Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 231629
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1129 AM CDT FRI SEP 23 2016
.UPDATE...906 AM CDT
Band of deeper though weakening moisture convergence is focused to
our west along with some elevated showers to our north. In between
across northeast IL/northwest IN weak shortwave energy across the
now stalled out frontal boundary will continue the chance for very
light showers or even some drizzle as the moisture layer is
shallow. Gradual dissipation is expected through the day.
Otherwise expect largely cloudy conditions with lower clouds along
and north of the front, with higher clouds drifting through the
southern half. Expect temps to only climb a little bit north of
the front today as well.
315 AM CDT
Early this morning a frontal boundary was oriented over Northern
Illinois stretching east through Northern Indiana. Along and north
of the boundary surface flow has become northeast/east, and allowed
cooler air to drift south/southwest. With shallow moisture lingering
over Lake Michigan and far Northeast Illinois, the cooler air and
light winds has allowed some areas of fog to develop and reduce
visibilities to between 1-3sm in spots. IR imagery does indicate
better vertical depth to clouds further west across Northwest
Illinois where some scattered showers to persist. With the lack of
forcing and mid-lvl support, expect only some weak warm-air
advection to allow scattered showers to persist through early this
afternoon before lifting north into Wisconsin. There is some weak
instability through this morning; however, not confident that there
will be enough vertical component to develop thunderstorms. In
addition mid-lvl heights begin to rise throughout the day and
provide enough diffluent flow within the mid-levels to prevent the
vertical lift needed. So have opted to leave precip as just showers.
The continued onshore component will create a weak lake shadow
effect; however, with water temps remaining very mild, temps
adjacent to Lake MI will still be in the mid 70s. Elsewhere temps
should be able to warm to around low/mid 80s.
Mid-lvl ridging continues to amplify as the upstream trough/cutoff
low further develops. With heights rising overhead, expect the weak
frontal boundary to remain over the region but lift north by early
Saturday. This setups will likely maintain a dry overnight period
with temps remaining mild in the lower 60s.
315 AM CDT
Saturday through Thursday...
Ridging will remain overhead through
daybreak Sunday, keeping the bulk of the weekend dry but mild. With
a frontal boundary positioned to the north, flow will be southerly
with shallow moisture pushing dewpoints back into the lower/middle
60s. Guidance continues to strengthen a surface low over the
Central/Northern Plains early Sunday, with a frontal boundary
becoming active and stretching south through the Southern Plains Sun
morning. The better vorticity will continue to be lifting
north/northeast through the upper midwest, so confidence in
convection for Sun is challenging as is the coverage for precip.
Have maintained chance POPs with the frontal boundary passage. The
timing looks to be out of phase, so could see the precip being
delayed until Sun eve or possibly early Mon.
Temps through the weekend will continue to be mild with afternoon
highs back into the lower to middle 80s, and overnight lows in the
Monday through Thursday: Early next week the 500mb trough/shortwave
will be lifting northeast to the Upper Midwest, with operational
solutions showing a robust vorticity lobe lagging and dropping
southeast across Iowa/Southwest Wisc midday Mon. This appears to be
translating into the transition of the developed low north of
Minnesota, to a secondary developing low near Green Bay. With the
downstream mid-lvl ridging over Western Quebec, it is likely that a
cutoff low will stall over the Great Lakes region Mon eve/Tue and
possibly linger into late Tue/early Wed.
This scenario will keep considerable cloud cover over the forecast
area, with considerably cooler air aloft through the middle of next
week and periodic chances for showers. Temps will struggle to warm
due to the solar shielding, with guidance keeping temps in the upper
60s to around 70. Depending on placement of the surface low and
cloud cover, it is possible highs could be several degrees cooler
The shortwave will eventually lift northeast away from the Great
Lakes, with surface ridging arriving and bringing seasonal temps
back and dry conditions to the forecast area.
For the 18Z TAFs...
Forecast concern is around MVFR ceilings remaining, lowering to
IFR in spots tonight, and possible visibility reductions
Drizzle briefly lowering cigs will no longer be a concern, but a
low level inversion with a continued cool and moist low level wind
flow off the lake will allow stratus to linger through Saturday
morning. Expect some lifting today and some possible scattering of
the lowest cloud layer, but confidence on how long any scattering
is low confidence and therefore have held onto the idea of a
slowly lifting MVFR cloud deck, though it may hold pretty constant
here through the day. As low level flow shifts to more
east/southeast, the lingering frontal boundary will lift
northward on Saturday, which should allow the low cloud to erode.
This gradient looks to linger across the interstate 80 corridor.
The lowest cigs tonight, likely IFR, look to be focused across
north central Illinois tonight which will be the last to lift on
Saturday. Forecast soundings suggest that all areas will lift back
to VFR through the day Saturday. East winds remain in the 8-11 kt
range through the day.
216 am...High pressure centered over Ontario will slowly move
southeast to the eastern Great Lakes by Sunday. Northeast winds
today will turn more easterly tonight and then turn southeasterly
Sunday as low pressure moves across Manitoba and into western
Ontario. A trailing cold front from this low is expected to move
across the western lakes Sunday night into Monday morning...but
the low pressure may move southeast to the northern part of Lake
Michigan on Monday...complicating the wind forecast. If this low
moves east of the lakes...then west/northwest winds will prevail
through the middle of next week.
As cooler air advects south over the lake this morning and runs
into higher dewpoint air over the southern portions of the lake
areas of fog...perhaps dense...will be possible this morning...
especially over southwest parts of Lake Michigan. cms
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