Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171737

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 PM CDT Sun Sep 17 2017

249 AM CDT

Through tonight...

Post frontal convection continues over central Iowa under the
influence of a negatively tilted trough across South Dakota and
Minnesota.  The trough continues northeast into Canada this morning
while the cold front moves through the forecast area this afternoon.

Expecting a few showers and maybe an isolated thunderstorm mainly
around the I-39 corridor early this morning.  Given the recent
drought, showers may struggle to reach the surface especially east
of I-39, but kept a slight chance of showers and thunderstorms going.

I have medium-low confidence in thunderstorm coverage this
afternoon, but medium-high confidence in timing and general
location.  With dewpoints already in the low 60s, we should have
enough surface moisture to support convection along the front.  Hi-
res CAMS have a very minor cap that erodes early this afternoon
along and east of I-55.  Scattered showers and storms will form and
slowly push east into this evening. Shear is still minimal so severe
storms are not expected. However, PWATs are still forecast to be 1.6-
1.75 inches and steering flow will be 20 kt or less, so slow moving
storms capable of heavy downpours are possible.

Areas west of I-55 will be dry after any isolated morning rain
showers. Rain ends across the forecast area as the cold front pushes
southeast tonight. Cooler air will result in low temps in the low
50s near RFD to low 60s along and east of I-55.



249 AM CDT

Monday through Saturday...

Highs Monday will be cooler in the mid 70s to around 80. An upper
level wave arrives from the southwest in the afternoon kicking off
showers and storms Monday afternoon and evening.  CAPE and shear are
forecast to be marginal again, so severe storms are not expected.
PWATs remain around 1.5-1.75 inches, but steering flow will be
stronger.  Overall, Monday looks like the best chance we`ll get for
widespread measurable rainfall in the next 7 days. However, it will
likely not be enough to make a dent in the ongoing drought.

Another, weaker wave may force showers and storms late Tuesday
afternoon/evening, but have low confidence in precip coverage.
Temperatures also begin to climb back into the low 80s Tuesday.

The warming trend continues Wednesday with another slight chance to
chance of showers and storms. Thursday and Friday look drier with
highs in the mid 80s thanks to high pressure. Models differ on how
quickly a cold front will advance late this week, so I have low
confidence in precip chances Saturday.



For the 18Z TAFs...

A cold front is moving through nern IL at 1730Z and should push
through the Chicago area terminals by 18Z, turning winds to nwly.
While the front is moving quickly, there is not much of a
temperature drop-off behind the front, so do not expect much more
than the wind shift to nwly, with wind speeds arnd 10-12 kt.
Latest radar and satellite imagery shows no pcpn along the
boundary and just some flat cu development as the air mass is not
very unstable at the time of fropa. There is still a chance that
there may be some shra/tsra development along the front this
afternoon, but will likely not be until the front is well east of
the terminals and will carry dry TAFs. There is some cooler air
with a secondary surge or secondary cold front moving through RFD
at 1730Z. Cigs are a bit lower in the true cooler air mass and
there still is some potential for mvfr cigs for a few hours
following the passage of the secondary boundary, but confidence is
low that the mvfr cigs will make it all the way to ORD/MDW/GYY,
so will only canny a tempo mvfr at RFD/DPA. As high pressure
builds across the upper Great Lakes tonight, winds will continue
to veer through nely overnight to sely by tomorrow morning.


249 AM CDT

Early this morning, low pressure is over Lake Superior with a
trailing cold front draped across portions of the Upper Midwest.
Moderate southerly flow of 15 to 20 kt will be in place today then
the cold front will begin to move across the lake this evening and
overnight with winds veering to the north and eventually northeast.
High pressure will build across the northern Great Lakes region
Monday and Tuesday resulting in a period of easterly flow across
Lake Michigan. A warm front will lift across Lake Michigan Wednesday
with winds veering back to southerly which is expected to last
through the end of the week.






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