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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 171123

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 AM CDT Mon Jun 17 2019

308 AM CDT

Through Tuesday...

For the remainder of the overnight hours and through Monday, 
conditions will be relatively quiet through still on the cooler side 
as stratus and patchy fog have regenerated with overnight cooling 
and persistent nely flow off of Lake Michigan.  Do not expect any 
pcpn today as the main short wave tracks are well north and south of 
the region, with zonal flow aloft.  Have not made any significant 
changes to the going forecast for Monday with highs expected to be 
in the lower to middle 70s for much of the CWA.  However, once 
again, with flow off of the still cold Lake Michigan water, lake 
front locations should see temperatures top out only in the lower 
60s.  For Tuesday, a lingering old frontal boundary over the Ohio 
Valley will lift a little northward and become a bit more active as 
a southern stream shortwave tracks along the sfc front.  While mu ch 
of the activity should remain south of the CWA, there is a chance 
that a few stray showers or an isolated thunderstorm could sneak 
into the far sern portions of the CWA.  Elsewhere, a northern stream 
shortwave and associated sfc cold front should reach the Middle 
Mississippi Valley and approach far nwrn Illinois by late in the 
day.  This system will bring the next round of significant weather 
to the area, but will likely impact the area until later Tuesday 
night and into the longer term forecast period.  With weak short 
wave ridging aloft and, at least, partially clearing skies, 
temperatures Tuesday should be closer to seasonal normals with highs 
in the upper 70s to 80 F for much of the area.  For the lake front 
locations, some warming is likely into early afternoon as winds 
should generally be light and variable under a weak pressure 
gradient.  However, a lake breeze should push inland by early 
afternoon, limiting the warming.  The lake breeze should develop 
late enough in the day for temperatures to hit the middle 60s before 
dropping back into the upper 50s following the passage of the lake 


308 AM CDT

Tuesday night through Sunday...

The remainder of the week into this weekend's forecast can be 
looked at in three message parts. First, a probable wet 
Wednesday, possibly excessively wet in spots if sustained and 
heavy enough rainfall rates. Second, below normal daytime 
temperatures Wednesday and Thursday. Finally, more sustained 
normal June temperatures this weekend within an unsettled pattern 
of periodic thunderstorm chances. As for a severe weather threat, 
on Wednesday as it looks now there will be some at least in the 
southern CWA but how much and how far north into the CWA are low 
confidence at this time. Later Friday into the weekend also has 

A quasi-zonal flow pattern will prevail over the middle of the
country during midweek, characterized by almost a split jet flow
within this...with a broad belt of strong upper westerlies across
the Upper Midwest and another subtropical jet maximum moving into
the southern Plains by Wednesday. Multiple short wave troughs are
forecast to traverse eastward between these where modest mid-
level flow will exist. The first of these and its low-level jet 
and warm advection response look to spark convection across the 
Missouri River Valley region Tuesday night. Some of this may 
develop into the area late Tuesday night into Wednesday morning, 
with the higher chances across north central Illinois.

This first short wave trough is fairly well advertised in 
guidance to move eastward and to our longitude by Wednesday 
evening, however there are noteworthy differences on how far north
or south. Some guidance indicates a second more low amplitude 
wave and some slight phasing that occurs as these move into the
area Wednesday night, amplifying the trough some and 
strengthening the surface response. While not all that common in 
late June for us to be looking at a deformation-like area to the 
north of the low, there is that signature and one with sizable 
moisture replenishment into it. This would present a swath of 
strong deep layer moisture convergence and heavy rain, with enough
instability (MUCAPE 500-1500 J/kg) to warrant concerns for 
probably a one to three inch rain swath within the region. The 00Z
GEFS has 60 percent of its members with over one inch in the CWA 
and 30 percent with over two inches. 

Now there are some key uncertainties, including how much 
convection is ongoing across the warm sector from Tuesday 
night/Wednesday morning as that would influence the overall 
morphology of Wednesday convection. Also the more precise system 
track is key given what could be a compact area of focus for the 
heaviest rain. Nonetheless, will note heavy rain potential at 
this time in messaging and will wait for some higher certainty in 
footprint idea for ESF (Hydrologic Outlook). As for now in the
deterministic forecast, have rain chances peak in the p.m. hours 
on Wednesday, and on a broad scale 0.50-1.50 inches QPF, with the 
likelihood that somewhere in the region will see isolated higher 
if the system can amplify some as it moves over the area. 

As for the severe weather threat, the earlier mentioned 
uncertainties play into that. The low-level moisture flux 
response looks to be strong enough to bring upper 60s dew points 
near or into the southern CWA, ahead of a quickly moving sub-1003 
mb low. Those are mass field parameters of concern within deep 
layer shear in excess of 35 kt, if the higher instability can 
rebuild which again there are doubts in occurring at this time. 
Convection should end by later Wednesday night with some lingering
showers possibly into early Thursday.

For Wednesday temperatures, the clouds, likely rain, and east to 
northeast winds north of the warm front should keep temperatures 
below normal once again for most of the area with high confidence.
The earlier rain moves in the lower the temperatures will be. As 
for Thursday in the wake of this system, northerly winds support 
highs again in the 70s.

The flow pattern aloft then turns southwesterly ahead of a long
wave western CONUS trough. The Friday through at least early 
Sunday forecast positioning of short wave energy and low-level 
jet/moisture transport is juxtaposed nicely in the middle of the
country, and that raises concerns for multiple rounds of
convection occurring over the broad region. Much will depend on
where exactly that juxtapose is, and if further west, there may be 
enough downstream height rises over the area to keep the potent 
convection threat lower at least at first. But right now its a
period we will have to keep an eye on and resolve in time, and
climatologically late June is a period where more organized/MCS
like thunderstorms and their hazards become more common in the
Lower Great Lakes region.



For the 12Z TAFs...

IFR to lower end MVFR cigs have persisted through the night and
should continue through late morning. There should be some
improvement to cigs, but will likely remain at MVFR levels through
the day. Some guidance wants to improve cigs to vfr this afternoon
and evening, which would be more favorable climatologically, but
given no sgfnt change to the air mass across the region, have
bucked guidance and climatology and gone with more of a
persistence forecast for cigs, keeping MVFR until late tomorrow
morning when weak high pressure moves across the region. Some
patchy dense fog has developed across portions of central IL, but
is expected to remain well south of the terminals. Some ocnl mvfr
vis in BR and patchy drizzle is possible this morning, but should
not be a widespread prevailing condition at the terminals.
Otherwise, light nnely winds, less than 10 kt will prevail through
the period.