Area Forecast Discussion
Issued by NWS Chicago, IL

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807
FXUS63 KLOT 142107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 PM CST Sat Jan 14 2017

.SHORT TERM...
200 PM CST

Through Sunday...

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
quiet.

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.

MTF

&&

.LONG TERM...
306 PM CST

Sunday night through Saturday...

Main concern for the long term forecast period remains the timing,
coverage and duration of a wintry mix of precipitation shaping
up for Sunday night and into early next week.

Following a brief period of quiet weather, conditions will become
unsettled again, with a period of wintry pcpn setting up.  The
longer term model guidance, in general, is still pointing to a
period of wintry precipitation changing over to rain.

The main feature of concern will be the timing and path of the
deep closed low digging into nwrn Mexico and when it will begin
to lift newd into the plains. There is still some uncertainty as
to the details of the timing and track of the system, which in
turn brings some uncertainty to the timing of p-type transitions.
As is fairly typical, the models are trending slower in lifting
the system out of the southwest, and consequently, the timing of
pcpn spreading into the CWA has also been delayed a bit. Also,
while there is relatively high confidence in the general p-type
trend. Given the lingering uncertainty as to the track of the
upper low and the associated sfc features, details of the p-type
forecast will likely still need refinement before onset.

The latest indicators suggest that the onset of pcpn may be as
much as 12 hours later than previously anticipated, possible not
reaching the Pontiac area until after midnight Sunday night and
then spreading into the Rockford and Chicago areas until after
midnight or into the early morning hours. As has been discussed in
previous forecasts, still feel that pcpn will start out as a
short period of light snow while entire column remains below
freezing, but screaming warm advection at in the midlevels, with
temps of up to +5 C in the warm layer overspreading the cold air
at the sfc, still suggesting a transition from light snow on the
leading edge of the pcpn shield, transitioning to a mix of
snow/sleet to freezing rain/sleet before changing over to more
widespread freezing rain. The timing of the transition zone
lifting through the area is the most difficult to pin down at this
time, but feel relatively confident in the general trend. Also,
still feel relatively confident that the transition to all rain
should be relatively quick, with some freezing rain possibly
lingering into the morning rush hours Monday morning and then
quickly changing over to all rain by late morning for much of the
CWA, though the freezing pcpn could linger a little longer over
the Rockford area as that area will be closer to the cold core of
the upper low. As the system lifts away from the area, pcpn should
end from west to east overnight Tuesday night.

Will hold off on any type of winter weather headline with this
system for now, but current indicators would suggest that the
window of necessity for a Winter Weather Advisory for late
Sunday evening into Monday morning.

Have made little change to the going forecast for temperatures
through the extended forecast period, with temps climbing into the
upper 30s to low 40s early in the day on Monday.  Temperatures
Tuesday through Friday will be well above normal, with highs in
the upper 40s to middle 50s through the period.

&&

.AVIATION...
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.

MTF

&&

.MARINE...
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.

MTF

&&

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

$$

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