Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190539

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1139 PM CST Wed Jan 18 2017

949 PM CST

Cloud cover has been exiting to the northeast early this evening
and have updated sky grids to reflect this trend. Some high cloud
cover will continue across the far southern part of the forecast
area and may work further north later tonight. Otherwise the
exiting of the low clouds is supportive of a higher probability of
fog development across the area. Have seen visibility start to
lower across areas that have cleared out but some modest southerly
wind persists and may be helping to limit the rate of cooling and
therefore fog development. A look out the office door also reveals
that dew/frost has developed which can be a sign that visibility
may not completely drop off. Will continue to mention patchy/areas
of fog through the overnight but visibility may settle in the 1 to
2 mile range overall. Patchy dense fog may occur in lower lying
areas but widespread development may not be likely. Recent high
res guidance has been unhelpful as it shows a spread of
possibilities and run to run consistency has been low. Will need
to keep an eye on trends over the next few hours but at this time
there is little to support major changes in the above thinking.



226 PM CST

Through Thursday...

The main weather concerns center around the threat of dense fog

A surface ridge of high pressure currently resides across the Mid
Mississippi Valley to the lower Great Lakes. With low-level
moisture remaining trapped under a stable inversion aloft, cloud
cover has been slow to dissipate across the area. However,
improvement is beginning, with skies even scattering out into the
western suburbs of Chicago at this time. This trend will likely
continue for at least a few hours into the early evening. The
main concern, however, is what happens thereafter.

Low-level moisture remains quite high across the region, and as
skies scatter out for a brief period this evening, it will not
take much of a fall in temperatures to result in a quick
redevelopment of low clouds and fog. As a result, I think
conditions will deteriorate again by mid to late evening, with
dense fog and/or very low CIGs redeveloping over the area overnight.
This fog then looks to linger into at least early Thursday
morning, before improving.

Temperatures could end up quite mild across the area on Thursday,
with highs possibly reaching the middle 40s. At this time I have
not strayed from this thinking. However, if tonight`s low clouds
and fog hangs on longer than expected tomorrow, high temperatures
could be held down into the 30s. Currently I do not have enough
confidence to lower temperatures much. The warmer airmass moving
over the area will certainly support temperatures into the 40s on
Thursday. Rain showers are expected to overspread much of the
region from the south during the afternoon tomorrow, though the
best rain potential will come Thursday night.



226 PM CST

Thursday night through Wednesday...

The theme for the extended is active, warmer, and cloudy. The
first in a series of upper level lows shifts northeast into the
Midwest Thursday. Soundings continue to feature minor amounts of
elevated CAPE and a decent low-level jet. As such, kept a chance
of thunderstorms across the warning area. A bit concerned about
how much rain could fall Thursday night leading to another rise in
local waterways. Thinking the majority of rain will run straight
into the rivers and creeks. The ground has certainly thawed since
the heavy rain event Monday night, but a lot of locations still
have standing water. Forecast rainfall totals are 0.5-0.75 inches,
but could see much higher amounts from thunderstorms.

The warm up continues Friday with highs in 40s. Some spots in the
far southern forecast area may reach 50.  Lingering rain lifts north
in the morning as the weakening upper level low and its very weak
surface low shift north through the region.  Another round of light
rain is possible Friday evening/night as a weakening vorticity
streamer moves through overhead.

Saturday will be similar to Friday but warmer! Highs are forecast to
be in the mid to upper 50s. Local climatology suggests a bit cooler
high temps, and a lake breeze may form; however, did not have
enough confidence to lower temps. Another chance of rain arrives
Saturday afternoon and night. Chances for rain continue Sunday,
and Sunday looks to be the beginning of the end of the spring-
like warmth for a little bit. An elongated upper level trough
passes overhead Sunday night into Monday and we remain in its cold
pool through mid next week. Temps will be in the low to mid 40s
early next week, with lows in the 30s. Periods of precip continue
and look to be mainly rain.



For the 06Z TAFs...

Main forecast concerns center around vsby trends through
daybreak, timing of arrival of rain late this afternoon/early this
evening and timing of arrival of IFR/LIFR later this evening into
Friday morning. Clearing skies have led to some visibility
reduction across the area. Cirrus clouds to the south have started
to make a more northward push and look to move across the
terminals helping to limit vsby reduction or allow for improvement
into the wee hours of the morning. With that in mind have made
some adjustments to the vsby and sky cover through the overnight
with a lowering chance for major vsby reduction.

Winds will turn southerly then southeasterly into the afternoon
with clouds thickening as low pressure approaches from the
southwest. Rain will move into the area late in the afternoon or
very early in the evening and eventually bring IFR cigs and vsby
with LIFR possible. Timing of this may need to be tweaked but have
good confidence in the trend and that IFR may not occur out the
gate with the rainfall. Thunder will also be possible. Low
conditions look to continue into Friday morning.



226 PM CST

Relatively quiet conditions continue on the lake.  Southwest winds
weaken and become south to southeast through Thursday as high
pressure continues east.  A weak surface low travels up the
Mississippi Thursday before dissipating over the lake Friday
morning. East winds increase to 10-20 kt over the southern end of
the lake ahead of the low. Southeast winds 10-20 kt continue through
Saturday night. Areas of fog may develop over the far southern end
of the lake and nearshore waters Saturday. Winds become northeast
and may increase to 15-25 kt over the northern end of the lake as
a strong low shifts east across the gulf coast Sunday night into
Monday. The low continues up the east coast early next week and
winds become north. Guidance differs on the pattern early next






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