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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 191125

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 AM CDT Tue Sep 19 2017

222 AM CDT

Through tonight...

Early this morning, a shortwave trough is lifting across the
Midwest while a weak surface low is located over west central 
Illinois. Showers and a few thunderstorms will continue this 
morning out ahead of these features. HRRR has had a decent handle
on conditions overnight and have leaned heavily on the latest 
runs for timing PoPs through the remainder of the morning. Expect 
most of the area to be dry by mid to late this morning with 
perhaps a few showers hanging on over northwest Indiana by early 
afternoon, if any at all. Mid/upper level height rises overspread 
the region behind the departing trough helping to erode the cloud
cover through the afternoon. Expect afternoon highs in the mid 
70s east to around 80 west. Temperatures along the lake front will
be held in the low 70s with onshore flow through the day.



222 AM CDT

Wednesday through Monday...

Transient upper ridge will traverse the region midweek with a more 
blocky and amplified pattern developing late in the week and 
continuing through early next week resulting in a warm pattern 
throughout much of the extended time frame. 

On Wednesday, closed upper low will be in place over the Pacific 
Northwest with a lead shortwave ejecting east across the northern 
Plains. Out ahead of the this feature, deep southerly flow will help 
transport high theta-e air all the way north across the Great Plains 
and into the Canadian Prairies. The thermal ridge axis with 850mb 
temperatures of 19-20C will advect east across the local area 
Wednesday contributing to very warm surface temperatures
accompanied by high dewpoints. Afternoon highs on Wednesday are 
expected to be in the mid to upper 80s and a few spots may tag 90.
Meanwhile, dewpoints are expected to rise to around 70 resulting 
in muggy conditions.

Wednesday night into Thursday looks to bring the best chance for any 
precipitation over the area as the aforementioned upper level 
disturbance lifts from the Upper Midwest into Canada while an weak 
surface cold front pushes east across the Mississippi Valley. Models 
are a bit mixed on how far east and south to bring precipitation 
with this front, and for now would err on the drier end of the 
spectrum given how far displaced we are from the upper wave and 
forcing along the front may struggle to overcome warm mid level air 
and capping. Moderately strong instability will be in place to 
support thunderstorm development, though, if forcing ends up being 

Meanwhile, the upper low over the Pacific Northwest is progged to 
dig into the Intermountain West Wednesday through Friday with 
further downstream amplification of the ridge expected to occur. 
Friday through the weekend the pattern becomes very blocky with the 
local area falling on the western periphery of a strong upper 
ridge. While widespread convection is possible off to our west, it
looks like northern Illinois and northwest Indiana will remain 
close enough to the ridge axis to stay primarily dry with 
continued unseasonably warm temperatures.



For the 12Z TAFs...

Showers continue to push east. Light showers near DeKalb should
continue to weaken this morning, but kept a VCSH at ORD in case
the showers hold together and/or additional showers pop up. Dry
conditions are expected from mid morning onward. MVFR cigs should
also lift by mid morning. East winds become southeast this
evening. Guidance hints at another round of MVFR cigs or fog
tonight. Clouds should be scattered tonight so not concerned with
MVFR cigs. GYY will be the most likely location for fog tonight
given the recent rain, light winds, and few high level clouds. All
other sites should have enough flow and cloud cover to inhibit
significant fog development.

Winds continue to veer to southerly Wednesday morning and VFR
cloud cover increases.



222 AM CDT

High pressure over Quebec will continue northeast. East winds 
become southeast tonight, and speeds increase to 15-
25 kt as the pressure gradient tightens over the lake Wednesday. 
High pressure passes over Lake Superior Thursday, and winds will 
become north briefly as the high passes by.  Southerly winds return 
across the lake Friday and remain through early next week due to the 
blocky pattern. A high pressure ridge remains stationary over the 
eastern half of the U.S. while a low pressure trough extends from 
the northern Mississippi Valley to the southern Plains through early 
next week.