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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 190202

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CDT Sun Aug 18 2019

902 PM CDT

Have made a couple minor tweaks to going forecast. 

First, removed the small thunderstorm chances for tonight.
Atmosphere over northern IL was well worked over by Sunday morning
convection and never really recovered. Guidance has continued to
show a smattering of QPF here and there tonight and there have
been a few very small showers over eastern IA since late
afternoon. However, those showers have generally be decreasing in
coverage recently. In addition, water vapor imagery shows an
abundance of mid-upper level dry air overspreading the region with
no meaningful shortwave troughs evident in water vapor imagery.
Given the latest observational trends, felt the chance of thunder
tonight is very low and the chance of measurable rain from any
showers in any one given location appears to be solidly below 15%,
so removed precip chances from the forecast.

Second, have introduced some fog into the forecast for tonight,
hitting it hardest in the southern portions of our CWA where Tds
are the highest and T/Td spreads the lowest. Winds are already
light and forecast to only weaken further overnight, which means
minimal dry air advection. Skies are mostly clear and most cloud
cover overnight should be north of I-80. So with mostly clear
skies, light winds, recent rains, and already small dewpoint
depressions, it would seem fog is a threat tonight. Wouldn't be
surprised to see fog become dense, again especially south of I-80.
Have added the fog to the forecast, but confidence is well below
thresholds for any sort of headlines, just something for oncoming
overnight shift to keep an eye on.

Updated forecast products are out.

- Izzi


154 PM CDT

Through Monday night...

In the near term, focus revolves around the low potential for 
additional isolated to widely scattered showers and/or storms this 
afternoon and evening. Otherwise, focus transitions to perhaps a 
lingering precipitation threat early Monday.

Fairly tranquil sensible weather conditions across the forecast area 
early this afternoon in the wake of this morning's showers and 
storms. Guidance is in fairly decent agreement with this trend 
continuing for much of the afternoon; however, the atmosphere 
continues to gradually recover from morning convection and with high 
dewpoint air remaining in place this afternoon, suppose it remains a 
possibility to pop isolated to widely scattered showers and/or 
thunderstorms. Still think this will largely be the exception rather 
than the rule as the vast majority of the region stays dry, and any 
additional deep convection remains well to our south tied to the 
better instability gradient. Gusty southwest winds will continue 
this afternoon with gusts as high as 25-35 mph, gradually curtailing 
this evening.

By this evening into the overnight hours, the surface trough/front 
situated to our west-northwest will gradually sag southward, when 
combined with a strengthening LLJ, may prove to be enough for at 
least isolated shower/storm develop across parts of the forecast 
area. Don't foresee any of this development getting too out of hand, 
although may still pose a briefly heavy rain threat given the 
moisture-rich air in place. Isolated - widely scattered shower 
chances continue into the morning hours on Monday while gradually 
sagging even further south with the majority of any early day 
activity focused along and south of I-80 (deeper convection confined 
to central IL and points south and east). High temperatures Monday 
varying from the upper 70s to near 80 along the immediate lakeshore 
given onshore the mid 80s inland.

High pressure centered to our north Monday evening/overnight should 
be plenty close to aid in bringing tranquil conditions to the region 
for much of Monday night. Partly to mainly clear skies should allow 
temperatures to fall several degrees cooler than they have been the 
previous several nights with lows ranging from the low 60s north and 
west to the mid-upper 60s elsewhere.



203 PM CDT

Tuesday through Sunday...

Attention rather quickly turns toward Tuesday as thunderstorm
chances return to the region once again. Increasing moisture and
instability will be the rule throughout the day on Tuesday, along
with the arrival of steeper lapse rates. This could certainly
support another period with organized storms and is a time frame 
worth monitoring in future outlooks. Beyond this, a cold frontal
boundary is expected to drop south across the region midweek,
with a rather substantial pattern change for the end of the week
as drier and cooler conditions are expected to prevail right on
through the end of the week into next weekend. High temperatures
falling back into the 70s by Thursday and Friday before gradually
increasing once again next weekend.



For the 00Z TAFs...

648 PM...No significant forecast concerns through the period. 
Westerly winds around 10kts will diminish this evening and turn 
northwest overnight as a cold front moves across the area. Winds 
will eventually shift northeast Monday morning and then will 
remain northeast for the rest of the period.

There is a small chance for showers later this evening and
overnight across northern IL but confidence for coverage is too
low for any mention in the tafs. Mvfr cigs are possible across 
the area Monday morning but confidence is low. Some patchy fog is 
also possible toward sunrise mainly away from the Chicago 
terminals. cms