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000
FXUS63 KLOT 211946
AFDLOT

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

.SHORT TERM...
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.

Ratzer

&&

.LONG TERM...
246 PM CDT

Sunday night through Saturday...

While conditions will continue to be mild across inland areas
early in the week, an upper low is expected to track eastward from
the Ozarks Sunday night towards the Ohio Valley by Tuesday night.
While it appears that much of the precipitation with this system
will remain south and east of the area, I cant rule out a few
light rain showers from time to time over portions of the area
late Monday into Tuesday. For this reason, I am maintaining some
slight chances for precipitation during this time. Otherwise,
expect temperatures to continue to be fairly warm for inland
areas. High temperatures look to be in the lower to mid 60s,
especially on Monday. Only slightly cooler temperatures are
expected on Tuesday due to more cloud cover and some possible
showers as a cold front moves across the area. For areas closer
to the Lake Michigan shores, expect much cooler conditions (40s to
near 50 degrees) both days due to continued onshore flow.

A ridge of surface high pressure is forecast to build
southeastward from the Upper Midwest Tuesday night into the Mid-
Mississippi Valley and lower Great Lakes Region for midweek. This
should result in a dry day or two Wednesday into Thursday before
our next weather maker takes aim on the area by late Thursday or
Thursday night. While forecast guidance is still not in agreement
on the exact timing of this system, there is a strong signal for
a decent mid-level disturbance to dig southeastward over the
western Great Lakes later in the week. The main impacts of this
system would be increasing chances for a period of showers in
association with the approach of a surface cold front late
Thursday. It also appears that a period of cooler weather will set
up over the area for Friday, before temperatures quickly moderate
next weekend.

KJB

&&

.AVIATION...
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.

Ratzer

&&

.MARINE...
246 PM CDT

Continued quiet weather over Lake Michigan through late Monday
with high pressure prevailing. Overall, this will result in mainly
light and variable winds. An area of low pressure is expected to
track over the southeastern CONUS on into Tuesday as another area
of high pressure begins to build southeastward into the Upper
Midwest. This should produce more of a north-northeasterly wind
component over the lake at times through midweek. The strongest
period of northerly winds currently looks to be late Tuesday night
through and Wednesday following a cold frontal passage. Winds up
around 30 kt look probable during this period, and this may result
in small craft advisory conditions for the southern Lake Michigan
near shore waters.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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