Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211918

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
218 PM CDT Thu Sep 21 2017

1031 AM CDT

Minor adjustments to temps across the forecast area for this
afternoon (a tad warmer east and a bit lower west), and to
increase cloud cover through midday especially west.

Nocturnal MCS continues to weaken over southeast IA/northern MO
this morning, but making no progress eastward as additional
development has been into the low level jet axis west of the
Mississippi. Thick anvil cirrus has produced overcast conditions
over western portions of the cwa however, which although it will
thin with loss of convection, will likely have at least some
impact on high temps there this afternoon. Further east, temps
already in the mid-80`s at 10 AM, setting the stage for a very
warm afternoon where thin high clouds will decrease even more as
storms dissipate to the west.

Low level thermal fields would support solid low 90`s, perhaps an
isolated 94-95 if no cloud cover was present. Given warm start
have edged temps up a degree or two (92-93) in the east, but
biggest change was to lower temps a couple degrees (88-89)for
Rockford and western tier or two of counties where denser high
cloudy will slow the warm up and prevent maximizing afternoon
temps. Weak southeasterly lake breeze will likely keep temps in
the mid-upper 80`s along the lake north of Chicago, especially
along the lakeshore in Lake County IL.



244 AM CDT

Through tonight...

A few festering showers will continue early this morning near the
I-39 corridor ahead of a weakening cold front as it encroaches on
an expansive ridge centered along the Appalachians. Overnight
satellite imagery shows convection back-building across Iowa with
diminishing chances for thunders across the local CWA. Hi-res
models are in good agreement on precipitation ending around or
shortly after daybreak today.

Meanwhile, an upper low is digging across the Pacific Northwest
this morning and will approach the Intermountain West this
evening. In response, southwest flow will increase across the
Great Plains and Mississippi Valley today lifting a warm front
into the far Upper Midwest which should be the focus for
convection today. This leaves the local area well entrenched in
the warm sector and capped by around +20C 850mb temps. Assuming
the broad cirrus shield this morning is able to clear out, this
should leave the area under partly cloudy to mostly sunny skies
this afternoon which will contribute to another day of
temperatures well into the 80s and low 90s. Record high
temperatures today are 92F for Chicago and 94F for Rockford, the
former having the best chance of being tied or broken today.
Dewpoints in the mid to upper 60s today will result in heat
indices in the upper 90s to around 100F this afternoon.




Friday night through Thursday...

218 pm...The models continue to show slight thermal cooling this
weekend but this will likely have little if any noticeable
affects as overnight lows remain in the 60s...low 70s
downtown...and highs in the upper 80s/near 90 Saturday/Sunday.
Bumped temps up a few degrees to near current guidance values.
Lake breezes are possible both days which will keep the immediate
lakeshore areas cooler but confidence is low regarding how strong
these lake breezes may be and how far inland they will move.
Forecast highs are cooler near the lake but trends will need to be
monitored as these time periods approach. Highs may still reach
the upper 80s before temps cool and winds shift off the lake.

There is still some potential that the models are cooling the
airmass too fast and if that were to occur...highs could still be
a few degrees warmer than currently expected this weekend.
However by early next week it does appear that the entire airmass
will cool enough for highs to remain in the 80s. Additional cloud
cover...perhaps a more broken afternoon cu field...may also help
put a lid on high temps...keeping them in the 80s.

A cold front will move across the area Tuesday night into
Wednesday next week. While an isolated shower is possible any
afternoon from Sunday onward next week...this front will be the
next best chance of rain for the entire area. However...not
confident on how widespread precip will become and its possible
just scattered precip in a narrow time window moves across the
area ahead of the front. Though a slower front...that arrives
later into Wednesday could allow for more daytime heating and
thus better precip coverage. This front will allow much cooler air
to spread across the area with highs back into the 70s. A second
push of colder air is possible late next week. cms


For the 18Z TAFs...

Weak surface pressure gradient across the area this afternoon will
allow a lake breeze to move close to ORD/MDW and go through GYY,
with current variable winds expected to become east-southeast less
than 10 kt. Direction becomes a bit uncertain with little push to
the lake breeze, though southeast appears to be most likely for
ORD/MDW. Winds will become more southerly again this evening with
loss of lake breeze circulation. Slightly stronger low level flow
Friday should keep lake breeze east of the Chicago metro

Otherwise, VFR conditions will prevail with gradually decreasing
high cloud cover associated with dissipating thunderstorms over
southeast IA/northeast MO. Some patchy shallow MVFR fog is
possible toward sunrise primarily for DPA in the Fox River Valley.




218 pm...A weak cold front has stalled over south/central
portions of Lake Michigan this afternoon. This front will lift
back north as a warm front tonight...shifting light winds back
southerly. A large trough of low pressure will remain nearly
stationary over the plains through early next week maintaining
southerly winds over the lake. This trough/front will move across
the Great Lakes region Tuesday night into Wednesday shifting winds
northwest with a period of 15-25 kts expected. cms





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