Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211130

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

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 12Z TAFs...

Convection continues to dissipate as it encounters more stable
air over IL. No additional rain is expected at any of the TAF
sites. Looking at southerly winds and VFR conditions through
Friday morning. Cloud cover may linger longer than currently
forecast, but it will be VFR.



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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