Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 142002
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
202 PM CST Sat Jan 14 2017
200 PM CST
There is potential for fog tonight in the far southern CWA
(along/south of U.S. Hwy 24), which if it materializes could make
for spotty slick road conditions. Otherwise through the day
Sunday, the weather for northern Illinois and northwest Indiana is
The persistent Upper Mississippi Valley high pressure is
rebuilding across Iowa this afternoon and will expand over the
forecast area late tonight. Light northwest winds on the eastern
periphery of this feature are gradually nudging clouds southeast
this afternoon, however we are not expecting them to clear
southern areas tonight. This will prevent dew points in the upper
20s to 30 degrees across the south from falling much. Forecast
relative humidity values approach or reach 100 percent late this
evening into overnight, so have included fog mention along/south
of U.S. Highway 24. Conceptually, dense fog is possible, and
trends through the evening will have to be monitored. Further
north, lower to mid teens are expected in outlying areas as any
high clouds should be transient and thin, allowing for more
pronounced radiational cooling.
Broad lift will be on the increase across the middle of the
country Sunday in response to upper diffluence and increased flow
ahead of a closed upper low in the far southwest states. While
high and mid clouds will be thickening, expecting precipitation to
likely hold off entering the CWA during the day. Have forecast
highs a couple degrees warmer than today.
222 AM CST
Sunday night through Friday...
As the big upper low over the southwest begins to lift out into
the plains, guidance suggests a band of light precip will move
across our CWA later Sunday night into Monday morning. Forecast
soundings suggest precip could briefly start as some snow, before
transitioning over to mostly light freezing rain and/or sleet.
Amounts at this distance look to be light, though the threshold
for impact with wintry mix events (particularly freezing rain) is
quite low and would anticipate a winter wx advisory eventually
becoming necessary if current trends in model guidance holds.
It appears as though could be a break in the precip for a time
Monday morning, during which time temps will likely inch up above
freezing most locations, setting the stage for appears to be
primarily a rain event by the time the main precip shield arrives
with the system Monday afternoon and especially Monday evening.
Rain with this system should end overnight Monday or early Tuesday
morning, but a northern stream shortwave fast on its heels could
provide for at least a threat of some of isolated showers Tuesday
as lapse rates steepen.
The quick transition to zonal flow and an extended period of above
average temps still looks quite likely through the rest of the
week into next weekend. In the latest medium range models, there
are more differences in the smaller scale features later in the
week and over the weekend, which will impact how extreme the late
week warmth will be and dictate precip chances. At this distance,
the pattern evolution looks pretty complex and confidence is low
in the details, but confidence is high in above (probably much
above) average temperatures Thur through the weekend.
For the 18Z TAFs...
Winds are turning northwest and should maintain that direction
this afternoon and evening as a secondary surface high builds
across Iowa. The northwest flow is light through the low-levels,
thus not giving a good push to the 4,000 ft clouds. These clouds
should want to advect south of the area by late afternoon/early
evening, but cannot rule out few/scattered lower clouds this
evening. A patch of IFR clouds in Wisconsin on the back edge of
the moisture should also fade this afternoon, but may clip RFD
near 20Z/21Z. Confidence in this is low, but if the IFR occurs it
will be very short lasting.
Winds probably will be near calm at daybreak Sunday, eventually
turning light southeast later Sunday afternoon. Precipitation is
presently not expected through Sunday evening.
200 PM CST
Northwest winds across the lake tonight will diminish tomorrow
morning before turning southwest, as high pressure moves over and
then east of the lake. Southerly flow will then prevail over the
lake through Tuesday morning. Low pressure is expected to track
northeast over or near the central part of the lake during midday
Tuesday into Tuesday evening. Westerly winds south of this center
should be at least 30 kt if the forecast low track and strength
pans out...so this is presently most favored across the southern
part of the lake Tuesday night. General southerly flow continues
to prevail over the lake from Wednesday through next weekend.
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