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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 220527

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1227 AM CDT Thu Aug 22 2019

122 PM CDT

Through Thursday ...

Water vapor pictures a bit more amplified flow across the northern 
Great Lakes courtesy of a closed upper low south of James Bay. This 
will drive a cold front south into northern Illinois this evening. 
Upstream to the west there are several convectively altered waves 
embedded in the westerly flow, and an MCV present in the radar loop 
across Iowa. 

SPC Mesoanalysis depicts and axis of MLCAPE is currently capped 
along and ahead of the front that according to the RAP plan view may 
erode. But there is an axis of deep layer moisture convergence, and 
this combined with upper level energy from Iowa is enough forcing 
for elevated thunderstorms above the capping inversion. Small hail
and heavy downpours would be the more likely hazards given the
elevated nature of the storms. This wave, while not easy to pick
out on satellite, is in an area well depicted by the RAP, and this
wave is projected to continue across the northern metro area this

Meanwhile the Iowa convection and associated MCV should be able 
maintain additional showers and thunderstorms into our western 
area this afternoon as they continue to follow the 850-300mb wind 
and forward propogating vectors to the northeast. We will need to 
watch this axis as well as the mid level flow does increase and 
mid level lapse rates are fairly impressive above the elevated 
mixed layer here as well, and shear is increasing which may allow
storms to maintain some organization. 

For tonight the evolution and coverage is still slightly muddled,
and there is a short gap between the current wave and continued 
shortwave energy farther upstream in Iowa. This energy will shift 
across central IL should maintain and axis of showers and some 
thunderstorms along and south of I-80, with some activity possibly
hanging back to the northeast across the metro area. Instability 
will wane quickly tonight thus thunder coverage should trend 

High pressure behind the cold front and drier dewpoints suggest a 
mostly dry and slightly cooler day Thursday, though will hang onto 
some lower precip chances near central IL close to the front, though 
some guidance tries to trigger a shower closer to the upper trough 
toward northern IL. 



146 PM CDT

Friday through Wednesday...

Surface high pressure is expected to dominate across the Great
Lakes region Friday through much of the upcoming weekend. Overall,
this is expected to result in pleasant (early autumn-like) weather
across the area through this period. High temperatures are
expected to be mainly in the 70s, with overnight lows in the 50s.
Winds will primarily be northeasterly off the lake, especially 
Friday and Saturday and this may result in a few lake effect 
clouds, especially on Friday. However, of more concern is the 
increased threat of rip currents at Lake Michigan beaches Friday 
due to building waves up around 4 foot due to the breezy 
northeasterly winds over the lake. This may result in the need for
a beach hazard statement for the southern Lake Michigan beaches.

As the surface high over the Great Lakes gradually shifts towards
New England later in the weekend and early next week expect the
surface winds to shift southeasterly on Sunday, then southerly on
Monday. These southerly winds will allow a warmer and higher
dewpoint airmass to shift northward back into the area by Monday,
resulting in a return to 80 degree temperatures. Our chances for
showers and thunderstorms will also return to the forecast by
early next week with this increase in warm moist air ahead of
another storm system, which looks to move across the Midwest.



For the 06Z TAFs...

VFR and dry conditions expected through the period, with any 
precip development today expected to be well south of the 
terminals. Varying winds expected to be the trend through midday,
with a light north/northwest wind expected to continue early this
morning. A wind shift to the north northeast is then likely by 
around 12z, but with winds still likely staying on the lighter 
side. Winds expected to then increase out of the northeast by mid 
to late morning with sustained winds likely above 10 kt. Some 
gusts will also likely be observed, especially this afternoon. 
Winds will diminish early this evening, but remain out of the