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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 182351

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
651 PM CDT Tue Jun 18 2019

255 PM CDT

Through Wednesday...

Early morning stratus and fog has since diminished but in its 
wake, have seen diurnal CU development across the CWA. This is 
likely at its peak, with sunny/clear conditions likely for the 
remainder of the afternoon into this evening. Surface ridge still 
in place and in this pattern, have not seen any real change to the
overall air mass. With light winds and clearer skies into this 
evening, could once again see stratus/fog development tonight. 
Have trended the forecast this way once again tonight into early 
Wednesday morning, keeping at least patchy fog in the forecast. 
Patchy dense fog will be possible across the entire area, and 
includes possible redeveloping fog over the lake spilling inland.

Focus for precip development still in place across the region. 
Development occurring to the south along surface trough/front, and
to the northwest along a separate trough/front. In the near term 
and going into this evening, expect both areas of precip to remain
just outside of the CWA. Additional development expected to 
remain focused on the feature to the northwest, but expect this 
axis to begin drifting into parts of northern IL tonight into 
Wednesday morning. Forcing does not appear to be overly strong, 
and expect coverage to stay low and likely focused across northern
IL near the IL/WI border. Thunder will be possible, mainly by 
Wednesday morning. However, instability is not looking overly high
at this time and expect this to be limited. Precip chances 
increase during the day Wednesday with the approach of a mid level
trough and deepening surface low. Coverage will likely stay 
limited in the morning and focused across north central IL, but 
should see this become more widespread later in the afternoon. 
Instability will be on the increase and expect the likelihood of 
thunder to increase, especially south of I-80. At this time, the 
threat of any strong/severe storms does appear low.



314 PM CDT

Wednesday night through Tuesday...

Wednesday night: A mid-level shortwave trough and associated compact 
but strong surface low is forecast to be in the vicinity of St. 
Louis early Wednesday evening. Unseasonably strong dynamics in 
response to strong mid/upper-level flow combined with Pwats over 
1.5" and associated low-level moisture transport support a heavy 
rain threat across central IL that may clip the southern tier of 
counties in the CWA primarily Thursday evening. However, CAMs offer 
a wide range of solutions, all of which hinge on the intensity of 
the surface low. There remains the potential that a weaker solution 
will allow the heavier rain potential to spread northward into at 
least the central CWA. Most deterministic lower-res guidance that 
contains somewhat less convective feedback would suggest a more 
southern solution. Will therefore maintain forecast consistency with 
previous forecasts and an overall trend for guidance to suggest the 
heaviest rain remaining across the far southern CWA. After 
collaboration with surrounding offices, decided to hold off on a 
flash flood watch for the southern counties at this time given 
continued uncertainty.

Thursday: Much of the activity from Wednesday night will have exited 
to the east by around daybreak Thursday, leaving the area in deep 
NNW flow through the day. Low-level moisture may remain sufficient 
to support deep enough diurnal cumulus to generate isolated showers 
roughly east of I-57. However, stabilization downwind of Lake MI 
will likely limit chances in NW Indiana.

Friday through Tuesday: Forecast guidance has continued to indicate 
an increasingly active period across the western Great Lakes and mid-
Mississippi Valley during this time. Mid-level ridging will amplify 
Thursday into Saturday along the Mississippi River Valley, with the 
ridge axis traversing the CWA on Saturday. An increasingly unstable 
airmass will expand NE under this ridge in response to increasing 
low-level moisture. A series of convectively enhanced/induced 
shortwave troughs forming over the central and southern Plains are 
expected to round the ridge Friday afternoon into Monday. Given the 
uncertainty and significant dependency on how convection evolves 
during this period, specifics in the forecast for the CWA are 
difficult if not near impossible to provide at this time. However, 
trends in data suggest focusing on the potential for a convective 
complex shifting across the area late Friday night, with multiple 
additional rounds Saturday evening through Sunday night. While 
different severe parameters are ideal at varying times throughout 
the weekend, pattern recognition suggests an least some inherent 
severe thunderstorm risk with with any convection.



For the 00Z TAFs...

651 PM...Forecast concerns include...

Chance of fog overnight/Wednesday morning.
Chance of showers Wednesday morning/afternoon.
Possible mvfr cigs Wednesday.
Potential for prevailing rain/embedded thunder Wednesday evening.

Fog is expected to redevelop over the lake tonight but confidence
is low for fog development over land overnight into Wednesday
morning. After a sunny afternoon helping to dry out the low
levels...a mid deck will be increasing through the night. 
Guidance doesn/t show much fog development over land but opted to 
maintain previous tempo mention until trends become more clear. 
Gyy would be most likely to have fog that moves in from the lake.

A weak cold front/trough will sag into northern IL overnight which
may lead to a few showers across northwest IL and rfd. Some of
this shower activity may move across the terminals in the late
morning/early afternoon Wednesday but confidence is low. An area
of low pressure will move across central IL Wednesday night and
additional showers and thunderstorms are possible. But there
remains uncertainty regarding how far north this activity will
reach and it may remain south of the terminals. So for now have
just included vicinity mention. Cigs may trend into mvfr on
Wednesday but may also be dependent on how much shower activity
develops and this too has low confidence.

Light easterly winds at the Chicago terminals will likely become
light and variable tonight and then become northeast to 10kts on
Wednesday. West/northwest winds at rfd may turn a bit more
northerly this evening but speeds should be light. cms