Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180535

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 AM CDT Tue Jul 18 2017

215 PM CDT

Through Tomorrow...

With high pressure centered over lower Michigan and a ridge axis
extending southwest through the middle Mississippi Valley,
conditions will remain unseasonably cool and dry through the night
tonight with temperatures topping out in the upper 70s to around
80 F. With dewpoints in the middle to upper 50s, mostly clear
skies and light winds, lows tonight should range from the middle
to upper 50s over much of the area to the middle 60s in downtown
Chicago. Temperatures and dewpoints will be on the rise tomorrow
as the high shifts to the east and low level swly flow sets up a
return flow of moisture. Dewpoints should rebound back into the
middle 60s. The pressure gradient will remain relatively weak
across the region tomorrow, so lake breeze development is likely.
This should keep lakefront locations in the middle to upper 70s,
while inland locations should see more seasonable temperatures as
highs reach into the middle to upper 80s. No pcpn is expected
through tomorrow as upper ridging remains over the region. A
strong shortwave is expected to cross through the upper
Mississippi Valley tomorrow, but any impacts will not be until at
least tomorrow night.


339 PM CDT

Tuesday night through Monday...

Tuesday night through the weekend...

Models are in reasonable agreement about a transition to a warmer
and more humid pattern from midweek through the weekend, with the
local area positioned on the interface between an expanding upper
ridge and the stronger westerlies around the northern side of the
ridge.  The proximity of the ridge to our south will support heat
index values exceeding 100 degrees for southern portions of the
area, while shortwave energy embedded within the westerlies will
support periods of thunderstorms, especially for the northern half
of the CWA.

The better moisture starts to work its way into the area late
Tuesday night ahead of a cold front dropping down from the north.
The chance of thunderstorms increases with the frontal passage
overnight but daytime on Wednesday appears to be mostly dry with
only slight chance POPs given little low level convergence and a
lack of significant support aloft.

Thunderstorm chances increase again as the ridge breaks down a bit
on Thursday and Friday, allowing shortwave energy in the westerlies
and another surface frontal zone to push farther south toward the
area.  The GFS and ECMWF also agree on bringing a surface low across
the area on Saturday which would extend thunderstorm chances into a
third day.  Areas of excessive rainfall are not out of the question,
so hopefully the higher amounts will not align with areas still in

The other question for this period is the increasing heat and
humidity.  The southern third of the forecast area appears most
likely to experience heat indicies over 100 degrees on successive
days, but at this point it is not clear if indicies will exceed the
105 degree advisory threshold for long enough over a wide enough
area to support heat advisory headlines.

The models are suggesting that Sunday won`t be quite as warm and
Monday will be considerably cooler, so a break does appear likely
before additional mid to late summer heat would continue.



For the 06Z TAFs...

1235 am...Forecast concerns this period include fog early this
morning...a lake breeze this afternoon and a chance of
thunderstorms early Wednesday morning.

Patchy fog has developed early this morning...especially over
parts of northwest Indiana where gyy briefly tanked. Confidence
is fairly low for fog through sunrise but expect some areas of
fog in the usual locations. For now will include tempo mention at
gyy/dpa and monitor trends for the rest of the area. Increasing
high clouds may have some minor influence toward morning as well.

Light/calm winds early this morning with high pressure moving
across the area. Winds will shift southerly later this morning and
may increase up toward 10kts by this afternoon. The light gradient
will allow a lake breeze to develop which will move through gyy
but only medium confidence at ord/mdw and it will likely slow into
the late afternoon as low level winds increase. Winds will shift
back southerly after sunset when the lake breeze dissipates.

Convection is expected to develop over the upper midwest later
today and move southeast this evening...possibly reaching northern
IL into early Wednesday morning...most likely after 06z. This
activity will also likely be in a weakening phase as it arrives.
Confidence is still low so no mention with the 30 hr ord taf but
some mention of precip/thunder Wednesday morning may be needed
with later forecasts. cms


242 PM CDT

Our sympathies go to the Mack racers who are strung out from
the Manitou Islands into the straits with very little wind this
afternoon, in sharp contrast to the conditions they fought most
of Sunday. It appears that some of them are starting to manage 3-5
knots, and while local thermal effects will most likely dominate
through this evening, the overall gradient appears to be
strengthening and offering hope for a finish tonight or early
tomorrow for most participants. Moderate south to southwesterly
flow develops across the lake Tuesday and continues to increase
into late Tuesday evening, though wave growth does not get out of
hand, peaking at 3 to 5 ft or maybe a bit higher in the open
waters toward the north end. A front dropping down the lake early
Wednesday turns winds north again and eventually relaxes the
gradient by Wednesday afternoon.






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