Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150539

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1139 PM CST Sat Jan 14 2017

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

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.



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.


For the 06Z TAFs...

Two main forecast concerns this period...fog through Sunday
morning and then the potential for mixed wintry precip for the 30
hour ord taf...early Monday morning.

High pressure will settle across the region tonight and slowly
move east Sunday afternoon. This will result in mainly calm winds
overnight into Sunday morning that will eventually become light
southeasterly Sunday afternoon with speeds around 5kt.

Patchy fog has developed over much of the area and latest guidance
suggests at least mvfr vis with fog will continue across much of
the area through Sunday morning. However...confidence is low
regarding how low visibilities may become and its possible that
some brief/patchy ifr vis may develop. Also low confidence
regarding how much...if any fog may develop at ord/mdw and trends
will need to be monitored.

Precipitation is expected to spread north across the region late
Sunday night into early Monday morning but forecast soundings show
a large dry layer that will need to saturate before precip can
reach the surface. How fast this occurs is uncertain but will
likely occur before sunrise Monday morning. Precip type also is
somewhat uncertain as it may begin as light snow/sleet but quickly
changed over to light freezing rain. Despite this uncertainty...
prob mention for the end of the 30 hour ord taf appears
reasonable but changes can be expected with later forecasts. cms


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 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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