Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211912

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
212 PM CDT Sat Apr 21 2018

211 PM CDT

Tonight through Sunday...

A narrow axis of light rain/sprinkles persists this afternoon, in
association with a weak mid-level short wave traversing an
elevated baroclinic zone which stretches from northeast IA through
northwest IN. While light low level easterly flow continues to
supply relatively dry air near the surface, forcing aloft induced
by frontogenesis in the 925-850 mb layer and co-located beneath
the divergent right entrance region of an upper jet streak over
the northern Lakes, was persistent enough to produce precip at the
surface. High-res guidance continues to indicate an overall
weakening trend and diminishing coverage along this axis through
late afternoon, though have adjusted to carry light rain and
sprinkles at least in isolated to scattered fashion through late
afternoon for the Rockford-Chicago area. Skies are expected to
remain mostly cloudy into tonight, especially across the southern
parts of the forecast area, while far northeast IL may see some
decrease in cloud cover overnight. Coolest overnight lows, in the
mid 30`s, should be across the far north/northeast parts of the
cwa where less cloud cover is likely, with temps in the upper 30`s
to around 40 in the south.

On Sunday, the closed upper low drifting slowly east across the
Plains is progged to track into the lower/middle Mississippi
Valley, with surface low pressure nearing Memphis TN by evening.
The surface pressure gradient tightens a bit during the day
between this low and the lingering high pressure ridge over the
Great Lakes, increasing easterly winds a bit across the local
area. While keeping dry low level air in place across the area, it
will also result in another day of cooling off the Lakes, with
temps in the 40`s once again along the Lake Michigan shore. A
little more sunshine filtering through high level cloud cover
combined with weak warm advection aloft is expected to allow temps
in the upper 50`s to lower 60`s farther inland however.
Precipitation is expected to remain well south of the cwa, mainly
south of the I-72/I-70 corridors across downstate IL/IN.



224 AM CDT

Sunday through Saturday...

Quiet weather is expected to continue Sunday into Monday as
mid/upper closed low moves from the central Great Plains to the
Tennessee Valley. Expect the local area to fall on the northern
periphery of the clouds from this system, but dry weather will
prevail with temperatures expected to be near the seasonal norms.

Upper low will meander north towards the Ohio Valley Tuesday and
some models show areas east of the I-57 corridor getting some
light rain on Tuesday. Meanwhile, another disturbance is progged
to dig from the Northern Plains Tuesday across the Mid Mississippi
Valley late Wednesday. More widespread showers appear possible
somewhere in the region, but there are large discrepancies in
location, so for now will maintain slight chance/low chance PoPs
Tuesday night into Wednesday.

Yet another wave takes aim on the area late in the week with a
more organized cold front expected to sweep across the region
Thursday night bringing another chance for showers. Looks like
only a glancing blow of cooler temps will occur Friday then
temperatures moderate again by Saturday. Broad upper troughing
sets up behind the front over the Great Lakes next weekend so the
cold air won`t be too far away next weekend.



For the 18Z TAFs...

Surface high pressure centered just to our east across the Great
Lakes maintains control over local weather during the period, with
dry easterly flow in low levels. Mid/high cloud cover had spread
across the region however, with a few sprinkles reaching the
surface from northern IA into far northern IL in association with
an elevated warm/stationary frontal zone, though forcing will
weaken along this feature over the next several hours and any
precip is expected to be light with no vis restrictions, and
mainly west/northwest of the Chicago terminals. Ceilings and
visibility are expected to remain VFR.

With the surface high to our east, and an area of low pressure
moving into the lower Missouri Valley on Sunday, surface winds
will remain generally east or east-northeast. Speed should
generally be less than 10 kts into tonight, with a gradual
increase into the 10-15 kt range Sunday. Precip with the southern
low should remain well south of the terminals.



101 AM CDT

Continued quiet weather over Lake Michigan through Monday night
with high pressure prevailing. This will present a variable wind
direction at times but with minimal speeds. A cold front will move
southward across the lake during or around Tuesday night bringing
northerly winds in its wake. This may present Small Craft
Advisory criteria conditions in the Illinois and Indiana
nearshores into Wednesday.






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