Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241805

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
105 PM CDT SAT SEP 24 2016

249 AM CDT

Through tonight...

Early this morning a shallow moist layer lingers with a light
northeast flow along the western edge of the surface ridge
positioned over Eastern Ontario. This has allowed patchy fog to
develop, but with the weak gradient in the mid-lvls, visibilities
are expected to only see minor reductions. The northeast to east
flow is progged to persist through much of today, despite the
surface ridge beginning to drift northeast away from the Great Lakes
region. Expect the shallow/moist stratus layer to linger and limit
surface heating across much of the forecast area. The exception will
be areas south of a Lee/Ogle to Kankakee line where temps should
return to the lower 80s, while further north temps will remain in
the 70s. Mid-lvl heights will continue to rise this afternoon and
further amplify into the Upper Midwest, which will inhibit any
precip from developing over Northern Illinois/Northwest Indiana. So
have continued to keep a dry forecast for today and tonight.

Focus overnight will be on the return flow moisture and the quasi-
stationary boundary that will begin to lift north. Guidance
continues to maintain the mid-lvl ridging overhead into daybreak
Sun, so expect a dry period tonight with temps remaining mild with
lows in the low/mid 60s.



249 AM CDT

Sunday through Friday...

Sunday through Monday: 500mb vort will be shifting east across
Montana into the Upper Midwest Sun ngt, with the trough axis
approaching the Central Plains midday Sun. Guidance continues to
hold heights over the forecast area Sun morning; however, quickly
fall through the afternoon with a weak convergence zone along the
warm/moist advection wind arriving. The concern is that the best
forcing will be focused well northwest of the forecast area Sun
aftn, and its possible enough diffluence in the mid-lvls could
linger that prevent increased coverage/development to the precip
shield Sun aftn/eve. Some of the hi-res solutions are beginning to
pick-up on this feature through early aftn, with the best chance for
precip arriving late aftn/eve Sun. The forward speed of the frontal
boundary does appear to accelerate in advance of the 500mb trough
axis arriving Mon. Instability Sun aftn/eve through early Mon is
expected to remain minimal, so have limited thunder chances despite
likely rain for a period Sun eve.

With frontal passage progged for early in the day Mon, temps will
struggle to warm throughout the day with continued northwest flow as
surface ridging builds into the Central/Southern Plains. Temps mon
will struggle to warm beyond the mid/upr 60s. One concern is that
with cooler air aloft and any clearing that can take place early in
the day mon could setup for some instability showers/thunder to
develop mon aftn. At this time have maintained a slight chance for
rain across far Northeast Il, then enough subsidence will arrive
from the surface ridge Mon ngt. Temps will then radiate down to the
40s to mid 50s closer to Lake Michigan.

Tuesday through Saturday: Ensembles begin to quiet the pattern mid-
week through the end of the week, with the 500mb trough axis slowly
weakening as the longwave pattern transitions to weak ridging over
the Upper Midwest/Great Lakes by weeks end. Temps will start
slightly below climo in the 60s for highs, then by the end of the
week moderate back into the mid/upr 70s. Dry conds are presently
expected for mid-week through weeks end from broad surface ridging
that becomes centered over the Great Lakes by Thur.



For the 18Z TAFs...

Forecast concern will initially be with the eventual scattering
of the MVFR cloud deck across the region. Concerns will shift to
increasing shower and storm chances, with a smaller chance in the
morning, and a better chance later in the day/evening.

We are beginning to see some erosion to the stratus deck across the
area this afternoon. The low level thermal inversion will remain
in place as surface heating is slow and we turn flow at 5000 ft
around to southerly through the afternoon. Forecast soundings do
suggest that we will eventually able scatter/lift the MVFR deck
with the last areas to scatter in north central IL. ORD/MDW should
see some lifting in the coming hours.

With high pressure sinking southeast into the eastern Great Lakes
and low pressure migrating east-northeast across the Upper
Midwest, winds will shift southeasterly overnight, probably
increasing to 10 kt or greater as a prefontal trough approaches.
There is an elevated warm front that will lift thorugh late
tonight into early Sunday that could trigger a shower or storm
across north central (move favored) and northeast IL. After a
quiet period through midday, chances will increase a strongly
forced line of showers and storms approaches, though this detail
will need to wait for the next few sets of TAFs.




215 am...Large high pressure over eastern Ontario will move east
to Quebec and New England Sunday into Sunday night as low pressure
over the northern plains lifts northeast into Manitoba tonight.
This low will then slowly move across Ontario Monday into Tuesday.
A trailing cold front will move across the western lakes Sunday
night. Easterly winds today will turn southeast tonight and more
southerly ahead of the front by Sunday afternoon. Speeds of 15-25
kts will diminish some later today but then return tonight into
Sunday with perhaps a few higher gusts. Winds will shift westerly
behind the front Sunday night and steadily increase to 30 kts by
midday Monday and with much cooler air spreading across the
region...its possible a period of gales may develop Monday
afternoon and persist into Monday night. Winds will remain strong
into Tuesday before slowly diminishing Tuesday night into they slowly turn more northerly. cms





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