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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 221852

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
152 PM CDT Thu Aug 22 2019

325 AM CDT

Through Friday...

We are right on the cusp of a considerable airmass change across 
northeast Illinois and northwest Indiana this morning, sandwiched 
between a much drier environment to our north and considerable 
moisture to our south. A cold front is presently sneaking south of 
the Wisconsin/Illinois state line, and upstream observations 
indicate dewpoints are falling into the low to mid 50s with this 
feature's passage. This drying process will probably be held up a 
little bit today by lake influences as some low 60s dewpoints spill 
in on the northeasterly winds. A broad region of precipitation 
continues immediately to our south, forced ahead of a shortwave 
slicing into northwest Illinois with an assist from the right 
entrance region of a 80 kt jet streak. The main moist axis 
supporting this area of showers and thunderstorms currently pushing 
into the I-72 corridor looks to just barely graze parts of Ford, 
Iroquois, and Benton counties just before daybreak. As a result, 
we'll show a brief period of higher PoPs before chances quickly 
diminish through the morning hours. 

Think that the vast majority of our CWA will then remain dry today, 
although have held onto a sliver of low (20%) PoPs across the far 
southeast into the mid afternoon as the aforementioned cold front 
will not totally clear until then. Can't rule out a spit of rain 
trying to develop with heating along the lake-enhanced push, but 
we'll be capping PoPs at 10% with no weather mention given the 
anticipated very limited potential. 

It also appears we'll see a pretty decent push of northeasterly 
winds down the lake later this morning and into the afternoon with
mixing. The ESRL HRRRx indicates notably stronger winds today 
than the operational NCEP HRRR which is pretty intriguing. We can 
only surmise that the warmer indicated lake and surface 
temperatures in the HRRRx are altering the low-level static 
stability profiles to support a gustier solution. Based on 
upstream observations, it does look like the ESRL HRRRx is doing a
better job capturing ongoing wind trends, and we've followed it 
more closely for winds across our nearshore waters as a result. A 
persistent northeast wind of 15 to 20 kts with gusts to 25 kts or 
so should result in steadily building wave action later this 
afternoon and through tonight. As a result, we'll be hosting a 
Beach Hazards Statement for an increased risk of rip currents for 
all of our Lake Michigan Beaches. At present we don't think the 
winds look quite strong enough to result in lakeshore flood 
issues, but the high lake levels have recently resulted in 
flooding in otherwise very marginal events. We'll keep an eye on 
observations and model trends today and tonight as a result, but 
don't have plans right now to hoist a Lakeshore Flood Advisory. 

As morning clouds vacate the region from north to south through the 
morning, it's shaping up to be a pretty spectacular day here. High 
temperatures will range from the mid to upper 70s inland, to the low 
to mid 70s closer to the lake as dewpoints fall into the upper 50s 
and lower 60s with mixing. A mostly clear sky tonight, combined with 
the very dry airmass will set the stage for temperatures to drop 
pretty quickly with the loss of heating. Have favored the cooler MOS 
guidance for lows tonight, and wouldn't be surprised to see perhaps 
some near 50 degree readings in the coolest outlying sheltered 
locations. Looks like we'll continue the fine stretch of weather 
into Friday to close out the week with highs ranging from the lower 
70s near the lake into the mid and upper 70s inland. 



151 PM CDT

Saturday through Thursday...

Surface high pressure is expected to dominate across the Great
Lakes region through much of the upcoming weekend. Overall, this 
is expected to result in pleasant (early autumn-like) weather 
across the area. High temperatures are expected to be mainly in 
the 70s, with overnight lows in the 50s Saturday night. Winds 
will primarily be onshore, especially Saturday. 

Early next week forecast guidance continues to be in good 
agreement with a well defined mid/upper level trough digging over
the Upper Midwest. With an associated surface frontal trough 
likely to approach the area later Monday and Monday night, we will
have some decent chances for some showers and thunderstorms. 
However, these chances should abate quickly during the day Tuesday
as the surface front shifts east-southeast of the area. This 
looks to result in mainly dry and pleasant weather with seasonal
temperatures again for much of Tuesday and at least Wednesday. 



For the 18Z TAFs...

There are no significant aviation concerns.

Main highlights over the next 24-30 hours:
* Northeast winds gusting to 15-20 kt at times through this
  afternoon at ORD, MDW and GYY. Sustained speeds 10-17 kt.
* Northeast winds 10-15 kt on Friday.

High pressure will influence the region through Friday, bringing
quiet weather conditions. With the high pressure centered north of
the Great Lakes today and over the northern Great Lakes on Friday,
expect steady northeast winds. There will be occasional gusts 
through late this afternoon at ORD, MDW and GYY, with gusts likely
diminishing around sunset. Gusts may be less prevalent on Friday.
SCT to BKN lower VFR CIGs this afternoon inland of Lake Michigan 
should be mostly FEW-SCT VFR on Friday. 



IL...Beach Hazards Statement...ILZ006-ILZ014 until 10 AM Friday.

IN...Beach Hazards Statement...INZ001-INZ002 until 10 AM Friday.

LM...Small Craft Advisory...nearshore waters 
     until 10 AM Friday.