing may take place just past the diurnal minimum, but it is
possible that things drop quickly to ifr or lifr at or around
sunrise and this will bear watching given the light winds that
will arrive in a few hours once the wake low eases.

Next concerns center around convective chances. Have hung onto the
Prob30 timing for now, but confidence is low. Forcing mechanisms
center around a south ward moving frontal boundary or possible
lake breeze, and any subtle west-southwest moving shortwaves. The
RUC/HRRR dampen the waves in the plains as they approach, and
convergence on the front/lake breeze appears weak, such that a cap
in forecast soundings may not be broken. Will re-evaluate in the
9z and 12z AMDs to see if thunder chances can be pulled.

If no convection occurs, light winds and moist conditions will
likely result in fog Friday night.



259 AM CDT

Winds this morning should ease as high pressure gradually shifts
east over the lake later today into tonight. A weak cold front
will also move south across the lake today but winds should remain
relatively weak. The high will shift east Saturday, and this
coupled with low pressure that will move to south of Hudson Bay
Sunday will allow a warm front to lift north across the lake
Sunday. Winds may gust to 25 possibly 30 kt Sunday. A weak cold
front will return Sunday night followed by high pressure through
mid week.

Waves are still a bit disturbed this morning following
thunderstorms last night, but expect that they will continue to
ease this morning.



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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