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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 251740

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM 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.



434 AM CDT

Monday through Saturday...

The long term forecast period will start out active with periods of 
showers and thunderstorms lingering into Tuesday morning. Much of 
the remainder of the period should be dry, though there may be 
another chance for precipitation for Friday and into next weekend. 

Showers and thunderstorms should be ongoing across eastern IL and 
northern IN Monday morning in response to a southern stream 
shortwave lifting from the Lower Missouri Valley and across central 
IN and northern IN. Showers and thunderstorms will be mostly south 
of I-80 in IL but all of nwrn IN will like see some activity. 
However, some scattered showers and thunderstorms cannot be ruled 
out for nrn IL.  As this initial wave continues to lift newd, weak 
shortwave ridging and mid-level drying may bring a lull in the pcpn 
for Monday afternoon, though some sct showers may be possible.

A strong northern stream shortwave will drop into the well Upper 
Midwest by early Monday evening with an associated cold front moving 
from ern IA into nwrn IL. In advance of the system, deep layer 
moisture will continue to increase, with pwats just under 2 inches 
pooling ahead of the sfc cold front.  Steep mid-level lapse rates, a 
40 kt low-level jet combined with the high moisture level and 
moderate instability will bring both chances for severe 
thunderstorms and localized flooding.  Strong winds will be the 
primary severe threat.  Flooding potential should be limited due to 
the relatively dry conditions over the past few weeks, giving area 
rivers ample storage capacity.  However, some of the latest guidance 
is suggesting that the progression of the cold front across the 
region may be slower than previously anticipated, so any heavy rain 
producing storms may have a longer residence time over a given area, 
increasing the flooding potential.  The cold front is expected to 
push east of the CWA by late Tuesday morning so there could be some 
lingering showers over nwrn IN through the morning.  The entire CWA 
should be rain-free by the afternoon.

Dry and pleasant conditions should return to the area for midweek as 
a surface high builds across the Middle Mississippi Valley through 
the Ohio Valley. High temperatures will be around 80 F for Monday 
and Tuesday.  As the upper level pattern trends toward more high 
amplitude long wavelength, with upper riding over the West Coast and 
broad upper troughing over the ern 2/3 CONUS, the resultant nwly 
flow general troughiness aloft will keep temperatures below seasonal 
normals, with highs in the middle to upper 70s for much of the 
remainder of the period, though highs could reach 80 F on Thursday 
as weak shortwave riding moves across the area in between a series 
of weak shortwaves rotating around the base of the main upper low 
over scntrl/sern Canada.  The next chance for precipitation may come 
by the end of the week into next weekend with some of the long range 
guidance suggesting the next frontal passage.  However, this 
guidance is also trending the upper level flow pattern to move zonal 
across much of the nrn CONUS by the end of the week, so timing of 
any shortwaves and associated features are low confidence at this 
time range.


For the 18Z TAFs...

The primary forecast concerns will be the timing of rain late
tonight into Monday, along with deteriorating CIGs during this

While conditions will be quiet today, outside of a possible
sprinkle or two this afternoon, things will change tonight into
Monday. Moisture will be on the increase tonight in association
with an approaching weather disturbance. This is likely to result
in periods of rain showers shifting over northern IL later
tonight, especially after 09z, and persisting through much of the
morning on Monday. With this rain we are also likely to see some
lower MVFR CIGs, with a period of IFR not out of the question
through the morning. In addition to the clouds, visibilities could
become reduced in there heavier rain showers. We also cannot rule
out the potential for some embedded thunderstorms, but we have 
opted to leave this mention out of the TAFs at this time. 

It appears that the shower activity will wane some during the
afternoon, with any remaining activity remaining widely 
scattered. However, MVFR CIGs could hold over the area through the
day. There is another chance of showers and thunderstorms Monday 

Otherwise, expect easterly winds tonight, shifting to the
southeast during the day on Monday.