Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211729

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1229 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.



244 AM CDT

Friday through Wednesday...

Large blocking high will be in place over the eastern half of the
U.S. Friday through the middle of next week resulting in a stretch
of above normal temperatures and primarily dry conditions. Friday
and Saturday in particular look very warm with record or near
record highs possible. Record highs for September 22nd are 92F for
Chicago and 93F for Rockford. Record highs for September 23rd are
91F for Chicago and 92F for Rockford. In addition, dewpoints in
the mid to upper 60s these days will make for continued muggy

Upper ridge axis shifts slightly farther east Monday and Tuesday.
Temperatures remain well above normal this time frame, but should
be tempered some as the air mass modifies and cloud cover
increases from upstream convection. GFS indicates the potential
for showers and thunderstorms Monday afternoon due in part to
steep low level lapse rates. Not ready to bite off on this though
as forcing is minimal and forecast soundings show overall weak
instability due to poor mid and upper level lapse rates and some
capping from the 700-800mb warm nose. Meanwhile, ECMWF maintains
a dry forecast on Monday.

GFS and ECMWF remain in reasonable agreement that the strong upper
ridge will begin to breakdown late Tuesday or Wednesday allowing
a cold front to sweep across the local area. This looks to be the
next decent shot for showers and thunderstorms and temperatures
will moderate behind the front closer to seasonal norms.



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.



244 AM CDT

A cold front will move over central Lake Michigan today and then
lift north as a warm front tonight. Wind direction will vary on
either side of the cold front, but southerly winds follow the warm
front. A blocky pattern then sets up and remains in place through
early next week. A high pressure ridge will sit over the eastern
U.S. while a low pressure trough extending from the northern
Mississippi Valley to the southern Plains remains stationary as it
weakens. This pattern will produce southerly winds through
Tuesday. While guidance differs on how the pattern breaks, the
models agree that winds will shift to northwest Wednesday.






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