Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190553

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1253 AM CDT Tue Sep 19 2017

920 PM CDT

Didn`t make any big changes to the going forecast, as the the idea
of showers and thunderstorms moving across the area still appears
to be on track. Did increase pops given overall trends, and made
the bigger changes across the southern CWA along/south of I80.
This is where surface trough and boundary, along with approaching
mid level wave will help to focus the greater ascent and more
widespread precip tonight. Precip chances will lower the further
north, but with locations in the I80 to I88 corridor still having
a chance at seeing precip. Instability axis will stay mainly over
the southern CWA, and have continued the higher chances of thunder
here. With higher PWATs shifting overhead, brief heavy downpours
are likely, and have increased QPF overnight as well.



228 PM CDT

Through Tuesday...

The main weather story through the period will be with the shower
and thunderstorm chances over the area tonight into early Tuesday.

Early this afternoon, a decent mid-level disturbance was noted in
the water vapor imagery shifting eastward over western Missouri.
This feature has been driving an area of showers, with some
thunderstorms, since last night. Therefore, it appears likely now
that a period of some much needed rain will occur over most of
the area tonight. Currently it appears the best timing will be
from later this evening through early Tuesday morning. During this
period, a corridor of better moisture (featuring Precipitable
water values up around 1.6") is expected to shift over the region.
While lapse rates will be marginal for charge separation, it does
appear that a few thunderstorms will be possible within the area
of showers overnight, especially over my southern CWA. Given the
likely presence of decent moisture over the area, it appears that
at least some areas could see a quarter to possibly a half inch
of rain with this activity tonight, especially in areas of any
better convective activity in my south tonight.

Some of these showers may linger over eastern sections of the area
early Tuesday. It also appears that a period of low clouds and
possibly some fog will be possible, especially during the early
morning on Tuesday. Clouds may linger across the area into the
afternoon, and as a result, this could result in slightly cooler
high temperatures than previously forecast over northern Illinois.
As a result, temperatures may remain in the 70s for daytime highs
on Tuesday.



207 PM CDT

Tuesday night through Monday...

As the active weather continues out west thanks to a
strong upper level jet stream and deep upper trough, the midwest
will remain on the warm and drier side of things as the strong
upper jet will dig the backside of the upstream trough and
amplify the upper flow in our area, though there will be period of
showers and storms at least close by if not in some areas for a
few days.

Tuesday night, weak upper ridging will develop, while a strong low
level jet will stir up convection to our west. This ridge appears
like it will amplify some as this convection nears, and with the
main upper forcing expected to shift northward with the surface
low, the convection will likely be in a decaying state as it gets
close to the area. Have to hold onto some lower precip chances
across north central IL Wednesday morning. PWATs and dewpoints
soar to summer like levels with unimpeded south-southwest flow.
Any debris cloud cover may cap temps a bit, but otherwise
Wednesday will be warm and humid.

Storms may again initiate to our northwest ahead of a cold front
Wednesday afternoon. With the high PWAT air in place and the low
level jet expected to flare back up to our west Wednesday night,
there is a bit better chance that some of these may get into the
area late Wednesday, but model guidance is still trying to hold
onto the ridge. We will continue the highest storm chances during
this period of time (Wed night - Thurs morning) for this
potential, but still only in the chance range, and much lower the
farther east you may be.

If we get a bit more convection or the front sags south like the
NAM shows, Thurs could start with some stratus northeast, but will
continue with the global models idea that this remains
north. After this point the moisture convergence axis will shift
even farther north and west as the deep western upper low will dig
into the four corners region through the weekend, and we will
remain well under the strong ridge. No precipitation is expected
with continued well above normal temperatures through the 80s.
The most uncomfortable air will be Thursday-Friday, with some
decrease in dewpoints through the weekend, but still very warm.
The pattern will lend itself to some cooling right near the IL
lakeshore each day. KMD


For the 06Z TAFs...

Rain showers will continue to shift northeast and impact the
terminals early this morning. I have high confidence in measurable
rainfall at all of the eastern terminals, but medium confidence in
measurable precip at RFD. Lightning will stay south of the
terminals as should lower cigs. MVFR cigs are being reported at
PNT and Morris, IL, but thinking the better saturation and lower
cigs will remain south of the terminals. Showers push east this
morning and low end VFR cigs will hang around through the day.
Winds will be northeast to east winds at less than 10 kt.

Clouds scatter out this evening and winds become southeast as the
high pushes east. Guidance suggests MVFR or lower cigs and vsby
could occur tonight, but thinking the models are too saturated.
Kept conditions VFR tonight.



207 PM CDT

Once high pressure north of Lake Michigan shifts east to
Quebec, its associated ridge axis will remain in place across the
eastern third of the country through the remainder of the week. A
A series of low pressure systems will form across the plains, but
with the high in place, most of these will track north and west of
the lake, resulting in an extended period of southerly winds. The
pressure gradient will tighten at times when these lows pass by to
the north, with occasional winds up to 25 kt, generally less. KMD





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