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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 250659

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
159 AM CDT Sun Aug 25 2019

158 AM CDT

Through Tonight...

A still mainly quiet weather pattern continues into today, though
a cloudier one and a sign of gradually increasing ascent and
moisture. With this some spotty sprinkles are possible this 
afternoon into early evening before scattered showers are likely 
late this evening into the overnight.

Upper and surface ridging are slowly moving east from the region
this morning. The upper ridging has been stout with 500 mb 
heights of 589 dam last evening over the area. This has served as
blocking to the west, however a stronger jet of 110+ kt surging 
across the northwest U.S. this morning will being to impinge into 
and weaken this ridge as it shifts east. Clouds will gradually 
thicken today in response and a subtle disturbance near St. Louis 
this morning will slowly meander north-northeast toward the area. 
Last evening's soundings showed substantial dry air above 850 mb 
which will take time to saturate, and thus believe through early 
evening it will be difficult to get more than spotty sprinkles.

The clouds and continued easterly winds will prevent temperatures 
from moderating much from yesterday, with a 2-3 degree increase 
expected. On the note of easterly winds, these once again will be 
providing onshore flow into northeast Illinois and Lake Michigan 
beach conditions will be close to a high rip current risk. At 
this time expecting more of a moderate risk, but for locations 
north of Chicago it will be close with 3-4 ft waves forecast.

The upper pattern begins to show a little more cohesiveness 
tonight with a potential vorticity perturbation developing over 
or just west of the region and low-level flow turning more 
southerly steering in higher precipitable water air. Moistening on
the isentropic surfaces with the stronger lift still looks 
marginal, likely resulting in only scattered showers. The eastern 
half of the forecast area may be a little more favored with the 
aforementioned wave over St. Louis this morning slowly lifting 
north-northeast serving as one focus, with the greater synoptic 
forcing still west. Elevated lapse rates are weak because of this
so the thunder threat looks limited though cannot rule out an 
isolated non-severe storm or two overnight south and west.



215 PM CDT

Monday through Saturday...

The period is expected to begin Monday on the unsettled side as an
upper level trough digs over the north central CONUS. However, by
Tuesday another few day period of mainly dry and cool weather is 

Showers, and possibly a few thunderstorms, may be ongoing across 
eastern IL and northern IN Monday morning in response to an 
lead mid-level disturbance shifting northeastward over the area.
While skies will remain cloudy through the day Monday, it does 
appear that we could have a period of rain-free time from the 
late morning and into the afternoon following the passage of this 
initial disturbance. However, the chances for showers and 
thunderstorms will increase across the area again by Monday 

A well defined mid/upper level trough will set up over the Upper 
Midwest later Monday and Monday night. As it does so, an 
associated sub 996 MB surface low is expected to develop late Monday
and Monday night over far western Ontario. It will be this storm
systems cold front that we will be focusing on for our next chance
for shower and thunderstorm activity over the area Monday night
through early Tuesday. Currently it appears that this cold front
will shift over north central IL by late Monday evening, then over
northeastern IL during the overnight hours and then exiting my
southeast Tuesday morning. For this reason, our best chances for
decent rainfall amounts will be Monday night. Severe weather is
not expected with this activity. 

Any lingering showers should end quickly Tuesday morning 
following the passage of this cold front. Dry and pleasant 
conditions should return to the area for midweek as a surface high
settles over the lower Missouri Valley. High temperatures are 
likely to remain in the 70s through late week.

Overall, mainly dry conditions are expected mid to late week.
However, forecast guidance suggests that another cold front could
move over the area sometime later in the week, and this could 
result in another small chance for showers and storms. However,
given that this is several days away, I have not strayed from the
blended guidance during this period.



For the 06Z TAFs...

Forecast items to note for today and tonight's Chicago area TAFs:

- Easterly winds again increasing to just above 10 kt with 
  occasional gusts

- Chance of showers late tonight into early Monday morning

High pressure across the eastern Great Lakes will continue to
gradually move away from the area and as it does so easterly winds
will increase. By late morning confidence is high in speeds of
10+ kt at ORD and MDW. Otherwise for today and this evening, dry
air aloft is likely to prevent showers from reaching the TAFs
during that time. Going into the overnight, lift will continue to
saturate the atmosphere and showers are likely to near and maybe
even move over the TAF sites into the Monday morning period. The
metrics for thunderstorms in the local area look limited through 
early Monday morning.