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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 161948

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 PM CDT Sun Jun 16 2019

223 PM CDT

Through Monday night...

Surface low/trough and front continue to slowly push through the 
region this afternoon. Dry conditions currently in place, though 
much of the area remains under solid stratus with even some fog 
still lingering this afternoon. Some continued clearing possible 
south of I-80 this afternoon/evening, but would expect any clearer
skies later today to be filled back in with cloud cover as 
stratus slides south on the back side of this system. Think the 
area will stay fairly dry for the remainder of the day into 
tonight, as focus looks to stay just south of the CWA. It is still
possible for a stray shower or storm to move across areas 
along/south of the Kankakee river valley this afternoon, but 
expect chances to remain low. Will need to monitor fog trends once
again tonight into Monday morning, as some patchy dense fog is 
possible. Some CAA/dry advection is likely later this evening, but
I wonder if it will be enough to scour out lower level moisture 
in some spots. If patchy dense fog were to develop tonight, the 
highest chances would be for areas near Lake Michigan and then 
along/south of I-80. Mainly dry conditions expected Monday, but 
with cloud cover to likely remain. Did lower high temps slightly, 
with low to mid 70s expected Monday.



248 PM CDT

Tuesday through Sunday...

A quasi-zonal and not overly fast-paced upper flow will be present
during midweek, before both slightly more amplified and stronger 
flow are indicated, which could make for an active end of week 
and especially next weekend.

Tuesday will find the remnant surface front to the south of the 
area with weakening high pressure over the CWA. While some weak
instability is forecast, there looks to be little in the way of
forcing aloft or low-level focus. Late Tuesday night through
Wednesday, a gradually amplifying/strengthening short wave is
forecast to move across the middle of the country in the GFS and
EC solutions. The present track of this would keep the moderate
instability values just south of the CWA for a severe 
thunderstorm threat, and possibly keep thunder more isolated to
widely scattered. 

However, precipitable waters in excess of 1.5 inches return 
northward and the 925-700 mb moisture transport nose and 
convergence along it could be centered over the CWA potentially 
bringing a heavy rain risk. Over half of the 00Z GFS ensemble 
members had provide in excess of an inch of rain in 24 hours 
across the southern half of the CWA...a decent warm season signal 
for three-four days out. Did want to note that subsequent GEFS 
runs and the European ensemble shifted south with the heaviest 
precip axis, which is something to watch. Further southward shift 
would take the heavy precip/localized flooding risk potentially 
south of the southern CWA counties altogether.

At some point Thursday, possibly even beforehand, the area looks 
to get on the backside of this system with diminished rain 
chances. A surge of stronger northerly winds on the backside of
this system could bring dangerous swimming conditions to the Lake
Michigan beaches.

Temperatures should be able to bounce back to closer to normal 
range on Tuesday (except lakeside), and if Wednesday has 
prevalent clouds and showers then highs would probably dip back 
into the 70-75 arena. Thursday will also likely feature cooler
than normal temps area wide again, coolest and only in the mid 60s

For Friday into next weekend there remains a fair amount of 
spread in the GFS and EC deterministic solutions and ensembles, 
however both would provide a convectively active pattern in the 
middle of the country, especially the EC with a longer wave trough
over the western CONUS, though today's GFS leaned a bit in that
direction. For now have trended a little warmer Saturday and 
preserve thunderstorm chances within that time. Mass fields of 
flow and moisture would support some severe weather threat, 
possibly multi-day if the EC were to pan out. In addition to the
severe threat, there is a strong signal on the European ensemble
for PWAT over 1.5", with even some 2"+ potential indicated. Thus,
any well organized MCS would likely have a noteworthy flooding
risk associated with it as well. The increase in moisture will
also be felt in much higher humidity than we've had this season,
with 70+ degree surface dew points possible. 



For the 18Z TAFs...

Concerns include:
* Continued low CIGs through Monday morning including LIFR likely
  returning tonight
* Chance for IFR (LIFR) VSBY returning overnight into Monday AM

Unusually persistent stratus deck for mid June remains in place. 
ORD finally came up over 500 ft and MDW has improved to MVFR. 
With conditions upstream of ORD still lower and northeast flow, 
they may not improve to MVFR, so have gone with temporary MVFR 
CIGs there late afternoon-early evening. Have similarly low 
confidence for GYY, DPA and RFD regarding whether they observe 
MVFR CIGs today. Conditions should trend back downward tonight. 
However, unlike last night and this morning, a slightly drier air 
mass is being advected southward. This may help keep VSBY from 
tanking at more sites and possibly CIGs from getting quite as 
low. Maintained temporary IFR VSBY and 002 CIGs early Monday at 
ORD/MDW, but confidence is low in both, especially the 200 ft 
CIGs. It appears GYY will have the best chance for LIFR VSBY early
Monday. RFD may only have occasional MVFR VSBY and 500 ft CIGs 
early Monday. 

VSBY should improve to VFR at all sites by late Monday AM, with 
CIGs improving to MVFR from the late morning into the early 
afternoon. Confidence is low on whether VFR CIGs will be observed 
through Monday, so have left out mention in the ORD/MDW TAFs.
North-northeast winds near or above 10 kt will diminish tonight
and then northeast winds may get close to 10 kt Monday afternoon.



IL...Beach Hazards Statement...ILZ006-ILZ014 until 10 PM Sunday.

LM...Small Craft Advisory...IL nearshore waters until 10 PM Sunday.

     Dense Fog Advisory...IN nearshore waters until 10 PM Sunday.