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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 221134

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 AM CDT Wed May 22 2019

338 AM CDT

Through Thursday night...

Showers and scattered thunderstorms from overnight should be 
clearing the eastern edge of the area toward daybreak. The warm 
front associated with this activity is a little tough to pick out 
in the sfc obs but likely runs from central Iowa across central 
Illinois as we write. During the morning this front will lift 
across the forecast area and support the return of a warmer and 
more humid airmass by this afternoon, along with gusty southerly 
winds. The strong winds will be aided by deep mixing during the 
afternoon, but there does not appear to be much support for 
convective development due to a capping inversion above the mixing
level, relatively warm temperatures aloft, and lack of forcing.

The picture for tonight and Thursday is looking a bit different than 
it did a day ago, but the afternoon discussion and recent SPC 
outlooks have addressed this nicely.  A cold front associated with 
the deep surface low lifting across the Plains toward Lake Superior 
still is expected to drop through the area, but instead of the 
resulting thunderstorm activity being favored during the daytime 
Thursday it now appears to be focused in the overnight hours 
tonight. Supporting factors include shortwave disturbances climbing 
up the west side of an upper level ridge, together with considerable 
moisture and temperature advection on the nose of a strong low level 
jet.  Forecast soundings suggest a small potential for severe 
weather early in the period, most likely strong winds, but a 
transition to more of a heavy rainfall profile as the event 

Thursday now is looking drier but still warm as the frontal zone 
pushes the better moisture to our south ahead of the front. Lows 
Thursday night likely will be a few degrees cooler than tonight, 
with lower dewpoints and less cloud cover.



338 AM CDT

Friday through Tuesday...

The evolution for Friday is looking similar to previous forecasts, 
with Thursday's frontal zone stalled to our south and then lifting 
back north as a warm front ahead of the next low pressure system 
developing over the Plains.  Models suggest the best chances for 
thunderstorms would be across the northern or northwestern portions 
of the forecast area Friday, likely due to better frontal 
convergence nearer to the low and away from the center of the upper 
ridge, along with cooler midlevel temperatures and better support 
aloft from the right entrance region of an upper jet.  SPC has the 
area in a Day 3 Slight Risk as of the 0742Z update.

Confidence on the timing and location of additional convection 
decreases into the weekend.  It is possible there could be a period 
of quieter weather at some point Friday night as the area again 
moves into the warm sector behind Friday's advancing warm front and 
the next cold front.  Saturday then could be another active day as 
the cold front drops through the area.  Alternatively, late Friday 
night could see the active weather if the front advances more 
quickly, as we are seeing with the activity expected tonight.

The remainder of the weekend into early next week looks to bring 
more of the same: a warm and moist airmass ahead of a fluctuating 
frontal boundary pushing through, stalling out, then lifting back 
north through the area. This would remain the focus for convective 
development, along with shortwave energy ejecting out of the western 
CONUS upper trough and over the broad southeast CONUS ridge where we 
will be positioned along its northern periphery.



For the 12Z TAFs...

634 AM...Forecast concerns include...

Mvfr/ifr cigs this morning.
Strong/gusty south/southwesterly winds late morning thru sunset.
Scattered thunderstorms late evening/early Thursday morning.

Low mvfr cigs have spread across the entire area with ifr cigs
west of the Chicago terminals and patchy lifr cigs further west.
These cigs will continue through mid morning and then lift and
scatter and end times may need some adjustment. Its possible 
some brief ifr cigs may develop at ord/mdw but confidence is low.

Southeast winds 10-15kt will turn southerly this morning and
increase in speed and gusts. Eventually wind direction is expected
to become south/southwest. Gusts into the 30kt range this
afternoon still look on track. Winds will diminish this evening
and turn westerly overnight.

Scattered thunderstorms are possible tonight...perhaps by mid
evening across northwest IL and then mainly overnight for the
Chicago terminals. Confidence for timing remains low and continued
prob mention. As trends emerge...thunder timing should be able to
be better pinned down and likely to a smaller time period. cms


IL...Beach Hazards Statement...ILZ014 until 10 AM Wednesday.

     Beach Hazards Statement...ILZ006 until 1 PM Wednesday.

LM...Small Craft Advisory...nearshore waters until 9 PM